diff --git a/tests/data/other_cert.pem b/tests/data/other_cert.pem
new file mode 100644
index 0000000..6895d1e
--- /dev/null
+++ b/tests/data/other_cert.pem
@@ -0,0 +1,33 @@
+-----BEGIN CERTIFICATE-----
+MIIFtTCCA52gAwIBAgIJAPBsLZmNGfKtMA0GCSqGSIb3DQEBBQUAMEUxCzAJBgNV
+BAYTAkFVMRMwEQYDVQQIEwpTb21lLVN0YXRlMSEwHwYDVQQKExhJbnRlcm5ldCBX
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+/KpGEs5UgKY+P9NulYpTDwCU/6SomuQpP795wbG6sm7Hzq82r2RmB61GupNRGeqi
+pXKsy69T388zBxYu6zQrosXiDl5YzaViH7tm0J7opye8dCWjjpnahki0vq2znti7
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+lSrwNr3qbGTEJX8JqpDBNVAd96XkMvDNyA==
+-----END CERTIFICATE-----
diff --git a/tests/data/pem_from_pkcs12.pem b/tests/data/pem_from_pkcs12.pem
new file mode 100644
index 0000000..2d77e10
--- /dev/null
+++ b/tests/data/pem_from_pkcs12.pem
@@ -0,0 +1,32 @@
+Bag Attributes
+    friendlyName: key
+    localKeyID: 22 7E 04 FC 64 48 20 83 1E C1 BD E3 F5 2F 44 7D EA 99 A5 BC 
+Key Attributes: <No Attributes>
+-----BEGIN PRIVATE KEY-----
+MIIEvgIBADANBgkqhkiG9w0BAQEFAASCBKgwggSkAgEAAoIBAQDh6PSnttDsv+vi
+tUZTP1E3hVBah6PUGDWZhYgNiyW8quTWCmPvBmCR2YzuhUrY5+CtKP8UJOQico+p
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++qTGDk5hAgMBAAECggEBAMYYfNDEYpf4A2SdCLne/9zrrfZ0kphdUkL48MDPj5vN
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++DrjGSIkTfhzbLa20Vt3AFM+LFK0ZoXT2dRnjbYPjQECgYEA+9XJFGwLcEX6pl1p
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+DwPY80cw0M0k9UeMnBxHt25JMDrDan/iTbxu++T/jlNrdebOXFlxlI5y3c7fULDS
+LZcNVzTXwhjlt7yp6d0NgzTyJw2ju9BiREfnTiRBAoGBAOPHrTOnPyjO+bVcCPTB
+WGLsbBd77mVPGIuL0XGrvbVYPE8yIcNbZcthd8VXL/38Ygy8SIZh2ZqsrU1b5WFa
+XUMLnGEODSS8x/GmW3i3KeirW5OxBNjfUzEF4XkJP8m41iTdsQEXQf9DdUY7X+CB
+VL5h7N0VstYhGgycuPpcIUQa
+-----END PRIVATE KEY-----
diff --git a/tests/data/privatekey.p12 b/tests/data/privatekey.p12
new file mode 100644
index 0000000..c369ecb
--- /dev/null
+++ b/tests/data/privatekey.p12
Binary files differ
diff --git a/tests/data/privatekey.pem b/tests/data/privatekey.pem
new file mode 100644
index 0000000..5744354
--- /dev/null
+++ b/tests/data/privatekey.pem
@@ -0,0 +1,27 @@
+-----BEGIN RSA PRIVATE KEY-----
+MIIEpAIBAAKCAQEA4ej0p7bQ7L/r4rVGUz9RN4VQWoej1Bg1mYWIDYslvKrk1gpj
+7wZgkdmM7oVK2OfgrSj/FCTkInKPqaCR0gD7K80q+mLBrN3PUkDrJQZpvRZIff3/
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+HD9CI1IcqqBRR2AFscsmmX6YzPLTuEKBGMW8twaYy3utlFxElMwoUEsrSWRcCA1y
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+UqWWSRedVBg4Ijbw3nWwYDECgYB0pLew4z4bVuvdt+HgnJA9n0EuYowVdadpTEJg
+soBjNHV4msLzdNqbjrAqgz6M/n8Ztg8D2PNHMNDNJPVHjJwcR7duSTA6w2p/4k28
+bvvk/45Ta3XmzlxZcZSOct3O31Cw0i2XDVc018IY5be8qendDYM08icNo7vQYkRH
+504kQQKBgQDjx60zpz8ozvm1XAj0wVhi7GwXe+5lTxiLi9Fxq721WDxPMiHDW2XL
+YXfFVy/9/GIMvEiGYdmarK1NW+VhWl1DC5xhDg0kvMfxplt4tynoq1uTsQTY31Mx
+BeF5CT/JuNYk3bEBF0H/Q3VGO1/ggVS+YezdFbLWIRoMnLj6XCFEGg==
+-----END RSA PRIVATE KEY-----
diff --git a/tests/data/privatekey.pub b/tests/data/privatekey.pub
new file mode 100644
index 0000000..11fdaa4
--- /dev/null
+++ b/tests/data/privatekey.pub
@@ -0,0 +1,8 @@
+-----BEGIN RSA PUBLIC KEY-----
+MIIBCgKCAQEA4ej0p7bQ7L/r4rVGUz9RN4VQWoej1Bg1mYWIDYslvKrk1gpj7wZg
+kdmM7oVK2OfgrSj/FCTkInKPqaCR0gD7K80q+mLBrN3PUkDrJQZpvRZIff3/xmVU
+1WeruQLFJjnFb2dqu0s/FY/2kWiJtBCakXvXEOb7zfbINuayL+MSsCGSdVYsSliS
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+/fy3ZpsL7WqgsZS7Q+0VRK8gKfqkxg5OYQIDAQAB
+-----END RSA PUBLIC KEY-----
diff --git a/tests/data/public_cert.pem b/tests/data/public_cert.pem
new file mode 100644
index 0000000..7af6ca3
--- /dev/null
+++ b/tests/data/public_cert.pem
@@ -0,0 +1,19 @@
+-----BEGIN CERTIFICATE-----
+MIIDIzCCAgugAwIBAgIJAMfISuBQ5m+5MA0GCSqGSIb3DQEBBQUAMBUxEzARBgNV
+BAMTCnVuaXQtdGVzdHMwHhcNMTExMjA2MTYyNjAyWhcNMjExMjAzMTYyNjAyWjAV
+MRMwEQYDVQQDEwp1bml0LXRlc3RzMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIB
+CgKCAQEA4ej0p7bQ7L/r4rVGUz9RN4VQWoej1Bg1mYWIDYslvKrk1gpj7wZgkdmM
+7oVK2OfgrSj/FCTkInKPqaCR0gD7K80q+mLBrN3PUkDrJQZpvRZIff3/xmVU1Wer
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+gyDap+8b5fpXZVJkq92hrcNtbkg7hCYUJczt8n9hcCTJCfUpApvaFQ18pe+zpyl4
++WzkP66I28hniMQyUlA1hBiskT7qiouq0m8IOodhv2fagSZKjOTTU2xkSBc//fy3
+ZpsL7WqgsZS7Q+0VRK8gKfqkxg5OYQIDAQABo3YwdDAdBgNVHQ4EFgQU2RQ8yO+O
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+AQH/MA0GCSqGSIb3DQEBBQUAA4IBAQBRv+M/6+FiVu7KXNjFI5pSN17OcW5QUtPr
+odJMlWrJBtynn/TA1oJlYu3yV5clc/71Vr/AxuX5xGP+IXL32YDF9lTUJXG/uUGk
++JETpKmQviPbRsvzYhz4pf6ZIOZMc3/GIcNq92ECbseGO+yAgyWUVKMmZM0HqXC9
+ovNslqe0M8C1sLm1zAR5z/h/litE7/8O2ietija3Q/qtl2TOXJdCA6sgjJX2WUql
+ybrC55ct18NKf3qhpcEkGQvFU40rVYApJpi98DiZPYFdx1oBDp/f4uZ3ojpxRVFT
+cDwcJLfNRCPUhormsY7fDS9xSyThiHsW9mjJYdcaKQkwYZ0F11yB
+-----END CERTIFICATE-----
diff --git a/tests/test__helpers.py b/tests/test__helpers.py
new file mode 100644
index 0000000..b7e0bab
--- /dev/null
+++ b/tests/test__helpers.py
@@ -0,0 +1,50 @@
+# Copyright 2016 Google Inc.
+#
+# 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
+#
+#      http://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.
+
+
+import pytest
+
+from google.auth import _helpers
+
+
+def test_to_bytes_with_bytes():
+    value = b'bytes-val'
+    assert _helpers.to_bytes(value) == value
+
+
+def test_to_bytes_with_unicode():
+    value = u'string-val'
+    encoded_value = b'string-val'
+    assert _helpers.to_bytes(value) == encoded_value
+
+
+def test_to_bytes_with_nonstring_type():
+    with pytest.raises(ValueError):
+        _helpers.to_bytes(object())
+
+
+def test_from_bytes_with_unicode():
+    value = u'bytes-val'
+    assert _helpers.from_bytes(value) == value
+
+
+def test_from_bytes_with_bytes():
+    value = b'string-val'
+    decoded_value = u'string-val'
+    assert _helpers.from_bytes(value) == decoded_value
+
+
+def test_from_bytes_with_nonstring_type():
+    with pytest.raises(ValueError):
+        _helpers.from_bytes(object())
diff --git a/tests/test_crypt.py b/tests/test_crypt.py
new file mode 100644
index 0000000..9069216
--- /dev/null
+++ b/tests/test_crypt.py
@@ -0,0 +1,186 @@
+# Copyright 2016 Google Inc.
+#
+# 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
+#
+#      http://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.
+
+import os
+
+import mock
+from pyasn1_modules import pem
+import pytest
+import rsa
+import six
+
+from google.auth import _helpers
+from google.auth import crypt
+
+
+DATA_DIR = os.path.join(os.path.dirname(__file__), 'data')
+
+# To generate privatekey.pem, privatekey.pub, and public_cert.pem:
+#   $ openssl req -new -newkey rsa:1024 -x509 -nodes -out public_cert.pem \
+#   >    -keyout privatekey.pem
+#   $ openssl rsa -in privatekey.pem -pubout -out privatekey.pub
+
+with open(os.path.join(DATA_DIR, 'privatekey.pem'), 'rb') as fh:
+    PRIVATE_KEY_BYTES = fh.read()
+    PKCS1_KEY_BYTES = PRIVATE_KEY_BYTES
+
+with open(os.path.join(DATA_DIR, 'privatekey.pub'), 'rb') as fh:
+    PUBLIC_KEY_BYTES = fh.read()
+
+with open(os.path.join(DATA_DIR, 'public_cert.pem'), 'rb') as fh:
+    PUBLIC_CERT_BYTES = fh.read()
+
+# To generate other_cert.pem:
+#   $ openssl req -new -newkey rsa:1024 -x509 -nodes -out other_cert.pem
+
+with open(os.path.join(DATA_DIR, 'other_cert.pem'), 'rb') as fh:
+    OTHER_CERT_BYTES = fh.read()
+
+# To generate pem_from_pkcs12.pem and privatekey.p12:
+#   $ openssl pkcs12 -export -out privatekey.p12 -inkey privatekey.pem \
+#   >    -in public_cert.pem
+#   $ openssl pkcs12 -in privatekey.p12 -nocerts -nodes \
+#   >   -out pem_from_pkcs12.pem
+
+with open(os.path.join(DATA_DIR, 'pem_from_pkcs12.pem'), 'rb') as fh:
+    PKCS8_KEY_BYTES = fh.read()
+
+with open(os.path.join(DATA_DIR, 'privatekey.p12'), 'rb') as fh:
+    PKCS12_KEY_BYTES = fh.read()
+
+
+def test_verify_signature():
+    to_sign = b'foo'
+    signer = crypt.Signer.from_string(PRIVATE_KEY_BYTES)
+    signature = signer.sign(to_sign)
+
+    assert crypt.verify_signature(
+        to_sign, signature, PUBLIC_CERT_BYTES)
+
+    # List of certs
+    assert crypt.verify_signature(
+        to_sign, signature, [OTHER_CERT_BYTES, PUBLIC_CERT_BYTES])
+
+
+def test_verify_signature_failure():
+    to_sign = b'foo'
+    signer = crypt.Signer.from_string(PRIVATE_KEY_BYTES)
+    signature = signer.sign(to_sign)
+
+    assert not crypt.verify_signature(
+        to_sign, signature, OTHER_CERT_BYTES)
+
+
+class TestVerifier(object):
+    def test_verify_success(self):
+        to_sign = b'foo'
+        signer = crypt.Signer.from_string(PRIVATE_KEY_BYTES)
+        actual_signature = signer.sign(to_sign)
+
+        verifier = crypt.Verifier.from_string(PUBLIC_KEY_BYTES)
+        assert verifier.verify(to_sign, actual_signature)
+
+    def test_verify_unicode_success(self):
+        to_sign = u'foo'
+        signer = crypt.Signer.from_string(PRIVATE_KEY_BYTES)
+        actual_signature = signer.sign(to_sign)
+
+        verifier = crypt.Verifier.from_string(PUBLIC_KEY_BYTES)
+        assert verifier.verify(to_sign, actual_signature)
+
+    def test_verify_failure(self):
+        verifier = crypt.Verifier.from_string(PUBLIC_KEY_BYTES)
+        bad_signature1 = b''
+        assert not verifier.verify(b'foo', bad_signature1)
+        bad_signature2 = b'a'
+        assert not verifier.verify(b'foo', bad_signature2)
+
+    def test_from_string_pub_key(self):
+        verifier = crypt.Verifier.from_string(PUBLIC_KEY_BYTES)
+        assert isinstance(verifier, crypt.Verifier)
+        assert isinstance(verifier._pubkey, rsa.key.PublicKey)
+
+    def test_from_string_pub_key_unicode(self):
+        public_key = _helpers.from_bytes(PUBLIC_KEY_BYTES)
+        verifier = crypt.Verifier.from_string(public_key)
+        assert isinstance(verifier, crypt.Verifier)
+        assert isinstance(verifier._pubkey, rsa.key.PublicKey)
+
+    def test_from_string_pub_cert(self):
+        verifier = crypt.Verifier.from_string(PUBLIC_CERT_BYTES)
+        assert isinstance(verifier, crypt.Verifier)
+        assert isinstance(verifier._pubkey, rsa.key.PublicKey)
+
+    def test_from_string_pub_cert_unicode(self):
+        public_cert = _helpers.from_bytes(PUBLIC_CERT_BYTES)
+        verifier = crypt.Verifier.from_string(public_cert)
+        assert isinstance(verifier, crypt.Verifier)
+        assert isinstance(verifier._pubkey, rsa.key.PublicKey)
+
+    def test_from_string_pub_cert_failure(self):
+        cert_bytes = PUBLIC_CERT_BYTES
+        true_der = rsa.pem.load_pem(cert_bytes, 'CERTIFICATE')
+        with mock.patch('rsa.pem.load_pem',
+                        return_value=true_der + b'extra') as load_pem:
+            with pytest.raises(ValueError):
+                crypt.Verifier.from_string(cert_bytes)
+            load_pem.assert_called_once_with(cert_bytes, 'CERTIFICATE')
+
+
+class TestSigner(object):
+    def test_from_string_pkcs1(self):
+        signer = crypt.Signer.from_string(PKCS1_KEY_BYTES)
+        assert isinstance(signer, crypt.Signer)
+        assert isinstance(signer._key, rsa.key.PrivateKey)
+
+    def test_from_string_pkcs1_unicode(self):
+        key_bytes = _helpers.from_bytes(PKCS1_KEY_BYTES)
+        signer = crypt.Signer.from_string(key_bytes)
+        assert isinstance(signer, crypt.Signer)
+        assert isinstance(signer._key, rsa.key.PrivateKey)
+
+    def test_from_string_pkcs8(self):
+        signer = crypt.Signer.from_string(PKCS8_KEY_BYTES)
+        assert isinstance(signer, crypt.Signer)
+        assert isinstance(signer._key, rsa.key.PrivateKey)
+
+    def test_from_string_pkcs8_extra_bytes(self):
+        key_bytes = PKCS8_KEY_BYTES
+        _, pem_bytes = pem.readPemBlocksFromFile(
+            six.StringIO(_helpers.from_bytes(key_bytes)),
+            crypt._PKCS8_MARKER)
+
+        with mock.patch('pyasn1.codec.der.decoder.decode') as mock_decode:
+            key_info, remaining = None, 'extra'
+            mock_decode.return_value = (key_info, remaining)
+            with pytest.raises(ValueError):
+                crypt.Signer.from_string(key_bytes)
+            # Verify mock was called.
+            mock_decode.assert_called_once_with(
+                pem_bytes, asn1Spec=crypt._PKCS8_SPEC)
+
+    def test_from_string_pkcs8_unicode(self):
+        key_bytes = _helpers.from_bytes(PKCS8_KEY_BYTES)
+        signer = crypt.Signer.from_string(key_bytes)
+        assert isinstance(signer, crypt.Signer)
+        assert isinstance(signer._key, rsa.key.PrivateKey)
+
+    def test_from_string_pkcs12(self):
+        with pytest.raises(ValueError):
+            crypt.Signer.from_string(PKCS12_KEY_BYTES)
+
+    def test_from_string_bogus_key(self):
+        key_bytes = 'bogus-key'
+        with pytest.raises(ValueError):
+            crypt.Signer.from_string(key_bytes)