A field of study and practice to secure information, preventing third parties from reading information to which they are not privy.
For a long period of history, cryptography dealt exclusively with the technique of ciphers — algorithms that were used to turn regular messages (called “plaintext” in cryptography) into ostensible gibberish (called “ciphertext”).
These messages could then be sent via letters, whereby the receiver would use an auxiliary piece of information (called “the key”) to decrypt the ciphertext back into readable plaintext. The information communicated this way was useless to any third-party adversaries without the knowledge of the key, even if the letter was physically intercepted — that is, until the development of cryptanalysis, the discipline that deals with cracking encryption algorithms.
The earliest types of ciphers, called substitution and transposition and known collectively as classical ciphers, could be both encrypted and decrypted by hand. This meant that they did not scramble the information enough to resist the methods of cryptanalysis available at the time.
The emergence of radio and, later, internet communications has made the need for strong encryption more acute than ever, as messages were now broadcast indiscriminately and could be intercepted at will.
Through the use of computers, new methods of plaintext scrambling became available that are effectively unbreakable: while in theory an advanced encryption algorithm can be deciphered without a key, it is infeasible to do so in a reasonable amount of time with the resources available to the adversaries.
Today, cryptography is a default in a lot of computer communication. Among its numerous applications are hash functions, like the SHA-256, which is used to secure the operation of the network of Bitcoin (BTC) and other cryptocurrencies.