Implementation of AES with counter (CTR) and cipher-block-chaining (CBC) modes. Based on spec at http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf. ** Disclaimer: For educational purposes only. Obviously, nobody should ever use a hacky un-vetted non-standard (not to mention, non-optimized) implementation of crypto like this **

aes_ctr_cbc.rb

require File.expand_path('../../utilities', __FILE__)
require 'openssl'

# Set to true to see debug output
DEBUG = false
def debug_puts(s=nil); puts(s) if DEBUG; end
def debug_print(s=nil); print(s) if DEBUG; end

# Encrypt data using given `mode`, `key_b`, `iv_b` and `data_b`, all as byte arrays
# Only uses padding in CBC mode
def openssl_aes_128(encrypt_or_decrypt, mode, key_b, iv_b, data_b)
  key_s, data_s = b2s(key_b), b2s(data_b)

  aes = OpenSSL::Cipher::AES.new(128, mode)
  aes.send(encrypt_or_decrypt)
  aes.padding = mode == 'CBC' ? 1 : 0
  aes.key = key_s
  aes.iv = b2s(iv_b) if iv_b
  encrypted_s = aes.update(data_s) + aes.final

  s2b(encrypted_s)
end

SBOX = {
  :encrypt => [
    0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76,
    0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0,
    0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15,
    0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75,
    0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84,
    0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF,
    0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8,
    0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2,
    0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73,
    0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB,
    0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79,
    0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08,
    0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A,
    0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E,
    0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF,
    0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16
  ],
  :decrypt => [
    0x52, 0x09, 0x6A, 0xD5, 0x30, 0x36, 0xA5, 0x38, 0xBF, 0x40, 0xA3, 0x9E, 0x81, 0xF3, 0xD7, 0xFB,
    0x7C, 0xE3, 0x39, 0x82, 0x9B, 0x2F, 0xFF, 0x87, 0x34, 0x8E, 0x43, 0x44, 0xC4, 0xDE, 0xE9, 0xCB,
    0x54, 0x7B, 0x94, 0x32, 0xA6, 0xC2, 0x23, 0x3D, 0xEE, 0x4C, 0x95, 0x0B, 0x42, 0xFA, 0xC3, 0x4E,
    0x08, 0x2E, 0xA1, 0x66, 0x28, 0xD9, 0x24, 0xB2, 0x76, 0x5B, 0xA2, 0x49, 0x6D, 0x8B, 0xD1, 0x25,
    0x72, 0xF8, 0xF6, 0x64, 0x86, 0x68, 0x98, 0x16, 0xD4, 0xA4, 0x5C, 0xCC, 0x5D, 0x65, 0xB6, 0x92,
    0x6C, 0x70, 0x48, 0x50, 0xFD, 0xED, 0xB9, 0xDA, 0x5E, 0x15, 0x46, 0x57, 0xA7, 0x8D, 0x9D, 0x84,
    0x90, 0xD8, 0xAB, 0x00, 0x8C, 0xBC, 0xD3, 0x0A, 0xF7, 0xE4, 0x58, 0x05, 0xB8, 0xB3, 0x45, 0x06,
    0xD0, 0x2C, 0x1E, 0x8F, 0xCA, 0x3F, 0x0F, 0x02, 0xC1, 0xAF, 0xBD, 0x03, 0x01, 0x13, 0x8A, 0x6B,
    0x3A, 0x91, 0x11, 0x41, 0x4F, 0x67, 0xDC, 0xEA, 0x97, 0xF2, 0xCF, 0xCE, 0xF0, 0xB4, 0xE6, 0x73,
    0x96, 0xAC, 0x74, 0x22, 0xE7, 0xAD, 0x35, 0x85, 0xE2, 0xF9, 0x37, 0xE8, 0x1C, 0x75, 0xDF, 0x6E,
    0x47, 0xF1, 0x1A, 0x71, 0x1D, 0x29, 0xC5, 0x89, 0x6F, 0xB7, 0x62, 0x0E, 0xAA, 0x18, 0xBE, 0x1B,
    0xFC, 0x56, 0x3E, 0x4B, 0xC6, 0xD2, 0x79, 0x20, 0x9A, 0xDB, 0xC0, 0xFE, 0x78, 0xCD, 0x5A, 0xF4,
    0x1F, 0xDD, 0xA8, 0x33, 0x88, 0x07, 0xC7, 0x31, 0xB1, 0x12, 0x10, 0x59, 0x27, 0x80, 0xEC, 0x5F,
    0x60, 0x51, 0x7F, 0xA9, 0x19, 0xB5, 0x4A, 0x0D, 0x2D, 0xE5, 0x7A, 0x9F, 0x93, 0xC9, 0x9C, 0xEF,
    0xA0, 0xE0, 0x3B, 0x4D, 0xAE, 0x2A, 0xF5, 0xB0, 0xC8, 0xEB, 0xBB, 0x3C, 0x83, 0x53, 0x99, 0x61,
    0x17, 0x2B, 0x04, 0x7E, 0xBA, 0x77, 0xD6, 0x26, 0xE1, 0x69, 0x14, 0x63, 0x55, 0x21, 0x0C, 0x7D
  ]
}

def sub_bytes(state, encrypt_or_decrypt)
  state.each_with_index do | column, column_index |
    column.each_with_index do | byte, row_index |
      state[column_index][row_index] = SBOX[encrypt_or_decrypt][byte]
    end
  end
  print_aes_state('subbytes', state)

  state
end

def shift_rows(state, encrypt_or_decrypt)
  direction = encrypt_or_decrypt == :encrypt ? 1 : -1
  state = state.transpose.each_with_index.map do | row, index |
    # Rotate the i'th row by i in the proper direction
    row.rotate(index * direction)
  end.transpose
  print_aes_state('shiftrows', state)

  state
end

# Multiplication of `a` and `b` in GF(2^n) for given `n` and
# characteristic polynomial `characteristic` given in binary
def galois_2_mult(a, b, n, characteristic)
  p = 0
  low_mask  = 1
  high_mask = (1 << (n-1))
  n.times do
    break if a.zero? or b.zero?

    if (b & low_mask) == low_mask
      # Add a to product
      p ^= a
    end
    # Divide b by x
    b >>= 1

    # If the coefficient of x^(n-1) is 1, the multiplication below will overflow
    carry = a & high_mask
    # Multiply a by x
    a <<= 1

    if carry == high_mask
      # Modulo the characteristic polynomial
      a ^= characteristic
    end
  end
  p
end

def aes_gmult(a, b)
  galois_2_mult(a, b, 8, 0x11b)
end

# AES rcon value for i. This is 2^(i-1) in GF(2^8) with characteristic 0x11b
def rcon(i)
  return 0 if i == 0
  exp = 1
  while i > 1
    exp = aes_gmult(exp, 2)
    i -= 1
  end
  exp
end

def mix_column(column, encrypt_or_decrypt)
  matrix = if encrypt_or_decrypt == :encrypt
   [[ 0x02, 0x03, 0x01, 0x01 ],
    [ 0x01, 0x02, 0x03, 0x01 ],
    [ 0x01, 0x01, 0x02, 0x03 ],
    [ 0x03, 0x01, 0x01, 0x02 ]]
  else
   [[ 0x0e, 0x0b, 0x0d, 0x09 ],
    [ 0x09, 0x0e, 0x0b, 0x0d ],
    [ 0x0d, 0x09, 0x0e, 0x0b ],
    [ 0x0b, 0x0d, 0x09, 0x0e ]]
  end

  matrix.reduce([]) do | result, row |
    result << row.each_with_index.reduce(0) do | acc, (coeff, index) |
      acc ^= aes_gmult(coeff, column[index])
    end
  end
end

def mix_columns(state, encrypt_or_decrypt)
  state.each_with_index do | column, index |
    state[index] = mix_column(column, encrypt_or_decrypt)
  end
  print_aes_state('mixcolumns', state)

  state
end

def print_word(which, word)
  debug_puts "#{which}: #{b2h(word)}"
end

# Core key expansion transformation
def transform_word(word_b, round)
  temp = word_b
  temp = temp.rotate
  print_word('rotword', temp)
  temp = temp.map { | b | SBOX[:encrypt][b] }
  print_word('subword', temp)
  rcon = rcon(round)
  print_word('rcon', [rcon, 0, 0, 0])
  temp[0] ^= rcon
  print_word('afterrcon', temp)
  temp
end

# Takes 16-byte key and expands to 10 16-byte round keys
def expand_key(key_b)
  debug_puts "--- KEY EXPANSION ---"
  key_words = key_b.each_slice(4).to_a
  # First four words of expansion is the cipher key
  expanded = key_words
  (1..10).each do | round |
    "--- KEY #{round} ---"
    # Start with transformation of previous word
    temp = expanded[-1]
    print_word('temp', temp)
    temp = transform_word(temp, round)
    4.times do
      # Append four words
      prev = expanded[-4]
      print_word('prev', prev)
      current = xorb(temp, prev)
      print_word('current', current)
      expanded << current
      temp = current
    end
  end
  # Return 11 16-byte blocks
  expanded.each_slice(4).map(&:flatten)
end

def add_round_key(state, round_key)
  round_key_state = round_key.each_slice(4).to_a
  print_aes_state('roundkey', round_key_state)

  state.each_with_index do | column, index |
    state[index] = xorb(column, round_key_state[index])
  end
  print_aes_state('addroundkey', state)

  state
end

def print_aes_state(which, state)
  debug_puts "#{which}:"
  (0...4).each { | row | (0...4).each { | column | debug_print "%02x " % state[column][row] }; debug_puts };nil
end

def custom_aes_128_decrypt(rounds, state, key_schedule)
  debug_puts "--- START DECRYPTION ---"

  # Run keys in reverse order for decryption
  key_schedule = key_schedule.reverse
  add_round_key(state, key_schedule[0])

  (1..rounds).each do | round |
    debug_puts "--- ROUND #{round} ---"

    state = shift_rows(state, :decrypt)
    state = sub_bytes(state, :decrypt)
    state = add_round_key(state, key_schedule[round])
    state = mix_columns(state, :decrypt) unless round == rounds # No MixColumns on last round
  end

  print_aes_state('final', state)
  state.flatten
end

def custom_aes_128_encrypt(rounds, state, key_schedule)
  debug_puts "--- START ENCRYPTION ---"

  add_round_key(state, key_schedule[0])

  (1..rounds).each do | round |
    debug_puts "--- ROUND #{round} ---"

    state = sub_bytes(state, :encrypt)
    state = shift_rows(state, :encrypt)
    state = mix_columns(state, :encrypt) unless round == rounds # No MixColumns on last round
    state = add_round_key(state, key_schedule[round])
  end

  print_aes_state('final', state)
  state.flatten
end

# Encrypt or decrypt one block of data using AES without openssl
def custom_aes_128(encrypt_or_decrypt, key_b, block_b)
  debug_puts "--- START ---"

  # AES-128 uses 10 rounds
  rounds = 10

  # First index into `state` is column, second is row
  state = block_b.each_slice(4).to_a
  print_aes_state('start', state)

  key_schedule = expand_key(key_b)

  case encrypt_or_decrypt
  when :encrypt
    custom_aes_128_encrypt(rounds, state, key_schedule)
  when :decrypt
    custom_aes_128_decrypt(rounds, state, key_schedule)
  else
    raise "Must either encrypt or decrypt"
  end
end


# Encrypt or decrypt one AES block using `strategy` of either openssl or custom
def aes_128_block(encrypt_or_decrypt, key_b, block_b, strategy = :custom)
  raise "16-byte block required" unless block_b.size == 16
  raise "16-byte key required" unless key_b.size == 16

  case strategy
  when :openssl
    openssl_aes_128(encrypt_or_decrypt, 'ECB', key_b, nil, block_b)
  when :custom
    custom_aes_128(encrypt_or_decrypt, key_b, block_b)
  else
    raise "Unknown AES strategy #{strategy}"
  end
end

# Generate a stream cipher key of length `length` bytes
# using given AES `key_b` and initialization vector `iv_b`
def aes_ctr_stream_key(key_b, iv_b, length)
  iv_i = b2i(iv_b)
  stream_key_b = []
  while stream_key_b.length < length
    stream_key_b += aes_128_block(:encrypt, key_b, i2b(iv_i))
    iv_i += 1
  end
  stream_key_b.take(length)
end

def do_ctr(key_b, iv_b, data_b)
  # Get a properly sized stream cipher key using AES-CTR
  stream_key_b = aes_ctr_stream_key(key_b, iv_b, data_b.length)
  # Stream-cipher decryption - simple xor
  xorb(stream_key_b, data_b)
end

# Encrypt plaintext with AES-128-CTR using given key and IV
# Input: 16-byte AES key `key_h` as hex string,
#        16-byte initialization vector `iv_h` as hex string,
#        Arbitrary length plaintext `pt_s` as ASCII text
# Output: Encrypted text with prepended initialization vector as hex string
def encrypt_ctr(key_h, iv_h, pt_s)
  key_b, iv_b, pt_b = h2b(key_h), h2b(iv_h), s2b(pt_s)

  encrypted_b = do_ctr(key_b, iv_b, pt_b)

  b2h(iv_b + encrypted_b)
end

# Decrypt AES-128-CTR encrypted ciphertext
# Input: 16-byte AES hex string `key_h`,
#        Arbitrary length ciphertext hex string `ct_h`
# Output: Plain text decryption
def decrypt_ctr(key_h, ct_h)
  key_b, ct_b = h2b(key_h), h2b(ct_h)

  # Pull off initialization vector in first 16 bytes of ciphertext
  iv_b, ct_b = ct_b.take(16), ct_b.drop(16)

  plaintext_b = do_ctr(key_b, iv_b, ct_b)

  b2s(plaintext_b)
end

# Encrypt plaintext with AES-128-CBC using given key and IV
# Input: 16-byte AES key `key_h` as hex string,
#        16-byte initialization vector `iv_h` as hex string,
#        Arbitrary length plaintext `pt_s` as ASCII text
# Output: Encrypted text with prepended initialization vector as hex string
def encrypt_cbc(key_h, iv_h, pt_s)
  key_b, iv_b, pt_b = h2b(key_h), h2b(iv_h), s2b(pt_s)
  ct_b = []
  last_block_b = iv_b
  padded = false
  pt_b.each_slice(16) do | block_b |
    if (pad_amount = (16 - block_b.size)) > 0
      padded = true
      block_b += [ pad_amount ] * pad_amount
    end
    encrypted_block_b = aes_128_block(:encrypt, key_b, xorb(last_block_b, block_b))
    ct_b += encrypted_block_b
    last_block_b = encrypted_block_b
  end

  # Add dummy block if needed
  ct_b += [ 16 ] * 16 unless padded

  b2h(iv_b + ct_b)
end

# Decrypt AES-128-CBC encrypted ciphertext
# Input: 16-byte AES hex string `key_h`,
#        Arbitrary length ciphertext hex string `ct_h`
# Output: Plain text decryption
def decrypt_cbc(key_h, ct_h)
  key_b, ct_b = h2b(key_h), h2b(ct_h)
  last_block_b, *blocks_b = ct_b.each_slice(16).to_a
  plaintext_b = []
  blocks_b.each do | block_b |
    plaintext_b += xorb(aes_128_block(:decrypt, key_b, block_b), last_block_b)
    last_block_b = block_b
 end

 # Strip padding
 pad_amount = plaintext_b.last
 unpadded_b = plaintext_b[0...(-pad_amount)]

 b2s(unpadded_b)
end

# TESTS

FAIL = RuntimeError.new('Failed assertion')

def test_ctr
  pt_s = 'CTR mode lets you build a stream cipher from a block cipher.'; pt_b = s2b(pt_s)
  key_h = '36f18357be4dbd77f050515c73fcf9f2'; key_b = h2b(key_h)
  ct_h = '69dda8455c7dd4254bf353b773304eec0ec7702330098ce7f7520d1cbbb20fc388d1b0adb5054dbd7370849dbf0b88d393f252e764f1f5f7ad97ef79d59ce29f5f51eeca32eabedd9afa9329'
  iv_h = ct_h[0...32]; iv_b = h2b(iv_h) # 16 bytes

  openssl_decrypted_s = b2s(openssl_aes_128(:decrypt, 'CTR', key_b, iv_b, h2b(ct_h[32..-1])))
  our_decrypted_s = decrypt_ctr(key_h, ct_h)
  raise FAIL unless our_decrypted_s == openssl_decrypted_s and our_decrypted_s == pt_s

  openssl_encrypted_h = (iv_h + b2h(openssl_aes_128(:encrypt, 'CTR', key_b, iv_b, pt_b)))
  our_encrypted_h = encrypt_ctr(key_h, iv_h, pt_s)
  raise FAIL unless our_encrypted_h == openssl_encrypted_h and our_encrypted_h == ct_h

  raise FAIL unless decrypt_ctr(key_h, encrypt_ctr(key_h, iv_h, 'some other string')) == 'some other string'

  "CTR works!"
end

def test_cbc
  pt_s = 'Basic CBC mode encryption needs padding.'; pt_b = s2b(pt_s)
  key_h = '140b41b22a29beb4061bda66b6747e14'; key_b = h2b(key_h)
  ct_h = '4ca00ff4c898d61e1edbf1800618fb2828a226d160dad07883d04e008a7897ee2e4b7465d5290d0c0e6c6822236e1daafb94ffe0c5da05d9476be028ad7c1d81'
  iv_h = ct_h[0...32]; iv_b = h2b(iv_h) # 16 bytes

  openssl_decrypted_s = b2s(openssl_aes_128(:decrypt, 'CBC', key_b, iv_b, h2b(ct_h[32..-1])))
  our_decrypted_s = decrypt_cbc(key_h, ct_h)
  raise FAIL unless our_decrypted_s == openssl_decrypted_s and our_decrypted_s == pt_s

  openssl_encrypted_h = (iv_h + b2h(openssl_aes_128(:encrypt, 'CBC', key_b, iv_b, pt_b)))
  our_encrypted_h = encrypt_cbc(key_h, iv_h, pt_s)
  raise FAIL unless our_encrypted_h == openssl_encrypted_h and our_encrypted_h == ct_h

  raise FAIL unless decrypt_cbc(key_h, encrypt_cbc(key_h, iv_h, 'some other string')) == 'some other string'

  "CBC works!"
end

puts test_ctr
puts test_cbc

utilities.rb

# Turn a byte array into a number
def b2i(b)
  # This bugs me but it seems like the easiest way to do this is to go to hex first
  h2i(b2h(b))
end

# Turn a number into a byte array
def i2b(i)
  # This bugs me but it seems like the easiest way to do this is to go to hex first
  h2b(i2h(i))
end

# Turn a byte array into a string
def b2s(b)
  b.pack('C*')
end

# Turn a string into a byte array
def s2b(s)
  s.unpack('C*')
end

# Turn a hex string into a number
def h2i(h)
  h.hex
end

# Turn a number into a hex string
def i2h(i)
  "%02x" % i
end


# Turn a hex string into a byte buffer
def h2b(h)
  h.scan(/../).map { | h | h2i(h) }
end

# Turn a byte buffer into a hex string
def b2h(b)
  b.map { | i | i2h(i) }.join
end

# Turn a byte array into a byte string
def b2s(b)
  b.pack('C*')
end

# Turn a byte string into a byte array
def s2b(s)
  s.unpack('C*')
end

# Turn a hex string into a byte string
def h2s(h)
  [h].pack('H*')
end

# Turn a byte string into a hex string
def s2h(s)
  s.unpack('H*').first
end

# Input byte arrays to be xor'd
# Output byte array of the result
def xorb(buf1, buf2)
  buf1.zip(buf2).map { | b1, b2 | (b1 ^ b2) if b1 && b2 }.compact
end

# Input two hex strings
# Output their xor as hex string
def xorh(h1, h2)
  b2h(xorb(h2b(h1), h2b(h2)))
end

# Input actual text for pt, ct as hex string
# Output xor as hex string
def xorsh(pt, ct)
  xorh(s2h(pt), ct)
end

# Return `howmany` hex "bytes" from `h`, starting at the `which`th
def subh(h, which, howmany = nil)
  if howmany
    h[which*2...howmany*2]
  else
    h[which*2..-1]
  end
end