package token

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"strconv"
)

// Token derives one Reader's userscript credential from the deployment
// secret, the Reader's Discord id and their token epoch.
//
// The credential is deterministic rather than stored random because the
// server must be able to rebuild the install URL after a restart while the
// database holds only hashes: a random token with no plaintext copy anywhere
// would be unreconstructible, and keeping plaintext in memory would break
// every install link on restart. HMAC output is high-entropy, indistinguishable
// from random to anyone without the secret, and changes whenever the epoch
// does — which is what rotation is. The stored form is Hash of this value,
// so a database leak yields nothing but hashes of unguessable strings.
func Token(key []byte, discordID string, epoch int64) string {
	mac := hmac.New(sha256.New, key)
	// The separator is unambiguous: discord ids are decimal snowflakes and
	// epochs are plain integers, so no two (id, epoch) pairs can collide.
	mac.Write([]byte(discordID))
	mac.Write([]byte{0})
	mac.Write([]byte(strconv.FormatInt(epoch, 10)))
	return hex.EncodeToString(mac.Sum(nil))
}

// Hash is the SHA-256 of a credential — the only form that ever touches the
// database (readers.token_sha256). SHA-256 rather than a password hash is
// deliberate: these are unguessable values with nothing to brute-force, so a
// slow hash would only add per-request cost.
func Hash(cred string) [32]byte {
	return sha256.Sum256([]byte(cred))
}