The decryptor takes in the 64 bit blocks of cipher text and reverses the mapping provided the same key is used.
With modern day computer power, a 56 bit key is relatively short and can be broken quickly with a powerful enough computer. Another feature of DES is that as a block cipher, the same input will deliver the same cipher text output each time. This makes alignment of the decryptor very easy but means that an adversary can defeat the system by studying the frequency of occurrence of 64 bit blocks of cipher text and mapping these back to the most likely most popular input. Having said that, it is generally reckoned to be secure from attack by individuals but not by organisations or governments with large financial resources.
By adding something known as Cipher Block Chaining, DES can be made much more secure, and this is done by feeding the output from one encoding of DES and feeding it back as the input for the next block. Putting the cipher text output through an Exlusive OR gate, the 64 bit cipher blocks are much more difficult to identify. The key length is also extended through the process of delaying the 64 bits in the feedback loop. In order for this to be effective there has to be no bit errors lost between the source and destination devices. This means that error detection and recovery must be effective over any link.
Triple DES is a method whereby the key length is effectively increased by a factor of 3 by applying the DES process three times. This greatly improves the security of the DES system.
DES is discussed in a number of our instructor-led training courses, and we will be adding VPN topics to our training portfolio soon.