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Objectives:

-Applications and uses of cryptography

-Encryption methodologies

-Cryptanalysis

-Management of Cryptography

-Key Management

Application and uses of Cryptography

What is Cryptography?

Cryptography is the science of hiding information in plain sight, in order to conceal it from unauthorized parties. -Substitution cipher first used by Caesar

for battlefield communications

Encryption Terms and Operations

•Plaintext – an original message

•Ciphertext – an encrypted message

•Encryption – the process of transforming plaintext into ciphertext (also encipher) •Decryption – the process of transforming ciphertext into plaintext (also decipher) •Encryption key – the text value required to encrypt and decrypt data Encryption Methodologies

•Substitution Cipher

–Plaintext characters are substituted to form ciphertext •Transposition Cipher

–Plaintext messages are transposed into ciphertext

–Subject to frequency analysis

attack

•Monoalphabetic Cipher

–One alphabetic character is substituted or another

–Subject to frequency analysis

attack

•Polyalphabetic Cipher

–Two or more substitution alphabets

–Not subject to frequency attack

•Running-key Cipher

–Plaintext letters converted to numeric (A=0, B=1, etc.)

–Plaintext values “added” to key values giving ciphertext –Modulo arithmetic is used to keep results in range 0-26

•One-time Pad

–Works like running key cipher, except

that key is length of plaintext,

and is used only once

–Highly resistant to cryptanalysis

Types of Encryption

•Block cipher

–Encrypts blocks of data, often 128 bits

•Stream cipher

–Operates on a continuous stream of data

Block Ciphers

•Encrypt and decrypt a block of data

at a time

–Typically 128 bits

•Typical uses for block ciphers

–Files, e-mail messages, text communications, web

•Well known encryption algorithms

–DES, 3DES, AES, CAST, Twofish, Blowfish, Serpent

Block Cipher Modes of Operation

•Electronic Code Book (ECB)

•Cipher-block chaining (CBC)

•Cipher feedback (CFB)

•Output feedback (OFB)

•Counter (CTR)

Initialization Vector (IV)

•Starting block of information needed to encrypt the first block of data •IV must be random and should not be re-used

–WEP wireless encryption is weak because it re-uses the IV, in addition to making other errors

Block Cipher: Electronic Code Book

•Simplest block cipher mode

•Each block encrypted separately

–Like plaintext encrypts to like ciphertext

–Vulnerable to a dictionary attack

–WEP does this

–Microsoft made this error in their password hashes

–Microsoft also made this error in Microsoft Office document encryption

ECB Mode

•Images from NIST (link Ch 5d)

Block Cipher: Cipher-block Chaining (CBC)

•Ciphertext output from each encrypted plaintext block is used in the encryption for the next block –First block encrypted with IV

(initialization vector)

Block Cipher: Cipher Feedback (CFB)

•Plaintext for block N is XOR’d

with the ciphertext from block N-1.

•In the first block, the plaintext

XOR’d with the encrypted IV

Block Cipher: Output Feedback (OFB)

•Plaintext is XOR’d with the encrypted material in the previous block to produce ciphertext

Block Cipher: Counter (CTR)

•Uses a “nonce” (a random number

that is used once) that is concatenated with a counter or other simple function, to create a series of keys –Allows parallel computation

Stream Ciphers

•Used to encrypt a continuous stream

of data, such as an audio or video transmission

–A stream cipher is a substitution cipher that typically uses an exclusive-or (XOR) operation that can be performed very quickly by a computer. •Most common stream cipher is RC4

•Other stream ciphers

–A5/1, FISH, Phelix1, ISAAC, MUGI, Panama, Phelix, Pike, Sapphire-II. SEAL, SOBER-128, and WAKE

Types of Encryption Keys

•Symmetric key

–A common secret that all parties...