Decode MS-MPPE-Send-Key / MS-MPPE-Recv-Key

key_decode.py

from hashlib import md5
from binascii import unhexlify, hexlify

seq_xor = lambda c, b: ''.join(
    chr(x) for x in map(lambda X: ord(X[0]) ^ ord(X[1]), zip(c, b))
    )

#
# RFC-2548: 2.4.3. MS-MPPE-Recv-Key
#
# C: Cyphertext
# S: Secret
# R: Request Authenticator
# A: Salt field
# P: Plaintext - key-len + key + padding
#
def rad_tunnel_pwdecode(C, S, R, A):

    c = [C[i:i+16] for i in range(0, len(C), 16)]
    p = ['\0'] * len(c)
    b = ['\0'] * len(c)
    
    b[0] = md5(S + R + A).digest()
    p[0] = seq_xor(c[0], b[0])
    
    for i in range(1, len(p)):

        b[i] = md5(S + c[i-1]).digest()
        p[i] = seq_xor(c[i], b[i])

    P = ''.join(p)
    return P

Secret = "xxxxxx"
RequestAuthenticator = "182d8f5a54cbfe158ff6239f52dd2e30"

Cyphertext ="f1b310d0d0e66dcaba76135acdj90bc9d6k10b009b94a3eceb0dac583a3fed11518fe3818303b9ba31510aae37fa25e6bd7b"

print "Cypher:%s"%Cyphertext

Salt = Cyphertext[:4]

Plaintext = rad_tunnel_pwdecode(
    unhexlify(Cyphertext[4:]),
    Secret,
    unhexlify(RequestAuthenticator),
    unhexlify(Salt)
)
Plaintext = hexlify(Plaintext)
print "Plain: %s"%Plaintext