// Package auth implements identity: password hashing, sessions, per-library
// grants, and the middleware that turns a cookie into a current user.
//
// Nothing outside this package may inspect a password or a raw session token.
package auth

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

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

// Argon2id parameters. These are the OWASP-recommended baseline for
// interactive logins (memory cost is the dominant factor). They are encoded
// into every hash, so changing them here does not invalidate old passwords —
// existing hashes keep verifying with their own stored parameters.
const (
	argonTime    = 2
	argonMemory  = 64 * 1024 // KiB, i.e. 64 MiB
	argonThreads = 4
	argonKeyLen  = 32
	argonSaltLen = 16
)

var (
	// ErrInvalidHash means a stored password hash is not in the PHC format
	// this package writes. It indicates database corruption, not a bad login.
	ErrInvalidHash = errors.New("auth: password hash is not in PHC format")
	// ErrIncompatibleVersion means the hash was made by a newer argon2 version.
	ErrIncompatibleVersion = errors.New("auth: incompatible argon2 version")
)

// HashPassword derives an argon2id hash and returns it as a PHC string:
//
//	$argon2id$v=19$m=65536,t=2,p=4$<salt>$<hash>
//
// The parameters travel with the hash, so verification never needs them passed
// in and parameter changes are backward compatible.
func HashPassword(password string) (string, error) {
	salt := make([]byte, argonSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("auth: read salt: %w", err)
	}
	key := argon2.IDKey([]byte(password), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version, argonMemory, argonTime, argonThreads,
		base64.RawStdEncoding.EncodeToString(salt),
		base64.RawStdEncoding.EncodeToString(key),
	), nil
}

// VerifyPassword reports whether password matches the PHC-encoded hash.
// It always performs the full derivation, so a malformed hash and a wrong
// password cost the same — there is no timing oracle for "user exists".
func VerifyPassword(password, encoded string) (bool, error) {
	salt, want, params, err := decodeHash(encoded)
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(password), salt, params.time, params.memory, params.threads, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}

type argonParams struct {
	time    uint32
	memory  uint32
	threads uint8
}

func decodeHash(encoded string) (salt, hash []byte, p argonParams, err error) {
	parts := strings.Split(encoded, "$")
	// Leading empty field from the leading '$' gives 6 fields.
	if len(parts) != 6 || parts[0] != "" || parts[1] != "argon2id" {
		return nil, nil, p, ErrInvalidHash
	}

	var version int
	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil {
		return nil, nil, p, ErrInvalidHash
	}
	if version != argon2.Version {
		return nil, nil, p, ErrIncompatibleVersion
	}

	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &p.memory, &p.time, &p.threads); err != nil {
		return nil, nil, p, ErrInvalidHash
	}

	salt, err = base64.RawStdEncoding.Strict().DecodeString(parts[4])
	if err != nil {
		return nil, nil, p, ErrInvalidHash
	}
	hash, err = base64.RawStdEncoding.Strict().DecodeString(parts[5])
	if err != nil {
		return nil, nil, p, ErrInvalidHash
	}
	if len(hash) == 0 || len(salt) == 0 {
		return nil, nil, p, ErrInvalidHash
	}
	return salt, hash, p, nil
}