diff --git a/.codeclimate.yml b/.codeclimate.yml
index 907c00b..077382c 100644
--- a/.codeclimate.yml
+++ b/.codeclimate.yml
@@ -15,3 +15,13 @@
   - "**.py"
 exclude_paths:
 - tests/**/*
+checks:
+  argument-count:
+    config:
+      threshold: 5
+  file-lines:
+    config:
+      threshold: 1000
+  method-complexity:
+    config:
+      threshold: 17
diff --git a/.gitignore b/.gitignore
index 1f5a640..e31443a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,6 +12,7 @@
 .coverage
 .coverage.*
 .cache/
+.mypy_cache/
 .pytest_cache/
 __pycache__/
 
diff --git a/.travis.yml b/.travis.yml
index 9b4da02..0fed68b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,30 +1,32 @@
 language: python
+dist: bionic   # required for Python >= 3.7
 cache: pip
 
 # Environment changes have to be manually synced with 'tox.ini'.
 # See: https://github.com/travis-ci/travis-ci/issues/3024
 
-# Python 3.7 is not yet supported by Travis CI.
-# See: https://github.com/travis-ci/travis-ci/issues/9815
-
 python:
-  - "2.7"
-  - "3.4"
-  - "3.5"
   - "3.6"
-  - "3.7-dev"
+  - "3.7"
+  - "3.8"
 
-# This is blocked by https://github.com/pypa/pipenv/issues/2449,
-# also see https://github.com/pypa/pipenv/projects/7
-#  - "pypy"
-  - "pypy3.5"
+matrix:
+  include:
+    - python: 3.5
+      dist: xenial  # Bionic has no Python 3.5
+      script: pip install zipp
+
+    # Disabled, see https://github.com/sybrenstuvel/python-rsa/issues/131
+    #- python: pypy3.5
+    #  dist: xenial  # Bionic has no Python 3.5
 
 install:
+  - pip install -U pip setuptools  # https://github.com/pypa/virtualenv/issues/1630
   - pip install pipenv
   - pipenv install --dev
 
 script:
-  - pipenv run py.test
+  - pipenv run py.test tests/
 
 after_success:
   - pipenv run coveralls
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..0fa3054
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,201 @@
+# Python-RSA changelog
+
+
+## Version 4.4 & 4.6 - released 2020-06-12
+
+Version 4.4 and 4.6 are almost a re-tagged release of version 4.2. It requires
+Python 3.5+. To avoid older Python installations from trying to upgrade to RSA
+4.4, this is now made explicit in the `python_requires` argument in `setup.py`.
+There was a mistake releasing 4.4 as "3.5+ only", which made it necessary to
+retag 4.4 as 4.6 as well.
+
+No functional changes compared to version 4.2.
+
+
+## Version 4.3 - released 2020-06-12
+
+Version 4.3 and 4.5 are almost a re-tagged release of version 4.0. It is the
+last to support Python 2.7. This is now made explicit in the `python_requires`
+argument in `setup.py`. Python 3.4 is not supported by this release. There was a
+mistake releasing 4.4 as "3.5+ only", which made it necessary to retag 4.3 as
+4.5 as well.
+
+Two security fixes have also been backported, so 4.3 = 4.0 + these two fixes.
+
+- Choose blinding factor relatively prime to N. Thanks Christian Heimes for pointing this out.
+- Reject cyphertexts (when decrypting) and signatures (when verifying) that have
+  been modified by prepending zero bytes. This resolves CVE-2020-13757. Thanks
+  Carnil for pointing this out.
+
+
+## Version 4.2 - released 2020-06-10
+
+- Rolled back the switch to Poetry, and reverted back to using Pipenv + setup.py
+  for dependency management. There apparently is an issue no-binary installs of
+  packages build with Poetry. This fixes
+  [#148](https://github.com/sybrenstuvel/python-rsa/issues/148)
+- Limited SHA3 support to those Python versions (3.6+) that support it natively.
+  The third-party library that adds support for this to Python 3.5 is a binary
+  package, and thus breaks the pure-Python nature of Python-RSA.
+  This should fix [#147](https://github.com/sybrenstuvel/python-rsa/issues/147).
+
+
+## Version 4.1 - released 2020-06-10
+
+- Added support for Python 3.8.
+- Dropped support for Python 2 and 3.4.
+- Added type annotations to the source code. This will make Python-RSA easier to use in
+  your IDE, and allows better type checking.
+- Added static type checking via [MyPy](http://mypy-lang.org/).
+- Fix [#129](https://github.com/sybrenstuvel/python-rsa/issues/129) Installing from source
+  gives UnicodeDecodeError.
+- Switched to using [Poetry](https://poetry.eustace.io/) for package
+  management.
+- Added support for SHA3 hashing: SHA3-256, SHA3-384, SHA3-512. This
+  is natively supported by Python 3.6+ and supported via a third-party
+  library on Python 3.5.
+- Choose blinding factor relatively prime to N. Thanks Christian Heimes for pointing this out.
+- Reject cyphertexts (when decrypting) and signatures (when verifying) that have
+  been modified by prepending zero bytes. This resolves CVE-2020-13757. Thanks
+  Adelapie for pointing this out.
+
+
+## Version 4.0 - released 2018-09-16
+
+- Removed deprecated modules:
+    - rsa.varblock
+    - rsa.bigfile
+    - rsa._version133
+    - rsa._version200
+- Removed CLI commands that use the VARBLOCK/bigfile format.
+- Ensured that PublicKey.save_pkcs1() and PrivateKey.save_pkcs1() always return bytes.
+- Dropped support for Python 2.6 and 3.3.
+- Dropped support for Psyco.
+- Miller-Rabin iterations determined by bitsize of key.
+  [#58](https://github.com/sybrenstuvel/python-rsa/pull/58)
+- Added function `rsa.find_signature_hash()` to return the name of the hashing
+  algorithm used to sign a message. `rsa.verify()` now also returns that name,
+  instead of always returning `True`.
+  [#78](https://github.com/sybrenstuvel/python-rsa/issues/13)
+- Add support for SHA-224 for PKCS1 signatures.
+  [#104](https://github.com/sybrenstuvel/python-rsa/pull/104)
+- Transitioned from `requirements.txt` to Pipenv for package management.
+
+
+## Version 3.4.2 - released 2016-03-29
+
+- Fixed dates in CHANGELOG.txt
+
+
+## Version 3.4.1 - released 2016-03-26
+
+- Included tests/private.pem in MANIFEST.in
+- Included README.md and CHANGELOG.txt in MANIFEST.in
+
+
+## Version 3.4 - released 2016-03-17
+
+- Moved development to GitHub: https://github.com/sybrenstuvel/python-rsa
+- Solved side-channel vulnerability by implementing blinding, fixes #19
+- Deprecated the VARBLOCK format and rsa.bigfile module due to security issues, see
+    https://github.com/sybrenstuvel/python-rsa/issues/13
+- Integration with Travis-CI [1], Coveralls [2] and Code Climate [3]
+- Deprecated the old rsa._version133 and rsa._version200 submodules, they will be
+  completely removed in version 4.0.
+- Add an 'exponent' argument to key.newkeys()
+- Switched from Solovay-Strassen to Miller-Rabin primality testing, to
+  comply with NIST FIPS 186-4 [4] as probabilistic primality test
+  (Appendix C, subsection C.3):
+- Fixed bugs #12, #14, #27, #30, #49
+
+[1] https://travis-ci.org/sybrenstuvel/python-rsa
+[2] https://coveralls.io/github/sybrenstuvel/python-rsa
+[3] https://codeclimate.com/github/sybrenstuvel/python-rsa
+[4] http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
+
+
+## Version 3.3 - released 2016-01-13
+
+- Thanks to Filippo Valsorda: Fix BB'06 attack in verify() by
+  switching from parsing to comparison. See [1] for more information.
+- Simplified Tox configuration and dropped Python 3.2 support. The
+  coverage package uses a u'' prefix, which was reintroduced in 3.3
+  for ease of porting.
+
+[1] https://blog.filippo.io/bleichenbacher-06-signature-forgery-in-python-rsa/
+
+
+## Version 3.2.3 - released 2015-11-05
+
+- Added character encoding markers for Python 2.x
+
+
+## Version 3.2.1 - released 2015-11-05
+
+- Added per-file licenses
+- Added support for wheel packages
+- Made example code more consistent and up to date with Python 3.4
+
+
+## Version 3.2 - released 2015-07-29
+
+- Mentioned support for Python 3 in setup.py
+
+
+## Version 3.1.4 - released 2014-02-22
+
+- Fixed some bugs
+
+
+## Version 3.1.3 - released 2014-02-02
+
+- Dropped support for Python 2.5
+
+
+## Version 3.1.2 - released 2013-09-15
+
+- Added Python 3.3 to the test environment.
+- Removed dependency on Distribute
+- Added support for loading public keys from OpenSSL
+
+
+## Version 3.1.1 - released 2012-06-18
+
+- Fixed doctests for Python 2.7
+- Removed obsolete unittest so all tests run fine on Python 3.2
+
+## Version 3.1 - released 2012-06-17
+
+- Big, big credits to Yesudeep Mangalapilly for all the changes listed
+  below!
+- Added ability to generate keys on multiple cores simultaneously.
+- Massive speedup
+- Partial Python 3.2 compatibility (core functionality works, but
+  saving or loading keys doesn't, for that the pyasn1 package needs to
+  be ported to Python 3 first)
+- Lots of bug fixes
+
+
+
+## Version 3.0.1 - released 2011-08-07
+
+- Removed unused import of abc module
+
+
+## Version 3.0 - released 2011-08-05
+
+- Changed the meaning of the keysize to mean the size of ``n`` rather than
+  the size of both ``p`` and ``q``. This is the common interpretation of
+  RSA keysize. To get the old behaviour, double the keysize when generating a
+  new key.
+- Added a lot of doctests
+- Added random-padded encryption and decryption using PKCS#1 version 1.5
+- Added hash-based signatures and verification using PKCS#1v1.5
+- Modeling private and public key as real objects rather than dicts.
+- Support for saving and loading keys as PEM and DER files.
+- Ability to extract a public key from a private key (PEM+DER)
+
+
+## Version 2.0
+
+- Security improvements by Barry Mead.
diff --git a/CHANGELOG.txt b/CHANGELOG.txt
deleted file mode 100644
index f8ed650..0000000
--- a/CHANGELOG.txt
+++ /dev/null
@@ -1,164 +0,0 @@
-Python-RSA changelog
-========================================
-
-Version 4.0 - released 2018-09-16
-----------------------------------------
-
-- Removed deprecated modules:
-    - rsa.varblock
-    - rsa.bigfile
-    - rsa._version133
-    - rsa._version200
-- Removed CLI commands that use the VARBLOCK/bigfile format.
-- Ensured that PublicKey.save_pkcs1() and PrivateKey.save_pkcs1() always return bytes.
-- Dropped support for Python 2.6 and 3.3.
-- Dropped support for Psyco.
-- Miller-Rabin iterations determined by bitsize of key.
-  [#58](https://github.com/sybrenstuvel/python-rsa/pull/58)
-- Added function `rsa.find_signature_hash()` to return the name of the hashing
-  algorithm used to sign a message. `rsa.verify()` now also returns that name,
-  instead of always returning `True`.
-  [#78](https://github.com/sybrenstuvel/python-rsa/issues/13)
-- Add support for SHA-224 for PKCS1 signatures.
-  [#104](https://github.com/sybrenstuvel/python-rsa/pull/104)
-- Transitioned from `requirements.txt` to Pipenv for package management.
-
-
-Version 3.4.2 - released 2016-03-29
-----------------------------------------
-
-- Fixed dates in CHANGELOG.txt
-
-
-Version 3.4.1 - released 2016-03-26
-----------------------------------------
-
-- Included tests/private.pem in MANIFEST.in
-- Included README.md and CHANGELOG.txt in MANIFEST.in
-
-
-Version 3.4 - released 2016-03-17
-----------------------------------------
-
-- Moved development to GitHub: https://github.com/sybrenstuvel/python-rsa
-- Solved side-channel vulnerability by implementing blinding, fixes #19
-- Deprecated the VARBLOCK format and rsa.bigfile module due to security issues, see
-    https://github.com/sybrenstuvel/python-rsa/issues/13
-- Integration with Travis-CI [1], Coveralls [2] and Code Climate [3]
-- Deprecated the old rsa._version133 and rsa._version200 submodules, they will be
-  completely removed in version 4.0.
-- Add an 'exponent' argument to key.newkeys()
-- Switched from Solovay-Strassen to Miller-Rabin primality testing, to
-  comply with NIST FIPS 186-4 [4] as probabilistic primality test
-  (Appendix C, subsection C.3):
-- Fixed bugs #12, #14, #27, #30, #49
-
-[1] https://travis-ci.org/sybrenstuvel/python-rsa
-[2] https://coveralls.io/github/sybrenstuvel/python-rsa
-[3] https://codeclimate.com/github/sybrenstuvel/python-rsa
-[4] http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
-
-
-Version 3.3 - released 2016-01-13
-----------------------------------------
-
-- Thanks to Filippo Valsorda: Fix BB'06 attack in verify() by
-  switching from parsing to comparison. See [1] for more information.
-- Simplified Tox configuration and dropped Python 3.2 support. The
-  coverage package uses a u'' prefix, which was reintroduced in 3.3
-  for ease of porting.
-
-[1] https://blog.filippo.io/bleichenbacher-06-signature-forgery-in-python-rsa/
-
-
-Version 3.2.3 - released 2015-11-05
-----------------------------------------
-
-- Added character encoding markers for Python 2.x
-
-
-Version 3.2.1 - released 2015-11-05
-----------------------------------------
-
-- Added per-file licenses
-- Added support for wheel packages
-- Made example code more consistent and up to date with Python 3.4
-
-
-Version 3.2 - released 2015-07-29
-----------------------------------------
-
-- Mentioned support for Python 3 in setup.py
-
-
-Version 3.1.4 - released 2014-02-22
-----------------------------------------
-
-- Fixed some bugs
-
-
-Version 3.1.3 - released 2014-02-02
-----------------------------------------
-
-- Dropped support for Python 2.5
-
-
-Version 3.1.2 - released 2013-09-15
-----------------------------------------
-
-- Added Python 3.3 to the test environment.
-- Removed dependency on Distribute
-- Added support for loading public keys from OpenSSL
-
-
-Version 3.1.1 - released 2012-06-18
-----------------------------------------
-
-- Fixed doctests for Python 2.7
-- Removed obsolete unittest so all tests run fine on Python 3.2
-
-Version 3.1 - released 2012-06-17
-----------------------------------------
-
-- Big, big credits to Yesudeep Mangalapilly for all the changes listed
-  below!
-- Added ability to generate keys on multiple cores simultaneously.
-- Massive speedup
-- Partial Python 3.2 compatibility (core functionality works, but
-  saving or loading keys doesn't, for that the pyasn1 package needs to
-  be ported to Python 3 first)
-- Lots of bug fixes
-
-
-
-Version 3.0.1 - released 2011-08-07
-----------------------------------------
-
-- Removed unused import of abc module
-
-
-Version 3.0 - released 2011-08-05
-----------------------------------------
-
-- Changed the meaning of the keysize to mean the size of ``n`` rather than
-  the size of both ``p`` and ``q``. This is the common interpretation of
-  RSA keysize. To get the old behaviour, double the keysize when generating a
-  new key.
-
-- Added a lot of doctests
-
-- Added random-padded encryption and decryption using PKCS#1 version 1.5
-
-- Added hash-based signatures and verification using PKCS#1v1.5
-
-- Modeling private and public key as real objects rather than dicts.
-
-- Support for saving and loading keys as PEM and DER files.
-
-- Ability to extract a public key from a private key (PEM+DER)
-
-
-Version 2.0
-----------------------------------------
-
-- Security improvements by Barry Mead.
diff --git a/METADATA b/METADATA
index 98a8942..5c228ef 100644
--- a/METADATA
+++ b/METADATA
@@ -9,11 +9,11 @@
     type: GIT
     value: "https://github.com/sybrenstuvel/python-rsa/"
   }
-  version: "version-4.0"
+  version: "version-4.6"
   license_type: NOTICE
   last_upgrade_date {
-    year: 2019
-    month: 2
-    day: 1
+    year: 2020
+    month: 7
+    day: 10
   }
 }
diff --git a/Pipfile b/Pipfile
index 89ec5fd..a6f846c 100644
--- a/Pipfile
+++ b/Pipfile
@@ -7,13 +7,14 @@
 "pyasn1" = ">=0.1.3"
 
 [dev-packages]
-tox = "*"
-mock = ">=2.0.0"
-Sphinx = "*"
-coveralls = "*"
-pytest = "*"
-pytest-cov = "*"
-pathlib2 = {version = "*", markers="python_version < '3.6'"}
+coveralls = "~=1.8, >=1.8"
+"Sphinx" = "~=2.1, >=2.1"
+"pathlib2" = {version = "~=2.3, >=2.3.4",markers = "python_version < '3.6'"}
+"pytest" = "~=5.0, >=5.0"
+"pytest-cov" = "~=2.7, >=2.7"
+"tox" = "~=3.13, >=3.13"
+"mypy" = "~=0.720, >=0.720"
+"flake8" = "~=3.7, >=3.7"
 
 [requires]
-python_version = "3.6"
+python_version = "3.7"
diff --git a/Pipfile.lock b/Pipfile.lock
index 03fc240..702edae 100644
--- a/Pipfile.lock
+++ b/Pipfile.lock
@@ -1,11 +1,11 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "a86e76a85c3a86f6a44f1b5f48205749c451c830746cbc535c66e72d8f5313cb"
+            "sha256": "4df253faa2a1f6d6665fddc4c13f5e278a4127c27d7b76e59607a8154f96b1ab"
         },
         "pipfile-spec": 6,
         "requires": {
-            "python_version": "3.6"
+            "python_version": "3.7"
         },
         "sources": [
             {
@@ -18,49 +18,61 @@
     "default": {
         "pyasn1": {
             "hashes": [
-                "sha256:b9d3abc5031e61927c82d4d96c1cec1e55676c1a991623cfed28faea73cdd7ca",
-                "sha256:f58f2a3d12fd754aa123e9fa74fb7345333000a035f3921dbdaa08597aa53137"
+                "sha256:014c0e9976956a08139dc0712ae195324a75e142284d5f87f1a87ee1b068a359",
+                "sha256:03840c999ba71680a131cfaee6fab142e1ed9bbd9c693e285cc6aca0d555e576",
+                "sha256:0458773cfe65b153891ac249bcf1b5f8f320b7c2ce462151f8fa74de8934becf",
+                "sha256:08c3c53b75eaa48d71cf8c710312316392ed40899cb34710d092e96745a358b7",
+                "sha256:39c7e2ec30515947ff4e87fb6f456dfc6e84857d34be479c9d4a4ba4bf46aa5d",
+                "sha256:5c9414dcfede6e441f7e8f81b43b34e834731003427e5b09e4e00e3172a10f00",
+                "sha256:6e7545f1a61025a4e58bb336952c5061697da694db1cae97b116e9c46abcf7c8",
+                "sha256:78fa6da68ed2727915c4767bb386ab32cdba863caa7dbe473eaae45f9959da86",
+                "sha256:7ab8a544af125fb704feadb008c99a88805126fb525280b2270bb25cc1d78a12",
+                "sha256:99fcc3c8d804d1bc6d9a099921e39d827026409a58f2a720dcdb89374ea0c776",
+                "sha256:aef77c9fb94a3ac588e87841208bdec464471d9871bd5050a287cc9a475cd0ba",
+                "sha256:e89bf84b5437b532b0803ba5c9a5e054d21fec423a89952a74f87fa2c9b7bce2",
+                "sha256:fec3e9d8e36808a28efb59b489e4528c10ad0f480e57dcc32b4de5c9d8c9fdf3"
             ],
             "index": "pypi",
-            "version": "==0.4.4"
+            "version": "==0.4.8"
         }
     },
     "develop": {
         "alabaster": {
             "hashes": [
-                "sha256:674bb3bab080f598371f4443c5008cbfeb1a5e622dd312395d2d82af2c54c456",
-                "sha256:b63b1f4dc77c074d386752ec4a8a7517600f6c0db8cd42980cae17ab7b3275d7"
+                "sha256:446438bdcca0e05bd45ea2de1668c1d9b032e1a9154c2c259092d77031ddd359",
+                "sha256:a661d72d58e6ea8a57f7a86e37d86716863ee5e92788398526d58b26a4e4dc02"
             ],
-            "version": "==0.7.11"
+            "version": "==0.7.12"
         },
-        "atomicwrites": {
+        "appdirs": {
             "hashes": [
-                "sha256:0312ad34fcad8fac3704d441f7b317e50af620823353ec657a53e981f92920c0",
-                "sha256:ec9ae8adaae229e4f8446952d204a3e4b5fdd2d099f9be3aaf556120135fb3ee"
+                "sha256:7d5d0167b2b1ba821647616af46a749d1c653740dd0d2415100fe26e27afdf41",
+                "sha256:a841dacd6b99318a741b166adb07e19ee71a274450e68237b4650ca1055ab128"
             ],
-            "markers": "python_version >= '2.7' and python_version != '3.1.*' and python_version != '3.3.*' and python_version != '3.0.*' and python_version != '3.2.*'",
-            "version": "==1.2.1"
+            "version": "==1.4.4"
         },
         "attrs": {
             "hashes": [
-                "sha256:10cbf6e27dbce8c30807caf056c8eb50917e0eaafe86347671b57254006c3e69",
-                "sha256:ca4be454458f9dec299268d472aaa5a11f67a4ff70093396e1ceae9c76cf4bbb"
+                "sha256:08a96c641c3a74e44eb59afb61a24f2cb9f4d7188748e76ba4bb5edfa3cb7d1c",
+                "sha256:f7b7ce16570fe9965acd6d30101a28f62fb4a7f9e926b3bbc9b61f8b04247e72"
             ],
-            "version": "==18.2.0"
+            "markers": "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'",
+            "version": "==19.3.0"
         },
         "babel": {
             "hashes": [
-                "sha256:6778d85147d5d85345c14a26aada5e478ab04e39b078b0745ee6870c2b5cf669",
-                "sha256:8cba50f48c529ca3fa18cf81fa9403be176d374ac4d60738b839122dfaaa3d23"
+                "sha256:1aac2ae2d0d8ea368fa90906567f5c08463d98ade155c0c4bfedd6a0f7160e38",
+                "sha256:d670ea0b10f8b723672d3a6abeb87b565b244da220d76b4dba1b66269ec152d4"
             ],
-            "version": "==2.6.0"
+            "markers": "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'",
+            "version": "==2.8.0"
         },
         "certifi": {
             "hashes": [
-                "sha256:376690d6f16d32f9d1fe8932551d80b23e9d393a8578c5633a2ed39a64861638",
-                "sha256:456048c7e371c089d0a77a5212fb37a2c2dce1e24146e3b7e0261736aaeaa22a"
+                "sha256:5ad7e9a056d25ffa5082862e36f119f7f7cec6457fa07ee2f8c339814b80c9b1",
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-            "version": "==2018.8.24"
+            "version": "==2020.4.5.2"
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@@ -69,58 +81,56 @@
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@@ -130,190 +140,385 @@
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+                "sha256:79375666b9954d4a1a10739315816324c3e73110af9d0e102d906fdb0aec009f",
+                "sha256:8c6b5b6ee1360b842645f336d9e5d68c55817c26d3050f46b235ef2bc650e48f"
+            ],
+            "version": "==0.2.4"
+        },
+        "zipp": {
+            "hashes": [
+                "sha256:aa36550ff0c0b7ef7fa639055d797116ee891440eac1a56f378e2d3179e0320b",
+                "sha256:c599e4d75c98f6798c509911d08a22e6c021d074469042177c8c86fb92eefd96"
+            ],
+            "markers": "python_version >= '3.6'",
+            "version": "==3.1.0"
         }
     }
 }
diff --git a/README.md b/README.md
index c5e0f2d..ea24210 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@
 [![PyPI](https://img.shields.io/pypi/v/rsa.svg)](https://pypi.org/project/rsa/)
 [![Build Status](https://travis-ci.org/sybrenstuvel/python-rsa.svg?branch=master)](https://travis-ci.org/sybrenstuvel/python-rsa)
 [![Coverage Status](https://coveralls.io/repos/github/sybrenstuvel/python-rsa/badge.svg?branch=master)](https://coveralls.io/github/sybrenstuvel/python-rsa?branch=master)
-[![Code Climate](https://img.shields.io/codeclimate/github/sybrenstuvel/python-rsa.svg)](https://codeclimate.com/github/sybrenstuvel/python-rsa)
+[![Code Climate](https://api.codeclimate.com/v1/badges/a99a88d28ad37a79dbf6/maintainability)](https://codeclimate.com/github/codeclimate/codeclimate/maintainability)
 
 [Python-RSA](https://stuvel.eu/rsa) is a pure-Python RSA implementation. It supports
 encryption and decryption, signing and verifying signatures, and key
@@ -12,7 +12,7 @@
 library as well as on the commandline. The code was mostly written by
 Sybren A.  Stüvel.
 
-Documentation can be found at the [Python-RSA homepage](https://stuvel.eu/rsa).
+Documentation can be found at the [Python-RSA homepage](https://stuvel.eu/rsa). For all changes, check [the changelog](https://github.com/sybrenstuvel/python-rsa/blob/master/CHANGELOG.md).
 
 Download and install using:
 
@@ -23,6 +23,11 @@
 The source code is maintained at [GitHub](https://github.com/sybrenstuvel/python-rsa/) and is
 licensed under the [Apache License, version 2.0](https://www.apache.org/licenses/LICENSE-2.0)
 
+Major changes in 4.1
+--------------------
+
+Version 4.0 was the last version to support Python 2 and 3.4. Version 4.1 is compatible with Python 3.5+ only.
+
 
 Major changes in 4.0
 --------------------
diff --git a/create_timing_table.py b/create_timing_table.py
index 6163916..498e6a0 100755
--- a/create_timing_table.py
+++ b/create_timing_table.py
@@ -1,6 +1,4 @@
 #!/usr/bin/env python
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/doc/compatibility.rst b/doc/compatibility.rst
index be4d295..1429553 100644
--- a/doc/compatibility.rst
+++ b/doc/compatibility.rst
@@ -16,7 +16,7 @@
 
 Signatures:
     PKCS#1 v1.5 using the following hash methods:
-    MD5, SHA-1, SHA-224, SHA-256, SHA-384, SHA-512
+    MD5, SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA3-256, SHA3-384, SHA3-512
 
 Private keys:
     PKCS#1 v1.5 in PEM and DER format, ASN.1 type RSAPrivateKey
diff --git a/doc/conf.py b/doc/conf.py
index 3331a86..60201b1 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 # Python-RSA documentation build configuration file, created by
 # sphinx-quickstart on Sat Jul 30 23:11:07 2011.
 #
@@ -46,7 +44,7 @@
 
 # General information about the project.
 project = u'Python-RSA'
-copyright = u'2011-2018, Sybren A. Stüvel'
+copyright = u'2011-2019, Sybren A. Stüvel'
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
diff --git a/doc/installation.rst b/doc/installation.rst
index 32dc257..3ab3ab1 100644
--- a/doc/installation.rst
+++ b/doc/installation.rst
@@ -1,13 +1,13 @@
 Installation
 ============
 
-Installation can be done in various ways. The simplest form uses pip
-or easy_install. Either one will work::
+Installation can be done in various ways. The simplest form uses pip::
 
     pip install rsa
 
 Depending on your system you may need to use ``sudo pip`` if you want to install
-the library system-wide.
+the library system-wide, or use ``pip install --user rsa`` to install the
+library in your home directory.
 
 Installation from source is also quite easy. Download the source and
 then type::
@@ -25,6 +25,9 @@
 Dependencies
 ------------
 
+Python-RSA is compatible with Python versions 3.5 and newer. The last
+version with Python 2.7 support was Python-RSA 4.0.
+
 Python-RSA has very few dependencies. As a matter of fact, to use it
 you only need Python itself. Loading and saving keys does require an
 extra module, though: pyasn1. If you used pip or easy_install like
@@ -34,17 +37,15 @@
 Development dependencies
 ------------------------
 
-In order to start developing on Python-RSA you need a bit more. Use
-pip to install the development requirements in a virtual environment::
-
-    virtualenv -p /path/to/your-python-version python-rsa-venv
-    . python-rsa-venv/bin/activate
-    pip install -r python-rsa/requirements.txt
-
-
-Once these are installed, use Git_ to get a copy of the source::
+In order to start developing on Python-RSA, use Git_ to get a copy of
+the source::
 
     git clone https://github.com/sybrenstuvel/python-rsa.git
-    python setup.py develop
+
+Use Poetry_ to install the development requirements in a virtual environment::
+
+    cd python-rsa
+    poetry install
 
 .. _Git: https://git-scm.com/
+.. _Poetry: https://poetry.eustace.io/
diff --git a/rsa/__init__.py b/rsa/__init__.py
index 9b05c6c..1567dc1 100644
--- a/rsa/__init__.py
+++ b/rsa/__init__.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -28,8 +26,8 @@
     VerificationError, find_signature_hash,  sign_hash, compute_hash
 
 __author__ = "Sybren Stuvel, Barry Mead and Yesudeep Mangalapilly"
-__date__ = "2018-09-16"
-__version__ = '4.0'
+__date__ = '2020-06-12'
+__version__ = '4.6'
 
 # Do doctest if we're run directly
 if __name__ == "__main__":
@@ -39,4 +37,4 @@
 
 __all__ = ["newkeys", "encrypt", "decrypt", "sign", "verify", 'PublicKey',
            'PrivateKey', 'DecryptionError', 'VerificationError',
-           'compute_hash', 'sign_hash']
+           'find_signature_hash', 'compute_hash', 'sign_hash']
diff --git a/rsa/_compat.py b/rsa/_compat.py
index 71197a5..050e81b 100644
--- a/rsa/_compat.py
+++ b/rsa/_compat.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -16,84 +14,14 @@
 
 """Python compatibility wrappers."""
 
-from __future__ import absolute_import
-
-import itertools
-import sys
 from struct import pack
 
-MAX_INT = sys.maxsize
-MAX_INT64 = (1 << 63) - 1
-MAX_INT32 = (1 << 31) - 1
-MAX_INT16 = (1 << 15) - 1
 
-PY2 = sys.version_info[0] == 2
-
-# Determine the word size of the processor.
-if MAX_INT == MAX_INT64:
-    # 64-bit processor.
-    MACHINE_WORD_SIZE = 64
-elif MAX_INT == MAX_INT32:
-    # 32-bit processor.
-    MACHINE_WORD_SIZE = 32
-else:
-    # Else we just assume 64-bit processor keeping up with modern times.
-    MACHINE_WORD_SIZE = 64
-
-if PY2:
-    integer_types = (int, long)
-    range = xrange
-    zip = itertools.izip
-else:
-    integer_types = (int, )
-    range = range
-    zip = zip
-
-
-def write_to_stdout(data):
-    """Writes bytes to stdout
-
-    :type data: bytes
-    """
-    if PY2:
-        sys.stdout.write(data)
-    else:
-        # On Py3 we must use the buffer interface to write bytes.
-        sys.stdout.buffer.write(data)
-
-
-def is_bytes(obj):
-    """
-    Determines whether the given value is a byte string.
-
-    :param obj:
-        The value to test.
-    :returns:
-        ``True`` if ``value`` is a byte string; ``False`` otherwise.
-    """
-    return isinstance(obj, bytes)
-
-
-def is_integer(obj):
-    """
-    Determines whether the given value is an integer.
-
-    :param obj:
-        The value to test.
-    :returns:
-        ``True`` if ``value`` is an integer; ``False`` otherwise.
-    """
-    return isinstance(obj, integer_types)
-
-
-def byte(num):
+def byte(num: int) -> bytes:
     """
     Converts a number between 0 and 255 (both inclusive) to a base-256 (byte)
     representation.
 
-    Use it as a replacement for ``chr`` where you are expecting a byte
-    because this will work on all current versions of Python::
-
     :param num:
         An unsigned integer between 0 and 255 (both inclusive).
     :returns:
@@ -102,7 +30,7 @@
     return pack("B", num)
 
 
-def xor_bytes(b1, b2):
+def xor_bytes(b1: bytes, b2: bytes) -> bytes:
     """
     Returns the bitwise XOR result between two bytes objects, b1 ^ b2.
 
@@ -117,46 +45,4 @@
     :returns:
         Bytes object, result of XOR operation.
     """
-    if PY2:
-        return ''.join(byte(ord(x) ^ ord(y)) for x, y in zip(b1, b2))
-
     return bytes(x ^ y for x, y in zip(b1, b2))
-
-
-def get_word_alignment(num, force_arch=64,
-                       _machine_word_size=MACHINE_WORD_SIZE):
-    """
-    Returns alignment details for the given number based on the platform
-    Python is running on.
-
-    :param num:
-        Unsigned integral number.
-    :param force_arch:
-        If you don't want to use 64-bit unsigned chunks, set this to
-        anything other than 64. 32-bit chunks will be preferred then.
-        Default 64 will be used when on a 64-bit machine.
-    :param _machine_word_size:
-        (Internal) The machine word size used for alignment.
-    :returns:
-        4-tuple::
-
-            (word_bits, word_bytes,
-             max_uint, packing_format_type)
-    """
-    max_uint64 = 0xffffffffffffffff
-    max_uint32 = 0xffffffff
-    max_uint16 = 0xffff
-    max_uint8 = 0xff
-
-    if force_arch == 64 and _machine_word_size >= 64 and num > max_uint32:
-        # 64-bit unsigned integer.
-        return 64, 8, max_uint64, "Q"
-    elif num > max_uint16:
-        # 32-bit unsigned integer
-        return 32, 4, max_uint32, "L"
-    elif num > max_uint8:
-        # 16-bit unsigned integer.
-        return 16, 2, max_uint16, "H"
-    else:
-        # 8-bit unsigned integer.
-        return 8, 1, max_uint8, "B"
diff --git a/rsa/asn1.py b/rsa/asn1.py
index b724b8f..32b1eb4 100644
--- a/rsa/asn1.py
+++ b/rsa/asn1.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/rsa/cli.py b/rsa/cli.py
index 6450af4..3166150 100644
--- a/rsa/cli.py
+++ b/rsa/cli.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -19,24 +17,25 @@
 These scripts are called by the executables defined in setup.py.
 """
 
-from __future__ import with_statement, print_function
-
 import abc
 import sys
-from optparse import OptionParser
+import typing
+import optparse
 
 import rsa
+import rsa.key
 import rsa.pkcs1
 
 HASH_METHODS = sorted(rsa.pkcs1.HASH_METHODS.keys())
+Indexable = typing.Union[typing.Tuple, typing.List[str]]
 
 
-def keygen():
+def keygen() -> None:
     """Key generator."""
 
     # Parse the CLI options
-    parser = OptionParser(usage='usage: %prog [options] keysize',
-                          description='Generates a new RSA keypair of "keysize" bits.')
+    parser = optparse.OptionParser(usage='usage: %prog [options] keysize',
+                                   description='Generates a new RSA keypair of "keysize" bits.')
 
     parser.add_option('--pubout', type='string',
                       help='Output filename for the public key. The public key is '
@@ -83,17 +82,15 @@
             outfile.write(data)
     else:
         print('Writing private key to stdout', file=sys.stderr)
-        rsa._compat.write_to_stdout(data)
+        sys.stdout.buffer.write(data)
 
 
-class CryptoOperation(object):
+class CryptoOperation(metaclass=abc.ABCMeta):
     """CLI callable that operates with input, output, and a key."""
 
-    __metaclass__ = abc.ABCMeta
-
     keyname = 'public'  # or 'private'
     usage = 'usage: %%prog [options] %(keyname)s_key'
-    description = None
+    description = ''
     operation = 'decrypt'
     operation_past = 'decrypted'
     operation_progressive = 'decrypting'
@@ -104,15 +101,16 @@
     expected_cli_args = 1
     has_output = True
 
-    key_class = rsa.PublicKey
+    key_class = rsa.PublicKey  # type: typing.Type[rsa.key.AbstractKey]
 
-    def __init__(self):
+    def __init__(self) -> None:
         self.usage = self.usage % self.__class__.__dict__
         self.input_help = self.input_help % self.__class__.__dict__
         self.output_help = self.output_help % self.__class__.__dict__
 
     @abc.abstractmethod
-    def perform_operation(self, indata, key, cli_args):
+    def perform_operation(self, indata: bytes, key: rsa.key.AbstractKey,
+                          cli_args: Indexable) -> typing.Any:
         """Performs the program's operation.
 
         Implement in a subclass.
@@ -120,7 +118,7 @@
         :returns: the data to write to the output.
         """
 
-    def __call__(self):
+    def __call__(self) -> None:
         """Runs the program."""
 
         (cli, cli_args) = self.parse_cli()
@@ -135,13 +133,13 @@
         if self.has_output:
             self.write_outfile(outdata, cli.output)
 
-    def parse_cli(self):
+    def parse_cli(self) -> typing.Tuple[optparse.Values, typing.List[str]]:
         """Parse the CLI options
 
         :returns: (cli_opts, cli_args)
         """
 
-        parser = OptionParser(usage=self.usage, description=self.description)
+        parser = optparse.OptionParser(usage=self.usage, description=self.description)
 
         parser.add_option('-i', '--input', type='string', help=self.input_help)
 
@@ -160,7 +158,7 @@
 
         return cli, cli_args
 
-    def read_key(self, filename, keyform):
+    def read_key(self, filename: str, keyform: str) -> rsa.key.AbstractKey:
         """Reads a public or private key."""
 
         print('Reading %s key from %s' % (self.keyname, filename), file=sys.stderr)
@@ -169,7 +167,7 @@
 
         return self.key_class.load_pkcs1(keydata, keyform)
 
-    def read_infile(self, inname):
+    def read_infile(self, inname: str) -> bytes:
         """Read the input file"""
 
         if inname:
@@ -178,9 +176,9 @@
                 return infile.read()
 
         print('Reading input from stdin', file=sys.stderr)
-        return sys.stdin.read()
+        return sys.stdin.buffer.read()
 
-    def write_outfile(self, outdata, outname):
+    def write_outfile(self, outdata: bytes, outname: str) -> None:
         """Write the output file"""
 
         if outname:
@@ -189,7 +187,7 @@
                 outfile.write(outdata)
         else:
             print('Writing output to stdout', file=sys.stderr)
-            rsa._compat.write_to_stdout(outdata)
+            sys.stdout.buffer.write(outdata)
 
 
 class EncryptOperation(CryptoOperation):
@@ -202,9 +200,10 @@
     operation_past = 'encrypted'
     operation_progressive = 'encrypting'
 
-    def perform_operation(self, indata, pub_key, cli_args=None):
+    def perform_operation(self, indata: bytes, pub_key: rsa.key.AbstractKey,
+                          cli_args: Indexable = ()) -> bytes:
         """Encrypts files."""
-
+        assert isinstance(pub_key, rsa.key.PublicKey)
         return rsa.encrypt(indata, pub_key)
 
 
@@ -219,9 +218,10 @@
     operation_progressive = 'decrypting'
     key_class = rsa.PrivateKey
 
-    def perform_operation(self, indata, priv_key, cli_args=None):
+    def perform_operation(self, indata: bytes, priv_key: rsa.key.AbstractKey,
+                          cli_args: Indexable = ()) -> bytes:
         """Decrypts files."""
-
+        assert isinstance(priv_key, rsa.key.PrivateKey)
         return rsa.decrypt(indata, priv_key)
 
 
@@ -241,8 +241,10 @@
     output_help = ('Name of the file to write the signature to. Written '
                    'to stdout if this option is not present.')
 
-    def perform_operation(self, indata, priv_key, cli_args):
+    def perform_operation(self, indata: bytes, priv_key: rsa.key.AbstractKey,
+                          cli_args: Indexable) -> bytes:
         """Signs files."""
+        assert isinstance(priv_key, rsa.key.PrivateKey)
 
         hash_method = cli_args[1]
         if hash_method not in HASH_METHODS:
@@ -266,8 +268,10 @@
     expected_cli_args = 2
     has_output = False
 
-    def perform_operation(self, indata, pub_key, cli_args):
+    def perform_operation(self, indata: bytes, pub_key: rsa.key.AbstractKey,
+                          cli_args: Indexable) -> None:
         """Verifies files."""
+        assert isinstance(pub_key, rsa.key.PublicKey)
 
         signature_file = cli_args[1]
 
diff --git a/rsa/common.py b/rsa/common.py
index f7aa2d1..e7df21d 100644
--- a/rsa/common.py
+++ b/rsa/common.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -14,21 +12,20 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 
-from rsa._compat import zip
-
 """Common functionality shared by several modules."""
 
+import typing
+
 
 class NotRelativePrimeError(ValueError):
-    def __init__(self, a, b, d, msg=None):
-        super(NotRelativePrimeError, self).__init__(
-            msg or "%d and %d are not relatively prime, divider=%i" % (a, b, d))
+    def __init__(self, a: int, b: int, d: int, msg: str = '') -> None:
+        super().__init__(msg or "%d and %d are not relatively prime, divider=%i" % (a, b, d))
         self.a = a
         self.b = b
         self.d = d
 
 
-def bit_size(num):
+def bit_size(num: int) -> int:
     """
     Number of bits needed to represent a integer excluding any prefix
     0 bits.
@@ -56,7 +53,7 @@
         raise TypeError('bit_size(num) only supports integers, not %r' % type(num))
 
 
-def byte_size(number):
+def byte_size(number: int) -> int:
     """
     Returns the number of bytes required to hold a specific long number.
 
@@ -81,7 +78,7 @@
     return ceil_div(bit_size(number), 8)
 
 
-def ceil_div(num, div):
+def ceil_div(num: int, div: int) -> int:
     """
     Returns the ceiling function of a division between `num` and `div`.
 
@@ -105,7 +102,7 @@
     return quanta
 
 
-def extended_gcd(a, b):
+def extended_gcd(a: int, b: int) -> typing.Tuple[int, int, int]:
     """Returns a tuple (r, i, j) such that r = gcd(a, b) = ia + jb
     """
     # r = gcd(a,b) i = multiplicitive inverse of a mod b
@@ -130,7 +127,7 @@
     return a, lx, ly  # Return only positive values
 
 
-def inverse(x, n):
+def inverse(x: int, n: int) -> int:
     """Returns the inverse of x % n under multiplication, a.k.a x^-1 (mod n)
 
     >>> inverse(7, 4)
@@ -147,7 +144,7 @@
     return inv
 
 
-def crt(a_values, modulo_values):
+def crt(a_values: typing.Iterable[int], modulo_values: typing.Iterable[int]) -> int:
     """Chinese Remainder Theorem.
 
     Calculates x such that x = a[i] (mod m[i]) for each i.
diff --git a/rsa/core.py b/rsa/core.py
index b3114d9..23032e3 100644
--- a/rsa/core.py
+++ b/rsa/core.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -20,17 +18,15 @@
 mathematically on integers.
 """
 
-from rsa._compat import is_integer
 
-
-def assert_int(var, name):
-    if is_integer(var):
+def assert_int(var: int, name: str) -> None:
+    if isinstance(var, int):
         return
 
     raise TypeError('%s should be an integer, not %s' % (name, var.__class__))
 
 
-def encrypt_int(message, ekey, n):
+def encrypt_int(message: int, ekey: int, n: int) -> int:
     """Encrypts a message using encryption key 'ekey', working modulo n"""
 
     assert_int(message, 'message')
@@ -46,7 +42,7 @@
     return pow(message, ekey, n)
 
 
-def decrypt_int(cyphertext, dkey, n):
+def decrypt_int(cyphertext: int, dkey: int, n: int) -> int:
     """Decrypts a cypher text using the decryption key 'dkey', working modulo n"""
 
     assert_int(cyphertext, 'cyphertext')
diff --git a/rsa/key.py b/rsa/key.py
index 1004412..b1e2030 100644
--- a/rsa/key.py
+++ b/rsa/key.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -34,9 +32,9 @@
 """
 
 import logging
+import typing
 import warnings
 
-from rsa._compat import range
 import rsa.prime
 import rsa.pem
 import rsa.common
@@ -48,17 +46,17 @@
 DEFAULT_EXPONENT = 65537
 
 
-class AbstractKey(object):
+class AbstractKey:
     """Abstract superclass for private and public keys."""
 
     __slots__ = ('n', 'e')
 
-    def __init__(self, n, e):
+    def __init__(self, n: int, e: int) -> None:
         self.n = n
         self.e = e
 
     @classmethod
-    def _load_pkcs1_pem(cls, keyfile):
+    def _load_pkcs1_pem(cls, keyfile: bytes) -> 'AbstractKey':
         """Loads a key in PKCS#1 PEM format, implement in a subclass.
 
         :param keyfile: contents of a PEM-encoded file that contains
@@ -70,7 +68,7 @@
         """
 
     @classmethod
-    def _load_pkcs1_der(cls, keyfile):
+    def _load_pkcs1_der(cls, keyfile: bytes) -> 'AbstractKey':
         """Loads a key in PKCS#1 PEM format, implement in a subclass.
 
         :param keyfile: contents of a DER-encoded file that contains
@@ -81,14 +79,14 @@
         :rtype: AbstractKey
         """
 
-    def _save_pkcs1_pem(self):
+    def _save_pkcs1_pem(self) -> bytes:
         """Saves the key in PKCS#1 PEM format, implement in a subclass.
 
         :returns: the PEM-encoded key.
         :rtype: bytes
         """
 
-    def _save_pkcs1_der(self):
+    def _save_pkcs1_der(self) -> bytes:
         """Saves the key in PKCS#1 DER format, implement in a subclass.
 
         :returns: the DER-encoded key.
@@ -96,7 +94,7 @@
         """
 
     @classmethod
-    def load_pkcs1(cls, keyfile, format='PEM'):
+    def load_pkcs1(cls, keyfile: bytes, format: str = 'PEM') -> 'AbstractKey':
         """Loads a key in PKCS#1 DER or PEM format.
 
         :param keyfile: contents of a DER- or PEM-encoded file that contains
@@ -118,7 +116,8 @@
         return method(keyfile)
 
     @staticmethod
-    def _assert_format_exists(file_format, methods):
+    def _assert_format_exists(file_format: str, methods: typing.Mapping[str, typing.Callable]) \
+            -> typing.Callable:
         """Checks whether the given file format exists in 'methods'.
         """
 
@@ -129,7 +128,7 @@
             raise ValueError('Unsupported format: %r, try one of %s' % (file_format,
                                                                         formats))
 
-    def save_pkcs1(self, format='PEM'):
+    def save_pkcs1(self, format: str = 'PEM') -> bytes:
         """Saves the key in PKCS#1 DER or PEM format.
 
         :param format: the format to save; 'PEM' or 'DER'
@@ -146,7 +145,7 @@
         method = self._assert_format_exists(format, methods)
         return method()
 
-    def blind(self, message, r):
+    def blind(self, message: int, r: int) -> int:
         """Performs blinding on the message using random number 'r'.
 
         :param message: the message, as integer, to blind.
@@ -163,7 +162,7 @@
 
         return (message * pow(r, self.e, self.n)) % self.n
 
-    def unblind(self, blinded, r):
+    def unblind(self, blinded: int, r: int) -> int:
         """Performs blinding on the message using random number 'r'.
 
         :param blinded: the blinded message, as integer, to unblind.
@@ -204,21 +203,21 @@
 
     __slots__ = ('n', 'e')
 
-    def __getitem__(self, key):
+    def __getitem__(self, key: str) -> int:
         return getattr(self, key)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return 'PublicKey(%i, %i)' % (self.n, self.e)
 
-    def __getstate__(self):
+    def __getstate__(self) -> typing.Tuple[int, int]:
         """Returns the key as tuple for pickling."""
         return self.n, self.e
 
-    def __setstate__(self, state):
+    def __setstate__(self, state: typing.Tuple[int, int]) -> None:
         """Sets the key from tuple."""
         self.n, self.e = state
 
-    def __eq__(self, other):
+    def __eq__(self, other: typing.Any) -> bool:
         if other is None:
             return False
 
@@ -227,14 +226,14 @@
 
         return self.n == other.n and self.e == other.e
 
-    def __ne__(self, other):
+    def __ne__(self, other: typing.Any) -> bool:
         return not (self == other)
 
-    def __hash__(self):
+    def __hash__(self) -> int:
         return hash((self.n, self.e))
 
     @classmethod
-    def _load_pkcs1_der(cls, keyfile):
+    def _load_pkcs1_der(cls, keyfile: bytes) -> 'PublicKey':
         """Loads a key in PKCS#1 DER format.
 
         :param keyfile: contents of a DER-encoded file that contains the public
@@ -260,7 +259,7 @@
         (priv, _) = decoder.decode(keyfile, asn1Spec=AsnPubKey())
         return cls(n=int(priv['modulus']), e=int(priv['publicExponent']))
 
-    def _save_pkcs1_der(self):
+    def _save_pkcs1_der(self) -> bytes:
         """Saves the public key in PKCS#1 DER format.
 
         :returns: the DER-encoded public key.
@@ -278,7 +277,7 @@
         return encoder.encode(asn_key)
 
     @classmethod
-    def _load_pkcs1_pem(cls, keyfile):
+    def _load_pkcs1_pem(cls, keyfile: bytes) -> 'PublicKey':
         """Loads a PKCS#1 PEM-encoded public key file.
 
         The contents of the file before the "-----BEGIN RSA PUBLIC KEY-----" and
@@ -292,7 +291,7 @@
         der = rsa.pem.load_pem(keyfile, 'RSA PUBLIC KEY')
         return cls._load_pkcs1_der(der)
 
-    def _save_pkcs1_pem(self):
+    def _save_pkcs1_pem(self) -> bytes:
         """Saves a PKCS#1 PEM-encoded public key file.
 
         :return: contents of a PEM-encoded file that contains the public key.
@@ -303,7 +302,7 @@
         return rsa.pem.save_pem(der, 'RSA PUBLIC KEY')
 
     @classmethod
-    def load_pkcs1_openssl_pem(cls, keyfile):
+    def load_pkcs1_openssl_pem(cls, keyfile: bytes) -> 'PublicKey':
         """Loads a PKCS#1.5 PEM-encoded public key file from OpenSSL.
 
         These files can be recognised in that they start with BEGIN PUBLIC KEY
@@ -322,14 +321,12 @@
         return cls.load_pkcs1_openssl_der(der)
 
     @classmethod
-    def load_pkcs1_openssl_der(cls, keyfile):
+    def load_pkcs1_openssl_der(cls, keyfile: bytes) -> 'PublicKey':
         """Loads a PKCS#1 DER-encoded public key file from OpenSSL.
 
         :param keyfile: contents of a DER-encoded file that contains the public
             key, from OpenSSL.
         :return: a PublicKey object
-        :rtype: bytes
-
         """
 
         from rsa.asn1 import OpenSSLPubKey
@@ -370,7 +367,7 @@
 
     __slots__ = ('n', 'e', 'd', 'p', 'q', 'exp1', 'exp2', 'coef')
 
-    def __init__(self, n, e, d, p, q):
+    def __init__(self, n: int, e: int, d: int, p: int, q: int) -> None:
         AbstractKey.__init__(self, n, e)
         self.d = d
         self.p = p
@@ -381,21 +378,21 @@
         self.exp2 = int(d % (q - 1))
         self.coef = rsa.common.inverse(q, p)
 
-    def __getitem__(self, key):
+    def __getitem__(self, key: str) -> int:
         return getattr(self, key)
 
-    def __repr__(self):
-        return 'PrivateKey(%(n)i, %(e)i, %(d)i, %(p)i, %(q)i)' % self
+    def __repr__(self) -> str:
+        return 'PrivateKey(%i, %i, %i, %i, %i)' % (self.n, self.e, self.d, self.p, self.q)
 
-    def __getstate__(self):
+    def __getstate__(self) -> typing.Tuple[int, int, int, int, int, int, int, int]:
         """Returns the key as tuple for pickling."""
         return self.n, self.e, self.d, self.p, self.q, self.exp1, self.exp2, self.coef
 
-    def __setstate__(self, state):
+    def __setstate__(self, state: typing.Tuple[int, int, int, int, int, int, int, int]) -> None:
         """Sets the key from tuple."""
         self.n, self.e, self.d, self.p, self.q, self.exp1, self.exp2, self.coef = state
 
-    def __eq__(self, other):
+    def __eq__(self, other: typing.Any) -> bool:
         if other is None:
             return False
 
@@ -411,13 +408,20 @@
                 self.exp2 == other.exp2 and
                 self.coef == other.coef)
 
-    def __ne__(self, other):
+    def __ne__(self, other: typing.Any) -> bool:
         return not (self == other)
 
-    def __hash__(self):
+    def __hash__(self) -> int:
         return hash((self.n, self.e, self.d, self.p, self.q, self.exp1, self.exp2, self.coef))
 
-    def blinded_decrypt(self, encrypted):
+    def _get_blinding_factor(self) -> int:
+        for _ in range(1000):
+            blind_r = rsa.randnum.randint(self.n - 1)
+            if rsa.prime.are_relatively_prime(self.n, blind_r):
+                return blind_r
+        raise RuntimeError('unable to find blinding factor')
+
+    def blinded_decrypt(self, encrypted: int) -> int:
         """Decrypts the message using blinding to prevent side-channel attacks.
 
         :param encrypted: the encrypted message
@@ -427,13 +431,13 @@
         :rtype: int
         """
 
-        blind_r = rsa.randnum.randint(self.n - 1)
+        blind_r = self._get_blinding_factor()
         blinded = self.blind(encrypted, blind_r)  # blind before decrypting
         decrypted = rsa.core.decrypt_int(blinded, self.d, self.n)
 
         return self.unblind(decrypted, blind_r)
 
-    def blinded_encrypt(self, message):
+    def blinded_encrypt(self, message: int) -> int:
         """Encrypts the message using blinding to prevent side-channel attacks.
 
         :param message: the message to encrypt
@@ -443,13 +447,13 @@
         :rtype: int
         """
 
-        blind_r = rsa.randnum.randint(self.n - 1)
+        blind_r = self._get_blinding_factor()
         blinded = self.blind(message, blind_r)  # blind before encrypting
         encrypted = rsa.core.encrypt_int(blinded, self.d, self.n)
         return self.unblind(encrypted, blind_r)
 
     @classmethod
-    def _load_pkcs1_der(cls, keyfile):
+    def _load_pkcs1_der(cls, keyfile: bytes) -> 'PrivateKey':
         """Loads a key in PKCS#1 DER format.
 
         :param keyfile: contents of a DER-encoded file that contains the private
@@ -506,7 +510,7 @@
 
         return key
 
-    def _save_pkcs1_der(self):
+    def _save_pkcs1_der(self) -> bytes:
         """Saves the private key in PKCS#1 DER format.
 
         :returns: the DER-encoded private key.
@@ -544,7 +548,7 @@
         return encoder.encode(asn_key)
 
     @classmethod
-    def _load_pkcs1_pem(cls, keyfile):
+    def _load_pkcs1_pem(cls, keyfile: bytes) -> 'PrivateKey':
         """Loads a PKCS#1 PEM-encoded private key file.
 
         The contents of the file before the "-----BEGIN RSA PRIVATE KEY-----" and
@@ -559,7 +563,7 @@
         der = rsa.pem.load_pem(keyfile, b'RSA PRIVATE KEY')
         return cls._load_pkcs1_der(der)
 
-    def _save_pkcs1_pem(self):
+    def _save_pkcs1_pem(self) -> bytes:
         """Saves a PKCS#1 PEM-encoded private key file.
 
         :return: contents of a PEM-encoded file that contains the private key.
@@ -570,7 +574,9 @@
         return rsa.pem.save_pem(der, b'RSA PRIVATE KEY')
 
 
-def find_p_q(nbits, getprime_func=rsa.prime.getprime, accurate=True):
+def find_p_q(nbits: int,
+             getprime_func: typing.Callable[[int], int] = rsa.prime.getprime,
+             accurate: bool = True) -> typing.Tuple[int, int]:
     """Returns a tuple of two different primes of nbits bits each.
 
     The resulting p * q has exacty 2 * nbits bits, and the returned p and q
@@ -615,7 +621,7 @@
     log.debug('find_p_q(%i): Finding q', nbits)
     q = getprime_func(qbits)
 
-    def is_acceptable(p, q):
+    def is_acceptable(p: int, q: int) -> bool:
         """Returns True iff p and q are acceptable:
 
             - p and q differ
@@ -648,7 +654,7 @@
     return max(p, q), min(p, q)
 
 
-def calculate_keys_custom_exponent(p, q, exponent):
+def calculate_keys_custom_exponent(p: int, q: int, exponent: int) -> typing.Tuple[int, int]:
     """Calculates an encryption and a decryption key given p, q and an exponent,
     and returns them as a tuple (e, d)
 
@@ -678,7 +684,7 @@
     return exponent, d
 
 
-def calculate_keys(p, q):
+def calculate_keys(p: int, q: int) -> typing.Tuple[int, int]:
     """Calculates an encryption and a decryption key given p and q, and
     returns them as a tuple (e, d)
 
@@ -691,7 +697,10 @@
     return calculate_keys_custom_exponent(p, q, DEFAULT_EXPONENT)
 
 
-def gen_keys(nbits, getprime_func, accurate=True, exponent=DEFAULT_EXPONENT):
+def gen_keys(nbits: int,
+             getprime_func: typing.Callable[[int], int],
+             accurate: bool = True,
+             exponent: int = DEFAULT_EXPONENT) -> typing.Tuple[int, int, int, int]:
     """Generate RSA keys of nbits bits. Returns (p, q, e, d).
 
     Note: this can take a long time, depending on the key size.
@@ -719,7 +728,10 @@
     return p, q, e, d
 
 
-def newkeys(nbits, accurate=True, poolsize=1, exponent=DEFAULT_EXPONENT):
+def newkeys(nbits: int,
+            accurate: bool = True,
+            poolsize: int = 1,
+            exponent: int = DEFAULT_EXPONENT) -> typing.Tuple[PublicKey, PrivateKey]:
     """Generates public and private keys, and returns them as (pub, priv).
 
     The public key is also known as the 'encryption key', and is a
@@ -754,9 +766,9 @@
     # Determine which getprime function to use
     if poolsize > 1:
         from rsa import parallel
-        import functools
 
-        getprime_func = functools.partial(parallel.getprime, poolsize=poolsize)
+        def getprime_func(nbits: int) -> int:
+            return parallel.getprime(nbits, poolsize=poolsize)
     else:
         getprime_func = rsa.prime.getprime
 
diff --git a/rsa/machine_size.py b/rsa/machine_size.py
deleted file mode 100644
index 2a871b8..0000000
--- a/rsa/machine_size.py
+++ /dev/null
@@ -1,74 +0,0 @@
-# -*- coding: utf-8 -*-
-#
-#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
-#
-#  Licensed under the Apache License, Version 2.0 (the "License");
-#  you may not use this file except in compliance with the License.
-#  You may obtain a copy of the License at
-#
-#      https://www.apache.org/licenses/LICENSE-2.0
-#
-#  Unless required by applicable law or agreed to in writing, software
-#  distributed under the License is distributed on an "AS IS" BASIS,
-#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#  See the License for the specific language governing permissions and
-#  limitations under the License.
-
-"""Detection of 32-bit and 64-bit machines and byte alignment."""
-
-import sys
-
-MAX_INT = sys.maxsize
-MAX_INT64 = (1 << 63) - 1
-MAX_INT32 = (1 << 31) - 1
-MAX_INT16 = (1 << 15) - 1
-
-# Determine the word size of the processor.
-if MAX_INT == MAX_INT64:
-    # 64-bit processor.
-    MACHINE_WORD_SIZE = 64
-elif MAX_INT == MAX_INT32:
-    # 32-bit processor.
-    MACHINE_WORD_SIZE = 32
-else:
-    # Else we just assume 64-bit processor keeping up with modern times.
-    MACHINE_WORD_SIZE = 64
-
-
-def get_word_alignment(num, force_arch=64,
-                       _machine_word_size=MACHINE_WORD_SIZE):
-    """
-    Returns alignment details for the given number based on the platform
-    Python is running on.
-
-    :param num:
-        Unsigned integral number.
-    :param force_arch:
-        If you don't want to use 64-bit unsigned chunks, set this to
-        anything other than 64. 32-bit chunks will be preferred then.
-        Default 64 will be used when on a 64-bit machine.
-    :param _machine_word_size:
-        (Internal) The machine word size used for alignment.
-    :returns:
-        4-tuple::
-
-            (word_bits, word_bytes,
-             max_uint, packing_format_type)
-    """
-    max_uint64 = 0xffffffffffffffff
-    max_uint32 = 0xffffffff
-    max_uint16 = 0xffff
-    max_uint8 = 0xff
-
-    if force_arch == 64 and _machine_word_size >= 64 and num > max_uint32:
-        # 64-bit unsigned integer.
-        return 64, 8, max_uint64, "Q"
-    elif num > max_uint16:
-        # 32-bit unsigned integer
-        return 32, 4, max_uint32, "L"
-    elif num > max_uint8:
-        # 16-bit unsigned integer.
-        return 16, 2, max_uint16, "H"
-    else:
-        # 8-bit unsigned integer.
-        return 8, 1, max_uint8, "B"
diff --git a/rsa/parallel.py b/rsa/parallel.py
index a3fe312..f9afedb 100644
--- a/rsa/parallel.py
+++ b/rsa/parallel.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -24,16 +22,14 @@
 
 """
 
-from __future__ import print_function
-
 import multiprocessing as mp
+from multiprocessing.connection import Connection
 
-from rsa._compat import range
 import rsa.prime
 import rsa.randnum
 
 
-def _find_prime(nbits, pipe):
+def _find_prime(nbits: int, pipe: Connection) -> None:
     while True:
         integer = rsa.randnum.read_random_odd_int(nbits)
 
@@ -43,7 +39,7 @@
             return
 
 
-def getprime(nbits, poolsize):
+def getprime(nbits: int, poolsize: int) -> int:
     """Returns a prime number that can be stored in 'nbits' bits.
 
     Works in multiple threads at the same time.
diff --git a/rsa/pem.py b/rsa/pem.py
index 2ddfae8..1ffb446 100644
--- a/rsa/pem.py
+++ b/rsa/pem.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,45 +15,29 @@
 """Functions that load and write PEM-encoded files."""
 
 import base64
+import typing
 
-from rsa._compat import is_bytes, range
+# Should either be ASCII strings or bytes.
+FlexiText = typing.Union[str, bytes]
 
 
-def _markers(pem_marker):
+def _markers(pem_marker: FlexiText) -> typing.Tuple[bytes, bytes]:
     """
     Returns the start and end PEM markers, as bytes.
     """
 
-    if not is_bytes(pem_marker):
+    if not isinstance(pem_marker, bytes):
         pem_marker = pem_marker.encode('ascii')
 
     return (b'-----BEGIN ' + pem_marker + b'-----',
             b'-----END ' + pem_marker + b'-----')
 
 
-def load_pem(contents, pem_marker):
-    """Loads a PEM file.
+def _pem_lines(contents: bytes, pem_start: bytes, pem_end: bytes) -> typing.Iterator[bytes]:
+    """Generator over PEM lines between pem_start and pem_end."""
 
-    :param contents: the contents of the file to interpret
-    :param pem_marker: the marker of the PEM content, such as 'RSA PRIVATE KEY'
-        when your file has '-----BEGIN RSA PRIVATE KEY-----' and
-        '-----END RSA PRIVATE KEY-----' markers.
-
-    :return: the base64-decoded content between the start and end markers.
-
-    @raise ValueError: when the content is invalid, for example when the start
-        marker cannot be found.
-
-    """
-
-    # We want bytes, not text. If it's text, it can be converted to ASCII bytes.
-    if not is_bytes(contents):
-        contents = contents.encode('ascii')
-
-    (pem_start, pem_end) = _markers(pem_marker)
-
-    pem_lines = []
     in_pem_part = False
+    seen_pem_start = False
 
     for line in contents.splitlines():
         line = line.strip()
@@ -67,9 +49,10 @@
         # Handle start marker
         if line == pem_start:
             if in_pem_part:
-                raise ValueError('Seen start marker "%s" twice' % pem_start)
+                raise ValueError('Seen start marker "%r" twice' % pem_start)
 
             in_pem_part = True
+            seen_pem_start = True
             continue
 
         # Skip stuff before first marker
@@ -85,21 +68,44 @@
         if b':' in line:
             continue
 
-        pem_lines.append(line)
+        yield line
 
     # Do some sanity checks
-    if not pem_lines:
-        raise ValueError('No PEM start marker "%s" found' % pem_start)
+    if not seen_pem_start:
+        raise ValueError('No PEM start marker "%r" found' % pem_start)
 
     if in_pem_part:
-        raise ValueError('No PEM end marker "%s" found' % pem_end)
+        raise ValueError('No PEM end marker "%r" found' % pem_end)
+
+
+def load_pem(contents: FlexiText, pem_marker: FlexiText) -> bytes:
+    """Loads a PEM file.
+
+    :param contents: the contents of the file to interpret
+    :param pem_marker: the marker of the PEM content, such as 'RSA PRIVATE KEY'
+        when your file has '-----BEGIN RSA PRIVATE KEY-----' and
+        '-----END RSA PRIVATE KEY-----' markers.
+
+    :return: the base64-decoded content between the start and end markers.
+
+    @raise ValueError: when the content is invalid, for example when the start
+        marker cannot be found.
+
+    """
+
+    # We want bytes, not text. If it's text, it can be converted to ASCII bytes.
+    if not isinstance(contents, bytes):
+        contents = contents.encode('ascii')
+
+    (pem_start, pem_end) = _markers(pem_marker)
+    pem_lines = [line for line in _pem_lines(contents, pem_start, pem_end)]
 
     # Base64-decode the contents
     pem = b''.join(pem_lines)
     return base64.standard_b64decode(pem)
 
 
-def save_pem(contents, pem_marker):
+def save_pem(contents: bytes, pem_marker: FlexiText) -> bytes:
     """Saves a PEM file.
 
     :param contents: the contents to encode in PEM format
diff --git a/rsa/pkcs1.py b/rsa/pkcs1.py
index 84f0e3b..57b0276 100644
--- a/rsa/pkcs1.py
+++ b/rsa/pkcs1.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -30,9 +28,10 @@
 
 import hashlib
 import os
+import sys
+import typing
 
-from rsa._compat import range
-from rsa import common, transform, core
+from . import common, transform, core, key
 
 # ASN.1 codes that describe the hash algorithm used.
 HASH_ASN1 = {
@@ -54,6 +53,21 @@
 }
 
 
+if sys.version_info >= (3, 6):
+    # Python 3.6 introduced SHA3 support.
+    HASH_ASN1.update({
+        'SHA3-256': b'\x30\x31\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x08\x05\x00\x04\x20',
+        'SHA3-384': b'\x30\x41\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x09\x05\x00\x04\x30',
+        'SHA3-512': b'\x30\x51\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x0a\x05\x00\x04\x40',
+    })
+
+    HASH_METHODS.update({
+        'SHA3-256': hashlib.sha3_256,
+        'SHA3-384': hashlib.sha3_384,
+        'SHA3-512': hashlib.sha3_512,
+    })
+
+
 class CryptoError(Exception):
     """Base class for all exceptions in this module."""
 
@@ -66,7 +80,7 @@
     """Raised when verification fails."""
 
 
-def _pad_for_encryption(message, target_length):
+def _pad_for_encryption(message: bytes, target_length: int) -> bytes:
     r"""Pads the message for encryption, returning the padded message.
 
     :return: 00 02 RANDOM_DATA 00 MESSAGE
@@ -112,7 +126,7 @@
                      message])
 
 
-def _pad_for_signing(message, target_length):
+def _pad_for_signing(message: bytes, target_length: int) -> bytes:
     r"""Pads the message for signing, returning the padded message.
 
     The padding is always a repetition of FF bytes.
@@ -146,7 +160,7 @@
                      message])
 
 
-def encrypt(message, pub_key):
+def encrypt(message: bytes, pub_key: key.PublicKey) -> bytes:
     """Encrypts the given message using PKCS#1 v1.5
 
     :param message: the message to encrypt. Must be a byte string no longer than
@@ -178,7 +192,7 @@
     return block
 
 
-def decrypt(crypto, priv_key):
+def decrypt(crypto: bytes, priv_key: key.PrivateKey) -> bytes:
     r"""Decrypts the given message using PKCS#1 v1.5
 
     The decryption is considered 'failed' when the resulting cleartext doesn't
@@ -234,6 +248,12 @@
     decrypted = priv_key.blinded_decrypt(encrypted)
     cleartext = transform.int2bytes(decrypted, blocksize)
 
+    # Detect leading zeroes in the crypto. These are not reflected in the
+    # encrypted value (as leading zeroes do not influence the value of an
+    # integer). This fixes CVE-2020-13757.
+    if len(crypto) > blocksize:
+        raise DecryptionError('Decryption failed')
+
     # If we can't find the cleartext marker, decryption failed.
     if cleartext[0:2] != b'\x00\x02':
         raise DecryptionError('Decryption failed')
@@ -247,14 +267,13 @@
     return cleartext[sep_idx + 1:]
 
 
-def sign_hash(hash_value, priv_key, hash_method):
+def sign_hash(hash_value: bytes, priv_key: key.PrivateKey, hash_method: str) -> bytes:
     """Signs a precomputed hash with the private key.
 
     Hashes the message, then signs the hash with the given key. This is known
     as a "detached signature", because the message itself isn't altered.
 
-    :param hash_value: A precomputed hash to sign (ignores message). Should be set to
-        None if needing to hash and sign message.
+    :param hash_value: A precomputed hash to sign (ignores message).
     :param priv_key: the :py:class:`rsa.PrivateKey` to sign with
     :param hash_method: the hash method used on the message. Use 'MD5', 'SHA-1',
         'SHA-224', SHA-256', 'SHA-384' or 'SHA-512'.
@@ -281,7 +300,7 @@
     return block
 
 
-def sign(message, priv_key, hash_method):
+def sign(message: bytes, priv_key: key.PrivateKey, hash_method: str) -> bytes:
     """Signs the message with the private key.
 
     Hashes the message, then signs the hash with the given key. This is known
@@ -303,7 +322,7 @@
     return sign_hash(msg_hash, priv_key, hash_method)
 
 
-def verify(message, signature, pub_key):
+def verify(message: bytes, signature: bytes, pub_key: key.PublicKey) -> str:
     """Verifies that the signature matches the message.
 
     The hash method is detected automatically from the signature.
@@ -331,6 +350,9 @@
     cleartext = HASH_ASN1[method_name] + message_hash
     expected = _pad_for_signing(cleartext, keylength)
 
+    if len(signature) != keylength:
+        raise VerificationError('Verification failed')
+
     # Compare with the signed one
     if expected != clearsig:
         raise VerificationError('Verification failed')
@@ -338,7 +360,7 @@
     return method_name
 
 
-def find_signature_hash(signature, pub_key):
+def find_signature_hash(signature: bytes, pub_key: key.PublicKey) -> str:
     """Returns the hash name detected from the signature.
 
     If you also want to verify the message, use :py:func:`rsa.verify()` instead.
@@ -357,7 +379,7 @@
     return _find_method_hash(clearsig)
 
 
-def yield_fixedblocks(infile, blocksize):
+def yield_fixedblocks(infile: typing.BinaryIO, blocksize: int) -> typing.Iterator[bytes]:
     """Generator, yields each block of ``blocksize`` bytes in the input file.
 
     :param infile: file to read and separate in blocks.
@@ -378,7 +400,7 @@
             break
 
 
-def compute_hash(message, method_name):
+def compute_hash(message: typing.Union[bytes, typing.BinaryIO], method_name: str) -> bytes:
     """Returns the message digest.
 
     :param message: the signed message. Can be an 8-bit string or a file-like
@@ -395,18 +417,18 @@
     method = HASH_METHODS[method_name]
     hasher = method()
 
-    if hasattr(message, 'read') and hasattr(message.read, '__call__'):
+    if isinstance(message, bytes):
+        hasher.update(message)
+    else:
+        assert hasattr(message, 'read') and hasattr(message.read, '__call__')
         # read as 1K blocks
         for block in yield_fixedblocks(message, 1024):
             hasher.update(block)
-    else:
-        # hash the message object itself.
-        hasher.update(message)
 
     return hasher.digest()
 
 
-def _find_method_hash(clearsig):
+def _find_method_hash(clearsig: bytes) -> str:
     """Finds the hash method.
 
     :param clearsig: full padded ASN1 and hash.
diff --git a/rsa/pkcs1_v2.py b/rsa/pkcs1_v2.py
index 5f9c7dd..f780aff 100644
--- a/rsa/pkcs1_v2.py
+++ b/rsa/pkcs1_v2.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -20,7 +18,6 @@
 documentation is RFC 2437: https://tools.ietf.org/html/rfc2437
 """
 
-from rsa._compat import range
 from rsa import (
     common,
     pkcs1,
@@ -28,7 +25,7 @@
 )
 
 
-def mgf1(seed, length, hasher='SHA-1'):
+def mgf1(seed: bytes, length: int, hasher: str = 'SHA-1') -> bytes:
     """
     MGF1 is a Mask Generation Function based on a hash function.
 
diff --git a/rsa/prime.py b/rsa/prime.py
index 3d63542..853aca5 100644
--- a/rsa/prime.py
+++ b/rsa/prime.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -20,14 +18,13 @@
 Roberto Tamassia, 2002.
 """
 
-from rsa._compat import range
 import rsa.common
 import rsa.randnum
 
 __all__ = ['getprime', 'are_relatively_prime']
 
 
-def gcd(p, q):
+def gcd(p: int, q: int) -> int:
     """Returns the greatest common divisor of p and q
 
     >>> gcd(48, 180)
@@ -39,7 +36,7 @@
     return p
 
 
-def get_primality_testing_rounds(number):
+def get_primality_testing_rounds(number: int) -> int:
     """Returns minimum number of rounds for Miller-Rabing primality testing,
     based on number bitsize.
 
@@ -65,7 +62,7 @@
     return 10
 
 
-def miller_rabin_primality_testing(n, k):
+def miller_rabin_primality_testing(n: int, k: int) -> bool:
     """Calculates whether n is composite (which is always correct) or prime
     (which theoretically is incorrect with error probability 4**-k), by
     applying Miller-Rabin primality testing.
@@ -118,7 +115,7 @@
     return True
 
 
-def is_prime(number):
+def is_prime(number: int) -> bool:
     """Returns True if the number is prime, and False otherwise.
 
     >>> is_prime(2)
@@ -144,7 +141,7 @@
     return miller_rabin_primality_testing(number, k + 1)
 
 
-def getprime(nbits):
+def getprime(nbits: int) -> int:
     """Returns a prime number that can be stored in 'nbits' bits.
 
     >>> p = getprime(128)
@@ -172,7 +169,7 @@
             # Retry if not prime
 
 
-def are_relatively_prime(a, b):
+def are_relatively_prime(a: int, b: int) -> bool:
     """Returns True if a and b are relatively prime, and False if they
     are not.
 
diff --git a/rsa/randnum.py b/rsa/randnum.py
index 310acaa..a5bb850 100644
--- a/rsa/randnum.py
+++ b/rsa/randnum.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -19,12 +17,12 @@
 # Source inspired by code by Yesudeep Mangalapilly <yesudeep@gmail.com>
 
 import os
+import struct
 
 from rsa import common, transform
-from rsa._compat import byte
 
 
-def read_random_bits(nbits):
+def read_random_bits(nbits: int) -> bytes:
     """Reads 'nbits' random bits.
 
     If nbits isn't a whole number of bytes, an extra byte will be appended with
@@ -40,12 +38,12 @@
     if rbits > 0:
         randomvalue = ord(os.urandom(1))
         randomvalue >>= (8 - rbits)
-        randomdata = byte(randomvalue) + randomdata
+        randomdata = struct.pack("B", randomvalue) + randomdata
 
     return randomdata
 
 
-def read_random_int(nbits):
+def read_random_int(nbits: int) -> int:
     """Reads a random integer of approximately nbits bits.
     """
 
@@ -59,7 +57,7 @@
     return value
 
 
-def read_random_odd_int(nbits):
+def read_random_odd_int(nbits: int) -> int:
     """Reads a random odd integer of approximately nbits bits.
 
     >>> read_random_odd_int(512) & 1
@@ -72,7 +70,7 @@
     return value | 1
 
 
-def randint(maxvalue):
+def randint(maxvalue: int) -> int:
     """Returns a random integer x with 1 <= x <= maxvalue
 
     May take a very long time in specific situations. If maxvalue needs N bits
diff --git a/rsa/transform.py b/rsa/transform.py
index 628d0af..03c4a77 100644
--- a/rsa/transform.py
+++ b/rsa/transform.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -19,16 +17,10 @@
 From bytes to a number, number to bytes, etc.
 """
 
-from __future__ import absolute_import
-
-import binascii
-from struct import pack
-
-from rsa._compat import byte, is_integer
-from rsa import common, machine_size
+import math
 
 
-def bytes2int(raw_bytes):
+def bytes2int(raw_bytes: bytes) -> int:
     r"""Converts a list of bytes or an 8-bit string to an integer.
 
     When using unicode strings, encode it to some encoding like UTF8 first.
@@ -39,110 +31,14 @@
     8405007
 
     """
-
-    return int(binascii.hexlify(raw_bytes), 16)
+    return int.from_bytes(raw_bytes, 'big', signed=False)
 
 
-def _int2bytes(number, block_size=None):
-    r"""Converts a number to a string of bytes.
-
-    Usage::
-
-        >>> _int2bytes(123456789)
-        b'\x07[\xcd\x15'
-        >>> bytes2int(_int2bytes(123456789))
-        123456789
-
-        >>> _int2bytes(123456789, 6)
-        b'\x00\x00\x07[\xcd\x15'
-        >>> bytes2int(_int2bytes(123456789, 128))
-        123456789
-
-        >>> _int2bytes(123456789, 3)
-        Traceback (most recent call last):
-        ...
-        OverflowError: Needed 4 bytes for number, but block size is 3
-
-    @param number: the number to convert
-    @param block_size: the number of bytes to output. If the number encoded to
-        bytes is less than this, the block will be zero-padded. When not given,
-        the returned block is not padded.
-
-    @throws OverflowError when block_size is given and the number takes up more
-        bytes than fit into the block.
+def int2bytes(number: int, fill_size: int = 0) -> bytes:
     """
+    Convert an unsigned integer to bytes (big-endian)::
 
-    # Type checking
-    if not is_integer(number):
-        raise TypeError("You must pass an integer for 'number', not %s" %
-                        number.__class__)
-
-    if number < 0:
-        raise ValueError('Negative numbers cannot be used: %i' % number)
-
-    # Do some bounds checking
-    if number == 0:
-        needed_bytes = 1
-        raw_bytes = [b'\x00']
-    else:
-        needed_bytes = common.byte_size(number)
-        raw_bytes = []
-
-    # You cannot compare None > 0 in Python 3x. It will fail with a TypeError.
-    if block_size and block_size > 0:
-        if needed_bytes > block_size:
-            raise OverflowError('Needed %i bytes for number, but block size '
-                                'is %i' % (needed_bytes, block_size))
-
-    # Convert the number to bytes.
-    while number > 0:
-        raw_bytes.insert(0, byte(number & 0xFF))
-        number >>= 8
-
-    # Pad with zeroes to fill the block
-    if block_size and block_size > 0:
-        padding = (block_size - needed_bytes) * b'\x00'
-    else:
-        padding = b''
-
-    return padding + b''.join(raw_bytes)
-
-
-def bytes_leading(raw_bytes, needle=b'\x00'):
-    """
-    Finds the number of prefixed byte occurrences in the haystack.
-
-    Useful when you want to deal with padding.
-
-    :param raw_bytes:
-        Raw bytes.
-    :param needle:
-        The byte to count. Default \x00.
-    :returns:
-        The number of leading needle bytes.
-    """
-
-    leading = 0
-    # Indexing keeps compatibility between Python 2.x and Python 3.x
-    _byte = needle[0]
-    for x in raw_bytes:
-        if x == _byte:
-            leading += 1
-        else:
-            break
-    return leading
-
-
-def int2bytes(number, fill_size=None, chunk_size=None, overflow=False):
-    """
-    Convert an unsigned integer to bytes (base-256 representation)::
-
-    Does not preserve leading zeros if you don't specify a chunk size or
-    fill size.
-
-    .. NOTE:
-        You must not specify both fill_size and chunk_size. Only one
-        of them is allowed.
+    Does not preserve leading zeros if you don't specify a fill size.
 
     :param number:
         Integer value
@@ -150,15 +46,6 @@
         If the optional fill size is given the length of the resulting
         byte string is expected to be the fill size and will be padded
         with prefix zero bytes to satisfy that length.
-    :param chunk_size:
-        If optional chunk size is given and greater than zero, pad the front of
-        the byte string with binary zeros so that the length is a multiple of
-        ``chunk_size``.
-    :param overflow:
-        ``False`` (default). If this is ``True``, no ``OverflowError``
-        will be raised when the fill_size is shorter than the length
-        of the generated byte sequence. Instead the byte sequence will
-        be returned as is.
     :returns:
         Raw bytes (base-256 representation).
     :raises:
@@ -171,42 +58,12 @@
     if number < 0:
         raise ValueError("Number must be an unsigned integer: %d" % number)
 
-    if fill_size and chunk_size:
-        raise ValueError("You can either fill or pad chunks, but not both")
+    bytes_required = max(1, math.ceil(number.bit_length() / 8))
 
-    # Ensure these are integers.
-    number & 1
+    if fill_size > 0:
+        return number.to_bytes(fill_size, 'big')
 
-    raw_bytes = b''
-
-    # Pack the integer one machine word at a time into bytes.
-    num = number
-    word_bits, _, max_uint, pack_type = machine_size.get_word_alignment(num)
-    pack_format = ">%s" % pack_type
-    while num > 0:
-        raw_bytes = pack(pack_format, num & max_uint) + raw_bytes
-        num >>= word_bits
-    # Obtain the index of the first non-zero byte.
-    zero_leading = bytes_leading(raw_bytes)
-    if number == 0:
-        raw_bytes = b'\x00'
-    # De-padding.
-    raw_bytes = raw_bytes[zero_leading:]
-
-    length = len(raw_bytes)
-    if fill_size and fill_size > 0:
-        if not overflow and length > fill_size:
-            raise OverflowError(
-                    "Need %d bytes for number, but fill size is %d" %
-                    (length, fill_size)
-            )
-        raw_bytes = raw_bytes.rjust(fill_size, b'\x00')
-    elif chunk_size and chunk_size > 0:
-        remainder = length % chunk_size
-        if remainder:
-            padding_size = chunk_size - remainder
-            raw_bytes = raw_bytes.rjust(length + padding_size, b'\x00')
-    return raw_bytes
+    return number.to_bytes(bytes_required, 'big')
 
 
 if __name__ == '__main__':
diff --git a/rsa/util.py b/rsa/util.py
index 29d5eb1..cb31c46 100644
--- a/rsa/util.py
+++ b/rsa/util.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -16,15 +14,13 @@
 
 """Utility functions."""
 
-from __future__ import with_statement, print_function
-
 import sys
 from optparse import OptionParser
 
 import rsa.key
 
 
-def private_to_public():
+def private_to_public() -> None:
     """Reads a private key and outputs the corresponding public key."""
 
     # Parse the CLI options
diff --git a/setup.cfg b/setup.cfg
index ed8a958..e377bdb 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -3,3 +3,17 @@
 
 [metadata]
 license_file = LICENSE
+
+[flake8]
+max-line-length = 100
+max-complexity = 10
+
+[pep8]
+max-line-length = 100
+
+[mypy]
+python_version = 3.7
+warn_unused_ignores = True
+ignore_missing_imports = True
+follow_imports = skip
+incremental = True
diff --git a/setup.py b/setup.py
index 18f339e..2d22865 100755
--- a/setup.py
+++ b/setup.py
@@ -14,14 +14,18 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 
+# io.open is needed for projects that support Python 2.7. It ensures open()
+# defaults to text mode with universal newlines, and accepts an argument to
+# specify the text encoding Python 3 only projects can skip this import.
+from io import open
 from setuptools import setup
 
-with open('README.md') as f:
+with open('README.md', encoding='utf-8') as f:
     long_description = f.read()
 
 if __name__ == '__main__':
     setup(name='rsa',
-          version='4.0',
+          version='4.6',
           description='Pure-Python RSA implementation',
           long_description=long_description,
           long_description_content_type='text/markdown',
@@ -40,17 +44,16 @@
               'License :: OSI Approved :: Apache Software License',
               'Operating System :: OS Independent',
               'Programming Language :: Python',
-              'Programming Language :: Python :: 2',
-              'Programming Language :: Python :: 2.7',
               'Programming Language :: Python :: 3',
-              'Programming Language :: Python :: 3.4',
               'Programming Language :: Python :: 3.5',
               'Programming Language :: Python :: 3.6',
               'Programming Language :: Python :: 3.7',
+              'Programming Language :: Python :: 3.8',
               'Programming Language :: Python :: Implementation :: CPython',
               'Programming Language :: Python :: Implementation :: PyPy',
               'Topic :: Security :: Cryptography',
           ],
+          python_requires='>=3.5, <4',
           install_requires=[
               'pyasn1 >= 0.1.3',
           ],
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 7ce57eb..1cd92c2 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -4,47 +4,37 @@
 
 from __future__ import print_function
 
-import unittest
-import sys
 import functools
-from contextlib import contextmanager
-
+import io
 import os
-from io import StringIO, BytesIO
+import sys
+import typing
+import unittest
+from contextlib import contextmanager, redirect_stdout, redirect_stderr
 
 import rsa
 import rsa.cli
 import rsa.util
-from rsa._compat import PY2
-
-
-def make_buffer():
-    if PY2:
-        return BytesIO()
-    buf = StringIO()
-    buf.buffer = BytesIO()
-    return buf
-
-
-def get_bytes_out(out):
-    if PY2:
-        # Python 2.x writes 'str' to stdout
-        return out.getvalue()
-    # Python 3.x writes 'bytes' to stdout.buffer
-    return out.buffer.getvalue()
 
 
 @contextmanager
-def captured_output():
+def captured_output() -> typing.Generator:
     """Captures output to stdout and stderr"""
 
-    new_out, new_err = make_buffer(), make_buffer()
-    old_out, old_err = sys.stdout, sys.stderr
-    try:
-        sys.stdout, sys.stderr = new_out, new_err
-        yield new_out, new_err
-    finally:
-        sys.stdout, sys.stderr = old_out, old_err
+    # According to mypy, we're not supposed to change buf_out.buffer.
+    # However, this is just a test, and it works, hence the 'type: ignore'.
+    buf_out = io.StringIO()
+    buf_out.buffer = io.BytesIO()  # type: ignore
+
+    buf_err = io.StringIO()
+    buf_err.buffer = io.BytesIO()  # type: ignore
+
+    with redirect_stdout(buf_out), redirect_stderr(buf_err):
+        yield buf_out, buf_err
+
+
+def get_bytes_out(buf) -> bytes:
+    return buf.buffer.getvalue()
 
 
 @contextmanager
diff --git a/tests/test_common.py b/tests/test_common.py
index af13695..71b81d0 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -1,6 +1,4 @@
 #!/usr/bin/env python
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/tests/test_compat.py b/tests/test_compat.py
index 62e933f..e74f046 100644
--- a/tests/test_compat.py
+++ b/tests/test_compat.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,7 +15,7 @@
 import unittest
 import struct
 
-from rsa._compat import byte, is_bytes, range, xor_bytes
+from rsa._compat import byte, xor_bytes
 
 
 class TestByte(unittest.TestCase):
@@ -26,7 +24,7 @@
     def test_byte(self):
         for i in range(256):
             byt = byte(i)
-            self.assertTrue(is_bytes(byt))
+            self.assertIsInstance(byt, bytes)
             self.assertEqual(ord(byt), i)
 
     def test_raises_StructError_on_overflow(self):
diff --git a/tests/test_integers.py b/tests/test_integers.py
index fb29ba4..2ca0a9a 100644
--- a/tests/test_integers.py
+++ b/tests/test_integers.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/tests/test_load_save_keys.py b/tests/test_load_save_keys.py
index 55bd5a4..7892fb3 100644
--- a/tests/test_load_save_keys.py
+++ b/tests/test_load_save_keys.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,13 +15,12 @@
 """Unittest for saving and loading keys."""
 
 import base64
-import mock
 import os.path
 import pickle
 import unittest
 import warnings
+from unittest import mock
 
-from rsa._compat import range
 import rsa.key
 
 B64PRIV_DER = b'MC4CAQACBQDeKYlRAgMBAAECBQDHn4npAgMA/icCAwDfxwIDANcXAgInbwIDAMZt'
diff --git a/tests/test_mypy.py b/tests/test_mypy.py
new file mode 100644
index 0000000..8258e7e
--- /dev/null
+++ b/tests/test_mypy.py
@@ -0,0 +1,27 @@
+import pathlib
+import unittest
+
+import mypy.api
+
+test_modules = ['rsa', 'tests']
+
+
+class MypyRunnerTest(unittest.TestCase):
+    def test_run_mypy(self):
+        proj_root = pathlib.Path(__file__).parent.parent
+        args = ['--incremental', '--ignore-missing-imports'] + [str(proj_root / dirname) for dirname
+                                                                in test_modules]
+
+        result = mypy.api.run(args)
+
+        stdout, stderr, status = result
+
+        messages = []
+        if stderr:
+            messages.append(stderr)
+        if stdout:
+            messages.append(stdout)
+        if status:
+            messages.append('Mypy failed with status %d' % status)
+        if messages and not all('Success' in message for message in messages):
+            self.fail('\n'.join(['Mypy errors:'] + messages))
diff --git a/tests/test_pem.py b/tests/test_pem.py
index 5fb9600..dd03cca 100644
--- a/tests/test_pem.py
+++ b/tests/test_pem.py
@@ -1,6 +1,4 @@
 #!/usr/bin/env python
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,7 +15,6 @@
 
 import unittest
 
-from rsa._compat import is_bytes
 from rsa.pem import _markers
 import rsa.key
 
@@ -79,13 +76,13 @@
 
     def test_bytes_public(self):
         key = rsa.key.PublicKey.load_pkcs1_openssl_pem(public_key_pem)
-        self.assertTrue(is_bytes(key.save_pkcs1(format='DER')))
-        self.assertTrue(is_bytes(key.save_pkcs1(format='PEM')))
+        self.assertIsInstance(key.save_pkcs1(format='DER'), bytes)
+        self.assertIsInstance(key.save_pkcs1(format='PEM'), bytes)
 
     def test_bytes_private(self):
         key = rsa.key.PrivateKey.load_pkcs1(private_key_pem)
-        self.assertTrue(is_bytes(key.save_pkcs1(format='DER')))
-        self.assertTrue(is_bytes(key.save_pkcs1(format='PEM')))
+        self.assertIsInstance(key.save_pkcs1(format='DER'), bytes)
+        self.assertIsInstance(key.save_pkcs1(format='PEM'), bytes)
 
 
 class TestByteInput(unittest.TestCase):
@@ -93,10 +90,10 @@
 
     def test_bytes_public(self):
         key = rsa.key.PublicKey.load_pkcs1_openssl_pem(public_key_pem.encode('ascii'))
-        self.assertTrue(is_bytes(key.save_pkcs1(format='DER')))
-        self.assertTrue(is_bytes(key.save_pkcs1(format='PEM')))
+        self.assertIsInstance(key.save_pkcs1(format='DER'), bytes)
+        self.assertIsInstance(key.save_pkcs1(format='PEM'), bytes)
 
     def test_bytes_private(self):
         key = rsa.key.PrivateKey.load_pkcs1(private_key_pem.encode('ascii'))
-        self.assertTrue(is_bytes(key.save_pkcs1(format='DER')))
-        self.assertTrue(is_bytes(key.save_pkcs1(format='PEM')))
+        self.assertIsInstance(key.save_pkcs1(format='DER'), bytes)
+        self.assertIsInstance(key.save_pkcs1(format='PEM'), bytes)
diff --git a/tests/test_pkcs1.py b/tests/test_pkcs1.py
index 5377b30..f7baf7f 100644
--- a/tests/test_pkcs1.py
+++ b/tests/test_pkcs1.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,11 +15,12 @@
 """Tests string operations."""
 
 import struct
+import sys
 import unittest
 
 import rsa
 from rsa import pkcs1
-from rsa._compat import byte, is_bytes
+from rsa._compat import byte
 
 
 class BinaryTest(unittest.TestCase):
@@ -46,8 +45,8 @@
 
         # Alter the encrypted stream
         a = encrypted[5]
-        if is_bytes(a):
-            a = ord(a)
+        self.assertIsInstance(a, int)
+
         altered_a = (a + 1) % 256
         encrypted = encrypted[:5] + byte(altered_a) + encrypted[6:]
 
@@ -66,6 +65,32 @@
         self.assertNotEqual(encrypted1, encrypted2)
 
 
+class ExtraZeroesTest(unittest.TestCase):
+    def setUp(self):
+        # Key, cyphertext, and plaintext taken from https://github.com/sybrenstuvel/python-rsa/issues/146
+        self.private_key = rsa.PrivateKey.load_pkcs1(
+            "-----BEGIN RSA PRIVATE KEY-----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\nXVJl32HuKYPJOMIWfc7hIfxLRHnCN+coz2M6tgqMDs0E/OfjuqVZ\n-----END RSA PRIVATE KEY-----",
+            format='PEM')
+        self.cyphertext = bytes.fromhex(
+            "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"
+        )
+        self.plaintext = bytes.fromhex("54657374")
+
+    def test_unmodified(self):
+        message = rsa.decrypt(self.cyphertext, self.private_key)
+        self.assertEqual(message, self.plaintext)
+
+    def test_prepend_zeroes(self):
+        cyphertext = bytes.fromhex("0000") + self.cyphertext
+        with self.assertRaises(rsa.DecryptionError):
+            rsa.decrypt(cyphertext, self.private_key)
+
+    def test_append_zeroes(self):
+        cyphertext = self.cyphertext + bytes.fromhex("0000")
+        with self.assertRaises(rsa.DecryptionError):
+            rsa.decrypt(cyphertext, self.private_key)
+
+
 class SignatureTest(unittest.TestCase):
     def setUp(self):
         (self.pub, self.priv) = rsa.newkeys(512)
@@ -75,9 +100,17 @@
 
         message = b'je moeder'
         signature = pkcs1.sign(message, self.priv, 'SHA-256')
-
         self.assertEqual('SHA-256', pkcs1.verify(message, signature, self.pub))
 
+
+    @unittest.skipIf(sys.version_info < (3, 6), "SHA3 requires Python 3.6+")
+    def test_sign_verify_sha3(self):
+        """Test happy flow of sign and verify with SHA3-256"""
+
+        message = b'je moeder'
+        signature = pkcs1.sign(message, self.priv, 'SHA3-256')
+        self.assertEqual('SHA3-256', pkcs1.verify(message, signature, self.pub))
+
     def test_find_signature_hash(self):
         """Test happy flow of sign and find_signature_hash"""
 
@@ -132,3 +165,21 @@
         signature = pkcs1.sign_hash(msg_hash, self.priv, 'SHA-224')
 
         self.assertTrue(pkcs1.verify(message, signature, self.pub))
+
+    def test_prepend_zeroes(self):
+        """Prepending the signature with zeroes should be detected."""
+
+        message = b'je moeder'
+        signature = pkcs1.sign(message, self.priv, 'SHA-256')
+        signature = bytes.fromhex('0000') + signature
+        with self.assertRaises(rsa.VerificationError):
+            pkcs1.verify(message, signature, self.pub)
+
+    def test_apppend_zeroes(self):
+        """Apppending the signature with zeroes should be detected."""
+
+        message = b'je moeder'
+        signature = pkcs1.sign(message, self.priv, 'SHA-256')
+        signature = signature + bytes.fromhex('0000')
+        with self.assertRaises(rsa.VerificationError):
+            pkcs1.verify(message, signature, self.pub)
diff --git a/tests/test_pkcs1_v2.py b/tests/test_pkcs1_v2.py
index 1d8f001..bba525e 100644
--- a/tests/test_pkcs1_v2.py
+++ b/tests/test_pkcs1_v2.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/tests/test_prime.py b/tests/test_prime.py
index f3bda9b..5577f67 100644
--- a/tests/test_prime.py
+++ b/tests/test_prime.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -18,7 +16,6 @@
 
 import unittest
 
-from rsa._compat import range
 import rsa.prime
 import rsa.randnum
 
diff --git a/tests/test_strings.py b/tests/test_strings.py
index 28fa091..1090a8e 100644
--- a/tests/test_strings.py
+++ b/tests/test_strings.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -31,12 +29,12 @@
 
     def test_enc_dec(self):
         message = unicode_string.encode('utf-8')
-        print("\tMessage:   %s" % message)
+        print("\tMessage:   %r" % message)
 
         encrypted = rsa.encrypt(message, self.pub)
-        print("\tEncrypted: %s" % encrypted)
+        print("\tEncrypted: %r" % encrypted)
 
         decrypted = rsa.decrypt(encrypted, self.priv)
-        print("\tDecrypted: %s" % decrypted)
+        print("\tDecrypted: %r" % decrypted)
 
         self.assertEqual(message, decrypted)
diff --git a/tests/test_transform.py b/tests/test_transform.py
index fe0970c..7b9335e 100644
--- a/tests/test_transform.py
+++ b/tests/test_transform.py
@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-#
 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
 #
 #  Licensed under the Apache License, Version 2.0 (the "License");
@@ -15,37 +13,26 @@
 #  limitations under the License.
 
 import unittest
-from rsa.transform import int2bytes, bytes2int, _int2bytes
+from rsa.transform import int2bytes, bytes2int
 
 
 class Test_int2bytes(unittest.TestCase):
     def test_accuracy(self):
         self.assertEqual(int2bytes(123456789), b'\x07[\xcd\x15')
-        self.assertEqual(_int2bytes(123456789), b'\x07[\xcd\x15')
 
     def test_codec_identity(self):
         self.assertEqual(bytes2int(int2bytes(123456789, 128)), 123456789)
-        self.assertEqual(bytes2int(_int2bytes(123456789, 128)), 123456789)
 
     def test_chunk_size(self):
         self.assertEqual(int2bytes(123456789, 6), b'\x00\x00\x07[\xcd\x15')
         self.assertEqual(int2bytes(123456789, 7),
                          b'\x00\x00\x00\x07[\xcd\x15')
 
-        self.assertEqual(_int2bytes(123456789, 6),
-                         b'\x00\x00\x07[\xcd\x15')
-        self.assertEqual(_int2bytes(123456789, 7),
-                         b'\x00\x00\x00\x07[\xcd\x15')
-
     def test_zero(self):
         self.assertEqual(int2bytes(0, 4), b'\x00' * 4)
         self.assertEqual(int2bytes(0, 7), b'\x00' * 7)
         self.assertEqual(int2bytes(0), b'\x00')
 
-        self.assertEqual(_int2bytes(0, 4), b'\x00' * 4)
-        self.assertEqual(_int2bytes(0, 7), b'\x00' * 7)
-        self.assertEqual(_int2bytes(0), b'\x00')
-
     def test_correctness_against_base_implementation(self):
         # Slow test.
         values = [
@@ -54,26 +41,16 @@
             1 << 77,
         ]
         for value in values:
-            self.assertEqual(int2bytes(value), _int2bytes(value),
-                             "Boom %d" % value)
             self.assertEqual(bytes2int(int2bytes(value)),
                              value,
                              "Boom %d" % value)
-            self.assertEqual(bytes2int(_int2bytes(value)),
-                             value,
-                             "Boom %d" % value)
 
     def test_raises_OverflowError_when_chunk_size_is_insufficient(self):
         self.assertRaises(OverflowError, int2bytes, 123456789, 3)
         self.assertRaises(OverflowError, int2bytes, 299999999999, 4)
 
-        self.assertRaises(OverflowError, _int2bytes, 123456789, 3)
-        self.assertRaises(OverflowError, _int2bytes, 299999999999, 4)
-
     def test_raises_ValueError_when_negative_integer(self):
         self.assertRaises(ValueError, int2bytes, -1)
-        self.assertRaises(ValueError, _int2bytes, -1)
 
     def test_raises_TypeError_when_not_integer(self):
         self.assertRaises(TypeError, int2bytes, None)
-        self.assertRaises(TypeError, _int2bytes, None)
diff --git a/tox.ini b/tox.ini
index a3109e4..0552a17 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,20 +1,18 @@
 [tox]
 # Environment changes have to be manually synced with '.travis.yml'.
-envlist = py27,py34,py35,py36,p37,pypy
+envlist = py35,py36,p37,p38
 
 [pytest]
 addopts = -v --cov rsa --cov-report term-missing
 
 [testenv]
 deps = pipenv
-commands=
-    pipenv install --dev
-    pipenv run py.test tests
+commands =
+    pipenv install --dev --deploy
+    pipenv run py.test tests/
 
-[testenv:py36]
+[testenv:py37]
+whitelist_externals = pipenv
 commands=
-    pipenv install --dev --ignore-pipfile
-    pipenv run py.test --doctest-modules rsa tests
-
-[pep8]
-max-line-length = 100
+    pipenv install -v
+    pipenv run py.test --doctest-modules rsa tests/
diff --git a/update_version.sh b/update_version.sh
new file mode 100755
index 0000000..89c5447
--- /dev/null
+++ b/update_version.sh
@@ -0,0 +1,18 @@
+#!/bin/bash
+
+if [ -z "$1" ]; then
+    echo "Usage: $0 new-version" >&2
+    exit 1
+fi
+
+DATE=$(date +'%Y-%m-%d')
+
+sed "s/__date__\s=\s'[^']*'/__date__ = '$DATE'/" -i rsa/__init__.py
+sed "s/__version__\s=\s'[^']*'/__version__ = '$1'/" -i rsa/__init__.py
+sed "s/version='[^']*'/version='$1'/" -i setup.py
+
+git diff
+echo
+echo "Don't forget to commit and tag:"
+echo git commit -m \'Bumped version to $1\' rsa/__init__.py setup.py
+echo git tag -a version-$1 -m \'Tagged version $1\'