Basic Encryption and Decryption.pdf

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Chap 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
–
–
–
–
S: sender
R: receiver
T: transmission medium
O: outsider, interceptor, intruder, attacker, or, adversary
• S wants to send a message to R
– S entrusts the message to T who will deliver it to R
– Possible actions of O
• block(interrupt), intercept, modify, fabricate
• Chapter 1
2.1.1 Terminology
• Encryption and Decryption
– encryption: a process of encoding a message so that its meaning is
not obvious
– decryption: the reverse process
• encode(encipher) vs. decode(decipher)
– encoding: the process of translating entire words or phrases to
other words or phrases
– enciphering: translating letters or symbols individually
– encryption: the group term that covers both encoding and
enciphering
2.1.1 Terminology
• Plaintext vs. Ciphertext
– P(plaintext): the original form of a message
– C(ciphertext): the encrypted form
• Basic operations
– plaintext to ciphertext: encryption: C = E(P)
– ciphertext to plaintext: decryption: P = D(C)
– requirement: P = D(E(P))
2.1.1 Terminology
• Encryption with key
–
–
–
–
encryption key: K
E
decryption key: K
D
C = E(K
E
, P)
P = D(K
D
, E(K
E
, 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
– a cipher that does not require the use of a key
• key cannot be changed
2.1.1 Terminology
• Symmetric Cryptosystem: K
E
= K
D
• Asymmetric Cryptosystem: K
E
≠
K
D
2.1.1 Terminology
• Cryptography
– cryptography means hidden writing, the practice of using
encryption to conceal text
• Cryptanalysis
– cryptanalyst studies encryption and encrypted message, with the
goal of finding the hidden meaning of the messages
• Cryptology
– includes both cryptography and cryptanalysis
2.1.1 Terminology
• Cryptanalysis
– break an encryption
– 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
– An encryption algorithm may be breakable, meaning that given
enough time and data, an analyst could determine the algorithm
– practicality is an issue
• for a given cipher scheme, there may be 10
30
possible
decipherments, so the task is to select the right one out of the
10
30
• cryptanalyst cannot be expected to try just the hard, long way
– another efficient algorithm may exist
• estimates of breakability are based on current technology
– budget dependent

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