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Encryption Algorithms
Algorithms have a variety of uses in ensuring the
integrity of communications. Security is necessary when communicating over any
untrusted medium. This includes not only inherently insecure networks, such as
radio (broadcast transmission is, by nature, subject to interception), but also
the Internet. There are four commonly accepted security requirements, and
cryptographic algorithms can contribute to each.
- Authentication: The process of proving one's identity. (The primary forms of host-to-host authentication on the Internet today are name-based or address-based, both of which are notoriously weak.)
- Privacy/confidentiality: Ensuring that no one can read the message except the intended recipient.
- Integrity: Assuring the recipient that the received message has not been altered in any way from the original that was sent.
- Non-repudiation: A mechanism to prove that this message really originated from the claimed sender.
Types of Algorithm
Many different algorithms are used for encryption, but certain elements are common to all of them. Algorithms can be divided into classes depending on the technique and approach employed.Symmetric Algorithms
Symmetric algorithms use the same key for encryption and decryption. They usually operate at relatively high speed, and are suitable for bulk encryption of messages. There are many symmetric algorithms, the most common of which is the Data Encryption Algorithm, DEA (specified in the Data Encryption Standard, DES). A stronger variation of DES, called Triple-DES, has been developed which uses the DEA algorithm three times to provide a far stronger cipher. Other well known algorithms include the Japanese FEAL algorithm and the more recent US algorithm, SKIPJACK.The increasing power of computer systems means that algorithms provide less protection as time passes, and some popular algorithms are now considerably less effective. Approval for use of DES by US Government agencies was withdrawn in 2000 with the announcement of the Advanced Encryption Standard (AES), which is also symmetric.
Asymmetric Algorithms
There is a special class of transformations in mathematics, where a pair of related by independent keys can be used to perform asymmetric encryption and decryption. One key from the pair is used to encrypt information into a cipher, and the other key decrypts the cipher back to plaintext. This form of encryption is popularly known as public key cryptography, and interest in this method increased in 1978 when a paper was published introducing a public key scheme (known as RSA, after its developers Rivest, Shamir, and Adelman). Asymmetric schemes rely on the fact that it is relatively easy (computationally) to multiply two very large prime numbers together to produce a large number, but it is extremely difficult to factorise this product back into the original primes.Asymmetric encryption is relatively slow, and therefore unsuitable for encryption of large messages. However, a major advantage of asymmetric key systems is that one of the two components of the key pair can be made public (hence the phrase "public key"). This has two important benefits: firstly, anyone can send private information to a recipient 'A' by encrypting the information using A's public key but only A will be able to recover the information by decrypting the cipher using the related private key (which A must keep secret). Second, if some known information can be correctly recovered by decrypting with A's public key, it must have been encrypted with A's private key, and therefore by A. This means that asymmetric algorithms provide proof of origin.