Hashing Java

Last revision (mm/dd/yy): 01/17/2016

Introduction

This page helps Java developers hash passwords safely. We rely on OWASP's Password Storage Cheat Sheet to explain hashing best practice and theory.

Java Example

   public static byte[] hashPassword( final char[] password, final byte[] salt, final int iterations, final int keyLength ) {
 
       try {
           SecretKeyFactory skf = SecretKeyFactory.getInstance( "PBKDF2WithHmacSHA512" );
           PBEKeySpec spec = new PBEKeySpec( password, salt, iterations, keyLength );
           SecretKey key = skf.generateSecret( spec );
           byte[] res = key.getEncoded( );
           return res;
 
       } catch( NoSuchAlgorithmException | InvalidKeySpecException e ) {
           throw new RuntimeException( e );
       }
   }

Guidance

The password and salt arguments are arrays, as is the result of the hashPassword function. Sensitive data should be cleared after you have used it (set the array elements to zero).

The example uses a Password Based Key Derivation Function 2 (PBKDF2), as discussed in the Password Storage Cheat Sheet.

The salt argument should be random data and vary for each user. It should be at least 32 bytes long. Remember to save the salt with the hashed password!

The interations argument specifies how many times the PBKDF2 executes its underlying algorithm. A higher value is safer. You need to experiment on hardware equivalent to your production systems. As a starting point, find a value that requires one half second to execute. Scaling to huge number of users is beyond the scope of this document. Remember to save the value of iterations with the hashed password!

A keyLength of 256 is safe.

If the example code generates a NoSuchAlgorithmException, replace PBKDF2WithHmacSHA512 with PBKDF2WithHmacSHA1. Both are adequate to the task but you may be criticized when people see "SHA1" in the specification (SHA1 can be unsafe outside of the context of PBKDF2).

The SecretKeyFactory and PBEKeySpec classes have been part of Java SE since version 1.4.

Reference

See Iron-Clad Java: Building Secure Web Applications by Manico and Detlefsen, 2015, Oracle Press.