说明.txt

Crypto++软件包

Applied Crypto++: Block Ciphers

Crypto++ offers over 25 Block Ciphers, ranging from AES to XTEA. Modern block ciphers require the choice of an algorithm, mode, feedback size, padding, and sometimes rounds. 

Commercial block ciphers made their debut in the mid 1970s. IBM researchers Walter Tuchman and Horst Feistel were part of the team which produced Lucifer, a 128 bit block cipher with a 128 bit key. Lucifer was patented in 1971 by IBM (US Patent 3,798,359 issued in 1974). Lucifer laid the foundation for the Data Encryption Standard, which was proposed in 1975, and standardized in 1977 through FIPS 46. In 1980, four modes of operation for DES were specified in FIPS 81. It is noteworthy that 64 bit DES (with 56 bit keys) is stronger than the 128 bit Lucifer.

Symmetric Key is also known as Shared Key Cryptography since both parties use the same secret key. This is in contrast to Asymmetric Cryptography (also known as Public Key Cryptography), where the key is split into a public and private key pair.

Symmetric Ciphers solve each of the big three problems related to security. The three are collectively referred to as CIA, or Confidentiality, Integrity, and Authentication. The Confidentiality problem is ensuring our communications are private (secrecy), and needs no further explanation. The Authenticity problem is solved since we know who else has the key. Loosely speaking, by adding a final encryption operation to the cipher, message integrity can be realized.

Shared Key encryption systems are easier to implement and faster to use. In addition, symmetric ciphers are usually designed to encrypt messages of arbitrary length. However, to use the system, both parties must be able to securely share the key. This is known as the Key Distribution Problem.

A symmetric cipher uses linear and non-linear transformation to encrypt and decrypt messages. This is in contrast to an asymmetric cipher such as RSA Cryptography or Elliptic Curve Cryptography, which use a difficult problem in mathematics to develop the strength of the Cryptosystem. Linear Transformations are operations such as rotate and bit shifts. Non-linear transformations include XOR, substitutions using a S-box, and permutations using a P-box. Linear and non-linear transformations are sometimes referred to as confusion and diffusion.

Encryption and decryption are usually very closely related. For example, one cipher may run the encryption algorithm in reverse, while another may reverse the key schedule (discussed below in 'Transformations').