what is the primary weakness of the des block cipher
Block cipher Due to a block cipher's characteristic as an invertible function, its output becomes distinguishable from a truly random output string over time due to the birthday attack. This property results in the cipher's security degrading quadratically, and needs to be taken into account when selecting a block size. There is a trade-off though as large block sizes can result in the algorithm becoming inefficient to operate.[28] Earlier block ciphers such as the DES have typically selected a 64-bit block size, while newer designs such as the AES support block sizes of 128 bits or more, with some ciphers supporting a range of different block sizes.[29]