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2. Block Ciphers
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- 1What is padding?Padding is the process of ensuring that each block has the same length. This often results in additional characters being added to the end of the last block.
- 2What is the definition of a block cipher of block length n?
- 3What is the definition of a mono-alphabetic cipher?
- 4Prove that the mono-alphabetic substitution cipher is a cryptosystem.(e is a bijection by definition)
- 5What is the definition of a poly-alphabetic substitution cipher?
- 6What is the Vigenere cipher?
- 7How do you calculate the decryption key for the Vigenere cipher?The decryption key for the Vigenere cipher is the additive inverse of the encryption key (modulo 26)
e.g. for a encryption key (25,8,12,12,4,17)
there is corresponding decryption key (1,18,14,14,22,11) - 8What are the advantages of the Vigenere cipher over mono-alphabetic substitution methods?It is not as susceptible to simple frequency analysis techniques.
- 9How can one improve the distribution of letter frequencies when using the Vigenere cipher?Increase the key length
- 10Name a disadvantage of large keys.They are challenging to communicate securely, it can become as challenging as transmitting the original message.
- 11What is the Kasiski examination?The Kasiski examination involves looking for repeated blocks in the cipher-text. It is likely the distance between these repeated strings will be a multiple of the block length. Taking the gcd of many such quantities gives a likely candidate for key length. Once this has been established we can examine every nth character using frequency analysis, as they all correspond to the same application of a Caesar cipher.
- 12What other cryptographic attacks is the Vigenere cipher vulnerable to?Known-plain-text attacks.
- 13What is the Electronic codebook (ECB) mode of operation?
- 14In what way is the ECB method vulnerable?If the same plain-text repeats in the same location within two different block then multiple occurrences will be mapped to the same cipher-text. This is the principle upon which the Kasiski examination relies.
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- Cryptography Fichas sobre 2. Block Ciphers, creado por Alice Richens el 26/01/2018.
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- Alice Richens
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