package movement

import (
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"time"
)

// TokenLifetime is how long a confirmation link stays good. Long enough for
// somebody to come back to their inbox tomorrow; short enough that a link
// sitting in an archive forever is not a live credential.
const TokenLifetime = 24 * time.Hour

// mintToken returns the token to mail and the hash to store.
//
// The token is random, not derived from the instance key. Deriving it would
// mean a key rotation silently killed every link already in somebody's inbox,
// and would make the key a second thing that has to survive for the app to
// keep its promises. Only the hash is stored, so a database read does not
// hand somebody the ability to confirm other people's addresses.
func mintToken() (token string, hash []byte) {
	var raw [32]byte
	if _, err := rand.Read(raw[:]); err != nil {
		// Every property of the confirmation flow rests on this being
		// unguessable. There is no degraded mode worth having.
		panic("movement: crypto/rand unavailable")
	}
	token = hex.EncodeToString(raw[:])
	return token, hashToken(token)
}

// hashToken is how a presented token is looked up. Plain SHA-256 is right
// here and would be wrong for a password: the input is 256 bits of entropy we
// generated, so there is nothing to brute-force and nothing a slow hash would
// buy.
func hashToken(token string) []byte {
	sum := sha256.Sum256([]byte(token))
	return sum[:]
}