Cryptography

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Components of Cryptography Chapter 3

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Categories of Cryptosystems

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Organization of Cryptosystems

Two-Way Cryptography

Symmetric

Asymmetric

Steganography

Message Integrity Controls

Stream

Block

Factoring the Product of Large Primes

Discreet Logarithms

Steganography

Art of hiding information

Plaintext hidden/disguised

Prevents a third party from knowing that a secret message exists

Traditionally accomplished in a number of ways:

Physical techniques

Null ciphers

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Null Cipher

Example Cipher text:

News Eight Weather: Tonight increasing snow. Unexpected precipitation smothers eastern towns. Be extremely cautious and use snowtires especially heading east. The highway is not knowingly slippery. Highway evacuation is suspected. Police report emergency situations in downtown ending near Tuesday.

Taking the first letter in each word successively yields the real message: "Newt is upset because he thinks he is President."

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Image-based Steganography

RGB* values altered to contain a message

File sizes are identical

Different hash values

E1089197693F6C4C26E0033F8C8AF00C

57694B77DCB55C543C6C0BA8E1FF2D17

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Digital Watermarking

Digital watermarks are visible or invisible markings embedded within a digital file to indicate copyright or other handling instructions, or to embed a fingerprint to detect unauthorized copying and distribution of images

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Substitution and Transposition

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Cryptography

Substitution

Simple

Polyalphabetic

Transposition

Scytale

Running Key

Codes

Substitution Ciphers

Four-square

Playfair

Caesar Cipher:

Shift (rotate) alphabet (move letters three spaces)

A B C D E F ... BAD

D E F G H I ... EDG

Scramble alphabet:

Substitute one letter for another

A B C D E F ... BAD

Q E Y R T M ... EQR

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Polyalphabetic Ciphers – Vigenère Cipher

This is a simple substitution cipher that uses multiple alphabets rather than just one

Encrypt the plaintext ‘FEEDBACK’ using a key of 3241

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Running Key Ciphers

Use the value of plaintext letters and a value of key based on a shared book

Value of Message

‘THIS message …’

+ Value of Key

‘on periodic …’

= Value of Ciphertext

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One-time Pads (OTPs)

The keys are the same size (length) as the plaintext message, and the keys must be randomly generated for this scheme to be truly effective

It is the requirement for randomness that makes OTPs particularly challenging to generate, since the best that we can typically achieve when generating such pads using computer processors are pseudo-random values

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Codes

A group of navy ships is operating under conditions of radio silence to avoid detection by the enemy

Maneuvering signals and comments are communicated by flag signals

A separate flag is used for each letter

To send a paragraph of information would be a real workout for the signalmen, so codes are used to minimize the effort and time involved; for instance, the letters BZ are the signal that the addressee performed well or did a good job

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Transposition Ciphers

Columnar – rearranging the message in a table

Plaintext “This is an example of transposition”

Ciphertext “tsaonihamfstinptpiselrooixeasn”

Key: grid shape and reading direction

Example: the Spartan Scytale

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