// Package auth handles passwords, API tokens, sessions and permissions.
package auth

import (
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base32"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"strconv"
	"strings"

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

// Argon2id parameters (OWASP recommendation: m=19 MiB, t=2, p=1).
const (
	argonTime    = 2
	argonMemory  = 19 * 1024
	argonThreads = 1
	argonKeyLen  = 32
	saltLen      = 16
)

// MinPasswordLength is the shortest accepted password.
const MinPasswordLength = 8

// HashPassword returns an argon2id hash in PHC string format.
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=%d$m=%d,t=%d,p=%d$%s$%s", argon2.Version, argonMemory, argonTime, argonThreads,
		base64.RawStdEncoding.EncodeToString(salt), base64.RawStdEncoding.EncodeToString(key)), nil
}

// dummyHash is verified against when a user doesn't exist, so a login
// attempt takes the same time whether or not the username is valid.
var dummyHash, _ = HashPassword("not-a-real-password")

// CheckPassword reports whether password matches hash. An empty hash
// never matches (the time spent is the same).
func CheckPassword(hash, password string) bool {
	if hash == "" {
		hash = dummyHash
		password = "\x00" + password
	}
	parts := strings.Split(hash, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false
	}
	var m, t uint32
	var p uint8
	for _, kv := range strings.Split(parts[3], ",") {
		k, v, _ := strings.Cut(kv, "=")
		n, err := strconv.ParseUint(v, 10, 32)
		if err != nil {
			return false
		}
		switch k {
		case "m":
			m = uint32(n)
		case "t":
			t = uint32(n)
		case "p":
			p = uint8(n)
		}
	}
	salt, err1 := base64.RawStdEncoding.DecodeString(parts[4])
	want, err2 := base64.RawStdEncoding.DecodeString(parts[5])
	if err1 != nil || err2 != nil || m == 0 || t == 0 || p == 0 {
		return false
	}
	got := argon2.IDKey([]byte(password), salt, t, m, p, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1
}

var b32 = base32.NewEncoding("abcdefghijklmnopqrstuvwxyz234567").WithPadding(base32.NoPadding)

func randomString(nbytes int) string {
	b := make([]byte, nbytes)
	if _, err := rand.Read(b); err != nil {
		panic(err)
	}
	return b32.EncodeToString(b)
}

// TokenPrefix starts every Jarl API token, which makes leaked tokens easy
// to recognize (and to find with secret scanners).
const TokenPrefix = "jarl_"

// NewToken generates a token: the full secret string to give to the user,
// its public ID (stored in clear) and the hash of the secret part.
func NewToken() (token, publicID, secretHash string) {
	publicID = randomString(10) // 16 chars
	secret := randomString(20)  // 32 chars
	return TokenPrefix + publicID + "_" + secret, publicID, HashSecret(secret)
}

// RotateToken makes a new token with an existing public ID (a rotated
// token keeps its ID; only the secret changes).
func RotateToken(publicID string) (token, secretHash string) {
	secret := randomString(20)
	return TokenPrefix + publicID + "_" + secret, HashSecret(secret)
}

// ParseToken splits a token into its public ID and secret.
func ParseToken(token string) (publicID, secret string, err error) {
	rest, ok := strings.CutPrefix(strings.TrimSpace(token), TokenPrefix)
	if !ok {
		return "", "", errors.New("not a Jarl token")
	}
	publicID, secret, ok = strings.Cut(rest, "_")
	if !ok || len(publicID) != 16 || len(secret) != 32 {
		return "", "", errors.New("malformed token")
	}
	return publicID, secret, nil
}

// HashSecret hashes a high-entropy secret (tokens, session IDs).
func HashSecret(secret string) string {
	h := sha256.Sum256([]byte(secret))
	return hex.EncodeToString(h[:])
}

// CheckSecret compares a secret with a stored hash in constant time.
func CheckSecret(secret, hash string) bool {
	return subtle.ConstantTimeCompare([]byte(HashSecret(secret)), []byte(hash)) == 1
}

// NewSessionID returns a new session ID and CSRF token.
func NewSessionID() (id, csrf string) {
	return randomString(32), randomString(32)
}

// GenerateTSIGSecret returns a random base64 secret of n bytes.
func GenerateTSIGSecret(n int) string {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		panic(err)
	}
	return base64.StdEncoding.EncodeToString(b)
}