fix: reworked GeneratePassword to use secure crypt random generator

internal/api/magic_link.go

@@ -83,7 +83,11 @@ func (a *API) MagicLink(w http.ResponseWriter, r *http.Request) error {
 	if isNewUser {
 		// User either doesn't exist or hasn't completed the signup process.
 		// Sign them up with temporary password.
-		password := utilities.GeneratePassword(config.Password.RequiredCharacters, 33)
+		password, err := utilities.GeneratePassword(config.Password.RequiredCharacters, 33)
+		if err != nil {
+			// password generation must succeed
+			panic(err)
+		}
 
 		signUpParams := &SignupParams{
 			Email:               params.Email,

internal/utilities/password.go

@@ -1,8 +1,9 @@
 package utilities
 
 import (
-	"math/rand"
+	"crypto/rand"
 	"strings"
+	"math/big"
 )
 
 // parseGroups processes the required character groups from a slice of strings.
@@ -12,29 +13,53 @@ func parseGroups(requiredChars []string) []string {
 	return groups
 }
 
-func GeneratePassword(requiredChars []string, length int) string {
-
+func GeneratePassword(requiredChars []string, length int) (string, error) {
 	groups := parseGroups(requiredChars)
 	passwordBuilder := strings.Builder{}
+	passwordBuilder.Grow(length)
 
+	// Add required characters
 	for _, group := range groups {
 		if len(group) > 0 {
-			passwordBuilder.WriteString(string(group[rand.Intn(len(group))]))
+			randomIndex, err := secureRandomInt(len(group))
+			if err != nil {
+				return "", err
+			}
+			passwordBuilder.WriteByte(group[randomIndex])
 		}
 	}
 
 	// Define a default character set for random generation (if needed)
 	allChars := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
 
+	// Fill the rest of the password
 	for passwordBuilder.Len() < length {
-		passwordBuilder.WriteString(string(allChars[rand.Intn(len(allChars))]))
+		randomIndex, err := secureRandomInt(len(allChars))
+		if err != nil {
+			return "", err
+		}
+		passwordBuilder.WriteByte(allChars[randomIndex])
 	}
 
-	password := passwordBuilder.String()
-	passwordBytes := []byte(password)
-	rand.Shuffle(len(passwordBytes), func(i, j int) {
+	// Convert to byte slice for shuffling
+	passwordBytes := []byte(passwordBuilder.String())
+
+	// Secure shuffling
+	for i := len(passwordBytes) - 1; i > 0; i-- {
+		j, err := secureRandomInt(i + 1)
+		if err != nil {
+			return "", err
+		}
 		passwordBytes[i], passwordBytes[j] = passwordBytes[j], passwordBytes[i]
-	})
+	}
 
-	return string(passwordBytes)
+	return string(passwordBytes), nil
 }
+
+func secureRandomInt(max int) (int, error) {
+	randomInt, err := rand.Int(rand.Reader, big.NewInt(int64(max)))
+	if err != nil {
+		return 0, err
+	}
+	return int(randomInt.Int64()), nil
+}