Merge pull request #1527 from alex/pr/1517

Added SSH public key loading

2 files changed, 81 insertions, 3 deletions

diff --git a/src/cryptography/hazmat/primitives/serialization.py b/src/cryptography/hazmat/primitives/serialization.py
index b9cf5967..0dbbc85c 100644
--- a/src/cryptography/hazmat/primitives/serialization.py
+++ b/src/cryptography/hazmat/primitives/serialization.py
@@ -4,9 +4,13 @@
 
 from __future__ import absolute_import, division, print_function
 
+import base64
+import struct
 import warnings
 
 from cryptography import utils
+from cryptography.exceptions import UnsupportedAlgorithm
+from cryptography.hazmat.primitives.asymmetric.rsa import RSAPublicNumbers
 
 
 def load_pem_traditional_openssl_private_key(data, password, backend):
@@ -39,3 +43,76 @@ def load_pem_private_key(data, password, backend):
 
 def load_pem_public_key(data, backend):
     return backend.load_pem_public_key(data)
+
+
+def load_ssh_public_key(data, backend):
+    key_parts = data.split(b' ')
+
+    if len(key_parts) != 2 and len(key_parts) != 3:
+        raise ValueError(
+            'Key is not in the proper format or contains extra data.')
+
+    key_type = key_parts[0]
+    key_body = key_parts[1]
+
+    if not key_type.startswith(b'ssh-'):
+        raise ValueError('SSH-formatted keys must begin with \'ssh-\'.')
+
+    if not key_type.startswith(b'ssh-rsa'):
+        raise UnsupportedAlgorithm('Only RSA keys are currently supported.')
+
+    return _load_ssh_rsa_public_key(key_body, backend)
+
+
+def _load_ssh_rsa_public_key(key_body, backend):
+    data = base64.b64decode(key_body)
+
+    key_type, rest = _read_next_string(data)
+    e, rest = _read_next_mpint(rest)
+    n, rest = _read_next_mpint(rest)
+
+    if key_type != b'ssh-rsa':
+        raise ValueError(
+            'Key header and key body contain different key type values.')
+
+    if rest:
+        raise ValueError('Key body contains extra bytes.')
+
+    return backend.load_rsa_public_numbers(RSAPublicNumbers(e, n))
+
+
+def _read_next_string(data):
+    """Retrieves the next RFC 4251 string value from the data."""
+    str_len, = struct.unpack('>I', data[:4])
+    return data[4:4 + str_len], data[4 + str_len:]
+
+
+def _read_next_mpint(data):
+    """
+    Reads the next mpint from the data.
+
+    Currently, all mpints are interpreted as unsigned.
+    """
+    mpint_data, rest = _read_next_string(data)
+
+    return _int_from_bytes(mpint_data, byteorder='big', signed=False), rest
+
+
+if hasattr(int, "from_bytes"):
+    _int_from_bytes = int.from_bytes
+else:
+    def _int_from_bytes(data, byteorder, signed=False):
+        assert byteorder == 'big'
+        assert not signed
+
+        if len(data) % 4 != 0:
+            data = (b'\x00' * (4 - (len(data) % 4))) + data
+
+        result = 0
+
+        while len(data) > 0:
+            digit, = struct.unpack('>I', data[:4])
+            result = (result << 32) + digit
+            data = data[4:]
+
+        return result
diff --git a/src/cryptography/utils.py b/src/cryptography/utils.py
index 63464dfa..78f73464 100644
--- a/src/cryptography/utils.py
+++ b/src/cryptography/utils.py
@@ -48,8 +48,9 @@ def verify_interface(iface, klass):
             )
 
 
-def bit_length(x):
-    if sys.version_info >= (2, 7):
+if sys.version_info >= (2, 7):
+    def bit_length(x):
         return x.bit_length()
-    else:
+else:
+    def bit_length(x):
         return len(bin(x)) - (2 + (x <= 0))