🐛
When implementing a cipher, it's useful to have a test vector with the cipher intermediate states (if you get it right the first time, you don't need it; if you get a detail wrong, it makes finding that detail a lot easier).
Julius Caesar is credited with perhaps the oldest known symmetric cipher algorithm. The so-called Caesar cipher — a variant of which you can probably find as a diversion in your local newspaper — assigns each letter, at random, to a number. This mapping of letters to numbers is the key in this simple algorithm.
思路：第一步，从Caesar cipher开始，引入symmetric cipher algorithm这个概念
Modern cipher algorithms must be much more sophisticated than Caesar’s in order to withstand automated attacks by computers. Although the basic premise remains — substituting one letter or symbol for another, and keeping track of that substitution for later — further elements of confusion and diffusion were added over the centuries to create modern cryptography algorithms. One such hardening technique is to operate on several characters at a time, rather than just one.
思路：第二步，随着时间的流逝，symmetric cipher algorithm也在不断的进行演变。
By far the most common category of symmetric encryption algorithm is the block cipher algorithm, which operates on a fixed range of bytes rather than on a single character at a time.
思路：第三步，在当代，symmetric cipher algorithm中最常使用的是block cipher algorithm。
Algorithm --> Language --> implementation
使用哪一个算法（Algorithm）是一回事，而使用哪一种编程语言（Language）编写这个算法是另一回事。那么，即使说确定了使用Java语言，那么，由谁去实现又是另外一回事，它可能是由JDK自己提供的实现，也可能是由BouncyCastle提供的实现，也可能是由我们自己来编写代码实现。
Standard: 强调标准
- NIST: National Institute of Standards and Technology 可以查询DES和AES的文档
Examples: 在写算法的过程中，这些示例可以帮助自己查看自己的程序是否正确
- Cryptographic Standards and Guidelines 这里是NIST网站提供的算法示例，包括Encryption-Block Ciphers(AES、TDES、Skipjack)、Block Cipher Modes(ECB、CBC、CFB、OFB、CTR)、Digital Signatures(DSA、RSA)、Secure Hashing(SHA1/SHA256)、Key Management(ECC)、Random Number Generation、Message Authentication。
Tutorial: Standard可能更专注于“专业术语”，而Tutorial能够帮助人们快速熟悉一些概念、原理
-
Cryptography Tutorials - Herong's Tutorial Examples 这个网站包含的内容很多，我学习过程中的一个好助手
-
Technology 这个网站对加密、数字签名、摘要做一个简要描述，对于整体上把握这些要素还是有帮助的
-
Journey into cryptography 我感觉，这个很有用，只是现在没有办法全部理解
EBook
- Java Security Solutions 这本书虽然是2001年写的，但是我想对于我探究这些安全算法是有帮助的。
- The Handbook of Applied Cryptography
javascript
- Crypto Academy: 这是用javascript来实现各种算法
- MD5/SHA-1 Hash Generator (with steps) 这个显示详细步骤