diff --git a/cryptography/hazmat/backends/commoncrypto/backend.py b/cryptography/hazmat/backends/commoncrypto/backend.py
index e5d4ee0..5c08a35 100644
--- a/cryptography/hazmat/backends/commoncrypto/backend.py
+++ b/cryptography/hazmat/backends/commoncrypto/backend.py
@@ -25,7 +25,7 @@
 from cryptography.hazmat.bindings.commoncrypto.binding import Binding
 from cryptography.hazmat.primitives import interfaces, constant_time
 from cryptography.hazmat.primitives.ciphers.algorithms import (
-    AES, Blowfish, TripleDES, ARC4
+    AES, Blowfish, TripleDES, ARC4, CAST5
 )
 from cryptography.hazmat.primitives.ciphers.modes import (
     CBC, CTR, ECB, OFB, CFB, GCM
@@ -198,6 +198,18 @@
                 mode_cls,
                 mode_const
             )
+        for mode_cls, mode_const in [
+            (CBC, self._lib.kCCModeCBC),
+            (ECB, self._lib.kCCModeECB),
+            (CFB, self._lib.kCCModeCFB),
+            (OFB, self._lib.kCCModeOFB)
+        ]:
+            self._register_cipher_adapter(
+                CAST5,
+                self._lib.kCCAlgorithmCAST,
+                mode_cls,
+                mode_const
+            )
         self._register_cipher_adapter(
             ARC4,
             self._lib.kCCAlgorithmRC4,
diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index fc3c3bd..ef34cb4 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -26,7 +26,7 @@
 from cryptography.hazmat.primitives import interfaces, hashes
 from cryptography.hazmat.primitives.asymmetric import rsa
 from cryptography.hazmat.primitives.ciphers.algorithms import (
-    AES, Blowfish, Camellia, TripleDES, ARC4,
+    AES, Blowfish, Camellia, TripleDES, ARC4, CAST5
 )
 from cryptography.hazmat.primitives.ciphers.modes import (
     CBC, CTR, ECB, OFB, CFB, GCM,
@@ -153,6 +153,12 @@
                 mode_cls,
                 GetCipherByName("bf-{mode.name}")
             )
+        for mode_cls in [CBC, CFB, OFB, ECB]:
+            self.register_cipher_adapter(
+                CAST5,
+                mode_cls,
+                GetCipherByName("cast5-{mode.name}")
+            )
         self.register_cipher_adapter(
             ARC4,
             type(None),
diff --git a/cryptography/hazmat/primitives/ciphers/algorithms.py b/cryptography/hazmat/primitives/ciphers/algorithms.py
index 19cf192..a5cfce9 100644
--- a/cryptography/hazmat/primitives/ciphers/algorithms.py
+++ b/cryptography/hazmat/primitives/ciphers/algorithms.py
@@ -90,6 +90,21 @@
         return len(self.key) * 8
 
 
+@utils.register_interface(interfaces.BlockCipherAlgorithm)
+@utils.register_interface(interfaces.CipherAlgorithm)
+class CAST5(object):
+    name = "CAST5"
+    block_size = 64
+    key_sizes = frozenset(range(40, 129, 8))
+
+    def __init__(self, key):
+        self.key = _verify_key_size(self, key)
+
+    @property
+    def key_size(self):
+        return len(self.key) * 8
+
+
 @utils.register_interface(interfaces.CipherAlgorithm)
 class ARC4(object):
     name = "RC4"
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 85d8e8e..210e656 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -99,7 +99,6 @@
     :param bytes key: The secret key, either ``128``, ``192``, or ``256`` bits.
                       This must be kept secret.
 
-
 .. class:: TripleDES(key)
 
     Triple DES (Data Encryption Standard), sometimes referred to as 3DES, is a
@@ -116,6 +115,15 @@
                       ``56`` bits long), they can simply be concatenated to
                       produce the full key. This must be kept secret.
 
+.. class:: CAST5(key)
+
+    CAST5 (also known as CAST-128) is a block cipher approved for use in the
+    Canadian government by the `Communications Security Establishment`_. It is
+    a variable key length cipher and supports keys from 40-128 bits in length.
+
+    :param bytes key: The secret key, 40-128 bits in length (in increments of
+                      8).  This must be kept secret.
+
 Weak Ciphers
 ------------
 
@@ -469,3 +477,4 @@
 .. _`described by Colin Percival`: http://www.daemonology.net/blog/2009-06-11-cryptographic-right-answers.html
 .. _`recommends 96-bit IV length`: http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-spec.pdf
 .. _`NIST SP-800-38D`: http://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf
+.. _`Communications Security Establishment`: http://www.cse-cst.gc.ca
diff --git a/tests/hazmat/primitives/test_cast5.py b/tests/hazmat/primitives/test_cast5.py
new file mode 100644
index 0000000..682b449
--- /dev/null
+++ b/tests/hazmat/primitives/test_cast5.py
@@ -0,0 +1,92 @@
+# 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.
+
+from __future__ import absolute_import, division, print_function
+
+import binascii
+import os
+
+import pytest
+
+from cryptography.hazmat.primitives.ciphers import algorithms, modes
+
+from .utils import generate_encrypt_test
+from ...utils import load_nist_vectors
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: backend.cipher_supported(
+        algorithms.CAST5("\x00" * 16), modes.ECB()
+    ),
+    skip_message="Does not support CAST5 ECB",
+)
+@pytest.mark.cipher
+class TestCAST5_ECB(object):
+    test_ECB = generate_encrypt_test(
+        load_nist_vectors,
+        os.path.join("ciphers", "CAST5"),
+        ["cast5-ecb.txt"],
+        lambda key, **kwargs: algorithms.CAST5(binascii.unhexlify((key))),
+        lambda **kwargs: modes.ECB(),
+    )
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: backend.cipher_supported(
+        algorithms.CAST5("\x00" * 16), modes.CBC("\x00" * 8)
+    ),
+    skip_message="Does not support CAST5 CBC",
+)
+@pytest.mark.cipher
+class TestCAST5_CBC(object):
+    test_CBC = generate_encrypt_test(
+        load_nist_vectors,
+        os.path.join("ciphers", "CAST5"),
+        ["cast5-cbc.txt"],
+        lambda key, **kwargs: algorithms.CAST5(binascii.unhexlify((key))),
+        lambda iv, **kwargs: modes.CBC(binascii.unhexlify(iv))
+    )
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: backend.cipher_supported(
+        algorithms.CAST5("\x00" * 16), modes.OFB("\x00" * 8)
+    ),
+    skip_message="Does not support CAST5 OFB",
+)
+@pytest.mark.cipher
+class TestCAST5_OFB(object):
+    test_OFB = generate_encrypt_test(
+        load_nist_vectors,
+        os.path.join("ciphers", "CAST5"),
+        ["cast5-ofb.txt"],
+        lambda key, **kwargs: algorithms.CAST5(binascii.unhexlify((key))),
+        lambda iv, **kwargs: modes.OFB(binascii.unhexlify(iv))
+    )
+
+
+@pytest.mark.supported(
+    only_if=lambda backend: backend.cipher_supported(
+        algorithms.CAST5("\x00" * 16), modes.CFB("\x00" * 8)
+    ),
+    skip_message="Does not support CAST5 CFB",
+)
+@pytest.mark.cipher
+class TestCAST5_CFB(object):
+    test_CFB = generate_encrypt_test(
+        load_nist_vectors,
+        os.path.join("ciphers", "CAST5"),
+        ["cast5-cfb.txt"],
+        lambda key, **kwargs: algorithms.CAST5(binascii.unhexlify((key))),
+        lambda iv, **kwargs: modes.CFB(binascii.unhexlify(iv))
+    )
diff --git a/tests/hazmat/primitives/test_ciphers.py b/tests/hazmat/primitives/test_ciphers.py
index 6a7b2f9..50cadf6 100644
--- a/tests/hazmat/primitives/test_ciphers.py
+++ b/tests/hazmat/primitives/test_ciphers.py
@@ -18,7 +18,7 @@
 import pytest
 
 from cryptography.hazmat.primitives.ciphers.algorithms import (
-    AES, Camellia, TripleDES, Blowfish, ARC4
+    AES, Camellia, TripleDES, Blowfish, ARC4, CAST5
 )
 
 
@@ -80,6 +80,19 @@
             Blowfish(binascii.unhexlify(b"0" * 6))
 
 
+class TestCAST5(object):
+    @pytest.mark.parametrize(("key", "keysize"), [
+        (b"0" * (keysize // 4), keysize) for keysize in range(40, 129, 8)
+    ])
+    def test_key_size(self, key, keysize):
+        cipher = CAST5(binascii.unhexlify(key))
+        assert cipher.key_size == keysize
+
+    def test_invalid_key_size(self):
+        with pytest.raises(ValueError):
+            CAST5(binascii.unhexlify(b"0" * 34))
+
+
 class TestARC4(object):
     @pytest.mark.parametrize(("key", "keysize"), [
         (b"0" * 10, 40),