Until modern times, cryptography referred almost exclusively to encryption, the process of converting ordinary information (plaintext) into unintelligible gibberish (ie, ciphertext). Decryption is the reverse, moving from unintelligible ciphertext to plaintext. A cipher (or cypher) is a pair of algorithms which creates the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and, in each instance, by a key. This is a secret parameter for a specific message exchange context.
Keys are important, as ciphers without variable keys are trivially breakable and therefore less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption, without additional procedures such as authentication or integrity checks.
In recent decades, the field has expanded beyond confidentiality concerns to include techniques for message integrity checking, sender/receiver identity authentication, digital signatures, interactive proofs, and secure computation, amongst others.
Symmetric Key Encryption
Symmetric-key cryptography refers to encryption methods in which both the sender
and receiver share the same key (or, less commonly, in which their keys are
different, but related in an easily computable way). This was the only kind
of encryption publicly known until June 1976.
Symmetric-key cryptosystems typically use the same key for encryption and decryption,
though this message or group of messages may have a different key than others.
A significant disadvantage of symmetric ciphers is the key management necessary
to use them securely. Each distinct pair of communicating parties must, ideally,
share a different key, and perhaps each ciphertext exchanged as well. The number
of keys required increases as the square of the number of network members, which
very quickly requires complex key management schemes to keep them all straight
and secret.
The difficulty of establishing a secret key between two communicating
parties, when a secure channel doesn't already exist between them, also presents
a chicken-and-egg problem which is a considerable practical obstacle for cryptography
users in the real world.
Asymmetric Key Encryptions
A public key system is so constructed that calculation of one key (the 'private key') is computationally infeasible from the other (the 'public key'), even though they are necessarily related. Instead, both keys are generated secretly, as an interrelated pair. The historian David Kahn described public-key cryptography as "the most revolutionary new concept in the field since polyalphabetic substitution emerged in the Renaissance".
In public-key cryptosystems, the public key may be freely distributed, while its paired private key must remain secret. The public key is typically used for encryption, while the private or secret key is used for decryption. Diffie and Hellman showed that public-key cryptography was possible by presenting the Diffie-Hellman key exchange protocol.
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Data Encryption
