Class: Miscreant::AES::PMAC

Inherits:
Object
  • Object
show all
Defined in:
lib/miscreant/aes/pmac.rb

Overview

The Parallel Message Authentication Code

Constant Summary collapse

PRECOMPUTED_BLOCKS =

Number of L blocks to precompute (i.e. ?? in the PMAC paper) TODO: dynamically compute these as needed

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Instance Method Summary collapse

Constructor Details

#initialize(key) ⇒ PMAC

Create a new PMAC instance

Parameters:

  • key (String) —

    16-byte or 32-byte Encoding::BINARY cryptographic key



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# File 'lib/miscreant/aes/pmac.rb', line 16

def initialize(key)
  @cipher = Internals::AES::BlockCipher.new(key)

  # L is defined as follows (quoted from the PMAC paper):
  #
  # Equation 1:
  #
  #     a · x =
  #         a<<1 if firstbit(a)=0
  #         (a<<1) ⊕ 0¹²⁰10000111 if firstbit(a)=1
  #
  # Equation 2:
  #
  #     a · x⁻¹ =
  #         a>>1 if lastbit(a)=0
  #         (a>>1) ⊕ 10¹²⁰1000011 if lastbit(a)=1
  #
  # Let L(0) ← L. For i ∈ [1..µ], compute L(i) ← L(i − 1) · x by
  # Equation (1) using a shift and a conditional xor.
  #
  # Compute L(−1) ← L · x⁻¹ by Equation (2), using a shift and a
  # conditional xor.
  #
  # Save the values L(−1), L(0), L(1), L(2), ..., L(µ) in a table.
  # (Alternatively, [ed: as we have done in this codebase] defer computing
  # some or  all of these L(i) values until the value is actually needed.)
  @l = []
  tmp = Internals::Block.new
  tmp.encrypt(@cipher)

  PRECOMPUTED_BLOCKS.times.each do
    block = Internals::Block.new(tmp.data.dup)
    block.data.freeze
    block.freeze

    @l << block
    tmp.dbl
  end

  @l.freeze

  # Compute L(−1) ← L · x⁻¹:
  #
  #     a>>1 if lastbit(a)=0
  #     (a>>1) ⊕ 10¹²⁰1000011 if lastbit(a)=1
  #
  @l_inv = Internals::Block.new(@l[0].data.dup)
  last_bit = @l_inv[Internals::Block::SIZE - 1] & 0x01

  (Internals::Block::SIZE - 1).downto(1) do |i|
    carry = Internals::Util.ct_select(@l_inv[i - 1] & 1, 0x80, 0)
    @l_inv[i] = (@l_inv[i] >> 1) | carry
  end

  @l_inv[0] >>= 1
  @l_inv[0] ^= Internals::Util.ct_select(last_bit, 0x80, 0)
  @l_inv[Internals::Block::SIZE - 1] ^= Internals::Util.ct_select(last_bit, Internals::Block::R >> 1, 0)
  @l_inv.freeze
  @l_inv.data.freeze
end

Instance Method Details

#digest(message) ⇒ String

Compute the PMAC of the given input message in a single shot, outputting the MAC tag.

Unlike other PMAC implementations, this one does not support incremental processing/IUF operation. (Though that would enable slightly more efficient decryption for AES-SIV)

Parameters:

  • message (String) —

    an Encoding::BINARY string to authenticate

Returns:

  • (String) —

    PMAC tag



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# File 'lib/miscreant/aes/pmac.rb', line 94

def digest(message)
  Internals::Util.validate_bytestring("message", message)

  offset = Internals::Block.new
  tmp = Internals::Block.new
  tag = Internals::Block.new
  ctr = 0
  remaining = message.bytesize

  while remaining > Internals::Block::SIZE
    offset.xor_in_place(@l.fetch(Internals::Util.ctz(ctr + 1)))

    tmp.copy(offset)
    tmp.xor_in_place(message[ctr * Internals::Block::SIZE, Internals::Block::SIZE])
    tmp.encrypt(@cipher)
    tag.xor_in_place(tmp)

    ctr += 1
    remaining -= Internals::Block::SIZE
  end

  if remaining == Internals::Block::SIZE
    tag.xor_in_place(message[(ctr * Internals::Block::SIZE)..-1])
    tag.xor_in_place(@l_inv)
  else
    remaining.times { |i| tag[i] ^= message.getbyte((ctr * Internals::Block::SIZE) + i) }
    tag[remaining] ^= 0x80
  end

  tag.encrypt(@cipher)
  tag.data
end

#inspect ⇒ String

Inspect this PMAC instance

Returns:

  • (String) —

    description of this instance



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# File 'lib/miscreant/aes/pmac.rb', line 80

def inspect
  to_s
end