// 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:])
}
// 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)
}