5

Same as the reason of why not switching to AES (or any other 80+ key bits algorithms available at the time). Because we want to reuse whatever "hardware" (chip design) resources available to do the blockcipher calculation, and re-designing the key schedule would prevent us from doing so since the existing hardware cannot compute the new key schedule.


1

The answer depends on the size of the plaintext you are attacking and the number of ciphertext blocks you have. Assume that you have a 64-bit block-sized cipher named C64 and 128-bit block cipher named C128. If you know that the $m$ is smaller than 64-bit than you will have only one ciphertext for C64 and C128. There is a need for padding at the end of the ...


1

Just changing the key schedule would improve the security of DES only moderately (at least in the model where high data complexity below exhausting the codebook does not render an attack impractical). For instance, chapter 4.6 of the book Differential Cryptanalysis of the Data Encryption Standard by Biham and Shamir describes an attack on DES with ...


1

The confidence that we have in the security of DES comes from years (decades) of cryptanalytic attempts and research. Any internal change would mean that we would have to re-analyze everything from scratch, and if we are doing that then we may as well just design a new block cipher that is better in many other ways. (For example: block size, speed, code size,...


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