LIGN 17 Midterm 1

Transposition Cipher

symbols maintain their identities but change their positions

Ex: Binary Transposition, just switching 0 and 1. So you should have the same number of characters as the original plaintext for each symbol

or ADTC

Substitution cipher

Characters staying in their places but changing identities

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

Define nomenclators

a combination of codes and ciphers when a single symbol can map to a single letter AND a symbol can map to a phrase

e.g. 12=t and 34=the

Let’s suppose that instead of using a simple monoalphabetic substitution cipher (MSC),  Alice decides to do something different and use three such ciphers, applying them in succession in the following way. First, she encrypts the plaintext using an MSC with her first key, to create the first ciphertext.  Then, she encrypts that ciphertext using an MSC with a second, different key, yielding a second ciphertext. Finally, she encrypts that ciphertext with a third key, yielding a third ciphertext that she sends to Bob.  

Which of the following statements is most true?

The resulting cipher is generally going to be equally secure as if Alice had used just one, since for any three monoalphabetic substitution cipher keys applied one after the other, there exists another, single monoalphabetic substitution cipher key that achieves the same result.

Kerckhoff’s Principle

The security of a cryptogr-system must not depend on keeping secret the crypto-algorithm. The security depends only on keeping secret the key.

I.e. assume that Eve knows everything about the system except for the key

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 with key phrase

Monoalphabetic Substitution Cipher w/ Homophones

Based on frequency, letter in plaintext maps to many symbols in ciphertext

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

•A method for breaking Vigenere Cipher

Analyze the text for repetitions, deduce the length of the keyword or phrase (call this N)
• Do frequency analysis on N different (Caesar Shift) ciphers

Binary Transposition Cipher

Reverse pairs of digits i.e. switch 1st and 2nd digits, 3rd and 4th digits

Binary Substitution Cipher

Plaintext and codeword, if digits are the same, put 0 in ciphertext, if different, put 1 in chiphertext
• DECRYPTION ALGORITHM IS SAME AS ENCRYPTION ALGORITHM!
• Reciprocal algorithm

First In First Out (Monoalphabetic Cipher)

• Alice's key to encrypt, Bob's key to encrypt again
• Alice's key to decrypt first, Bob's key to decrypt second

First In First Out (Caesar Cipher)

• First shift 4, then shift 2
• Shift 2 in opposite direction, shift another 4

One-Way Functions

• Easy to compute in a forward direction but difficult to reverse
• Metaphor: mixing paint. We don’t have a way of separating colors once mixed

Diffie-Helman protocol

Z = Y^X (mod P)

a solution to the Key Distribution problem. ALSO, not a form of cryptography!!!

Book cipher is most similar to

Monoalphabetic substitution cipher with homophones

Define Steganography

Secret communication is achieved by masking the existence of a message (hidden message)

Define cryptography

Secret communication is achieved by masking the content (scrambled message)

Null

a symbol in the ciphertext that corresponds to nothing in the plaintext

What ____% of letters in English are E?

12.7

What ____% of letters in English are T?

9.1

What is a Dowbleth?

A symbol in the ciphertext that means "repeat the letter beside me" on the left or right to eliminate double letter giveaways.

Cryptographic crib

When Eve hypothesizes possible pieces of the plaintext to deduce the key

Decimal

Base 10. What humans use

XOR Cipher

A type of Binary substitution cipher. Switching the zeros and ones that are next to each other.

bit

short for binary digit

DES (Data Encryption Standard)

In the 70s, computers used this combination of transposition and substitution. 56-bit keys. By 1999, it took less than a day for a computer to break this.

key distribution problem

How can Alice and Bob agree on a key when Eve is intercepting all communication?

Which are the two ciphers in which the padlock metaphor actually would work?

XOR and Caesar shift

Binary addition

Modular Arithmetic

You divide by the mod. Your answer is the remainder (if there is a remainder)