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package internal
//go:generate go run chacha_core_gen.go
import (
"encoding/binary"
)
const (
wordSize = 4 // the size of ChaCha20's words
stateSize = 16 // the size of ChaCha20's state, in words
blockSize = stateSize * wordSize // the size of ChaCha20's block, in bytes
)
type ChaCha20Stream struct {
state [stateSize]uint32 // the state as an array of 16 32-bit words
block [blockSize]byte // the keystream as an array of 64 bytes
offset int // the offset of used bytes in block
rounds int
}
func NewChaCha20Stream(key []byte, nonce []byte, rounds int) *ChaCha20Stream {
s := new(ChaCha20Stream)
// the magic constants for 256-bit keys
s.state[0] = 0x61707865
s.state[1] = 0x3320646e
s.state[2] = 0x79622d32
s.state[3] = 0x6b206574
for i := 0; i < 8; i++ {
s.state[i+4] = binary.LittleEndian.Uint32(key[i*4 : i*4+4])
}
switch len(nonce) {
case 8:
s.state[14] = binary.LittleEndian.Uint32(nonce[0:])
s.state[15] = binary.LittleEndian.Uint32(nonce[4:])
case 12:
s.state[13] = binary.LittleEndian.Uint32(nonce[0:4])
s.state[14] = binary.LittleEndian.Uint32(nonce[4:8])
s.state[15] = binary.LittleEndian.Uint32(nonce[8:12])
default:
panic("bad nonce length")
}
s.rounds = rounds
ChaCha20Block(&s.state, s.block[:], s.rounds)
return s
}
func (s *ChaCha20Stream) XORKeyStream(dst, src []byte) {
// Stride over the input in 64-byte blocks, minus the amount of keystream
// previously used. This will produce best results when processing blocks
// of a size evenly divisible by 64.
i := 0
max := len(src)
for i < max {
gap := blockSize - s.offset
limit := i + gap
if limit > max {
limit = max
}
o := s.offset
for j := i; j < limit; j++ {
dst[j] = src[j] ^ s.block[o]
o++
}
i += gap
s.offset = o
if o == blockSize {
s.offset = 0
s.state[12]++
ChaCha20Block(&s.state, s.block[:], s.rounds)
}
}
}
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