Purpose of DES parity bits DES has a 64-bit key size, but only 56 of those are used during encryption. The other 8 are "parity bits".

What was the intended purpose of the party bits, and why are they no longer used in modern ciphers?
They are there to check if the key was indeed correctly retrieved. It could for instance be that the key is a result of key decryption or key agreement. In that case, or simply during transmission, wrong keys are used. According to NIST FIPS 46-3:


  The 8 error detecting bits..."


Or even better, Wikipedia states ANSI INCITS 92-1981), section 3.5:


  One bit in each 8-bit byte of the KEY may be utilized for error detection in key generation, distribution, and storage. Bits 8, 16,..., 64 are for use in ensuring that each byte is of odd parity.


So you see, they're there to protect against errors reinstating/recreating the key.



Nowadays transmission errors are usually taken care of at the transport layer. As e.g. TCP/IP (and most other network protocols) already deliver a reliable transport mechanism the need for the parity bits has been strongly reduced. Things like parity checking are not seen as part of a cipher definition. Nowadays it is seen as an unnecessary nuisance, complexity where it isn't required.

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.