This episode provides a foundational survey of symmetric ciphers, also known as conventional encryption, which require the exact same key for both the encryption and decryption processes. The fundamental assumption is that the algorithm itself does not need to be kept secret, only the key.
The primary methods discussed fall into two categories: substitution techniques and transposition techniques. Substitution involves replacing plaintext symbols with ciphertext symbols. Early methods like the Caesar cipher apply a fixed cyclic shift and are easily broken by brute-force attack due to the small number of possible keys.
Monoalphabetic ciphers employ a consistent substitution rule, but they are vulnerable to cryptanalysis because they preserve the underlying frequency statistics of the language (e.g., letter frequencies). The Playfair cipher introduced complexity by encrypting pairs of letters (digrams) to flatten the frequency distribution somewhat.
More complex systems, known as polyalphabetic ciphers, use multiple substitution rules. The Vigenère cipher, a classic example, uses a repeating keyword to apply a sequence of Caesar ciphers, thereby obscuring the statistical frequency of single letters. However, analysis can reveal the keyword length, allowing the cipher to be broken.
The quest for ultimate security led to the Vernam cipher, which works with binary data using XOR. The improvement on this, the one-time pad, utilizes a truly random key that is as long as the message and is used only once. The one-time pad is unbreakable because the ciphertext output bears no statistical relationship to the plaintext, achieving perfect secrecy. Practical use is limited, though, due to the difficulty of securely distributing such large, unique keys.
In contrast to substitution, transposition techniques rearrange the order of the plaintext letters through a permutation. Simple examples include the rail fence technique.
Finally, this episode covers steganography, a method that conceals the existence of a secret message, differentiating it from cryptography, which only conceals the meaning. Modern examples involve hiding secret data invisibly within the least significant bits of digital files.
