Cryptography Tutorials - Herong's Tutorial Notes
Dr. Herong Yang, Version 4.00
 This site Web DES Algorithm - Stream Cipher Modes and JCE SUN Implementation Part:   1  2  3  4  (Continued from previous part...) With the above notations, a k-bit CFB operation mode variation can be described as: ```C[i] = P[i] XOR Fk(E(I[i])) I[i] = Sk(I[i-1], C[i-1]) I[1] = IV IV | I[1]------Sk()--I[2]------Sk()--I[3] | / | / | E() / E() / E() | / | / | 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 CFB variations: The 64-bit CFB variation is the original CFB operation mode. The 8-bit CFB variation is a stream cipher that takes 1 byte (8 bits) from the plaintext message at a time. The 1-bit CFB variation is a stream cipher that takes 1 bit from the plaintext message at a time. OFB (Output FeedBack) Operation Mode as a 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) 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 IV | I[1]------Sk()--I[2]------Sk()--I[3] | / | / | 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: The 64-bit OFB variation is the original OFB operation mode. The 8-bit OFB variation is a stream cipher that takes 1 byte (8 bits) from the plaintext message at a time. The 1-bit OFB variation is a stream cipher that takes 1 bit from the plaintext message at a time. Sun Java Implementation of DES Operation Modes Sun has implemented stream cipher modes for both CFB and OFB modes, but with restrictions that the feedback sizes must be multiples of 8 bits. To use CFB or OFB in a stream cipher mode, you nee to specify the feedback size in bits right after the mode name when calling Cipher.getInstance(algorithm) to create a cipher object like: ``` Cipher cObj1 = Cipher.getInstance("DES/CFB8/NoPadding"); Cipher cObj2 = Cipher.getInstance("DES/CFB16/NoPadding"); Cipher cObj3 = Cipher.getInstance("DES/CFB24/NoPadding"); ... Cipher cObja = Cipher.getInstance("DES/OFB8/NoPadding"); Cipher cObjb = Cipher.getInstance("DES/OFB16/NoPadding"); Cipher cObjc = Cipher.getInstance("DES/OFB24/NoPadding"); ... ``` (Continued on next part...) Part:   1  2  3  4

Dr. Herong Yang, updated in 2007
Cryptography Tutorials - Herong's Tutorial Notes - DES Algorithm - Stream Cipher Modes and JCE SUN Implementation