SYMMETRIC ENCRYPTION

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Process of converting one binary string to another binary string

INPUT - sequence of plaintext bits

PROCESS - perform series of operations

OUTPUT - series of cyphertext bits

STREAM CIPHER

INPUT - one bit of plaintext, key

PROCESS - perform series of operations

OUTPUT - one bit of cyphertext bits

Some stream ciphers process blocks of one byte and can be called Block ciphers

KEY - > Key is much shorter than the length of the plaintext

KEY STREAM GENERATOR -> Generates the keystream used for encryption

Both sides have the same key and the key stream generator

KEYSTREAM has to be as long as the plaintext

PsuedoRandom generator should be unpredictable to make cipher secure

BLOCK CIPHER

INPUT - a block of plaintext bits, key

PROCESS - perform series of operations

OUTPUT - a block of cyphertext bits

Properties

Plaintext Block size is fixed

Cyphertext Blocksize is also fixed

Block size too short (dictionary attacks) and too large can cause issues like padding of spaces unused in the black

Versatility - widely used, primitives like MAC and hash functions

Compatibility - since used in many places, easily compatible like AES

Adaptability

PROBLEMS of PADDING

introduces inefficiency, e.g. padding 8 bit in a 128-bit AES scheme

can cause security issue if a proper padding scheme is not chosen

DES -> 64 bits block

AEX -> 128 bits block

Vernam Cipher applies one-time pad on binary strings.

INPUT - sequence of plaintext bits

PROCESS - perform one-time pad operations (XOR)

OUTPUT - series of cyphertext bits

Uses pseudo random Key generator to generate a key stream

Length of the Key same as the Plaintext

Key >= Plaintext

DES

AES

IDEA

SERPENT

TWOFISH

IMPACT OF ERROR

Transmission Error bit flip, 0 becomes 1 and 1 becomes 0

Transmission Loss bits gets dropped

Computational Error incorrectly computed 1 became 0 and 0 became 1

Properties of STREAM CIPHERS

No Error Propagation

Speed

On the fly encryption

Examples of STREAM CIPHER

RC-4 (TLS/SSL), A5/1 (GSM), E0 (BLUETOOTH)

DES (Data Encryption Standard)

is also known as Feistel Cypher

block size is 64 bit

Technical Control - hardware to store the keys and protocol used to secure exchange the keys

Process Control - policies, practice & procedures for key management

Environmental Control - physical location plays very important role

Human Control - Managing keys is a human

SIMPLE HIERARCHY

Master key top level keys, longest lifetime, use Hardware security module to store the keys

Key encryption key long lifetime

Data Keys shortest lifetime

Definitions

Cipher

DES

Key Management