// setup produces a sub-key and Salsa20 counter given a nonce and key.
func setup(subKey *[32]byte, counter *[16]byte, nonce *[24]byte, key *[32]byte) {
// We use XSalsa20 for encryption so first we need to generate a
// key and nonce with HSalsa20.
var hNonce [16]byte
copy(hNonce[:], nonce[:])
salsa.HSalsa20(subKey, &hNonce, key, &salsa.Sigma)
// The final 8 bytes of the original nonce form the new nonce.
copy(counter[:], nonce[16:])
}
// XORKeyStream crypts bytes from in to out using the given key and nonce. In
// and out may be the same slice but otherwise should not overlap. Nonce must
// be either 8 or 24 bytes long.
func XORKeyStream(out, in []byte, nonce []byte, key *[32]byte) {
if len(out) < len(in) {
in = in[:len(out)]
}
var subNonce [16]byte
if len(nonce) == 24 {
var subKey [32]byte
var hNonce [16]byte
copy(hNonce[:], nonce[:16])
salsa.HSalsa20(&subKey, &hNonce, key, &salsa.Sigma)
copy(subNonce[:], nonce[16:])
key = &subKey
} else if len(nonce) == 8 {
copy(subNonce[:], nonce[:])
} else {
panic("salsa20: nonce must be 8 or 24 bytes")
}
salsa.XORKeyStream(out, in, &subNonce, key)
}
// Precompute calculates the shared key between peersPublicKey and privateKey
// and writes it to sharedKey. The shared key can be used with
// OpenAfterPrecomputation and SealAfterPrecomputation to speed up processing
// when using the same pair of keys repeatedly.
func Precompute(sharedKey, peersPublicKey, privateKey *[32]byte) {
curve25519.ScalarMult(sharedKey, privateKey, peersPublicKey)
salsa.HSalsa20(sharedKey, &zeros, sharedKey, &salsa.Sigma)
}