Symmetric cryptography
Overview
It involves sharing a key (cipher) with the person you want to communicate with.
The primary problems with symmetric:
- Trust in the counter-party.
- Overhead and maintenance of the list of authorized people.
- You may not know when a key has been compromised.
Examples
Caesar's Cipher
In Rome, circa 50 BC, Julius Caesar used this method of encryption for his personal correspondence.
It's a simple method of symmetric encryption that works by shifting the characters in a message by a fixed amount.
Ahead of time, Caesar would give the recipient the cipher (key) of say 3. And a D would become an A in the encrypted message.
The problem here is that the act of sending the cipher is insecure - any form of communication can be spied on and the key ascertained.
Stream Ciphers
Stream ciphers encrypt byte-by-byte.
The most common one you'll see is RC4, which is often used in SSL. The basic construction is essentially a random number generator seeded with your key, which generates bytes that are XORed with each byte of plaintext for encryption. Decryption is simply XORing the ciphertext instead. The strength of the algorithm is then solely dependent on the quality of rhe randomness.
Block Ciphers
AES (Rijndael), DES, 3DES, Twofish, are all block ciphers.
You split the data into N-byte blocks and encrypt those separately. Because we can't assume that all data is a multiple of N-bytes long, we have to pad data, introducing complexity. Encryption and decryption processes are not the same.
Electronic CodeBook (ECB) Mode
Electronic CodeBook mode is the simplest mode of operation for a block cipher. Each plaintext block is encrypted independently to produce a ciphertext block.
This means that if you see two blocks with the same ciphertext, you know that they must have had the same plaintext.
Cipher-Block Chaining (CBC) Mode
Cipher-Block Chaining is perhaps the most common form you will see. With CBC, each plaintext block is XORed with the ciphertext of the previous block before encryption. The opposite is performed for decryption. The first block is XORed with the Initialization Vector (IV).