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Crytographers
invent crytographic algorithms (secret codes)

Crytopanalysts
find ways to break codes

Decipher a message
find the plaintext without being given the key or secret algorithm

Break a code
 find a systematic way to decipher ciphertext
 created using the code with affordable resources.

encryption scheme has five ingredients
 1. plaintext
 2. encryption algorithm
 3. secret key
 4. ciphertext
 5. decryption algorithm

Secret Key (also "Conventional" or "Symmetric")
 1. Identical keys used to encrypt and decrypt data
 2. Ciphertext is same length as plaintext (+ padding)
 3.Used for transmission and storage for privacy
 4. Can be used for authentication

Public Key Cryptography ("PublicPrivate", "Asymmetric")
 1.Invented in 1975 (RSA)
 2.Public Key can be used by anyone to send a message
 3.Private Key can be used for a "Digital Signature“
 4. Hash Algorithms ("Message Digest" or "1Way Transform")
 5.Password hashing

Feistel Cipher Structure
 1. Block size
 2. Key size
 3. Number of rounds
 4. Subkey generation algorithm
 5. Fast software encryption/decrytion

DES
Data Encryption Standard

DES attributes
 1. The most widely used encryption scheme
 2. DES is a block cipher
 3. The plaintext is processed in 64bit blocks
 4. The key is 56bits in length

Triple DEA
Use three keys and three executions of the DES algorithm (encryptdecryptencrypt)
Effective key length of 168 bits

Triple DEA algorithm
C = EK3[DK2[EK1[P]]]

Other Symmetric Block Ciphers
 1. International Data Encryption Algorithm (IDEA)
 2.Blowfish
 3. RC5


Blowfish
 Easy to implement
 High execution speed
 Run in less than 5k of memory

RC5
 1.Suitable for hardware and software
 2.Fast, simple
 3.Adaptable to processors of different word lengths
 4.Variable number of rounds
 5.Variablelength key
 6.Low memory requirement
 7.High security
 8.Datadependent rotations

Location of encryption device
 1. Link encryption
 2. Endtoend encryption
 3. High security

Link encryption
 1. A lot of encryption devices
 2. High level of security
 3. Decrypt each packet at every switch

Endtoend encryption
 1. the source encrypts and the receiver decrypts
 2. payload encrypted
 3. header in the clear

high security
both link and endtoend encryption are needed

the use of two keys has consequences i
 1. key
 2. distribution
 3. confidentiality
 4. authentication

Publickey crypothography ingredients
 1. Plaintext
 2. Encryption algorithm
 3. Public key
 4. Private key
 5. Ciphertext
 6. Decryption algorithm

Applications for PublicKey Cryptosystems
 1.Encryption/decryption: The sender encrypts a message with the recipient’s public key.
 2.Digital signature: The sender ”signs” a message with its private key.
 3.Key echange: Two sides cooperate to exhange a session key.

PublicKey Cryptographic Algorithms

RSA
 1. RSA is a block cipher
 2. The most widely implemented

DiffieHellman
 1. Echange a secret key securely
 2. Compute discrete logarithms

Authentication requirements
 1. Message came from apparent source or author
 2. Contents have not been altered

Authentication
protection against active attack (falsification of data and transactions)

Authentication Using Conventional Encryption
Only the sender and receiver should share a key

Message Authentication without Message Encryption
An authentication tag is generated and appended to each message

Message Authentication Code
Calculate the MAC as a function of the message and the key. MAC = F(K, M)

Oneway HASH function
Secret value is added before the hash and removed before transmission.

Secure HASH Functions
Purpose of the HASH function is to produce a ”fingerprint”.

Properties of a HASH function H
1.H can be applied to a block of data of any size
2.H produces a fixed length output
3.H(x) is easy to compute for any given x.
4.For any given code h, it is computationally infeasible to find x such that H(x) = h
5.For any given block x, it is computationally infeasible to find with H(y) = H(x).
6.It is computationally infeasible to find any pair (x, y) such that H(x) = H(y)

Requirements for PublicKey Cryptography
 1.Computationally easy for a party B to
 generate a pair (public key KUb, private key KRb)
2.Easy for sender to generate ciphertext: C = Ekub(M)
 3.Easy for the receiver to decrypt ciphertect
 using private key: M = Dkrb(C) = Dkrb[Ekub(M)]
4.Computationally infeasible to determine private key (KRb) knowing public key (KUb)
5.Computationally infeasible to recover message M, knowing KUb and ciphertext C
 6.Either of the two keys can be used for
 encryption, with the other used for decryption:
 M = Dkrb[Ekub(M)] = Dkub[Ekrb(M)]

