Question

I want to implement Electronic codebook (ECB) Cipher-block chaining (CBC) Cipher feedback (CFB) Output feedback (OFB) Counter (CTR) modes either in c++ or in java. But I don’t know how and where to start. Can any one please suggest me the steps to do?

Answer 1

It is actually pretty simple, let’s assume you have a function called block_cipher_encrypt(plaintext, key) that takes a single block of plaintext and a key as input and returns a single block of ciphertext.

Now, say you have an array of blocks of plaintext (say pt[i] is the ith block of plaintext) and an array ct for ciphertext blocks. To do ECB, it would be:

for i from 0 to len(pt)-1:  
  ct[i] = block_cipher_encrypt(pt[i], key)

For CBC mode, you also need an IV, but it is still pretty simple:

ct[0] = block_cipher_encrypt(XOR(pt[0], IV), key)
for i from 1 to len(pt):
  ct[i] = block_cipher_encrypt(XOR(pt[i], ct[i-1]), key)

For the other modes, just look at Wikipedia’s article on modes of operation. They provide nice block diagrams for both encryption and decryption. That is what I did for the pseudocode above.