Ref: STAL03 More Concepts of Cryptography and Cryptanalysis

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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

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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

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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

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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

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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

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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.