SSH serialization for public keys (#2957)

* SSH serialization for public keys

* name errors ahoy!

* id, ego, superego

* dsa support

* EC support

* Don't keyerror

* Documentation OpenSSH

* flake8

* fix

* bytes bytes bytes

* skip curve unsupported

* bytes!

* Move a function

* reorganize code for coverage

5 files changed, 81 insertions, 14 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index d8a681e6..126a881a 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -4,6 +4,7 @@
 
 from __future__ import absolute_import, division, print_function
 
+import base64
 import calendar
 import collections
 import itertools
@@ -1698,11 +1699,23 @@ class Backend(object):
         self.openssl_assert(res == 1)
         return self._read_mem_bio(bio)
 
-    def _public_key_bytes(self, encoding, format, evp_pkey, cdata):
+    def _public_key_bytes(self, encoding, format, key, evp_pkey, cdata):
         if not isinstance(encoding, serialization.Encoding):
             raise TypeError("encoding must be an item from the Encoding enum")
 
-        if format is serialization.PublicFormat.SubjectPublicKeyInfo:
+        if (
+            format is serialization.PublicFormat.OpenSSH or
+            encoding is serialization.Encoding.OpenSSH
+        ):
+            if (
+                format is not serialization.PublicFormat.OpenSSH or
+                encoding is not serialization.Encoding.OpenSSH
+            ):
+                raise ValueError(
+                    "OpenSSH format must be used with OpenSSH encoding"
+                )
+            return self._openssh_public_key_bytes(key)
+        elif format is serialization.PublicFormat.SubjectPublicKeyInfo:
             if encoding is serialization.Encoding.PEM:
                 write_bio = self._lib.PEM_write_bio_PUBKEY
             else:
@@ -1732,6 +1745,44 @@ class Backend(object):
         self.openssl_assert(res == 1)
         return self._read_mem_bio(bio)
 
+    def _openssh_public_key_bytes(self, key):
+        if isinstance(key, rsa.RSAPublicKey):
+            public_numbers = key.public_numbers()
+            return b"ssh-rsa " + base64.b64encode(
+                serialization._ssh_write_string(b"ssh-rsa") +
+                serialization._ssh_write_mpint(public_numbers.e) +
+                serialization._ssh_write_mpint(public_numbers.n)
+            )
+        elif isinstance(key, dsa.DSAPublicKey):
+            public_numbers = key.public_numbers()
+            parameter_numbers = public_numbers.parameter_numbers
+            return b"ssh-dss " + base64.b64encode(
+                serialization._ssh_write_string(b"ssh-dss") +
+                serialization._ssh_write_mpint(parameter_numbers.p) +
+                serialization._ssh_write_mpint(parameter_numbers.q) +
+                serialization._ssh_write_mpint(parameter_numbers.g) +
+                serialization._ssh_write_mpint(public_numbers.y)
+            )
+        else:
+            assert isinstance(key, ec.EllipticCurvePublicKey)
+            public_numbers = key.public_numbers()
+            try:
+                curve_name = {
+                    ec.SECP256R1: b"nistp256",
+                    ec.SECP384R1: b"nistp384",
+                    ec.SECP521R1: b"nistp521",
+                }[type(public_numbers.curve)]
+            except KeyError:
+                raise ValueError(
+                    "Only SECP256R1, SECP384R1, and SECP521R1 curves are "
+                    "supported by the SSH public key format"
+                )
+            return b"ecdsa-sha2-" + curve_name + b" " + base64.b64encode(
+                serialization._ssh_write_string(b"ecdsa-sha2-" + curve_name) +
+                serialization._ssh_write_string(curve_name) +
+                serialization._ssh_write_string(public_numbers.encode_point())
+            )
+
 
 class GetCipherByName(object):
     def __init__(self, fmt):
diff --git a/src/cryptography/hazmat/backends/openssl/dsa.py b/src/cryptography/hazmat/backends/openssl/dsa.py
index 5abc3da9..1608df04 100644
--- a/src/cryptography/hazmat/backends/openssl/dsa.py
+++ b/src/cryptography/hazmat/backends/openssl/dsa.py
@@ -297,6 +297,7 @@ class _DSAPublicKey(object):
         return self._backend._public_key_bytes(
             encoding,
             format,
+            self,
             self._evp_pkey,
             None
         )
diff --git a/src/cryptography/hazmat/backends/openssl/ec.py b/src/cryptography/hazmat/backends/openssl/ec.py
index aa4a7a35..2f476031 100644
--- a/src/cryptography/hazmat/backends/openssl/ec.py
+++ b/src/cryptography/hazmat/backends/openssl/ec.py
@@ -299,6 +299,7 @@ class _EllipticCurvePublicKey(object):
         return self._backend._public_key_bytes(
             encoding,
             format,
+            self,
             self._evp_pkey,
             None
         )
diff --git a/src/cryptography/hazmat/backends/openssl/rsa.py b/src/cryptography/hazmat/backends/openssl/rsa.py
index d8458ccc..920bae06 100644
--- a/src/cryptography/hazmat/backends/openssl/rsa.py
+++ b/src/cryptography/hazmat/backends/openssl/rsa.py
@@ -642,6 +642,7 @@ class _RSAPublicKey(object):
         return self._backend._public_key_bytes(
             encoding,
             format,
+            self,
             self._evp_pkey,
             self._rsa_cdata
         )
diff --git a/src/cryptography/hazmat/primitives/serialization.py b/src/cryptography/hazmat/primitives/serialization.py
index d848e5d4..992fd42f 100644
--- a/src/cryptography/hazmat/primitives/serialization.py
+++ b/src/cryptography/hazmat/primitives/serialization.py
@@ -59,7 +59,7 @@ def load_ssh_public_key(data, backend):
     except TypeError:
         raise ValueError('Key is not in the proper format.')
 
-    inner_key_type, rest = _read_next_string(decoded_data)
+    inner_key_type, rest = _ssh_read_next_string(decoded_data)
 
     if inner_key_type != key_type:
         raise ValueError(
@@ -70,8 +70,8 @@ def load_ssh_public_key(data, backend):
 
 
 def _load_ssh_rsa_public_key(key_type, decoded_data, backend):
-    e, rest = _read_next_mpint(decoded_data)
-    n, rest = _read_next_mpint(rest)
+    e, rest = _ssh_read_next_mpint(decoded_data)
+    n, rest = _ssh_read_next_mpint(rest)
 
     if rest:
         raise ValueError('Key body contains extra bytes.')
@@ -80,10 +80,10 @@ def _load_ssh_rsa_public_key(key_type, decoded_data, backend):
 
 
 def _load_ssh_dss_public_key(key_type, decoded_data, backend):
-    p, rest = _read_next_mpint(decoded_data)
-    q, rest = _read_next_mpint(rest)
-    g, rest = _read_next_mpint(rest)
-    y, rest = _read_next_mpint(rest)
+    p, rest = _ssh_read_next_mpint(decoded_data)
+    q, rest = _ssh_read_next_mpint(rest)
+    g, rest = _ssh_read_next_mpint(rest)
+    y, rest = _ssh_read_next_mpint(rest)
 
     if rest:
         raise ValueError('Key body contains extra bytes.')
@@ -95,8 +95,8 @@ def _load_ssh_dss_public_key(key_type, decoded_data, backend):
 
 
 def _load_ssh_ecdsa_public_key(expected_key_type, decoded_data, backend):
-    curve_name, rest = _read_next_string(decoded_data)
-    data, rest = _read_next_string(rest)
+    curve_name, rest = _ssh_read_next_string(decoded_data)
+    data, rest = _ssh_read_next_string(rest)
 
     if expected_key_type != b"ecdsa-sha2-" + curve_name:
         raise ValueError(
@@ -121,7 +121,7 @@ def _load_ssh_ecdsa_public_key(expected_key_type, decoded_data, backend):
     return numbers.public_key(backend)
 
 
-def _read_next_string(data):
+def _ssh_read_next_string(data):
     """
     Retrieves the next RFC 4251 string value from the data.
 
@@ -137,22 +137,34 @@ def _read_next_string(data):
     return data[4:4 + str_len], data[4 + str_len:]
 
 
-def _read_next_mpint(data):
+def _ssh_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)
+    mpint_data, rest = _ssh_read_next_string(data)
 
     return (
         utils.int_from_bytes(mpint_data, byteorder='big', signed=False), rest
     )
 
 
+def _ssh_write_string(data):
+    return struct.pack(">I", len(data)) + data
+
+
+def _ssh_write_mpint(value):
+    data = utils.int_to_bytes(value)
+    if six.indexbytes(data, 0) & 0x80:
+        data = b"\x00" + data
+    return _ssh_write_string(data)
+
+
 class Encoding(Enum):
     PEM = "PEM"
     DER = "DER"
+    OpenSSH = "OpenSSH"
 
 
 class PrivateFormat(Enum):
@@ -163,6 +175,7 @@ class PrivateFormat(Enum):
 class PublicFormat(Enum):
     SubjectPublicKeyInfo = "X.509 subjectPublicKeyInfo with PKCS#1"
     PKCS1 = "Raw PKCS#1"
+    OpenSSH = "OpenSSH"
 
 
 @six.add_metaclass(abc.ABCMeta)