 on until L0 and RO are obtained. The L0 and RO elements are then concatenated into a 64-bit block. The bits are then reordered, according to the inverse of the shuffling algorithm. The resulting block is decrypted. Then all decrypted blocks are concatenated together and converted into plaintext. We can infer that the whole decrypting operation can be performed in a few milliseconds of computer time, depending on the complexity of the step-3 cipher and the mangling function. As I have stated it, the mangling function represents the key to the encryption and decryption process. Such a key can be defined by Classical and quantum cryptography a number, which represents the different integer parameters to be used for generating the mangling function. Thus, the number of possible keys is virtually infinite. To prevent any possibility that this code became absolutely unbreakable, it was agreed that the number of keys should be 215 2 72 X 1015 which corresponds to a 56-bit key. Twenty-five years later, the principle of the Lucifer system with a 56-bit key was officially endorsed in the USA to become the data encryption standard (DES), which is still widely in use today, albeit superseded by its own variants and other standards, such as AES (see further). Double-key encryption In spite of the tremendous levels of protection offered by algorithms like DES and Lucifer, a major weakness remains, which is the obligation for the originator to com- municate the key to the destinee. Like the Germans with Enigma, both parties could keep a secret copy of the same book, where the key to be used is indicated for each day of a month or year. A new set of books must be created and exchanged on this monthly or yearly basis. But a malicious third party may intercept the book and rapidly copy the contents, while making sure that no one notices anything. As a result, all secret communications based on this book's keys could be read "in the open" by this third party, the worst being that the two ends are unaware of it. In cryptography, th