Block Padding in mcrypt

This section describes the padding schema used in PHP mcrypt library for MCRYPT_MODE_ECB and MCRYPT_MODE_CBC modes. mcrypt simply pads 0x00 to the end of the plaintext.

If you use mcrypt DES to encrypt a stream of bytes in MCRYPT_MODE_CFB or MCRYPT_MODE_OFB mode, you don't need to worry about padding the byte stream. Because MCRYPT_MODE_CFB and MCRYPT_MODE_OFB are 1-byte ciphers.

But if you use mcrypt DES to encrypt a stream of bytes in MCRYPT_MODE_ECB or MCRYPT_MODE_CBC mode, you have to worry about padding the byte stream. Because MCRYPT_MODE_ECB and MCRYPT_MODE_CBC are 8-byte ciphers.

"mycypt" offers a very simple padding method which simply adds 0x00 to the end of the plaintext message to make it multiples of 8 bytes (64 bits) for MCRYPT_MODE_ECB and MCRYPT_MODE_CBC modes.

This will create problems if you encrypt a message with PHP mcrypt in MCRYPT_MODE_ECB mode and send it to someone else who tries to decrypt it with a JDK JCE program. Because the JDK JCE package uses the DES/ECB/PKCS5Padding mode by default. The last block will fail to decrypt because the padding schema is different.

So if you want exchange DES encrypted messages with a Java programmer, you need to handle the PKCS5Padding padding yourself as part of your PHP script. See the PKCS5Padding schema section for detailed information.

Table of Contents

 About This Book

 Cryptography Terminology

 Cryptography Basic Concepts

 Introduction to AES (Advanced Encryption Standard)

 Introduction to DES Algorithm

 DES Algorithm - Illustrated with Java Programs

 DES Algorithm Java Implementation

 DES Algorithm - Java Implementation in JDK JCE

 DES Encryption Operation Modes

 DES in Stream Cipher Modes

PHP Implementation of DES - mcrypt

 mcrypt Library for PHP

 mcrypt Encryption Functions

 mcrypt DES Encryption Testing Program

Block Padding in mcrypt

 Blowfish - 8-Byte Block Cipher

 Secret Key Generation and Management

 Cipher - Secret Key Encryption and Decryption

 Introduction of RSA Algorithm

 RSA Implementation using java.math.BigInteger Class

 Introduction of DSA (Digital Signature Algorithm)

 Java Default Implementation of DSA

 Private key and Public Key Pair Generation

 PKCS#8/X.509 Private/Public Encoding Standards

 Cipher - Public Key Encryption and Decryption

 MD5 Mesasge Digest Algorithm

 SHA1 Mesasge Digest Algorithm

 OpenSSL Introduction and Installation

 OpenSSL Generating and Managing RSA Keys

 OpenSSL Managing Certificates

 OpenSSL Generating and Signing CSR

 OpenSSL Validating Certificate Path

 "keytool" and "keystore" from JDK

 "OpenSSL" Signing CSR Generated by "keytool"

 Migrating Keys from "keystore" to "OpenSSL" Key Files

 Certificate X.509 Standard and DER/PEM Formats

 Migrating Keys from "OpenSSL" Key Files to "keystore"

 Using Certificates in IE (Internet Explorer)

 Using Certificates in Firefox

 Using Certificates in Google Chrome

 Outdated Tutorials

 References

 PDF Printing Version