OFB (Output FeedBack) as a Stream Cipher

This section describes how DES OFB (Output FeedBack) operation mode can be modified as a 1-bit stream cipher or a 1-byte stream cipher.

OFB (Output FeedBack) operation mode can be described with notations defined earlier as the following formula and diagram:

C[i] = P[i] XOR O[i]
O[i] = E(O[i-1])
O[1] = E(IV)

       |        -----|        -----| 
      E()     /     E()     /     E()
       |--O[1]       |--O[2]       |--O[3] 
       |             |             | 
P[1]--XOR     P[2]--XOR     P[3]--XOR
       |             |             | 
     C[1]          C[2]          C[3]

As you can see from the formula and the diagram, in OFB mode, each block of plaintext is XORed with the current output block to generate the current ciphertext block. The current output block is obtained by applying the encryption process on the previous output block. Note that for the first block, the Initial Vector (IV) is used as the previous output block.

In order to run the OFB operation mode as a stream cipher, FIPS 81 defines OFB variations where plaintext blocks can have any size less than 64 bits. To describe OFB variations, we need the following additional notations:

k - Representing the size plaintext blocks. k can have a value between 1 and 64.

Fk() - Representing a filter function that take the first k bits of a 64-bit block.

I = I[1], I[2], I[3], ..., I[i], ... - Representing the input block, I, used as input to the DES encryption process.

Sk() - Representing a shifting function that shifts k bits out of the input block from the left side. The missing k bits are taken from the output block.

With the above notations, a k-bit OFB operation mode variation can be described as:

C[i] = P[i] XOR Fk(O[i])
O[i] = E(I[i])
I[i] = Sk(I[i-1], O[i-1]) 
I[1] = IV

       |       /       |       /       | 
      E()     /       E()     /       E()
       |--O[1]         |--O[2]         |--O[3] 
     Fk()            Fk()            Fk()
       |               |               | 
P[1]--XOR       P[2]--XOR       P[3]--XOR
       |               |               | 
     C[1]            C[2]            C[3]

Note that there are some special cases of OFB variations:

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

 Introducting DES Stream Cipher Modes

 CFB (Cipher FeedBack) as a Stream Cipher

OFB (Output FeedBack) as a Stream Cipher

 CFB and OFB Stream Ciphers Implemented in JCE

 JCE DES Stream Ciphers Testing Program

 JCE DES Stream Ciphers Testing Program Result

 PHP Implementation of DES - 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

 Using Certificates in Google Chrome

 Using Certificates in Firefox

 Outdated Tutorials


 Full Version in PDF/EPUB