Merge branch '2.10'

2 files changed, 26 insertions, 1 deletions

diff --git a/paramiko/auth_handler.py b/paramiko/auth_handler.py
index c188242e..db89670a 100644
--- a/paramiko/auth_handler.py
+++ b/paramiko/auth_handler.py
@@ -22,6 +22,7 @@
 
 import weakref
 import time
+import re
 
 from paramiko.common import (
     cMSG_SERVICE_REQUEST,
@@ -298,6 +299,23 @@ class AuthHandler(object):
                 key_type
             ),
         )
+        # NOTE re #2017: When the key is an RSA cert and the remote server is
+        # OpenSSH 7.7 or earlier, always use ssh-rsa-cert-v01@openssh.com.
+        # Those versions of the server won't support rsa-sha2 family sig algos
+        # for certs specifically, and in tandem with various server bugs
+        # regarding server-sig-algs, it's impossible to fit this into the rest
+        # of the logic here.
+        if key_type.endswith("-cert-v01@openssh.com") and re.search(
+            r"-OpenSSH_(?:[1-6]|7\.[0-7])", self.transport.remote_version
+        ):
+            pubkey_algo = "ssh-rsa-cert-v01@openssh.com"
+            self.transport._agreed_pubkey_algorithm = pubkey_algo
+            self._log(DEBUG, "OpenSSH<7.8 + RSA cert = forcing ssh-rsa!")
+            self._log(
+                DEBUG, "Agreed upon {!r} pubkey algorithm".format(pubkey_algo)
+            )
+            return pubkey_algo
+        # Normal attempts to handshake follow from here.
         # Only consider RSA algos from our list, lest we agree on another!
         my_algos = [x for x in self.transport.preferred_pubkeys if "rsa" in x]
         self._log(DEBUG, "Our pubkey algorithm list: {}".format(my_algos))
diff --git a/paramiko/rsakey.py b/paramiko/rsakey.py
index f7971dca..000f41c5 100644
--- a/paramiko/rsakey.py
+++ b/paramiko/rsakey.py
@@ -141,9 +141,16 @@ class RSAKey(PKey):
         if isinstance(key, rsa.RSAPrivateKey):
             key = key.public_key()
 
+        # NOTE: pad received signature with leading zeros, key.verify()
+        # expects a signature of key size (e.g. PuTTY doesn't pad)
+        sign = msg.get_binary()
+        diff = key.key_size - len(sign) * 8
+        if diff > 0:
+            sign = b"\x00" * ((diff + 7) // 8) + sign
+
         try:
             key.verify(
-                msg.get_binary(),
+                sign,
                 data,
                 padding.PKCS1v15(),
                 self.HASHES[sig_algorithm](),