Its purpose is to ensure privacy by keeping information hidden from anyone for whom it is not intended, even those who have access to the encrypted data. Encryption and decryption generally require the use of some secret information, referred to as a key. Cryptography today is more than encryption and decryption. Authentication is as fundamentally a part of our lives as privacy. We use authentication throughout our everyday lives -when we sign our name to some document for instance and, as we move to a world where our decisions and agreements are communicated electronically, we need to have electronic techniques for providing authentication.
Cryptography provides mechanisms for such procedures.
A digital signature, for example, binds a document to the possessor of a particular key. A cryptographic algorithm, or cipher, is a mathematical function used in the encryption and decryption process. The same plaintext encrypts to different ciphertext with different keys. A cryptographic algorithm, plus all possible keys and all the protocols that make it work comprise a cryptosystem. PGP is a cryptosystem.