From 8e949f837bce8ba735c011be8137e414c2cbcd88 Mon Sep 17 00:00:00 2001
From: Yorick van Pelt <yorick@yorickvanpelt.nl>
Date: Sun, 30 Apr 2023 18:16:52 +0200
Subject: [PATCH] Use PHC hashes for password storage

---
 handlers.go         |  3 +++
 internal/db/user.go | 52 ++++++++++++++++++++++++++++++++++-----------
 2 files changed, 43 insertions(+), 12 deletions(-)

diff --git a/handlers.go b/handlers.go
index 0c273d5..35cb9e3 100644
--- a/handlers.go
+++ b/handlers.go
@@ -338,6 +338,9 @@ func (rl *rushlink) authenticateUser(w http.ResponseWriter, r *http.Request, sho
 		// Authentication failed, return a 401 Unauthorized response
 		w.Header().Set("WWW-Authenticate", `Basic realm="Restricted"`)
 		w.WriteHeader(http.StatusUnauthorized)
+		if err != nil {
+			log.Printf("authentication failure: %s", err)
+		}
 		return nil
 	}
 	return user
diff --git a/internal/db/user.go b/internal/db/user.go
index 67e1942..4c757d7 100644
--- a/internal/db/user.go
+++ b/internal/db/user.go
@@ -6,6 +6,7 @@ import (
 	"encoding/base64"
 	"errors"
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/google/uuid"
@@ -54,37 +55,64 @@ func Authenticate(db *gorm.DB, username string, password string) (*User, error)
 	}
 
 	// Compare the hashed password with the provided password
-	if !comparePassword(user.Password, password) {
+	valid, err := comparePassword(user.Password, password)
+	if err != nil {
+		return nil, err
+	}
+	if !valid {
 		return nil, errors.New("invalid password")
 	}
 
 	return &user, nil
 }
 
+const (
+	pwdSaltSize = 16
+	pwdHashSize = 32
+	pwdParams   = "m=65536,t=2,p=1"
+	pwdAlgo     = "argon2id"
+)
+
 func HashPassword(password string) (string, error) {
 	// Generate a salt for the password hash
-	salt := make([]byte, 16)
+	salt := make([]byte, pwdSaltSize)
 	if _, err := rand.Read(salt); err != nil {
 		return "", err
 	}
 
 	// Hash the password using argon2id
-	hash := argon2.IDKey([]byte(password), salt, 2, 64*1024, 1, 64)
+	hash := argon2.IDKey([]byte(password), salt, 2, 64*1024, 1, pwdHashSize)
 
-	// Encode the salt and hash as a string
-	return string(salt) + string(hash), nil
+	// Encode the salt and hash as a string in PHC format
+	encodedSalt := base64.RawStdEncoding.EncodeToString(salt)
+	encodedHash := base64.RawStdEncoding.EncodeToString(hash)
+	return fmt.Sprintf("$%s$%s$%s$%s", pwdAlgo, pwdParams, encodedSalt, encodedHash), nil
 }
 
-func comparePassword(hashedPassword string, password string) bool {
-	// Decode the salt and hash from the hashed password string
-	salt := []byte(hashedPassword)[:16]
-	hash := []byte(hashedPassword)[16:]
+func comparePassword(hashedPassword string, password string) (bool, error) {
+	// Extract the salt and hash from the hashed password string
+	fields := strings.Split(hashedPassword, "$")[1:]
+	if len(fields) != 4 || fields[0] != pwdAlgo || fields[1] != pwdParams {
+		return false, errors.New("invalid password format in db")
+	}
+	encodedSalt, encodedHash := fields[2], fields[3]
 
-	// Hash the password using the same salt and parameters
-	computedHash := argon2.IDKey([]byte(password), salt, 2, 64*1024, 1, 64)
+	// Decode the salt and hash from base64
+	salt, err := base64.RawStdEncoding.DecodeString(encodedSalt)
+	if err != nil {
+		return false, err
+	}
+	hash, err := base64.RawStdEncoding.DecodeString(encodedHash)
+	if err != nil {
+		return false, err
+	}
+
+	// Hash the password using the extracted salt and parameters
+	computedHash := argon2.IDKey([]byte(password), salt, 2, 64*1024, 1, pwdHashSize)
 
 	// Compare the computed hash with the stored hash
-	return bytes.Equal(hash, computedHash)
+	// todo constant time?
+	return bytes.Equal(hash, computedHash), nil
 }
 
 // DeleteUser deletes a user with the specified username from the database.