Class: Analyzers::CbcMac::VariableLength::Analyzer

Inherits:
Object
  • Object
show all
Defined in:
lib/crypto-toolbox/analyzers/cbc_mac/variable_length/analyzer.rb

Instance Method Summary collapse

Constructor Details

#initialize(oracle_class = ::Analyzers::CbcMac::VariableLength::Oracles::Tcp, block_length = 32) ⇒ Analyzer

This class implements an attack on CBC-MAC with variable length. This issue is known for a long time and thus should be avoided by any implementation. However this code shows how to forge a tag in this mode and can be seen das a PoC.

Thanks to Matthew Green for this great article about the potential implementation problems of CBC-MAC: blog.cryptographyengineering.com/2013/02/why-i-hate-cbc-mac.html

This class has the VL (variable length) suffix it its name to make100% clear that this attack works only on this condition



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# File 'lib/crypto-toolbox/analyzers/cbc_mac/variable_length/analyzer.rb', line 18

def initialize(oracle_class = ::Analyzers::CbcMac::VariableLength::Oracles::Tcp,block_length=32)
  @oracle = oracle_class.new
end

Instance Method Details

#analyze(target_message) ⇒ Object

NOTE: handle too short messages properly



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# File 'lib/crypto-toolbox/analyzers/cbc_mac/variable_length/analyzer.rb', line 23

def analyze(target_message)
  @oracle.connect

  #target_msg = "I, the server, hereby agree that I will pay $100 to this student"
  target_bufs = CryptBuffer(target_message).chunks_of(32)

  # add to_crypt_buffer to String!
  target_tag1 = CryptBuffer(@oracle.mac(target_bufs[0].chars,target_bufs[0].length)) #.split("").map{|i| i.bytes.first }

  # NOTE  t''  = m || [ (m_1' + t ) ||m_2'||...||m_x']
  m2_blocks = target_bufs[1].chunks_of(16)
  msg2 = CryptBuffer((m2_blocks[0].xor(target_tag1)).bytes + m2_blocks[1].bytes)

  # @oracle.tag_for(msg2.chars,msg2.length)
  forge_tag = @oracle.mac(msg2.chars,msg2.length)

  # @oracle.verify(target_msg.chars, target_msg.length, forge_tag)
  ret = @oracle.verify(target_message.chars, target_message.length, forge_tag)

  
  if forge_successfull?(ret)
    puts "result is: #{CryptBuffer(forge_tag).hex}"
    puts "Message verified successfully!"
  else
    puts "Message verification failed."
  end
  @oracle.disconnect
end