Cryptography

Encryption

- plain text of the messenger will be encrypted to cipher text decrypted back to its plain text

• Original Version - Plain text 

• Encrypted version - cipher text

  • Encryption - process of using an algorithm to transform into unreadable format

• Decryption - transforming information back into a readable format

• Cryptosystem - pair of algorithms; one for encryption another for decryption

Cryptography

: using science and mathematics to encrypt and decrypt data

• Confidentiality - sender and receiver assured no third party can read the message

• Integrity - ensures no this party can make change in the message

• Authenticity - Assures Bob that Alice is the one sending him the message

• Non-repudiation - Bob can prove to any third party that Alice sent the message

Simple Ciphers

• Pigpen Cipher 

  • Dancing Men 

  • Caesar’s Cipher (Shift)

• Vigenre Cipher (Polyalphabetic cipher system)

  • Created by French Diplomat Blaise de Vigenre. Viewed as a substitute cipher where letters are substitute but alphabets may be repeated periodically according to some key

Symmetric vs Asymmetric

Symmetric Encryption: secret key encryption

Asymmetric Encryption : public key encryption

• Introduced by W. Diffie and M. Hellman

• Each person gets two special keys — one public and one private — to lock and unlock messages.

• When someone encrypts a message using the public key, only the matching private key can open it again.

Modular Arithmetic - a = b mod n

• Read as a is congruent to b modulo n -> meaning a - b is divisible by integer n

RSA Crypto System (Rivest, Shamir, Adleman)

• Trap door function is like a math shortcut that’s easy to do in one direction but almost impossible to reverse; known as a oneway function

• Key Generation 

  • Take two large prime numbers and compute n = pq

  • Choose a number e less than n and relatively prime (common divisor is 1) to = (p–1) (q–1)

  • Find another number d such that ed – 1 is divisible by = (p–1) (q–1), that is 

ed 1 mode = (p–1) (q–1)

• Public key is (n,e), private is (n,d)

Encryption Process: C = Me mod n

Decryption Process: M = Cd mod n