Issue
I am writing a simple ssh-client in python following the specs specified here. While parsing the list of preferred kex_algorithms there are some random bytes at the start of the list, which should not be there?
I am following the structure defined over here for parsing the server response.
Here is the python code for the client which I am discussing.
import os
import socket
import io
import struct
HOST = 'localhost'
PORT = 22
MSG_KEXINIT = 20
_preferred_ciphers = (
"aes128-ctr-random-str",
"aes192-ctr-random-str",
"aes256-ctr-random-str",
"aes128-cbc-random-str",
"aes192-cbc-random-str",
"aes256-cbc-random-str",
"blowfish-cbc-random-str",
"3des-cbc-random-str",
)
_preferred_macs = (
"hmac-sha2-256",
"hmac-sha2-512",
"hmac-sha1",
"hmac-md5",
"hmac-sha1-96",
"hmac-md5-96",
)
_preferred_keys = (
"ssh-ed25519",
"ecdsa-sha2-nistp256",
"ecdsa-sha2-nistp384",
"ecdsa-sha2-nistp521",
"ssh-rsa",
"ssh-dss",
)
_preferred_kex = (
"ecdh-sha2-nistp256",
"ecdh-sha2-nistp384",
"ecdh-sha2-nistp521",
"diffie-hellman-group-exchange-sha256",
"diffie-hellman-group-exchange-sha1",
"diffie-hellman-group14-sha1",
"diffie-hellman-group1-sha1",
)
_preferred_compression = ("none",)
def to_name_list(namelist):
namelist_bytes = ",".join(namelist).encode('utf-8')
namelist_size = struct.pack(">I", len(namelist_bytes))
return namelist_size + namelist_bytes
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((HOST, PORT))
# Version exchange
local_version = b'SSH-2.0-MYSSH\r\n'
sock.sendall(local_version)
print("SENT : {}".format(local_version))
data = sock.recv(1024)
print("RECEIVED : {}".format(data))
print("-"*20)
# Key negotiation
kex_msg = io.BytesIO()
kex_msg.write(struct.pack("B", MSG_KEXINIT))
kex_msg.write(os.urandom(16))
kex_msg.write(to_name_list(_preferred_kex))
kex_msg.write(to_name_list(_preferred_keys))
kex_msg.write(to_name_list(_preferred_ciphers))
kex_msg.write(to_name_list(_preferred_ciphers))
kex_msg.write(to_name_list(_preferred_macs))
kex_msg.write(to_name_list(_preferred_macs))
kex_msg.write(to_name_list(_preferred_compression))
kex_msg.write(to_name_list(_preferred_compression))
kex_msg.write(bytes())
kex_msg.write(bytes())
kex_msg.write(struct.pack("B", 0))
kex_msg.write(struct.pack(">I", 0))
sock.sendall(kex_msg.getvalue())
print("SENT : {}".format(kex_msg.getvalue()))
data = sock.recv(1024*4)
print("RECEIVED : {}".format(data))
server_kex_bytes = io.BytesIO(data)
print("Parsing server response")
flag = struct.unpack(">B", server_kex_bytes.read(1))[0]
print("flag : {}".format(flag))
cookie = server_kex_bytes.read(16)
print("cookie : {}".format(cookie))
kex_algo_list_len = struct.unpack(">I", server_kex_bytes.read(4))[0]
print("length of kex algo list: {}".format(kex_algo_list_len))
kex_algo_list = server_kex_bytes.read(kex_algo_list_len)
print("kex_algo_list: {}".format(kex_algo_list))
The code mentioned above generates the following output.
SENT : b'SSH-2.0-MYSSH\r\n'
RECEIVED : b'SSH-2.0-OpenSSH_8.0\r\n'
--------------------
SENT : b'\x14\xc6\xff\x10r/W2c+\xf3\x15w\x93`\x92\xf5\x00\x00\x00\xb7ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1\x00\x00\x00Wssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa,ssh-dss\x00\x00\x00\xafaes128-ctr-random-str,aes192-ctr-random-str,aes256-ctr-random-str,aes128-cbc-random-str,aes192-cbc-random-str,aes256-cbc-random-str,blowfish-cbc-random-str,3des-cbc-random-str\x00\x00\x00\xafaes128-ctr-random-str,aes192-ctr-random-str,aes256-ctr-random-str,aes128-cbc-random-str,aes192-cbc-random-str,aes256-cbc-random-str,blowfish-cbc-random-str,3des-cbc-random-str\x00\x00\x00Ghmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-md5,hmac-sha1-96,hmac-md5-96\x00\x00\x00Ghmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-md5,hmac-sha1-96,hmac-md5-96\x00\x00\x00\x04none\x00\x00\x00\x04none\x00\x00\x00\x00\x00'
RECEIVED : b'\x00\x00\x044\x06\x14\x18!\xfa\xea\xc4Vv\xd2\xe6\xe5pg\xbc\x08\x8a\xa3\x00\x00\x01\x02curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1\x00\x00\x00Arsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256,ssh-ed25519\x00\x00\[email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]\x00\x00\[email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]\x00\x00\x00\[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1\x00\x00\x00\[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1\x00\x00\x00\x15none,[email protected]\x00\x00\x00\x15none,[email protected]\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
Parsing server response
flag : 0
cookie : b'\x00\x044\x06\x14\x18!\xfa\xea\xc4Vv\xd2\xe6\xe5p'
length of kex algo list: 1740376202
kex_algo_list: b'\xa3\x00\x00\x01\x02curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1\x00\x00\x00Arsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256,ssh-ed25519\x00\x00\[email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]\x00\x00\[email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]\x00\x00\x00\[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1\x00\x00\x00\[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1\x00\x00\x00\x15none,[email protected]\x00\x00\x00\x15none,[email protected]\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
Why there are some extra bytes \xa3\x00\x00\x01\x02 at the beginning of the kex-list?
Is server leaking those extra bytes or I have made a mistake to understand the protocol?
Solution
It looks like this parsing has skipped some of the fields and reordered others.
\x00\x044\x06 looks like the binary packet fields:
uint32 packet_length
byte padding_length
(I think sequences like \00\x044 make this a pretty difficult representation to count bytes in.)
The cookie contents end up with this and the kex message number which I think is \x14 where they should be the 16 bytes after the message type right up to the name-lists that included \xa3\x00\x00\x01\x02. Also the size as parsed looks impossibly high.
Answered By - lossleader