People usually think of cryptography as a means to secure private transmission of data.
However, as discussed in the section, Uses of Encryption , cryptography has become essential as personal communication and financial transactions are increasingly digital. In secret key the same key is used for both encryption and decryption. In public key encryption, the keys used for encryption and decryption are different. A problem may be difficult because its solution requires some secret knowledge, or a problem may be hard because it is intrinsically difficult to complete, such as determing a function that produces a given hash value. The RSA algorithm is based on the hard problem of finding the prime factorization of very large numbers.