Fix Support for ssh-certificates

Author: Senjuu

libagent/ssh/certificate.py

@@ -0,0 +1,76 @@
+"""Utilities for signing ssh-certificates."""
+import io
+
+from . import formats, util
+
+
+def _parse_stringlist(i):
+    res = []
+    length, = util.recv(i, '>L')
+    while length >= 4:
+        size, = util.recv(i, '>L')
+        length -= 4
+        size = min(size, length)
+        if size == 0:
+            continue
+        res.append(util.recv(i, size).decode('utf8'))
+        length -= size
+    return res
+
+
+def parse(blob):
+    """Parses a data blob to a to-be-signed ssh-certificate."""
+    # https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.certkeys
+    res = {}
+    i = io.BytesIO(blob)
+    firstString = util.read_frame(i)
+    if firstString.endswith(b'[email protected]'):
+        res['isCertificate'] = True
+        _certificate_key_type = firstString
+        _nonce = util.read_frame(i)
+        if (_certificate_key_type.startswith(b'ssh-rsa')):
+            _pub_key = {}
+            _pub_key['e'] = util.read_frame(i)
+            _pub_key['n'] = util.read_frame(i)
+        elif (_certificate_key_type.startswith(b'ssh-dsa')):
+            _pub_key = {}
+            _pub_key['p'] = util.read_frame(i)
+            _pub_key['q'] = util.read_frame(i)
+            _pub_key['g'] = util.read_frame(i)
+            _pub_key['y'] = util.read_frame(i)
+        elif (_certificate_key_type.startswith(b'ecdsa-sha2-nistp')):
+            _curve = util.read_frame(i)
+            _pub_key = util.read_frame(i)
+        elif (_certificate_key_type.startswith(b'ssh-ed25519')):
+            _pub_key = util.read_frame(i)
+        else:
+            raise ValueError('unknown certificate key type: '+_certificate_key_type.decode('utf8'))
+        _serial_number,  = util.recv(i, '>Q')
+        res['certificate_type'],  = util.recv(i, '>L')
+        _key_id_ = util.read_frame(i)
+        res['principals'] = _parse_stringlist(i)
+        res['principals'] = ', '.join(res['principals'])
+        _valid_after, = util.recv(i, '>Q')
+        _valid_before, = util.recv(i, '>Q')
+        _critical_options = _parse_stringlist(i)
+        _extensions = _parse_stringlist(i)
+        _reserved = util.read_frame(i)
+        _signature_key = util.read_frame(i)
+        assert not i.read()
+        return res
+    res['isCertificate'] = False
+    i.close()
+    return res
+
+
+def format(certificate):
+    """
+    Makes certificate better human readable.
+
+    Formats list properties to comma seperated strings and
+    the signature key to human readable string.
+    """
+    certificate['principals'] = ', '.join(certificate['principals'])
+    certificate['critical_options'] = ', '.join(certificate['critical_options'])
+    certificate['extensions'] = ', '.join(certificate['extensions'])
+    certificate['signature_key'] = formats.parse_pubkey(certificate['signature_key'])

libagent/ssh/client.py

@@ -6,7 +6,7 @@
 import io
 import logging
 
-from . import formats, util
+from . import certificate, formats, util
 
 log = logging.getLogger(__name__)
 
@@ -36,6 +36,15 @@ def sign_ssh_challenge(self, blob, identity):
         if msg['sshsig']:
             log.info('please confirm "%s" signature for "%s" using %s...',
                      msg['namespace'], identity.to_string(), self.device)
+        elif msg['sshcertsign']:
+            entity = 'unknown'
+            if msg['certificate_type'] == 1:
+                entity = 'user'
+            elif msg['certificate_type'] == 2:
+                entity = 'host'
+            log.info('please confirm signing public key for %s "%s" with "%s" using %s...',
+                     entity, msg['principals'], identity.to_string(),
+                     self.device)
         else:
             log.debug('%s: user %r via %r (%r)',
                       msg['conn'], msg['user'], msg['auth'], msg['key_type'])
@@ -59,22 +68,31 @@ def parse_ssh_blob(data):
         i = io.BytesIO(data[6:])
         # https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.sshsig
         res['sshsig'] = True
+        res['sshcertsign'] = False
         res['namespace'] = util.read_frame(i)
         res['reserved'] = util.read_frame(i)
         res['hashalg'] = util.read_frame(i)
         res['message'] = util.read_frame(i)
     else:
-        i = io.BytesIO(data)
         res['sshsig'] = False
-        res['nonce'] = util.read_frame(i)
-        i.read(1)  # SSH2_MSG_USERAUTH_REQUEST == 50 (from ssh2.h, line 108)
-        res['user'] = util.read_frame(i)
-        res['conn'] = util.read_frame(i)
-        res['auth'] = util.read_frame(i)
-        i.read(1)  # have_sig == 1 (from sshconnect2.c, line 1056)
-        res['key_type'] = util.read_frame(i)
-        public_key = util.read_frame(i)
-        res['public_key'] = formats.parse_pubkey(public_key)
+        _certificate = certificate.parse(data)
+        if _certificate['isCertificate']:
+            certificate.format(_certificate)
+            _certificate['sshsig'] = res['sshsig']
+            _certificate['sshcertsign'] = True
+            return _certificate
+        else:
+            res['sshcertsign'] = False
+            i = io.BytesIO(data)
+            res['nonce'] = util.read_frame(i)
+            i.read(1)  # SSH2_MSG_USERAUTH_REQUEST == 50 (from ssh2.h, line 108)
+            res['user'] = util.read_frame(i)
+            res['conn'] = util.read_frame(i)
+            res['auth'] = util.read_frame(i)
+            i.read(1)  # have_sig == 1 (from sshconnect2.c, line 1056)
+            res['key_type'] = util.read_frame(i)
+            public_key = util.read_frame(i)
+            res['public_key'] = formats.parse_pubkey(public_key)
 
     unparsed = i.read()
     if unparsed:

libagent/ssh/tests/test_client.py

@@ -101,6 +101,7 @@ def test_parse_ssh_challenge():
                        'fingerprint': '47:a3:26:af:0b:5d:a2:c3:91:ed:26:36:94:be:3a:d5',
                        'type': b'ssh-ed25519'},
         'sshsig': False,
+        'sshcertsign': False,
         'user': b'git',
     }
 
@@ -122,4 +123,5 @@ def test_parse_ssh_signature():
         'namespace': b'file',
         'reserved': b'',
         'sshsig': True,
+        'sshcertsign': False,
     }