agent.py

import os
from struct import pack, unpack
from socket import AF_UNIX, IPPROTO_TCP, SOCK_STREAM, socket
from base64 import b64encode


SSH_AGENT_FAILURE              = 5
SSH_AGENT_SUCCESS              = 6
SSH2_AGENTC_REQUEST_IDENTITIES = 11
SSH2_AGENTC_SIGN_REQUEST       = 13
SSH2_AGENT_IDENTITIES_ANSWER   = 12
SSH2_AGENT_SIGN_RESPONSE       = 14

def peek_str(b, o):
    i, = unpack('>L', b[o:o + 4])
    o += 4
    no = o + i
    return b[o:no], no

def main():
    s = socket(AF_UNIX, SOCK_STREAM, 0)
    s.connect(os.environ['SSH_AUTH_SOCK'])

    payload = pack('>B', SSH2_AGENTC_REQUEST_IDENTITIES)
    packet = pack('>L', len(payload)) + payload
    s.send(packet)
    l, = unpack('>L', s.recv(4))
    recv_payload = s.recv(l)
    c, num_keys = unpack('>BL', recv_payload[0:5])
    assert c == SSH2_AGENT_IDENTITIES_ANSWER
    o = 5
    keys = []
    for _ in range(0, num_keys):
        key_blob, o = peek_str(recv_payload, o)
        key_comment, o = peek_str(recv_payload, o)
        keys.append((key_blob, key_comment))

    key_blob, _ = keys[0]
    payload = pack('>B', SSH2_AGENTC_SIGN_REQUEST)
    payload += pack('>L', len(key_blob)) + key_blob
    data = b'a'
    payload += pack('>L', len(data)) + data
    payload += pack('>L', 0)
    packet = pack('>L', len(payload)) + payload
    s.send(packet)
    l, = unpack('>L', s.recv(4))
    recv_payload = s.recv(l)
    c, = unpack('>B', recv_payload[0:1])
    assert c == SSH2_AGENT_SIGN_RESPONSE
    signature_blob, _ = peek_str(recv_payload[1:], 0)
    print(['%02x' % x for x in signature_blob])

if __name__ == '__main__':
    main()