Modern ciphers work on blocks of plain text, not just a single symbol.
They are made up of a series of permutations and substitutions repeated on each block.
The key controls the exact nature of the permutations and substitutions.
Advanced Encryption Standard (AES)
AES is a state-of-the-art block cipher.
It works on blocks of 128 bits.
It generates 10 round keys from a single 128-bit key.
It uses one permutation: ShiftRows and three substitutions SubBytes, MixColumns, AddRoundKey.
Security of AES
No formal proof of security (P = NP?) but best known cryptographic attack requires 2126 key guesses - an (irrelevant) improvement of factor 4 compared to 2128 key guesses via brute force attack.
There are side channel attacks.
Key aspects of security :
Shuffling of rows and columns to ensure small change in input causes very big change in output.
Require at least one non-linear operation on the data - provided by the SubByte - operation.