Leon Battista Alberti developed one of the first concepts in machine-assisted encryption in XV century through the invention of the cipher disc. Basically it consists of two circular discs, diffferent in diamenter where the alphabetic scales are engraved. The smaller disc is mounted concentrically to the larger one in such away when moving the discs, the alphabetic scales are related among themselves.
Thus, cipher disc provided an easy way to change ciphers merely by moving the scales; so it was possible to arrange 26 differents ways to represent a letter, depending entirely on the position of the inner disc alphabetic scale.
When sending a message, the sender and the recipient would agree on a cypher key For instance the letter "A" in the regular alphabet would be positioned next to the
"G" in the ciphered alphabet.
Without a doubt the cipher disc invented by Alberti carved out the way for the practical use of the polyalphabetic ciphering, as well as later on inspiring other reseachers to employ his machine-assisted encryption concept as the one developed by the American state secretary, Thomas Jefferson in 1795.

The machine-assisted concept invented by the American secretary of state Jefferson in 1795
Courtesy: National Security Agency USA

The Jefferson machine-assisted encryption consisted twenty-five wooden discs mounted in a single spindle. On each disc the twenty-six letters of the alphabet were written in a jumble order in such a way that the order on each disc was different.
In spite of such advanced machine-assisted encryption concept it was not quite well- understood for the moment and so, its resurgence came out one century later by the researches of the French cryptologist Etienne Bazeries.
Known as Bazeries cylinder, it consists of a set of roughly 20 to 30 numbered discs with a different cipher alphabet on the edge of each disc, and a hole in the centre of the discs to allow them to be stacked on a spindle. The discs are removable and can be mounted on the spindle in any desired order. So, the order of the discs can be considered the cipher key for the Bazeries cylinder, which could be arranged the discs in the same predefined order.
In order to encode the message, the plain text is spelled out in one row of disc whose order was previously selected.Then the user can copy any row of text on the discs other than the one that contains the message. The recipient simply has to put the discs in the agreed-upon order, spell out the encrypted message by rotating the discs, and then look around the rows until the plaintext message could be read.
The only advantage that those relatively simple devices had over traditional ciphers was speed.
However, as afore seen considering the urgency of the plain text enciphering, generally required by the substantial volume of military communications, very soon armies and armed forces felt the necessity to employ new types of machine-assisted encryption.
In this way just after the WWI, that kind of machine was made in several countries and around 1935 they reached a high level of efficacy.
Most were German or Swiss -made and generally automated a polyalphabetic substitution concept with extremely long cipher-key using several hundred thousand incoherent series of letters. They were supplied in several types. While the smaller ones were manually operated and did not print out messages, the larger models were electric operated and had a typewriter keyboard like the ones used in a standard office typewriter machine.
In the later thirties most of the great nations had machine-assisted encryption systems some of them in a compact, size similar to a portable typewriter.

Machine-assisted encryption in a compact size similar to a portable typewriter at the end of WWII.
Courtesy: Antonio Fucci Collection.

During the WWII, was developed machine-assisted encryption systems using teletypewriting facilities connected to an automatic ciphering box. Among the most famous one is the Japanese machine-assisted encryption known by the codename "PURPLE".

The famous machine-assisted encryption developed by the Japanese known by the codename Purple.
Courtesy: National Security Agency USA.






  Fig. 215 a 6 - Display showing the eletromechanical operation encryption keys used in the Purple mahcine.
Courtesy: National Security Agency USA




However, all the codes and cyphers as well as the machine-assisted encryption aforementioned still used a logical system known as symmetric key algorithms in which the same cryptographic key is selected with the underlying algorithm by both the sender and the recipient, who must both keep it secret.

Algorithm (al-Khwärizmï), in simple words it consist in a set of well-defined instructions for accomplishing some task. Its Etymology is related with the surname of the Persian mathematician Abu 'Abd Allah Muhammad ibn Müsä al-Khwärizmï.

In mid seventies a new encryption system known as asymmetric key algorithms fundametally changed the way cryptosystems might work. In such new system it uses a pair of mathematically related keys, each of which decode the encrypted message performed using the other. Some, but not all, these algorithms have the additional property that one of the paired keys cannot be deduced from the other by any known method other than trial and error.
Furthermore, for a known encrypted messsage using such an algorithm, only one key pair is needed per user. So, by designating one key of the pair as private, that is always considered as secret, and the other as public domain, no secure channel is needed for key exchanging. Whereas the private key stays unknown, the public key can be widely used for a very long time without compromising its security, making it safe to reuse the same key pair indefinitely.
Thus, by using the asymmetric key algorithm it is possible a secure communication through an insecure channel.
It is interesting to note that in adopting this new cryptosystem developed in conjunction by Whitfield Diffie and Martin Hellman in 1976 it brought the principle of encyphering squarely into the public domain and in turn broke the near monopoly on cryptography held by government organizations worldwide.
Nowadays, the enciphering public domain can be found in several modern technological advancements. Among them are: the enciphering system A5/1 and A5/2 used in cell phone operating in the GSM technology; Content Scramble System (CSS) for encrypting and controlling recorded data in products such as DVD or digital video discs .