it mitigates the risk of accidental key alteration in particular, any all-zeros or all-ones byte of the key is rejected by the mandatory odd parity check, and any one-bit alteration is caught, which are advantages from a functionality perspective.Parity of DES key bytes was introduced on request of US authorities during the design of DES in the late 1970s: In the unlikely event that you'd ever need to perform parity or CRC checking it is of course easy to add the necessary bits as long as you strip them away again before using the key. Nowadays it is seen as an unnecessary nuisance, complexity where it isn't required. Things like parity checking are not seen as part of a cipher definition. TCP/IP (and most other network protocols) already deliver a reliable transport mechanism the need for the parity bits has been strongly reduced. Nowadays transmission errors are usually taken care of at the transport layer. So you see, they're there to protect against errors reinstating/recreating the key. Bits 8, 16., 64 are for use in ensuring that each byte is of odd parity. One bit in each 8-bit byte of the KEY may be utilized for error detection in key generation, distribution, and storage. Or even better, Wikipedia states ANSI INCITS 92-1981), section 3.5: In that case, or simply during transmission, wrong keys are used. It could for instance be that the key is a result of key decryption or key agreement. They are there to check if the key was indeed correctly retrieved.
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