A method of public key encryption (see cryptography) devised by Rivest, Shamir, and Adleman. A message is encrypted by mapping it onto an integer, M say, raising M to a (publicly known) power e and forming the remainder on division by a (publicly known) divisor, n, to give the encrypted message S. Decryption is achieved by similarly raising S to a (secret) power d, and again forming the remainder on division by n; the result will be the value of M. The method relies on the choice of n as the product of two large secret prime numbers, p and q. The values of e and d are chosen such that

Security is achieved largely by the difficulty of finding the prime factors of n.