Class: OpenSSL::CMAC

Inherits:
Object
  • Object
show all
Defined in:
lib/openssl/cmac.rb,
lib/openssl/cmac/version.rb

Overview

Abstract from tools.ietf.org/html/rfc4493:

The National Institute of Standards and Technology (NIST) has recently specified the Cipher-based Message Authentication Code (CMAC), which is equivalent to the One-Key CBC MAC1 (OMAC1) submitted by Iwata and Kurosawa. This memo specifies an authentication algorithm based on CMAC with the 128-bit Advanced Encryption Standard (AES). This new authentication algorithm is named AES-CMAC. The purpose of this document is to make the AES-CMAC algorithm conveniently available to the Internet Community.

tools.ietf.org/html/rfc4494 reduces the length of the result from 16 to 12 Byte.

tools.ietf.org/html/rfc4615 allows to use variable key sizes.

Constant Summary collapse

VERSION = 
'2.0.1'

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(cipher, key = '') ⇒ Object

Returns an instance of OpenSSL::CMAC set with the cipher algorithm and key to be used. The instance represents the initial state of the message authentication code before any data has been processed. To process data with it, use the instance method update with your data as an argument.

Parameters:

  • cipher (String) —

    entry of OpenSSL::CMAC.ciphers

  • key (String) (defaults to: '') —

    binary key string



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# File 'lib/openssl/cmac.rb', line 56

def initialize(cipher, key = '')
  unless CMAC.ciphers.include?(cipher.upcase)
    fail CMACError, "unsupported cipher algorithm (#{cipher})"
  end

  @keys = []
  @buffer = ''.force_encoding('ASCII-8BIT')
  @cipher = OpenSSL::Cipher.new("#{cipher.upcase}-128-CBC")

  self.key = key unless key == ''
end

Class Method Details

.ciphers ⇒ [String]

Searches for supported algorithms within OpenSSL

Returns:

  • ([String]) —

    supported algorithms



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# File 'lib/openssl/cmac.rb', line 28

def self.ciphers
  @ciphers ||= OpenSSL::Cipher.ciphers.select { |c| c.match(/-128-CBC$/i) }.map { |e| e[0..-9].upcase }.uniq
end

.digest(cipher, key, data, length = 16) ⇒ String

Returns the authentication code as a binary string. The cipher parameter must be an entry of OpenSSL::CMAC.ciphers.

Parameters:

  • cipher (String) —

    entry of OpenSSL::CMAC.ciphers

  • key (String) —

    binary key string

  • data (String) —

    binary data string

  • length (Number) (defaults to: 16) —

    length of the authentication code

Returns:

  • (String) —

    authentication code



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# File 'lib/openssl/cmac.rb', line 41

def self.digest(cipher, key, data, length = 16)
  CMAC.new(cipher, key).update(data).digest(length)
end

Instance Method Details

#<<(data) ⇒ Object

Alias for: update



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# File 'lib/openssl/cmac.rb', line 96

def <<(data)
  update(data)
end

#block_length ⇒ Number

Returns the block length of the used cipher algorithm.

Returns:

  • (Number) —

    length of the used cipher algorithm



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# File 'lib/openssl/cmac.rb', line 103

def block_length
  16
end

#digest(length = 16) ⇒ Object

Returns the authentication code an instance represents as a binary string.

Parameters:

  • length (Number) (defaults to: 16) —

    length of the authentication code



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# File 'lib/openssl/cmac.rb', line 150

def digest(length = 16)
  fail CMACError, 'no key is set' if @keys[0].nil?
  fail CMACError, 'no key is set' unless length.between?(1, 16)

  block = @buffer.bytes
  @buffer.clear
  k = @keys[block.length == 16 ? 1 : 2].dup
  i = block.length.times { |t| k[t] ^= block[t] }
  k[i] ^= 0x80 if i < 16
  mac = @cipher.update(k.pack('C*')) + @cipher.final
  @cipher.reset
  @cipher.encrypt
  @cipher.key = @keys[0]
  # Each block is 16-bytes and the last block will always be PKCS#7 padding
  # which we want to discard.  Take the last block prior to the padding for
  # the MAC.
  mac[-32...(-32 + length)]
end

#digest_max_length ⇒ Number

Returns the maximum length of the resulting digest.

Returns:

  • (Number) —

    maximum length of the resulting digest



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# File 'lib/openssl/cmac.rb', line 110

def digest_max_length
  16
end

#key=(key) ⇒ Object

Returns self as it was when it was first initialized with new key, with all processed data cleared from it.

Parameters:

  • key (String) —

    binary key string

Returns:

  • (Object) —

    self with initial state and new key



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# File 'lib/openssl/cmac.rb', line 74

def key=(key)
  reset
  key = CMAC.digest('AES', "\x00" * 16, key, 16) unless key.b.length == 16

  @keys[0] = key.dup
  @cipher.key = @keys[0]

  cipher = OpenSSL::Cipher.new(@cipher.name)
  cipher.encrypt
  cipher.key = @keys[0]
  k = (cipher.update("\x00" * 16) + cipher.final).bytes[0...16]
  1.upto(2) do |i|
    k = k.pack('C*').unpack('B*')[0]
    msb = k.slice!(0)
    k = [k, '0'].pack('B*').bytes
    k[15] ^= 0x87 if msb == '1'
    @keys[i] = k.dup
  end
  self
end

#name ⇒ String

Returns the name of the used authentication code algorithm.

Returns:

  • (String) —

    name of the used authentication code algorithm



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# File 'lib/openssl/cmac.rb', line 117

def name
  "CMAC with #{@cipher.name[0..-9]}"
end

#reset ⇒ Object

Returns self as it was when it was first initialized, with all processed data cleared from it.

Returns:

  • (Object) —

    self with initial state



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# File 'lib/openssl/cmac.rb', line 125

def reset
  @keys.clear
  @buffer.clear
  @cipher.reset unless @keys[0].nil?
  @cipher.encrypt
  self
end

#update(data) ⇒ Object

Returns self updated with the message to be authenticated. Can be called repeatedly with chunks of the message.

Parameters:

  • data (String) —

    binary data string

Returns:

  • (Object) —

    self with new state



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# File 'lib/openssl/cmac.rb', line 139

def update(data)
  fail CMACError, 'no key is set' if @keys[0].nil?

  @buffer += data
  @cipher.update(@buffer.slice!(0...16)) while @buffer.length > 16
  self
end