Module: Nuckle::Internals::Chacha20

Defined in:
lib/nuckle/internals/chacha20.rb

Overview

ChaCha20 stream cipher (DJB’s original 8-byte-nonce variant).

State layout (16 x 32-bit little-endian words):

[ 0.. 3] "expand 32-byte k" constants
[ 4..11] 32-byte key
[12..13] 64-bit block counter (low word first)
[14..15] 8-byte nonce

Quarter-round rotations: 16, 12, 8, 7.

References:

Constant Summary collapse

MASK32 = 
0xFFFFFFFF
SIGMA = 
"expand 32-byte k".b.freeze
SIGMA_V4 = 
SIGMA.unpack("V4").freeze

Class Method Summary collapse

Class Method Details

.block(key, nonce, counter) ⇒ String

ChaCha20 block function: 20 rounds on 16 x 32-bit words.

Parameters:

  • key (String) —

    32-byte key

  • nonce (String) —

    8-byte nonce

  • counter (Integer) —

    64-bit block counter

Returns:

  • (String) —

    64-byte keystream block



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# File 'lib/nuckle/internals/chacha20.rb', line 34

def block(key, nonce, counter)
  k0, k1, k2, k3, k4, k5, k6, k7 = key.unpack("V8")
  n0, n1 = nonce.unpack("V2")
  s0, s1, s2, s3 = SIGMA_V4
  c0 = counter & MASK32
  c1 = (counter >> 32) & MASK32

  x0  = s0; x1  = s1; x2  = s2; x3  = s3
  x4  = k0; x5  = k1; x6  = k2; x7  = k3
  x8  = k4; x9  = k5; x10 = k6; x11 = k7
  x12 = c0; x13 = c1; x14 = n0; x15 = n1

  z0 = x0; z1 = x1; z2 = x2; z3 = x3
  z4 = x4; z5 = x5; z6 = x6; z7 = x7
  z8 = x8; z9 = x9; z10 = x10; z11 = x11
  z12 = x12; z13 = x13; z14 = x14; z15 = x15

  10.times do
    # Column round: QR(0,4,8,12), QR(1,5,9,13), QR(2,6,10,14), QR(3,7,11,15)
    z0  = (z0  + z4)  & MASK32; z12 ^= z0;  z12 = ((z12 << 16) | (z12 >> 16)) & MASK32
    z8  = (z8  + z12) & MASK32; z4  ^= z8;  z4  = ((z4  << 12) | (z4  >> 20)) & MASK32
    z0  = (z0  + z4)  & MASK32; z12 ^= z0;  z12 = ((z12 <<  8) | (z12 >> 24)) & MASK32
    z8  = (z8  + z12) & MASK32; z4  ^= z8;  z4  = ((z4  <<  7) | (z4  >> 25)) & MASK32

    z1  = (z1  + z5)  & MASK32; z13 ^= z1;  z13 = ((z13 << 16) | (z13 >> 16)) & MASK32
    z9  = (z9  + z13) & MASK32; z5  ^= z9;  z5  = ((z5  << 12) | (z5  >> 20)) & MASK32
    z1  = (z1  + z5)  & MASK32; z13 ^= z1;  z13 = ((z13 <<  8) | (z13 >> 24)) & MASK32
    z9  = (z9  + z13) & MASK32; z5  ^= z9;  z5  = ((z5  <<  7) | (z5  >> 25)) & MASK32

    z2  = (z2  + z6)  & MASK32; z14 ^= z2;  z14 = ((z14 << 16) | (z14 >> 16)) & MASK32
    z10 = (z10 + z14) & MASK32; z6  ^= z10; z6  = ((z6  << 12) | (z6  >> 20)) & MASK32
    z2  = (z2  + z6)  & MASK32; z14 ^= z2;  z14 = ((z14 <<  8) | (z14 >> 24)) & MASK32
    z10 = (z10 + z14) & MASK32; z6  ^= z10; z6  = ((z6  <<  7) | (z6  >> 25)) & MASK32

    z3  = (z3  + z7)  & MASK32; z15 ^= z3;  z15 = ((z15 << 16) | (z15 >> 16)) & MASK32
    z11 = (z11 + z15) & MASK32; z7  ^= z11; z7  = ((z7  << 12) | (z7  >> 20)) & MASK32
    z3  = (z3  + z7)  & MASK32; z15 ^= z3;  z15 = ((z15 <<  8) | (z15 >> 24)) & MASK32
    z11 = (z11 + z15) & MASK32; z7  ^= z11; z7  = ((z7  <<  7) | (z7  >> 25)) & MASK32

    # Diagonal round: QR(0,5,10,15), QR(1,6,11,12), QR(2,7,8,13), QR(3,4,9,14)
    z0  = (z0  + z5)  & MASK32; z15 ^= z0;  z15 = ((z15 << 16) | (z15 >> 16)) & MASK32
    z10 = (z10 + z15) & MASK32; z5  ^= z10; z5  = ((z5  << 12) | (z5  >> 20)) & MASK32
    z0  = (z0  + z5)  & MASK32; z15 ^= z0;  z15 = ((z15 <<  8) | (z15 >> 24)) & MASK32
    z10 = (z10 + z15) & MASK32; z5  ^= z10; z5  = ((z5  <<  7) | (z5  >> 25)) & MASK32

    z1  = (z1  + z6)  & MASK32; z12 ^= z1;  z12 = ((z12 << 16) | (z12 >> 16)) & MASK32
    z11 = (z11 + z12) & MASK32; z6  ^= z11; z6  = ((z6  << 12) | (z6  >> 20)) & MASK32
    z1  = (z1  + z6)  & MASK32; z12 ^= z1;  z12 = ((z12 <<  8) | (z12 >> 24)) & MASK32
    z11 = (z11 + z12) & MASK32; z6  ^= z11; z6  = ((z6  <<  7) | (z6  >> 25)) & MASK32

    z2  = (z2  + z7)  & MASK32; z13 ^= z2;  z13 = ((z13 << 16) | (z13 >> 16)) & MASK32
    z8  = (z8  + z13) & MASK32; z7  ^= z8;  z7  = ((z7  << 12) | (z7  >> 20)) & MASK32
    z2  = (z2  + z7)  & MASK32; z13 ^= z2;  z13 = ((z13 <<  8) | (z13 >> 24)) & MASK32
    z8  = (z8  + z13) & MASK32; z7  ^= z8;  z7  = ((z7  <<  7) | (z7  >> 25)) & MASK32

    z3  = (z3  + z4)  & MASK32; z14 ^= z3;  z14 = ((z14 << 16) | (z14 >> 16)) & MASK32
    z9  = (z9  + z14) & MASK32; z4  ^= z9;  z4  = ((z4  << 12) | (z4  >> 20)) & MASK32
    z3  = (z3  + z4)  & MASK32; z14 ^= z3;  z14 = ((z14 <<  8) | (z14 >> 24)) & MASK32
    z9  = (z9  + z14) & MASK32; z4  ^= z9;  z4  = ((z4  <<  7) | (z4  >> 25)) & MASK32
  end

  [
    (z0  + x0)  & MASK32, (z1  + x1)  & MASK32, (z2  + x2)  & MASK32, (z3  + x3)  & MASK32,
    (z4  + x4)  & MASK32, (z5  + x5)  & MASK32, (z6  + x6)  & MASK32, (z7  + x7)  & MASK32,
    (z8  + x8)  & MASK32, (z9  + x9)  & MASK32, (z10 + x10) & MASK32, (z11 + x11) & MASK32,
    (z12 + x12) & MASK32, (z13 + x13) & MASK32, (z14 + x14) & MASK32, (z15 + x15) & MASK32,
  ].pack("V16")
end

.stream(key, nonce, length, counter = 0) ⇒ String

Generate ChaCha20 keystream bytes (equivalent to XOR against zeros).

Parameters:

  • key (String) —

    32-byte key

  • nonce (String) —

    8-byte nonce

  • length (Integer) —

    number of bytes to generate

  • counter (Integer) (defaults to: 0) —

    initial 64-bit block counter (default 0)

Returns:

  • (String) —

    keystream bytes



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# File 'lib/nuckle/internals/chacha20.rb', line 149

def stream(key, nonce, length, counter = 0)
  xor(key, nonce, ("\x00".b * length), counter)
end

.xor(key, nonce, message, counter = 0) ⇒ String

XOR message with ChaCha20 keystream.

Parameters:

  • key (String) —

    32-byte key

  • nonce (String) —

    8-byte nonce

  • message (String) —

    plaintext/ciphertext

  • counter (Integer) (defaults to: 0) —

    initial 64-bit block counter (default 0)

Returns:

  • (String) —

    XOR’d output (same length as message)



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# File 'lib/nuckle/internals/chacha20.rb', line 111

def xor(key, nonce, message, counter = 0)
  msg    = message.b
  len    = msg.bytesize
  out    = String.new(capacity: len, encoding: Encoding::BINARY)
  offset = 0

  while offset < len
    ks        = block(key, nonce, counter)
    remaining = len - offset
    take      = remaining < 64 ? remaining : 64

    if take == 64
      m = msg.byteslice(offset, 64).unpack("Q<8")
      b = ks.unpack("Q<8")
      out << [m[0] ^ b[0], m[1] ^ b[1], m[2] ^ b[2], m[3] ^ b[3],
              m[4] ^ b[4], m[5] ^ b[5], m[6] ^ b[6], m[7] ^ b[7]].pack("Q<8")
    else
      m = msg.byteslice(offset, take).unpack("C*")
      b = ks.unpack("C*")
      take.times { |i| m[i] ^= b[i] }
      out << m.pack("C*")
    end

    offset  += take
    counter += 1
  end

  out
end