Cryptography or cryptology is the practice and study of techniques for secure communication in the presence of third parties called adversaries. More generally, cryptography consists of constructing and analyzing protocols that prevent third parties or the public from reading private messages. Aspects of information security, such as data confidentiality, data integrity, authentication and non-repudiation, are fundamental to modern cryptography. Modern cryptography exists at the intersection of mathematics, computer science, electrical engineering and communication sciences. Cryptography applications include e-commerce, chip-based payment cards, digital coins, computer passwords, and military communications.
Cryptography before the modern age was effectively synonymous with encryption, converting information from a readable to seemingly meaningless state. The originator of an encrypted message shared the decoding technique necessary to retrieve the original information only with intended recipients, thus preventing unwanted persons from doing the same. Cryptography literature often uses the name Alice ("A") for the sender, Bob ("B") for the intended recipient, and Eve ("eavesdropper") for the adversary. Since the development of rotor encryption machines in World War I and the advent of computers in World War II, the methods used to carry out cryptology have become increasingly complex and their application more widespread.
Modern cryptography relies heavily on mathematical theory and computer practice; the cryptographic algorithms are designed around computational hardness assumptions, making such algorithms difficult to break in practice by any adversary. Theoretically it is possible to break such a system, but it is impossible to do so by any known practical means. Therefore, these schemes are called safe from the computational point of view; for example, improvements in integer factorization algorithms and faster computing technology require that these solutions be continuously adapted. There are safe schemes in theory that can not be broken even with unlimited computing power - an example is the one-time pad - but these schemes are more difficult to implement than the best theoretically breakable but computationally secure mechanisms.
The growth of cryptographic technology has raised a number of legal issues in the information age. The potential of cryptography as an instrument of espionage and sedition has led many governments to classify it as a weapon and to limit or even prohibit its use and export. In some jurisdictions where the use of cryptography is legal, the law allows researchers to force the disclosure of encryption keys for documents relevant to an investigation. Cryptography also plays an important role in the management of digital rights and copyright infringement of digital media.