Comsec

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  • Topic: Cryptography, Block cipher, Cipher
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Chapter 5: Cryptography
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...
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