Class: Megar::Crypto::AesCtr

Inherits:
Object
  • Object
show all
Defined in:
lib/megar/crypto/aes_ctr.rb

Overview

Implements AES COUNTER mode using base AES CBC implementation provided by OpenSSL

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(options = {}) ⇒ AesCtr

Consturcts a new AES CTR-mode object give options options = encryption key as binary string or array of 32-bit integer (required) options = initialisation vector as array of 32-bit integer (nulled by default)



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# File 'lib/megar/crypto/aes_ctr.rb', line 10

def initialize(options={})
  self.key = options[:key]
  self.iv = options[:iv]
end

Instance Attribute Details

#iv ⇒ Object

Returns the value of attribute iv.



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# File 'lib/megar/crypto/aes_ctr.rb', line 5

def iv
  @iv
end

#key ⇒ Object

Returns the value of attribute key.



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# File 'lib/megar/crypto/aes_ctr.rb', line 4

def key
  @key
end

Instance Method Details

#a32_to_str(a) ⇒ Object

TODO: refactor to pull this method from a shared lib.



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# File 'lib/megar/crypto/aes_ctr.rb', line 69

def a32_to_str(a)
  b = ''
  (a.size * 4).times { |i| b << ((a[i>>2] >> (24-(i & 3)*8)) & 255).chr }
  b
end

#packing ⇒ Object 



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# File 'lib/megar/crypto/aes_ctr.rb', line 15

def packing
  'l>*'
end

#str_to_a32(b, signed = true) ⇒ Object

TODO: refactor to pull this method from a shared lib.



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# File 'lib/megar/crypto/aes_ctr.rb', line 58

def str_to_a32(b,signed=true)
  a = Array.new((b.length+3) >> 2,0)
  b.length.times { |i| a[i>>2] |= (b.getbyte(i) << (24-(i & 3)*8)) }
  if signed
    a.pack('l>*').unpack('l>*')
  else
    a
  end
end

#update(chunk) ⇒ Object

Returns the encrypted binary string of chunk (provided as binary string). Repeated calls will continue the counter sequence.



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# File 'lib/megar/crypto/aes_ctr.rb', line 29

def update(chunk)
  a32 = str_to_a32(chunk)
  last_i = 0

  (0..a32.size - 3).step(4) do |i|
    enc = Megar::Crypto::Aes.new(key: key).encrypt(iv)
    4.times do |m|
      a32[i+m] = (a32[i+m] || 0) ^ (enc[m] || 0)
    end
    iv[3] += 1
    iv[2] += 1 if iv[3] == 0
    last_i = i + 4
  end

  remainder = a32.size % 4

  if remainder > 0
    v = [0, 0, 0, 0]
    (last_i..a32.size - 1).step(1) { |m| v[m-last_i] = a32[m] || 0 }
    enc = Megar::Crypto::Aes.new(key: key).encrypt(iv)
    4.times { |m| v[m] = v[m] ^ enc[m] }

    (last_i..a32.size - 1).step(1) { |j| a32[j] = v[j - last_i] || 0 }
  end

  a32_to_str(a32)[0..chunk.size - 1]
end