Cryptography is the study of techniques for securing information against adversaries. Its classic goal is confidentiality, keeping a message readable only by its intended recipient, but modern cryptography also provides integrity (detecting tampering), authentication (proving who sent something), and non-repudiation (preventing a sender from later denying a message). These goals are achieved by transforming data with mathematical operations whose security rests on a secret key.
The practice is ancient. Long before any theory existed, people protected messages with substitution and transposition schemes, shifting or rearranging letters by hand. For most of that history cryptography was an art of clever tricks, and the strength of a system was judged by whether anyone had managed to break it yet.
That changed in 1949 when Claude Shannon published “Communication Theory of Secrecy Systems” in the Bell System Technical Journal. Shannon applied his information theory to secrecy, defining what it means for a cipher to be perfectly secret and showing how the information leaked to an eavesdropper could be measured. His work put cryptography on a mathematical footing and gave designers a way to reason about security rather than simply hope it held.
Today cryptography is a standardized engineering discipline. NIST, the U.S. National Institute of Standards and Technology, publishes the cryptographic standards and guidelines, in its FIPS and Special Publication series, that specify the block ciphers, hash functions, digital signatures, and key-management schemes used to protect data across the internet. The field now spans symmetric encryption, public-key systems, and ongoing work on post-quantum algorithms.