challenge29.py

from Cryptodome.Random import get_random_bytes
from Cryptodome.Random.random import randint
import challenge28
import struct

def padSHA1(s):
    l = len(s) * 8
    s += b'\x80'
    s += b'\x00' * ((56 - (len(s) % 64)) % 64)
    s += struct.pack('>Q', l)
    return s

keylen = randint(0, 100)
key = get_random_bytes(keylen)

def validate(message, digest):
    return challenge28.authSHA1(key, message) == digest

message = b'comment1=cooking%20MCs;userdata=foo;comment2=%20like%20a%20pound%20of%20bacon'
messageDigest = challenge28.authSHA1(key, message)

def forgeHash(keylen, message, digest, suffix):
    paddedForgedMessageWithKey = padSHA1(key + message) + suffix
    forgedMessage = paddedForgedMessageWithKey[keylen:]
    h = struct.unpack('>5I', digest)
    forgedDigest = challenge28.SHA1(suffix, h[0], h[1], h[2], h[3], h[4], len(paddedForgedMessageWithKey) * 8).digest()
    return (forgedMessage, forgedDigest)

def forgeValidatingHash(maxkeylen, message, digest, suffix):
    for i in range(maxkeylen):
        (forgedMessage, forgedDigest) = forgeHash(i, message, digest, suffix)
        if validate(forgedMessage, forgedDigest):
            return(forgedMessage, forgedDigest)
    raise Exception('unexpected')

print(forgeValidatingHash(100, message, messageDigest, b';admin=true'))