A demonstration of length extention attack

length-extension-attack.rb

# # #
# /17/sha-256.rb
# (c) 2010 Jan Lelis <mail@janlelis.de>. MIT License.
# See: http://ruby.janlelis.de/17-sha-256
#
# May 2017, Modified by Panos Sakkos to demonstrate Length Extension Attack
# - Refactored so the initialization vector and message length can be injected.
# - Added example of length extension attack
# # #

# Helper method
class Integer
  def rotate(n=1)
    self >> n | self << (32 - n)
  end
end

class SHA256

  def self.digest(input)

    input = input.force_encoding('US-ASCII')

    return self.inner_digest(
            input,
            # Original initialization vector of SHA-256
            [0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19],
            input.length)
  end

  def self.inner_digest(input, z, length)

    # Note 1: All variables are unsigned 32 bits and wrap modulo 232 when calculating
    # Note 2: All constants in this pseudo code are in big endian

    # Initialize table of round constants
    # (first 32 bits of the fractional parts of the cube roots of the first 64 primes 2..311):
    k =
       0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
       0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
       0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
       0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
       0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
       0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
       0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
       0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2

    # Pre-processing:
    # append the bit '1' to the message
    # append k bits '0', where k is the minimum number >= 0 such that the resulting message
    #    length (in bits) is congruent to 448 (mod 512)
    # append length of message (before pre-processing), in bits, as 64-bit big-endian integer

    input << 0x80
    input << 0x00 while input.size % 64 != 56
    input += [length * 8].pack('Q').reverse

    # Process the message in successive 512-bit chunks:
    input.unpack('C*').each_slice(64){|chunk|

      w = []
      chunk.each_slice(4){|a,b,c,d| w << (((a<<8|b)<<8|c)<<8|d) }

      # Extend the sixteen 32-bit words into sixty-four 32-bit words:
      (16..63).map{|i|
        s0 = w[i-15].rotate(7) ^ w[i-15].rotate(18) ^ (w[i-15] >> 3)
        s1 = w[i-2].rotate(17) ^ w[i-2].rotate(19) ^ (w[i-2] >> 10)
        w[i] = w[i-16] + s0 + w[i-7] + s1 & 0xffffffff
      }

      # Initialize hash value for this chunk:
      a,b,c,d,e,f,g,h = z

      # Main loop:
      (0..63).each{|i|
        s0  = a.rotate(2) ^ a.rotate(13) ^ a.rotate(22)
        maj = (a & b) ^ (a & c) ^ (b & c)
        t2  = s0 + maj & 0xffffffff
        s1  = e.rotate(6) ^ e.rotate(11) ^ e.rotate(25)
        ch = (e & f) ^ ((~e) & g)
        t1 = h + s1 + ch + k[i] + w[i] & 0xffffffff

        h = g
        g = f
        f = e
        e = d + t1 & 0xffffffff
        d = c
        c = b
        b = a
        a = t1 + t2 & 0xffffffff
      }

      # Add this chunk's hash to result so far:
      z[0] = z[0] + a & 0xffffffff
      z[1] = z[1] + b & 0xffffffff
      z[2] = z[2] + c & 0xffffffff
      z[3] = z[3] + d & 0xffffffff
      z[4] = z[4] + e & 0xffffffff
      z[5] = z[5] + f & 0xffffffff
      z[6] = z[6] + g & 0xffffffff
      z[7] = z[7] + h & 0xffffffff
    }

    # Produce the final hash value (big-endian)
    hash = '%.8x'*8 % z

    return hash

  end
end

input = 'message'
puts 'Intercepted signature:  ' + SHA256.digest('secret' + input)
# Intercepted signature:  33dd93031495b1e73b345ef5b7f494146d6c361908b4f2ad9cf7bbd35cffaa26

input = 'forged'.force_encoding('US-ASCII')
puts 'Forged signature:       ' + SHA256.inner_digest(input, [0x33dd9303, 0x1495b1e7, 0x3b345ef5, 0xb7f49414, 0x6d6c3619, 0x08b4f2ad, 0x9cf7bbd3, 0x5cffaa26], 70)
# Forged signature:       f9f333d547088763f8767a241baae7b50532f95a5ad75071a8e2960bc430fd37

input = 'message'.force_encoding('US-ASCII')

# Construct the padding so when our message is appended on the secret, then our 'forged' string is pushed to the next block message
length = (input.length + 6) * 8
input << 0x80
input << 0x00 while (input.size + 6) % 64 != 56
input += [length].pack('Q').reverse
input += 'forged'

puts 'Authentic signature:    ' + SHA256.digest('secret' + input)
# Authentic signature:    f9f333d547088763f8767a241baae7b50532f95a5ad75071a8e2960bc430fd37