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DES Algorithm - Stream Cipher Modes and JCE SUN Implementation

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This tutorial helps you understand:

• DES in Stream Cipher Modes
• CFB (Cipher FeedBack) Operation Mode as a Stream Cipher
• OFB (Output FeedBack) Operation Mode as a Stream Cipher
• Sun Java Implementation of DES Operation Modes
• JceSunDesStreamCipherTest.java - JCE DES Stream Cipher Mode Testing Program
• Test Cases of DES Stream Cipher Modes

DES in Stream Cipher Modes

As we from previous tutorials, DES algorithm is a block cipher algorithm. But it can be operated in different ways to become stream ciphers.

(FIPS 81) Federal Information Processing Standards Publication 81 published in 1980 defined the following 4 operation modes:

• ECB - Electronic Code Book operation mode.
• CBC - Cipher Block Chaining operation mode.
• CFB - Cipher Feedback operation mode
• OFB - Output Feedback operation mode

See http://www.itl.nist.gov/fipspubs/fip81.htm for details.

In FIPS 81, the last two operation modes, CFB and OFB, are defined in ways so that they can be used as stream ciphers.

In order to describe these operation modes, we need to define the following notations:

P = P[1], P[2], P[3], ..., P[i], ... - Representing the original plaintext message, P, being arranged into multiple 64-bit plaintext blocks. P[i] represents plaintext block number i.

E(P[i]) - Representing the DES encryption algorithm applied on a single 64-bit plaintext block, P[i], with a predefined key, k.

C = C[1], C[2], C[3], ..., C[i], ... - Representing the final ciphertext message, C, being regrouped from multiple 64-bit ciphertext blocks. C[i] represents ciphertext block number i.

IV - Called "Initial Vector", representing a predefined 64-bit initial value.

CFB (Cipher FeedBack) Operation Mode as a Stream Cipher

CFB (Cipher FeedBack) operation mode as a block cipher can be described with notations defined earlier as the following formula and diagram:

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

      IV                             
       |        -----|        -----| 
      E()     /     E()     /     E()
       |     /       |     /       |
       |    /        |    /        | 
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 CBC mode, each block of plaintext is XORed with the encrypted version of the previous ciphertext to generate the current ciphertext block. In this way, each ciphertext block is depending on all plaintext blocks up to that point. Note that for the first block, the Initial Vector (IV) is used as the previous ciphertext block.

In order to run the CFB operation mode as a stream cipher, FIPS 81 defines CFB variations where plaintext blocks can have any size less than 64 bits. To describe CFB 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 ciphertext block.

(Continued on next part...)

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