Block Ciphers: Design and Security

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Student Presentation

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Liljana Babinkostova


The Data Encryption Standard (DES) is a block cipher standard published by the National Institute of Standards and Technology (NIST) in 1977. Block ciphers are a class of symmetric-key encryption algorithms that transform a fixed-length block of plaintext (unencrypted text) into a block of ciphertext (encrypted text) of the same length. This transformation takes place under the action of a user-provided secret key. Decryption is performed by applying the reverse transformation using the same secret key. DES, relies heavily on the ideas of Claude Shannon and the concepts of diffusion and confusion. The aim of diffusion is to spread the influence of all parts of the inputs to a block cipher (the plaintext and the key) to all parts of the output (ciphertext). The aim of confusion is to make the relationship between the plaintext, ciphertext and key complicated. The exploration of structural properties of a block cipher is important since it can give insights about the security the cipher. For most types of block ciphers it is common to investigate the structural properties of small scale variants of the cipher to provide a fully understandable framework for the analysis of the full cipher and its security. We study the design and the security of a simplified version of DES that has many features of DES.

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