AES, DES的实现

在Cortex-m3上实现AES或DES算法

DES

It was once a predominant symmetric-key algorithm for the encryption of electronic data.

AES

For AES, NIST selected three members of the Rijndael family, each with a block size of 128 bits, but three different key lengths: 128, 192 and 256 bits.

那DES和AES选择哪一个呢？

根据资料，DES较老，AES的速度和可靠性方面更好些，所以选择AES吧。

另一个角度，为什么UDP包在Wifi网络上是不加密的呢？

As far as I understand, WiFi networks that require no password send traffic through the air unencrypted. Those that require a password encrypt each connection uniquely, even if they’re all using the same password.

If this is true, I don’t understand why. Requiring a password for access and encrypting connections seem like totally separate issues.

合理解释了这个问题。因为连进同一个路由器的设备在链路层上是没有任何加密的，所以我们要在应用层上加上加密步骤。

AES的实现

步骤	解释
substitution	字符替代
permutation	字符打乱，分散

10 rounds for 128-bit key

加密

步骤	解释
Key expansion	生成 Key Schedule, expanding a short key into a larger key, whose parts are used during the different iterations.
SubBytes
ShiftRow
MixColumnz
Add Round Keys

解密

步骤	解释
Inverse Shift Row
Inverse SubBytes
Add Round Key
Inverse Mix Column

补充:

The last round for encryption does not involve the “Mix columns” step. The last round for decryption does not involve the “Inverse mix columns” step.

使用CBC mode. 但是当前的应用的话，使用ECB mode也没有什么问题。在Github上找到了一个范例，在Mac OSX上编译成功了。下一步再把代码移植到Cortex-M3环境。