ITIS 3200: Cryptography

Symmetric Cryptography

Parties share a secret key used for both encryption and decryption. Plain text typically the same length as cyphertext.

Brute-Force Attack

Try all possible keys and determine if message is a likely plaintext

Substitution Cyphers

Each letter is uniquely replaced by another. An example is a Caesar Cypher. (ex. ROT13)

Block Cipher

Plaintext and Cipher Text have fixed length. Each message is divided into blocks and sometimes padding is added to keep the length fixed. (Examples: DES, 3DES, AES)

DES

Data Encryption Standard

64 bit, small key space makes exhausted search attack feasible since late 90s

Triple DES

Nested application of DES with 3 different keys: KA, KB, KC

Effective key length is 168 making exhausting search attacks unfeasible.

AES

Advanced Encryption Standard

128 bit blocks, exhaustive search attack no feasible. AES-128 is the algorithm choice of most commercial apps.

ECB

Electronic Code Book

The simplest block cipher mode.

Adv: Allows parallel encryptions, and can tolerate the loss/damage of block.

Disadv: Same plaintext -> Same Cipher. Docs and Images are not suitable since patters are repeated.

CBC

Cipher Block Chaining. The previous cipher text block is combined with the current block.

Adv: Doesn't show patterns, is the most common mode, fast and simple.

Disadv: Requires reliable transmission of all blocks sequentially. Not suitable for applications that allow packet loss (streaming)

Symmetric Key Distribution

Required each pair of 'n' communicating parties to share a (separate) secret key. n(n-1)/2

Diffie-Hellman Key Exchange

Used to generate/share keys between two parties if you do not trust the other party's ability to generate a secure key.

Public-Key Encryption

Person has a pair of keys.

1) A private key, kept secret, used to sign messages sent.

2) A public key, which is public, used by others to decrypt messages.

Digital Signature

Uses Public-Key cryptography. Bob signs a message with private key. Anyone can verify authenticity by decrypting with public key since Bob should be the only one with the private key.

Hash Functions

One way function that is collision-resistant (meaning hard to find two messages that are the same). Its pretty much shredding the message to a fixed length in cipher-text.

Certificate Authority

Digitally signs a binding between an identity and the public key for that identity.

Public Key Distribution

Only one pair of keys is needed for each user.

Fundamental Theorem of Arithmetic

The prime decomposition of a positive integer is unique.

Hash Table

Search data structure based on storing items in locations associated with their hash value. Chaining or open addressing deals with collisions

MD5

Message-Digest Algorithm 5

Uses 128-bit hash values. Widely used in legacy apps although considered insecure.

Compression Function

Works on input values of fixed length.

Iterated Hash Function

extends a compression function to inputs of arbitrary length.

MAC

Message Authentication Code

Hash function with two inputs (secret K and the message M). Gives message integrity. Receiver recomputes MAC tag from received message and compares it with received MAC tag. More efficient than signing each message.

BitCoin

Crypto currency. Most well known digital cash.