PKCS#1 2.0: Implementation of MGF1 (#89)

Implementation of the Mask Generation Function `MGF1` used in the OAEP encoding step.
For more information, the MGF1 specification is at https://tools.ietf.org/html/rfc2437#section-10.2.1

3 files changed, 195 insertions, 1 deletions

diff --git a/rsa/pkcs1.py b/rsa/pkcs1.py
index 323bf48..59190ed 100644
--- a/rsa/pkcs1.py
+++ b/rsa/pkcs1.py
@@ -250,7 +250,7 @@ def sign_hash(hash_value, priv_key, hash_method):
 
     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 priv_key: the :py:class:`rsa.PrivateKey` to sign with
diff --git a/rsa/pkcs1_v2.py b/rsa/pkcs1_v2.py
new file mode 100644
index 0000000..4ae69b3
--- /dev/null
+++ b/rsa/pkcs1_v2.py
@@ -0,0 +1,111 @@
+# -*- 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.
+
+"""Functions for PKCS#1 version 2 encryption and signing
+
+This module implements certain functionality from PKCS#1 version 2. Main
+documentation is RFC 2437: https://tools.ietf.org/html/rfc2437
+"""
+
+from rsa._compat import range
+from rsa import (
+    common,
+    pkcs1,
+    transform,
+)
+
+HASH_METHOD_TO_BYTE_LENGTH = {
+    'MD5': 16,
+    'SHA-1': 20,
+    'SHA-256': 28,
+    'SHA-384': 48,
+    'SHA-512': 64,
+}
+
+
+def mgf1(seed, length, hasher='SHA-1'):
+    """
+    MGF1 is a Mask Generation Function based on a hash function.
+
+    A mask generation function takes an octet string of variable length and a
+    desired output length as input, and outputs an octet string of the desired
+    length. The plaintext-awareness of RSAES-OAEP relies on the random nature of
+    the output of the mask generation function, which in turn relies on the
+    random nature of the underlying hash.
+
+    :param bytes seed: seed from which mask is generated, an octet string
+    :param int length: intended length in octets of the mask, at most 2^32(hLen)
+    :param str hasher: hash function (hLen denotes the length in octets of the hash
+        function output)
+
+    :return: mask, an octet string of length `length`
+    :rtype: bytes
+
+    :raise OverflowError: when `length` is too large for the specified `hasher`
+    :raise ValueError: when specified `hasher` is invalid
+    """
+
+    try:
+        hash_length = HASH_METHOD_TO_BYTE_LENGTH[hasher]
+    except KeyError:
+        raise ValueError(
+            'Invalid `hasher` specified. Please select one of: {hash_list}'.format(
+                hash_list=', '.join(sorted(HASH_METHOD_TO_BYTE_LENGTH.keys()))
+            )
+        )
+
+    # If l > 2^32(hLen), output "mask too long" and stop.
+    if length > (2**32 * hash_length):
+        raise OverflowError(
+            "Desired length should be at most 2**32 times the hasher's output "
+            "length ({hash_length} for {hasher} function)".format(
+                hash_length=hash_length,
+                hasher=hasher,
+            )
+        )
+
+    # Looping `counter` from 0 to ceil(l / hLen)-1, build `output` based on the
+    # hashes formed by (`seed` + C), being `C` an octet string of length 4
+    # generated by converting `counter` with the primitive I2OSP
+    output = b''.join(
+        pkcs1.compute_hash(
+            seed + transform.int2bytes(counter, fill_size=4),
+            method_name=hasher,
+        )
+        for counter in range(common.ceil_div(length, hash_length) + 1)
+    )
+
+    # Output the leading `length` octets of `output` as the octet string mask.
+    return output[:length]
+
+
+__all__ = [
+    'mgf1',
+]
+
+if __name__ == '__main__':
+    print('Running doctests 1000x or until failure')
+    import doctest
+
+    for count in range(1000):
+        (failures, tests) = doctest.testmod()
+        if failures:
+            break
+
+        if count and count % 100 == 0:
+            print('%i times' % count)
+
+    print('Doctests done')
diff --git a/tests/test_pkcs1_v2.py b/tests/test_pkcs1_v2.py
new file mode 100644
index 0000000..1d8f001
--- /dev/null
+++ b/tests/test_pkcs1_v2.py
@@ -0,0 +1,83 @@
+# -*- 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.
+
+"""Tests PKCS #1 version 2 functionality.
+
+Most of the mocked values come from the test vectors found at:
+http://www.itomorrowmag.com/emc-plus/rsa-labs/standards-initiatives/pkcs-rsa-cryptography-standard.htm
+"""
+
+import unittest
+
+from rsa import pkcs1_v2
+
+
+class MGFTest(unittest.TestCase):
+    def test_oaep_int_db_mask(self):
+        seed = (
+            b'\xaa\xfd\x12\xf6\x59\xca\xe6\x34\x89\xb4\x79\xe5\x07\x6d\xde\xc2'
+            b'\xf0\x6c\xb5\x8f'
+        )
+        db = (
+            b'\xda\x39\xa3\xee\x5e\x6b\x4b\x0d\x32\x55\xbf\xef\x95\x60\x18\x90'
+            b'\xaf\xd8\x07\x09\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+            b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+            b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+            b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+            b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\xd4\x36\xe9\x95\x69'
+            b'\xfd\x32\xa7\xc8\xa0\x5b\xbc\x90\xd3\x2c\x49'
+        )
+        masked_db = (
+            b'\xdc\xd8\x7d\x5c\x68\xf1\xee\xa8\xf5\x52\x67\xc3\x1b\x2e\x8b\xb4'
+            b'\x25\x1f\x84\xd7\xe0\xb2\xc0\x46\x26\xf5\xaf\xf9\x3e\xdc\xfb\x25'
+            b'\xc9\xc2\xb3\xff\x8a\xe1\x0e\x83\x9a\x2d\xdb\x4c\xdc\xfe\x4f\xf4'
+            b'\x77\x28\xb4\xa1\xb7\xc1\x36\x2b\xaa\xd2\x9a\xb4\x8d\x28\x69\xd5'
+            b'\x02\x41\x21\x43\x58\x11\x59\x1b\xe3\x92\xf9\x82\xfb\x3e\x87\xd0'
+            b'\x95\xae\xb4\x04\x48\xdb\x97\x2f\x3a\xc1\x4f\x7b\xc2\x75\x19\x52'
+            b'\x81\xce\x32\xd2\xf1\xb7\x6d\x4d\x35\x3e\x2d'
+        )
+
+        # dbMask = MGF(seed, length(DB))
+        db_mask = pkcs1_v2.mgf1(seed, length=len(db))
+        expected_db_mask = (
+            b'\x06\xe1\xde\xb2\x36\x9a\xa5\xa5\xc7\x07\xd8\x2c\x8e\x4e\x93\x24'
+            b'\x8a\xc7\x83\xde\xe0\xb2\xc0\x46\x26\xf5\xaf\xf9\x3e\xdc\xfb\x25'
+            b'\xc9\xc2\xb3\xff\x8a\xe1\x0e\x83\x9a\x2d\xdb\x4c\xdc\xfe\x4f\xf4'
+            b'\x77\x28\xb4\xa1\xb7\xc1\x36\x2b\xaa\xd2\x9a\xb4\x8d\x28\x69\xd5'
+            b'\x02\x41\x21\x43\x58\x11\x59\x1b\xe3\x92\xf9\x82\xfb\x3e\x87\xd0'
+            b'\x95\xae\xb4\x04\x48\xdb\x97\x2f\x3a\xc1\x4e\xaf\xf4\x9c\x8c\x3b'
+            b'\x7c\xfc\x95\x1a\x51\xec\xd1\xdd\xe6\x12\x64'
+        )
+
+        self.assertEqual(db_mask, expected_db_mask)
+
+        # seedMask = MGF(maskedDB, length(seed))
+        seed_mask = pkcs1_v2.mgf1(masked_db, length=len(seed))
+        expected_seed_mask = (
+            b'\x41\x87\x0b\x5a\xb0\x29\xe6\x57\xd9\x57\x50\xb5\x4c\x28\x3c\x08'
+            b'\x72\x5d\xbe\xa9'
+        )
+
+        self.assertEqual(seed_mask, expected_seed_mask)
+
+    def test_invalid_hasher(self):
+        """Tests an invalid hasher generates an exception"""
+        with self.assertRaises(ValueError):
+            pkcs1_v2.mgf1(b'\x06\xe1\xde\xb2', length=8, hasher='SHA2')
+
+    def test_invalid_length(self):
+        with self.assertRaises(OverflowError):
+            pkcs1_v2.mgf1(b'\x06\xe1\xde\xb2', length=2**50)