在Python中验证ssh-agent签名

Question

Python's Paramiko library lets you request that the ssh-agent sign a bit of data for you. That's great! But how do I verify that signature when I only have access to the public key, not the agent?

from base64 import b64encode
from paramiko.agent import Agent
agent = Agent()
key = agent.get_keys()[0]
sig = key.sign_ssh_data(None, "Hello World!")
print b64encode(sig)
# AAAAB3NzaC1yc2EAAAEAqZFIGxm1Zz3ALeTa2YiiYX2rbr9Cnb3gj4tJj1Fl19sNsRy6UbbauYnmahYWl6mWqhOHJaj0sAtVqPab8U1RiyeZyQB+Bwp0MUal4ZGCmrJxJ7ykVE+BeJnpOxXyutHHV376TXC5l+tx0PjFsylL1vVbBm6J927Nyc/cq1uk0q2QpcRexg0iJ51i1WGyISVvu1PC+g0LmW1dGh2mXTo/fpt+Cu45/hjWpMlcBUg8O60+nkj2jYxxGVh/z7U6zTYrDSJU4hTEveuKG5I38gsaMRBw/YkSIaMtkGNJX36Ybc+/EVolL4Z/NOOCV+kd7WPHjSTf1hVZ02ulTTrnMN4HfA==

I've tried every solution I can come up with. OpenSSL can't even look at the key I'm using (generated via ssh-keygen):

$ ssh-keygen -f ~/.ssh/testssh.pub -e -m pem > test.pub
$ cat test.pub
-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEA7Zs6ZyrHk3RpDKv7Kzd7NCU2Qk5k5ajVJvntRPiEukWOIe4CkcBo
B76HSDyPvEDI50DUyebj2Qj77NDX/lh5AhDrVTEmX/Lfzx2iCwgpWwQcu8dgAaN5
qhsCMYOc77d2YykcnbDUhgxus/KiieLnruKmviVdSHoLBs6ygnVEa+8nx+DhJwSB
B3+6gClM89QnKrPYOonj9z4zi+UeWPX9TVwJVOAADnXEC6LCmDCgHDb+FrCaiNT9
d4yam3iZJ2RMQ+ajBj6JWvoig2LdUBI7eSgmrREjjEEskBCNRkD4YmjeT8mfZg9d
FhbGz0CQjnF31jiohIvhABTwcUQnDdrPEQIDAQAB
-----END RSA PUBLIC KEY-----
$ openssl rsa -inform PEM -in test.pub -pubin -text
unable to load Public Key
140735078689212:error:0906D06C:PEM routines:PEM_read_bio:no start line:pem_lib.c:696:Expecting: PUBLIC KEY

PyCrypto comes up with a completely different signature for the same data:

from base64 import b64encode
from Crypto.Hash import SHA
from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5

# PyCrypto
key = RSA.importKey(open('.ssh/testssh').read())
signer = PKCS1_v1_5.new(key)
h = SHA.new()
h.update("Hello World!")
sig = signer.sign(h)
print "CRYPTO: %s" % b64encode(sig)
# CRYPTO: qZFIGxm1Zz3ALeTa2YiiYX2rbr9Cnb3gj4tJj1Fl19sNsRy6UbbauYnmahYWl6mWqhOHJaj0sAtVqPab8U1RiyeZyQB+Bwp0MUal4ZGCmrJxJ7ykVE+BeJnpOxXyutHHV376TXC5l+tx0PjFsylL1vVbBm6J927Nyc/cq1uk0q2QpcRexg0iJ51i1WGyISVvu1PC+g0LmW1dGh2mXTo/fpt+Cu45/hjWpMlcBUg8O60+nkj2jYxxGVh/z7U6zTYrDSJU4hTEveuKG5I38gsaMRBw/YkSIaMtkGNJX36Ybc+/EVolL4Z/NOOCV+kd7WPHjSTf1hVZ02ulTTrnMN4HfA==

Obviously, I'm not a crytpologist. I've been reading code for the last 12 hours trying to figure out how to verify an ssh-agent signature. Is there something painfully obvious I'm missing? What's the "Right" way to do this?

Update

Following the M2Crypto verify process outlined here , I'm getting this error against the ssh-agent's signature:

Traceback (most recent call last):
  File "test.py", line 36, in <module>
    print key.verify(datasha.digest(), agentsig)
  File "/Users/shbeta/virtualenvs/dev/lib/python2.7/site-packages/M2Crypto-0.21.1-py2.7-macosx-10.7-intel.egg/M2Crypto/RSA.py", line 259, in verify
M2Crypto.RSA.RSAError: wrong signature length

Testing against the Crypto-generated signature, however, succeeds. Is the ssh-agent using a nonstandard signature algorithm?

Update 2: redacted. I misread.

Answer 1

It seems that the general ideas around building an RSA key object from the public SSH key are...not quite accurate. Again, I'm no cryptographer. But this works:

from paramiko import Message
from paramiko.rsakey import RSAKey
from sshkey import load_rsa_pub_key
with open('.ssh/testssh.pub') as fh:
  ketype, b64key, _ = fh.next().strip().split(None, 2)
decoded_key = base64.b64decode(b64key)
pkey = RSAKey(data=decoded_key)
psig = Message(agentsig)
print pkey.verify_ssh_sig(plaintext, psig)
# True
print pkey.verify_ssh_sig("bogus", psig)
# False

I'd love to understand why using the converted PEM format didn't work. Does ssh-keygen dump it incorrectly?