3.2 Communicating Securely

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Cryptology is the science of secure communications.

• Cryptography - is the process of inventing cipher systems (the secret ways of writing)

• Cryptoanalysis - is the process of  breaking cipher systems (decoding)

In cryptography encryption is the process of converting plain text to cipher text, while decryption is the process of reverting cipher test to its plain, original, text.

Cryptography is about constructing and analyzing protocols that prevent third parties from reading private or public messages. Encryption has four objectives:

1. Confidentiality

2. Integrity

3. Authenticity

4. Non-Repudiation

Cryptosystems to be discussed:

1. Pigpen Cipher

2. Shift Cipher

3. Substitution Cipher

4. Transposition Cipher

5. Affine Cipher

6. Vigenere Cipher

Cryptosystems are classified into two (2):

1. Monoalphabetic Cryptosystems

2. Polyalphabetic Cryptosystems

Monoalphabetic vs Polyalphabetic

1. Monoalphabetic Cryptosystems

• Substitution is fixed for each letter

• The problem with monoalphabetic cryptosystems is that they are vulnerable to brute force attack via frequency analysis

2. Polyalphabetic Cryptosystems

• Some letters can be mapped to two or more different letters

• For example, letter A can be mapped as letters D, Z, Y, T

1. Pigpen Cipher

• The pigpen cipher is a monoalphabetic, substitution cipher that echanges letters for symbols which are fragments of a grid. Below is the Pigpen Cipher.

2. Shift Cipher

• The shift cipher involves replacing each letter in the message by a letter that is some fixed number of positions further along in the alphabet. 

• The most common shift cipher is the Caesar Cipher. The table below is the Caesar Cipher with a shift key of 3 (k=3).

3. Substitution Cipher

• The substitution cipher is similar to the Caesar Cipher, the difference is, you can choose which letters substitute the alphabet.

4. Transposition Cipher

• Letters are moved around

• Each letter retains its identity but changes its position

• In most cases, there is a pattern/ rule applied to the letters in order to decode the message.

Example:

1. THAMSIYMETIROVAF - reverse letters of each word

2. ETIROVAFYMSIHTAM - reverse order of the letters

Try guessing the phrase! Are you done? You're right! The decypted phrase is MATH IS MY FAVORITE

POLYALPHABETIC CRYPTOSYSTEMS

1. Affine Cipher

2. Vigenere Cipher

For the polyalphabetic cryptosystems, we recommend you to watch our lecture video below!