More Concepts of Cryptography
and Cryptanalysis
• Reference
– William Stallings, Cryptography and Network
Security, 3rd Edition, Prentice Hall.
Ref: STAL03
More Concepts of Cryptography and
Cryptanalysis
1
Characterization of Cryptographic
Systems
• 1. Type of Operations Used
– Subsittution
– Transposition
– Product Systems--Multiple Stages of
Substituion and Transposition
Ref: STAL03
More Concepts of Cryptography and
Cryptanalysis
2
Characterization of Cryptographic
Systems
• 2. Number of Keys
– Same single or secret key
– Different keys for sender and receiver
• 3. Processing Technique
– Block cipher--one block at a time.
– Stream cipher--processes elements
continuously, one element at a time.
Ref: STAL03
More Concepts of Cryptography and
Cryptanalysis
3
Code Breaking Techniques
• Cryptanalysis
– Relies on the nature of the algorithm and
perhaps some knowledge of the characteristics
of the plaintext.
– Attempts to discover the ciphertext or the key.
• Brute-force Attacks
– Attacker tries all possible keys.
– On average, half of all possible keys must be
tried.
Ref: STAL03
More Concepts of Cryptography and
Cryptanalysis
4
Definitions of Security
• An encryption scheme is unconditionally
secure if the ciphertext generated by the
scheme does not contain enough
information to determine the corresponding
plaintext, no matter the amount of
ciphertext available.
• Besides the one-time pad, there is no
algorithm that is unconditionally secure.
Ref: STAL03
More Concepts of Cryptography and
Cryptanalysis
5
Definitions of Security (p.2)
• An encryption algorithm is said to be
computationally secure if:
– The cost of breaking the cipher exceeds the
value of the encrypted information.
– The time required to break the cipher exceeds
the useful lifetime of the information.
Ref: STAL03
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Cryptanalysis
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