Cryptography

Cryptography, also known as Cryptology, is the practice and study of techniques for converting information into a form for secure communication or storage in the face of a known or potential adversary.

Due to the covert, sensitive or hidden nature of certain subjects concerning them, various aspects of cryptography are utilized on both magic and science sides.

Principles
One variant of cryptography is, which involves concealing a hidden message within another message or an object. Rather than just protecting the contents of a message, steganography also involves concealing the fact that a secret message is being sent.

Background
Cryptography has been used in simple forms since antiquity, with examples including the used in  and the, a substitution cipher named after and used by Roman general  of Rome in private correspondence.

Due to the hidden nature of the magic side, forms of cryptography have been utilized at various points throughout its history. Many grimoires and magical texts are written in code (the infamous the Book of the Law among them),     and people like the Rosicrucians have used a form of code to concealed magical messages within paintings and other works of art.

One notable cryptographer who was also involved in the occult, was, a German abbot and polymath whose students included Heinrich Cornelius Agrippa and Paracelsus. Two of his famous cryptographic works are the three-volume and the five-volume. One of the ciphers in the latter was utilized in the that were involved in the founding of the Golden Dawn by William Wynn Westcott and Samuel Liddell MacGregor Mathers.

At some point, an absolute encryption contest was held in Academy City, with the random number encryption program Omega Secret winning the contest, subsequently gaining a reputation for being practically unbreakable.

Toaru Majutsu no Index Light Novel Volume SS2
On the third Friday of September, while hacking into the Bank for an unknown reason, Misaka Mikoto found the data inside the system becoming encrypted at a rate she couldn't keep up with even with her powers, and deduced that it was the work of Omega Secret from the random manner of encryption.

Toaru Majutsu no Index SS: Agnese's Magic Side Work Experience
While investigating a possible connection between the recent incidents they had resolved, Agnese Sanctis, Lucia and Angelene experienced frustration in dealing with ciphers on top of many languages when reading grimoires in the Anglican remote database in Zurich. Later, during the attack on the Guild's base, they found that the Guild was using a  for its encryptions, rather than a  as they expected from a group dating from ancient Greece, a fact they would use in figuring out how the Guild had changed with the times from Greek to Roman to Christian, their thinking and their goals.

Coronzon Arc
During the events surrounding the Crowley's Hazard invasion of the United Kingdom, the recreated William Wynn Westcott utilized a spell, verbally invoking a coded text he held, fragments said to reveal power when read using methods of ’s, and referred to the knowledge of the Golden Dawn's beginning.

Christmas Eve Arc
During the events on December 24th, Misaka Mikoto and Misaka 10032 had a brief competition over who could crack Maidono Hoshimi's phone, only to be surprised when Index beat them to the passcode - an incident which would later be referred to in a discussion about the vulnerability of electronic protection and encryption to magic.

Los Angeles Arc
During the events in Los Angeles on December 26th, Othinus had a discussion with Kamijou Touma about the vulnerability of firewalls and other forms of electronic protection, even quantum encryption, to magic, referencing how Index was previously able to break a phone's password even without magic, and raising the possibility that the Academy City forces' unmanned weapons may have been turned against them.

The practice of hiding messages (such as a recipe for the Philosopher's Stone) in paintings, and the possible simplicty of the code by using symbols only known to an intended recipient (such as the sun representing gold and a pelican for the stone) was also referenced while trying to solve a message left on Melzabeth Grocery's smartwatch. Stiyl Magnus also considered that the Citrinitas spell would have required the decoding of a German original grimoire.

Trivia

 * Although not a cryptographical device itself, the design of the John's Pen Remote Control was influenced by the device reminding Haimura Kiyotaka of the from the, a device whose name is said to derive from cryptology's crypto- and codex - using the "science of cryptology" to hide a codex, whose design in turn also bears similarities to historical wheel ciphers and wheel-based encryption devices, such as the.