Classical Cipher Taxonomy

Transposition, substitution, stream, and block ciphers

Before modern algorithms like AES, cryptography relied on classical cipher techniques that remain foundational to understanding how encryption works. These fall into two broad categories within symmetric-key cryptography.

Transposition vs. Substitution

Transposition ciphers rearrange the positions of plaintext characters without changing the characters themselves. Mathematically, a bijective function maps each character's original position to a new position. The Rail Fence cipher is a classic example.

Substitution ciphers replace plaintext characters with different characters according to a fixed system. The Caesar cipher — shifting each letter by a fixed number — is the simplest example; the Vigenère cipher uses a keyword to vary the shift per letter.

Stream Ciphers vs. Block Ciphers

Stream ciphers encrypt one bit or byte at a time using a pseudorandom keystream. The same plaintext bit encrypted at different positions produces different ciphertext, making them ideal for real-time communication. ChaCha20 is a widely deployed modern stream cipher.

Block ciphers encrypt fixed-size blocks of data (typically 128 bits). AES encrypts 128-bit blocks with key sizes of 128, 192, or 256 bits. DES (Data Encryption Standard) used 56-bit keys and was the U.S. government standard from 1977 until AES replaced it in 2001.