The Caesar Cipher, also known as the shift cipher, is one of the simplest and most well-known encryption techniques. It works by shifting each letter in the plaintext by a fixed number of positions down the alphabet. Decoding requires determining the shift value and applying the reverse shift to recover the original message.

The Atbash cipher is a simple substitution cipher where each letter is replaced by its mirror image in the alphabet: A becomes Z, B becomes Y, C becomes X, and so on. It is a symmetric cipher (self-inverse), meaning applying it twice returns the original text.

ROT13 (rotate by 13 places) is a special case of the Caesar cipher with a shift of 13. It is its own inverse (applying ROT13 twice returns the original text), making it convenient for simple obfuscation. It is commonly used in online forums to hide spoilers and puzzle solutions.

The Reverse Cipher (also known as the backwards cipher) is a simple transposition cipher where the order of letters in the word is reversed. For example, 'HELLO' becomes 'OLLEH'. This cipher is often combined with other ciphers in competitive exam problems.

The Keyword Cipher is a monoalphabetic substitution cipher where a keyword is used to generate the cipher alphabet. The keyword is written first (with duplicate letters removed), followed by the remaining letters of the alphabet in order. This creates a one-to-one mapping between plaintext and ciphertext letters.

Morse code is a method of encoding text characters as sequences of dots (.) and dashes (-). Each letter, number, and punctuation has a unique pattern. It was developed for telegraph communication and remains relevant in puzzle and competitive exam contexts.

The Rail Fence Cipher (also called the zigzag cipher) is a transposition cipher that writes the plaintext in a zigzag pattern across multiple rails (rows), then reads off each rail sequentially to produce the ciphertext. The number of rails determines the pattern complexity.

A Book Cipher is a cipher where the key is a specific book (or text). The ciphertext consists of references to positions in the book: page number, line number, and word number (or letter position). Only someone with the same book can decode the message.

The Vigenère Cipher is a method of encrypting alphabetic text using a series of Caesar ciphers based on the letters of a keyword. It is a polyalphabetic substitution cipher, meaning the same plaintext letter can be encoded to different ciphertext letters depending on its position.

The Playfair Cipher is a digraph substitution cipher that encrypts pairs of letters (digraphs) using a 5x5 matrix generated from a keyword. It treats I and J as the same letter. The cipher was used extensively in military applications and remains a popular puzzle in competitive exams.

The Columnar Transposition Cipher is a transposition cipher that writes the plaintext into a grid row by row, then reads the columns in an order determined by a keyword. The keyword letters determine the column order for reading the ciphertext.

The Autokey Cipher is a polyalphabetic substitution cipher that uses a priming key followed by the plaintext itself (or ciphertext) to generate the keystream. This addresses the periodicity weakness of the Vigenère cipher by making the key as long as the message.

The Hill Cipher is a polygraphic substitution cipher based on linear algebra. It uses matrix multiplication to transform blocks of letters (vectors) into ciphertext vectors. The key is an invertible matrix, and decoding requires the inverse of that matrix modulo 26.

The Baconian Cipher (also known as the Bacon's cipher) uses a 5-bit binary representation for each letter, where each bit is represented by two different typefaces (often 'A' and 'B' or '0' and '1'). Developed by Francis Bacon, it's a form of steganography.

Base64 and Hexadecimal (Hex) are encoding schemes used to represent binary data in ASCII text format. Base64 uses 64 characters (A-Z, a-z, 0-9, +, /) to encode data, while Hex uses 16 characters (0-9, A-F). These encodings are common in computer science and appear in competitive exams.

The Keyboard Shift Cipher replaces each letter with an adjacent key on a standard QWERTY keyboard. Common shifts include shifting left, right, up, or down. This cipher is based on physical keyboard layouts rather than alphabetical order.

A Date-Based Cipher uses a specific date (e.g., a birthday, historical event, or current date) to determine the encryption key. Common approaches use the sum of day+month+year (mod 26) as a Caesar shift, or use the date components to generate a keyword.