the practice and study of techniques for secure communication and sending of data, and the construction and analysis of protocols that accomplish this.

Cryptography is, to define it loosely, the study of sending stuff securely. It referred almost exclusively to encryption up until recently. Over the years, a variety of cryptography protocols have been invented: MD5, SHA-1, and RSA all have been used successfully.

From Wikipedia:

Cryptography is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). More generally, it is about constructing and analyzing protocols that overcome the influence of adversaries and which are related to various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation. Modern cryptography intersects the disciplines of mathematics, computer science, and electrical engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce.

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