Lecture 12 - Cryptography

Cryptography

the art of secret writing

converts data into unintelligible form

must be reversible

process of encryption and decryption

Plaintext

encryption (key)

ciphertext

decryption (key)

plaintext

plaintext

a message in its original form

Ciphertext

a message in the transformed, unrecognized form

Encryption

the process that transforms a plaintext into a ciphertextd

decryption

the process that transforms a ciphertext to the corresponding plaintextkey

Key

the value used to control encryption/decryption

Caesar Cipher

moving each letter of plaintext n number of times to the right

to decrypt, you move it backwards (left)

symmetric

single key

the key used to encrypt a message is the same as the one used to decrypt it

Asymmetric

Public key

key to decrypt a message is different from the key used to encrypt it

Algorithm & key

single key encryption

two pieces of identical information encrypted with the same algorithm but with different keys produce completely different ciphertexts

do not need to keep algorithm secret, just the key

key is small (number of bits)

brute-force attacks

try all possible keys on some ciphertext until get an intelligible translation into plaintext

on average half of all possible keys must be tried to achieve success

Key management

with symmetric encryption, the communicating parties must share the one key

Data Encryption Standard (DES)

Single Key

developed in mid 1970s

used 56-bit key

broken by experts in less than a day

Triple DES (3DES)

3 different keys to produce the encrypted text

Advanced encryption standard (AES)

most advanced computers and techniques available today

will require 150 trillion years to break by brute force

life span: 20 years

RSA (Rivest-Shamir-Adleman)

most popular form of asymmetric encryption

two keys:

one key (public key) is used to encrypt message

second very different private key is used to decrypt the message.

from 512, 1024, 2048 bits in length

one way function: once it is encrypted by the one way function, the message cannot be decrypted without the private key

RSA Key Pair

• Public key to encrypt the message

• Private key to decrypt the message

RSA Key Length

RSA key sizes can be 512, 1024, or 2048 bits long. Larger key sizes provide stronger security.

RSA One-Way Function

RSA uses a one-way function—once encrypted, the message cannot be decrypted without the private key, ensuring secure communication.

Public key encryption benefits

reduce the key management problem

key management problem is reduced to the on-site protection of the private key