A field of study and practice to secure information, preventing third parties from reading information to which they are not privy.
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.
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