How does a transposition cipher work?

The letters of the plaintext are rearranged according to a pattern or key — all original letters appear in the ciphertext, just in a different order Letters are transported from one alphabet to another, changing their appearance The message is split into parts and each part is transported separately to the recipient The entire message is flipped (reversed) so it reads backward

What is the Vigenère cipher and why was it called 'le chiffre indéchiffrable' (the unbreakable cipher)?

A polyalphabetic cipher using a keyword — each letter of the keyword shifts the corresponding plaintext letter by a different amount, defeating simple frequency analysis A cipher using 26 different substitution alphabets that change after every sentence A modern cipher named after Vigenère that uses very long random keys A cipher that was mathematically proven to be unbreakable before computers

How does frequency analysis break a monoalphabetic substitution cipher?

Count how often each ciphertext symbol appears; the most frequent symbol likely represents E (~12.7%), then T (~9.1%), and match remaining frequencies to known English letter frequencies Try all possible key mappings and see which one produces English text Find repeated patterns that match common words like 'THE' and 'AND' Compare the ciphertext to known messages encrypted with the same method

What is the Playfair cipher and what made it more secure than simple substitution?

A cipher encrypting pairs of letters (digraphs) rather than individual letters — using a 5×5 grid keyed with a keyword — harder to break because it has 26² = 676 possible digraph substitutions A cipher where the key changes after every word — 'playing' the key forward A cipher invented specifically for children's play codes that is easy to use but hard to break A cipher that uses a deck of playing cards to determine the key

What is a 'homophonic cipher'?

A cipher that maps each plaintext letter to multiple possible ciphertext symbols — common letters like E get many possible symbols, rare letters get fewer, flattening frequency distribution A cipher where the message sounds the same when read aloud in two different languages A cipher designed for telephone communication that sounds like normal conversation Encrypting a message so it sounds like music when decoded as notes

How does the 'rail fence cipher' work?

A transposition cipher where plaintext is written in a zigzag across multiple 'rails' (rows), then read off row by row A cipher used by railroad companies to protect train schedules and cargo manifests Writing a message on separate pieces of paper and mixing them up A cipher where every other letter is the 'rail' and the alternating letters carry the hidden message

What was the Enigma machine and what made it so difficult to crack?

An electromechanical cipher machine used by Nazi Germany — its rotors changed the substitution with every keypress, making each letter encoded by a different cipher, creating trillions of possible settings A code machine that encoded messages as random symbols that changed daily A device that scrambled radio signals so they couldn't be intercepted A mechanical typewriter with a special keyboard that automatically reversed messages

What is a 'one-time pad' and why is it the only theoretically unbreakable cipher?

A cipher where each letter is shifted by a completely random key letter that is never reused — it's provably unbreakable because for any ciphertext, every possible plaintext is equally likely A cipher that can only be used once — after one use, the algorithm is published A digital cipher that requires a new computer for each encryption A cipher pad that self-destructs after the message is read

What are 'Navajo Code Talkers' and why were their messages unbreakable in WWII?

Native American Marines who transmitted military messages in their native language — Japanese cryptanalysts couldn't break the code because Navajo is an extraordinarily complex language with no written form and almost no non-native speakers Native American military officers who invented a cipher used during the Civil War A group of linguists who translated captured enemy codes into English during WWII Special soldiers who used animal communication patterns as a code system

What is a 'polyalphabetic cipher' and how does it improve on monoalphabetic ciphers?

A cipher that uses multiple substitution alphabets, switching between them throughout the message — the same plaintext letter encrypts differently depending on its position A cipher that supports multiple languages — the same key works for English, French, and Spanish A cipher that encrypts a message multiple times for extra security Any cipher that uses more than 26 symbols in its ciphertext alphabet