Cryptography overview

• Process of hiding information
• Can protect e.g. emails, files, and other sensitive data.

Cryptography terms

• Cipher: an algorithm performing encryption and decryption
• Clear text / plaintext: unencrypted data
• Cipher text: encrypted data
• Key: specifies the transformation of data for encryption / decryption

Cipher types

• Cipher: algorithm performing encryption and decryption.

Classical ciphers

• Used historically but no longer used for the most part.

Substitution cipher

• Every character is substituted with another one

• E.g. Caesar Cipher (100 BC)

    Plaintext:  THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
    Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
    Key: right shift of 3

Polyalphabetic cipher

• Based on substitution
• Uses multiple substitution alphabets
• E.g. Vigenère cipher from 1467
  • Has cipher has several Caesar ciphers in sequence with different shift values.

Transposition cipher

• Plain text is repositioned (shifted) to create a ciphertext

• Also called a zigzag cipher

• E.g. Rail fence cipher (invented by ancient Greeks)

    Clear text: WE ARE DISCOVERED. FLEE AT ONCE
  
    W . . . E . . . C . . . R . . . L . . . T . . . E
    . E . R . D . S . O . E . E . F . E . A . O . C .
    . . A . . . I . . . V . . . D . . . E . . . N . .
  
    Ciphertext: WECRLTEERDSOEEFEAOCAIVDEN

Modern ciphers

Computational infeasibility

• Modern cryptographic systems are built on problems which are assumed to be computationally infeasible
• A computation which although computable would take far too many resources to actually compute.
• Cryptography tries to ensure an infeasible computation's cost is greater than the reward obtained by computing it
• "Secure" because it's "too slow" to achieve by computers.

Key-based ciphers

Symmetric encryption

• One (same) key to encrypt and decrypt
  • Known as single key or shared key.
• Either uses stream cipher or block cipher
• E.g. AES, DES
• Problems include key distribution and management
• Suitable for large amounts of data
• Harder for groups of people because more keys are needed as group increases
• Doing nothing for non-repudiation, only performs confidentiality

Asymmetric encryption

• Also known as public key cryptography
• 📝 Different keys to encrypt and decrypt
  • Public key to encrypt
    • Known by everyone
    • Can be issued by Public Key Infrastructure (PKI)
  • Private key to decrypt
    • Only known by the owner, secret to the public.
• It's slower than symmetric encryption
  • 📝 Hybrid encryption
    • Combining
      • public-key cryptography for ease of secure key exchange
      • symmetric-key cryptography for speed
    • E.g. SSL/TLS uses asymmetric encryption to create a key that's later used for encrypting/decrypting packets.
• 📝 Used in • digital certificates • PKI • SSH • PGP • IPSec
• Algorithms include • RSA • DSA • Diffie-Hellman

Forward secrecy

• Also known as perfect forward secrecy
• Property of cryptographic systems ensuring future disclosure of encryption keys cannot be used to decrypt communications in past.
• Ensures that a session key derived from a set of public and private keys will not be compromised if one of the private keys is compromised in the future
• E.g. web traffic cannot be decrypted with only server's private key through a court order.

Input-based ciphers

Block cipher

• 📝 Fixed-size blocks of data using a symmetric key
• Data bits are split up into blocks and fed into the cipher
• Each block of data (usually 64 bits) is encrypted with key and algorithm
• Simpler and slower than stream ciphers
• Key chosen for cipher must have a length larger than the data, if not, it is vulnerable to frequency attacks

Stream cipher

• 📝 One bit at a time using a symmetric key
• Combines each bit with a pseudorandom cipher digit stream (keystream)
• Works at a high rate of speed
• Usually done by an combining XOR with random generated key.

XOR cipher

• Also known as • exclusive OR • modulus 2 (addition/subtraction).

• 📝 If the inputs match, the output is a 0; otherwise, it is a 1.

  A	B	Output
  0	0	0
  0	1	1
  1	0	1
  1	1	0

• ❗ If the key chosen is actually smaller than the data, the cipher will be vulnerable to frequency attacks

• Uses "⊕" that denotes the exclusive disjunction (XOR) operation

• Applying XOR operator at every character encrypts, re-applying it decrypts.