Cryptography Week 3: CPA-Secure Encryptions from PRF's

Notes on block ciphers:

For large enough n, random permutation is indistinguishable from random functions, therefore, block ciphers are good pseudo-random functions, Assuming you have a n-bit key k that you put into a block cipher, you can encrypt a n-bit message m. This is the same as the One Time Pad; the benefit here is that because the block cipher is pseudo-random, you can encrypt multiple messages with the same key. **

Currently the trouble with this encryption scheme is that encrypting an n-bit string results in a 2n-bit cipher text:

an n-bit R and n-bit key are plugged into the block cipher the pseudo-random output of the block cipher is XOR’d with m R & the output of the XOR are sent as the 2n-bit ciphertext *** You can mitigate this in several ways; the first step is to break the message into many small blocks to allow for an arbitrary length of message. ***

Modes of Operation:

Stream Ciphers (not covered in this course): Synchronized and Unsynchronized

Block ciphers:

Counter Mode, or CTR: I have a bunch of n-bit blocks Define an n-bit number c0 (First block in cipher text) c0+i is used as input for pseudo-random, where i is the # of the block in the message so final message is original message+n bits long

Cipher Block Chaining, or CBC mode: Define an n-bit number c0(first block in cipher text) XOR with m1 Xor c1 with m2 -> c2, c2 with m3 ->c3, and so on

Never use Electronic Code book, EBC mode: Was defined before true security notions were understood use m1 to encrypt m2 and so forth if two blocks are the same in a long plaintext, it won’t work because in long amounts there will be repeated ciphertext