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Transposition Cipher

Characters remain the same

One-to-one Mapping

• Each symbol in the plain alphabet maps to a single
symbol in the cipher alphabet.
• Each symbol in the cipher alphabet maps to a single
symbol in the plain alphabet.

Ex: MSC

One-to-many Mapping

• Symbols in the plain alphabet can map to more than
one symbol in the cipher alphabet.
• Each symbol in the cipher alphabet maps to a single
symbol in the plain alphabet.

Ex: MSC w/ Homophones

Many-to-many Mapping

• Symbols in the plain alphabet can map to more than
one symbol in the cipher alphabet.
• Symbols in the cipher alphabet can map to more than
one symbol in the plain alphabet.

Ex: Vigenere

Monoalphabetic Substitution Cipher

Each letter in plaintext maps to one symbol in ciphertext.

Ex: Caesar Shift Cipher

Monoalphabetic Substitution Cipher w/ Homophones

Based on frequency

Polyalphabetic Substitution Cipher

The letters in the plaintext map to multiple symbols in the cipher text

Ex: Vigenere Cipher

One-time Pad Cipher

• Key has to be as long as plaintext
• Key is generated randomly
• Key is never repeated
• Unbreakable

Book Cipher

• A book or text is the key
• Number each word (or letter) in key

Kasiski-Babbage Method (for Breaking Vigenere Cipher)

• Analyze the text for repetitions

Binary Transposition Cipher

Reverse pairs of digits i.e. switch 1st and 2nd digits

Binary Substitution Cipher

Plaintext and codeword

First In First Out (Monoalphabetic Cipher)

• Alice's key to encrypt

First In First Out (Caesar Cipher)

• First shift 4

One-Way Functions

• Easy to compute in forward direction
• Difficult to reverse

Diffie-Helman

Z \= Y^X (mod P)

Book cipher is most similar to

Monoalphabetic substitution cipher with homophones

Credit Card number (Luhn Algorithm)

• Move right-to-left
• Digits in odd numbered positions: write that digit
• Digits in even numbered places

Universal Product Codes

Moving from left-to-right
• Odd position is tripled
• Take sum (mod 10)

ISBN

• Starting from right

Checksum (Luhn Algorithm)

• Luhn algorithm detects ALL single-digit errors
• Catches some transposition

Checksum (UPC)

• Catches ALL single-digit errors
• Won't detect errors that differ by exactly 5

Checksum (ISBN)

• Detects ALL single-digit errors and ALL transposition errors