package security

import (
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
)

const sessionTokenBytes = 32

// NewSessionToken returns a fresh opaque session token and its storage hash. The token is 32 random
// bytes rendered as base64url and is the only secret that goes to the browser (inside the __Host-
// cookie). The database stores only HashSessionToken(token), so a read of the sessions table never
// yields a usable token and a stolen backup cannot be replayed as a live session.
func NewSessionToken() (token string, tokenHash []byte, err error) {
	raw := make([]byte, sessionTokenBytes)
	if _, err := rand.Read(raw); err != nil {
		return "", nil, fmt.Errorf("security: read session token: %w", err)
	}
	token = base64.RawURLEncoding.EncodeToString(raw)
	return token, HashSessionToken(token), nil
}

// HashSessionToken returns the SHA 256 hash of a token, the form stored and looked up in the
// database.
func HashSessionToken(token string) []byte {
	sum := sha256.Sum256([]byte(token))
	return sum[:]
}