先介绍几个跟算法相关的基本概念。
block cipher
In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called blocks, with an unvarying transformation that is specified by a symmetric key. Block ciphers are important elementary components in the design of many cryptographic protocols, and are widely used to implement encryption of bulk data.
stream cipher
In cryptography, a stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the cyphertext stream. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state. In practice, a digit is typically a bit and the combining operation an exclusive-or (xor).
RC2/RC4/RC5/RC6
In cryptography, RC2 (also known as ARC2) is a block cipher designed by Ron Rivest in 1987. "RC" stands for "Ron's Code" or "Rivest Cipher"; other ciphers designed by Rivest include RC4, RC5 and RC6. RC4 是stream cipher; RC5和RC6是block cipher。
RSA
RSA is an algorithm for public-key cryptography that is based on the presumed difficulty of factoring large integers, the factoring problem. RSA stands for Ron Rivest, Adi Shamir and Leonard Adleman, who first publicly described the algorithm in 1977.
RSA是一种非对称加密算法。对极大整数做因数分解的难度决定了RSA算法的可靠性。换言之，对一极大整数做因数分解愈困难，RSA算法愈可靠。尽管如此，只有一些RSA算法的变种[来源请求]被证明为其安全性依赖于因数分解。假如有人找到一种快速因数分解的算法的话，那么用RSA加密的信息的可靠性就肯定会极度下降。但找到这样的算法的可能性是非常小的。今天只有短的RSA钥匙才可能被强力方式解破。到2008年为止，世界上还没有任何可靠的攻击RSA算法的方式。只要其钥匙的长度足够长，用RSA加密的信息实际上是不能被解破的。但在分布式计算和量子计算机理论日趋成熟的今天，RSA加密安全性受到了挑战。
Diffie-Hellman
Diffie-Hellman(DH) is a specific method of exchanging cryptographic keys. It is one of the earliest practical examples of key exchange implemented within the field of cryptography. The Diffie–Hellman key exchange method allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over an insecure communications channel. This key can then be used to encrypt subsequent communications using a symmetric key cipher.
DH还包含两个变种：DH (store and forward) key-exchange algorithm和DH (ephemeral) key-exchange algorithm，区别是前者在key handle被销毁后，exchange key在CSP中被保留，而后者则将在key handle被销毁后，exchange key被删除。
DSS
Digital Signature Standard (DSS) is the digital signature algorithm (DSA) developed by the U.S. National Security Agency (NSA) to generate a digital signature for the authentication of electronic documents. DSS was put forth by the National Institute of Standards and Technology (NIST) in 1994, and has become the United States government standard for authentication of electronic documents. DSS is specified in Federal Information Processing Standard (FIPS) 186.
DES
The Data Encryption Standard (DES), adopted by the American National Standards Institute (ANSI) in 1976, is a block cipher algorithm that has been widely used as a secure way to transfer data. The DES is based on an algorithm developed by IBM in the early 1970s, called LUCIFER. It was adopted by ANSI for its high level and key-based security (the security resides in the key), its efficiency, and its exportability.
triple DES
A variation of the DES block cipher algorithm that encrypts plain text with one key, encrypts the resulting ciphertext with a second key, and finally, encrypts the result of the second encryption with a third key. Triple DES is a symmetric algorithm that uses the same algorithm and keys for encryption and decryption.
AES
The Advanced Encryption Standard (AES) is an encryption algorithm for securing sensitive but unclassified material by U.S. Government agencies and, as a likely consequence, may eventually become the de facto encryption standard for commercial transactions in the private sector. AES is based on the Rijndael cipher and is a block cipher. 支持128,192,256 bit key length.
DES-X
In cryptography, DES-X (or DESX) is a variant on the DES (Data Encryption Standard) block cipher intended to increase the complexity of a brute force attack using a technique called key whitening.
The original DES algorithm was specified in 1976 with a 56-bit key size: 256 possibilities for the key. There was criticism that an exhaustive search might be within the capabilities of large governments, particularly the United States' National Security Agency (NSA). One scheme to increase the key size of DES without substantially altering the algorithm was DES-X, proposed by Ron Rivest in May 1984.
The algorithm has been included in RSA Security's BSAFE cryptographic library since the late 1980s.
Elliptic curve Diffie-Hellman key exchange algorithm
Elliptic curve Diffie–Hellman (ECDH) is an anonymous key agreement protocol that allows two parties, each having an elliptic curve public-private key pair, to establish a shared secret over an insecure channel. This shared secret may be directly used as a key, or better yet, to derive another key which can then be used to encrypt subsequent communications using a symmetric key cipher. It is a variant of the Diffie–Hellman protocol using elliptic curve cryptography.
One way function hashing algorithm
A one-way hash function, also known as a message digest, fingerprint or compression function, is a mathematical function which takes a variable-length input string and converts it into a fixed-length binary sequence. Furthermore, a one-way hash function is designed in such a way that it is hard to reverse the process, that is, to find a string that hashes to a given value (hence the name one-way.) A good hash function also makes it hard to find two strings that would produce the same hash value.
All modern hash algorithms produce hash values of 128 bits and higher.
Even a slight change in an input string should cause the hash value to change drastically. Even if 1 bit is flipped in the input string, at least half of the bits in the hash value will flip as a result. This is called an avalanche effect.
Since it is computationally infeasible to produce a document that would hash to a given value or find two documents that hash to the same value, a document's hash can serve as a cryptographic equivalent of the document. This makes a one-way hash function a central notion in public-key cryptography. When producing a digital signature for a document, we no longer need to encrypt the entire document with a sender's private key (which can be extremely slow). It is sufficient to encrypt the document's hash value instead.
Although a one-way hash function is used mostly for generating digital signatures, it can have other practical applications as well, such as secure password storage, file identification and message authentication code (MAC.)
Hughes MD5 hashing algorithm
MAC(Message Authentication Code)
In cryptography, a message authentication code (often MAC) is a short piece of information used to authenticate a message and to provide integrity and authenticity assurances on the message. Integrity assurances detects accidental and intentional message changes, while authenticity assurances affirms the message's origin.
A MAC algorithm, sometimes called a keyed (cryptographic) hash function (however, cryptographic hash function is only one of the possible ways to generate MACs), accepts as input a secret key and an arbitrary-length message to be authenticated, and outputs a MAC (sometimes known as a tag). The MAC value protects both a message's data integrity as well as its authenticity, by allowing verifiers (who also possess the secret key) to detect any changes to the message content.
HMAC(hash-based message authentication code)
In cryptography, a hash-based message authentication code (HMAC) is a specific construction for calculating a message authentication code (MAC) involving a cryptographic hash function in combination with a secret cryptographic key. As with any MAC, it may be used to simultaneously verify both the data integrity and the authentication of a message. Any cryptographic hash function, such as MD5 or SHA-1, may be used in the calculation of an HMAC; the resulting MAC algorithm is termed HMAC-MD5 or HMAC-SHA1 accordingly. The cryptographic strength of the HMAC depends upon the cryptographic strength of the underlying hash function, the size of its hash output, and on the size and quality of the key.
MD2/MD4/MD5
The MD2 Message-Digest Algorithm is a cryptographic hash function developed by Ronald Rivest in 1989. The algorithm is optimized for 8-bit computers. MD2 is specified in RFC 1319. Although MD2 is no longer considered secure, even as of 2010, it remains in use in public key infrastructures as part of certificates generated with MD2 and RSA.
The MD4 Message-Digest Algorithm is a cryptographic hash function developed by Ronald Rivest in 1990. The digest length is 128 bits. The algorithm has influenced later designs, such as the MD5, SHA-1 and RIPEMD algorithms.
Rivest在1989年开发出MD2算法。在这个算法中，首先对信息进行数据补位，使信息的字节长度是16的倍数。然后，以一个16位的检验和追加到信息末尾，并且根据这个新产生的信息计算出散列值。后来，Rogier和Chauvaud发现如果忽略了检验将和MD2产生冲突。MD2算法加密后结果是唯一的（即不同信息加密后的结果不同）。
为了加强算法的安全性，Rivest在1990年又开发出MD4算法。MD4算法同样需要填补信息以确保信息的比特位长度减去448后能被512整除（信息比特位长度mod 512 = 448）。然后，一个以64位二进制表示的信息的最初长度被添加进来。信息被处理成512位damg?rd/merkle迭代结构的区块，而且每个区块要通过三个不同步骤的处理。Den boer和Bosselaers以及其他人很快的发现了攻击MD4版本中第一步和第三步的漏洞。Dobbertin向大家演示了如何利用一部普通的个人电脑在几分钟内找到MD4完整版本中的冲突（这个冲突实际上是一种漏洞，它将导致对不同的内容进行加密却可能得到相同的加密后结果）。毫无疑问，MD4就此被淘汰掉了。
尽管MD4算法在安全上有个这么大的漏洞，但它对在其后才被开发出来的好几种信息安全加密算法的出现却有着不可忽视的引导作用。
1991年，Rivest开发出技术上更为趋近成熟的md5算法。它在MD4的基础上增加了"安全-带子"（safety-belts）的概念。虽然MD5比MD4复杂度大一些，但却更为安全。这个算法很明显的由四个和MD4设计有少许不同的步骤组成。在MD5算法中，信息-摘要的大小和填充的必要条件与MD4完全相同。Den boer和Bosselaers曾发现MD5算法中的假冲突（pseudo-collisions），但除此之外就没有其他被发现的加密后结果了。
SEAL
In cryptography, SEAL (Software-Optimized Encryption Algorithm) is a very fast stream cipher optimised for machines with a 32-bit word size and plenty of RAM. SEAL is actually a pseudorandom function family in that it can easily generate arbitrary portions of the keystream without having to start from the beginning. This makes it particularly well suited for applications like encrypting hard drives.
SHA(Secure Hash Algorithm)
Secure Hash Algorithm is a family of cryptographic hash functions published by the National Institute of Standards and Technology (NIST) as a U.S. Federal Information Processing Standard (FIPS), and may refer to SHA-0,SHA-1,SHA-2(SHA-256 and SHA-512， 2 truncated version SHA-224 and SHA-384),SHA-3.
Skipjack
In cryptography, Skipjack is a block cipher—an algorithm for encryption—developed by the U.S. National Security Agency (NSA). Initially classified, it was originally intended for use in the controversial Clipper chip. Subsequently, the algorithm was declassified and now provides a unique insight into the cipher designs of a government intelligence agency.