// Package auth implements local authentication: Argon2id passwords, opaque
// sessions, CSRF tokens, login throttling, roles and HTTP middleware
// (ADR-0006).
package auth

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"fmt"
	"strings"

	"golang.org/x/crypto/argon2"
)

// Argon2id parameters: 64 MiB, 3 passes, 2 lanes. Roughly 100 ms on the
// development machine, which throttles offline guessing without hurting login.
const (
	argonTime    = 3
	argonMemory  = 64 * 1024
	argonThreads = 2
	argonKeyLen  = 32
	saltLen      = 16
)

// MinPasswordLength is the shortest password accepted anywhere.
const MinPasswordLength = 12

// HashPassword returns a PHC-formatted Argon2id hash.
func HashPassword(password string) (string, error) {
	if len(password) < MinPasswordLength {
		return "", fmt.Errorf("password must be at least %d characters", MinPasswordLength)
	}
	salt := make([]byte, saltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := argon2.IDKey([]byte(password), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s", argonMemory, argonTime, argonThreads,
		base64.RawStdEncoding.EncodeToString(salt), base64.RawStdEncoding.EncodeToString(key)), nil
}

// VerifyPassword reports whether password matches the PHC hash. It parses the
// parameters from the hash so they can be raised later without invalidating
// existing users.
func VerifyPassword(hash, password string) (bool, error) {
	parts := strings.Split(hash, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, errors.New("unsupported hash format")
	}
	var m uint32
	var t uint32
	var p uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &m, &t, &p); err != nil {
		return false, fmt.Errorf("bad hash params: %w", err)
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, err
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(password), salt, t, m, p, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}

// dummyHash is verified against when a username does not exist so that the
// response time does not reveal whether the account exists.
var dummyHash, _ = HashPassword("noema-timing-equalisation-password")