func (n *noise255ctx) mac(keystream, ciphertext, authtext []byte) [16]byte {
var macKey [32]byte
var tag [16]byte
copy(macKey[:], keystream)
poly1305.Sum(&tag, n.authData(ciphertext, authtext), &macKey)
return tag
}
// Seal appends an encrypted and authenticated copy of message to out, which
// must not overlap message. The key and nonce pair must be unique for each
// distinct message and the output will be Overhead bytes longer than message.
func Seal(out, message []byte, nonce *[24]byte, key *[32]byte) []byte {
var subKey [32]byte
var counter [16]byte
setup(&subKey, &counter, nonce, key)
// The Poly1305 key is generated by encrypting 32 bytes of zeros. Since
// Salsa20 works with 64-byte blocks, we also generate 32 bytes of
// keystream as a side effect.
var firstBlock [64]byte
salsa.XORKeyStream(firstBlock[:], firstBlock[:], &counter, &subKey)
var poly1305Key [32]byte
copy(poly1305Key[:], firstBlock[:])
out = out[len(out):]
outLen := len(message) + poly1305.TagSize
if cap(out) >= outLen {
out = out[:outLen]
} else if out == nil {
out = make([]byte, outLen)
} else {
for i := 0; i < outLen; i++ {
out = append(out, 0)
}
}
// We XOR up to 32 bytes of message with the keystream generated from
// the first block.
firstMessageBlock := message
if len(firstMessageBlock) > 32 {
firstMessageBlock = firstMessageBlock[:32]
}
tagOut := out
out = out[poly1305.TagSize:]
for i, x := range firstMessageBlock {
out[i] = firstBlock[32+i] ^ x
}
message = message[len(firstMessageBlock):]
ciphertext := out
out = out[len(firstMessageBlock):]
// Now encrypt the rest.
counter[8] = 1
salsa.XORKeyStream(out, message, &counter, &subKey)
var tag [poly1305.TagSize]byte
poly1305.Sum(&tag, ciphertext, &poly1305Key)
copy(tagOut, tag[:])
return tagOut
}