Argon2 Password Hashing

package main

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

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

const (
	argonTime    = 1
	argonMemory  = 64 * 1024
	argonThreads = 4
	argonKeyLen  = 32
	argonSaltLen = 16
)

func HashPassword(password string) (string, error) {
	salt := make([]byte, argonSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("read salt: %w", 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
}

func VerifyPassword(password, encoded string) (bool, error) {
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, fmt.Errorf("unsupported hash format")
	}
	var memory uint32
	var timeCost uint32
	var threads uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &timeCost, &threads); err != nil {
		return false, fmt.Errorf("parse parameters: %w", err)
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, fmt.Errorf("decode salt: %w", err)
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, fmt.Errorf("decode hash: %w", err)
	}
	got := argon2.IDKey([]byte(password), salt, timeCost, memory, threads, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}

func main() {
	encoded, err := HashPassword("correct horse battery staple")
	if err != nil {
		panic(err)
	}
	fmt.Println("encoded prefix:", encoded[:10])
	match, err := VerifyPassword("correct horse battery staple", encoded)
	if err != nil {
		panic(err)
	}
	reject, err := VerifyPassword("wrong password", encoded)
	if err != nil {
		panic(err)
	}
	fmt.Println("match:", match, "reject:", reject)
}