Chap 2. Basic Encryption and Decryption
嚜澧hap 2. Basic Encryption and
Decryption
H. Lee Kwang
Department of Electrical Engineering
& Computer Science,
KAIST
Objectives
? Concepts of encryption
? Cryptanalysis: how encryption systems are ※broken§
2.1 Terminology and Background
? Notations
每
每
每
每
S: sender
R: receiver
T: transmission medium
O: outsider, interceptor, intruder, attacker, or, adversary
? S wants to send a message to R
每 S entrusts the message to T who will deliver it to R
每 Possible actions of O
? block(interrupt), intercept, modify, fabricate
? Chapter 1
2.1.1 Terminology
? Encryption and Decryption
每 encryption: a process of encoding a message so that its meaning is
not obvious
每 decryption: the reverse process
? encode(encipher) vs. decode(decipher)
每 encoding: the process of translating entire words or phrases to
other words or phrases
每 enciphering: translating letters or symbols individually
每 encryption: the group term that covers both encoding and
enciphering
2.1.1 Terminology
? Plaintext vs. Ciphertext
每 P(plaintext): the original form of a message
每 C(ciphertext): the encrypted form
? Basic operations
每 plaintext to ciphertext: encryption: C = E(P)
每 ciphertext to plaintext: decryption: P = D(C)
每 requirement: P = D(E(P))
2.1.1 Terminology
? Encryption with key
每
每
每
每
encryption key: KE
decryption key: KD
C = E(KE, P)
P = D(KD, E(KE, P))
If the encryption algorithm
should fall into the interceptor*s
hands, future messages can still
be kept secret because the
interceptor will not know the
key value
? Keyless Cipher
每 a cipher that does not require the use of a key
? key cannot be changed
2.1.1 Terminology
? Symmetric Cryptosystem: KE = KD
? Asymmetric Cryptosystem: KE ≧ KD
2.1.1 Terminology
? Cryptography
每 cryptography means hidden writing, the practice of using
encryption to conceal text
? Cryptanalysis
每 cryptanalyst studies encryption and encrypted message, with the
goal of finding the hidden meaning of the messages
? Cryptology
每 includes both cryptography and cryptanalysis
2.1.1 Terminology
? Cryptanalysis
每 break an encryption
每 cryptanalyst can do any or all of three different things:
? attempt to break a single message
? attempt to recognize patterns in encrypted messages, in order
to be able to break subsequent ones by applying a
straightforward decryption algorithm
? attempt to find general weakness in an encryption algorithm,
without necessarily having intercepted any messages
2.1.1 Terminology
? Breakable encryption
每 An encryption algorithm may be breakable, meaning that given
enough time and data, an analyst could determine the algorithm
每 practicality is an issue
? for a given cipher scheme, there may be 1030 possible
decipherments, so the task is to select the right one out of the
1030
? cryptanalyst cannot be expected to try just the hard, long way
每 another efficient algorithm may exist
? estimates of breakability are based on current technology
每 budget dependent
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