AES CBC和CTR加解密实例

AES（Advanced Encryption Standard，高级加密标准）又叫Rijndael加密法，用来替代DES算法。常见AES加密模式有ECB、CBC、CFB、OFB和CTR等五种， CFB、OFB都带反馈，做流加密用的多，CBC和CTR、ECB多用于独立block加密，由于ECB算法有点小缺点，所以CBC和CTR这两种加解密方式用的较多，也是很多标准规范要求的实现算法，下面看一下这两种算法原理。AES跟Rijndael相比有点小区别，就是使用固定块（block size）为128bits（16字节）（原Rijndael块大小更灵活），密钥长度支持128、192或256位。

一、AES CBC加解密原理

CBC加解密原理如下图所示（图片来源维基百科，参考文末地址）：CBC加密原理：明文跟向量异或，再用KEY进行加密，结果作为下个BLOCK的初始化向量。解密原理：使用密钥先对密文解密，解密后再同初始向量异或得到明文。

CBC需要对明文块大小进行Padding（补位），由于前后加密的相关性，只能实施串行化动作，无法并行运算。另外，CBC需要参量：密钥和初始化向量。

二、AES CTR加解密原理

CTR加密原理：用密钥对输入的计数器加密，然后同明文异或得到密文。解密原理：用密钥对输入计数器加密，然后同密文异或得到明文。

CTR不需要Padding，而且采用了流密钥方式加解密，适合于并行运算，CTR涉及参量：Nounce随机数、Counter计数器和密钥。Nounce随机数和Counter计数器整体可看作计数器，因为只要算法约定好，就可以回避掉串行化运算。

三、AES CBC和CTR加解密实例

下文实例使用了第三方开源源码，官方网址：http://www.gladman.me.uk/， 本文测试源码来源：plushost.co.uk - ，测试时，如下源码文件需加入工程：aes_modes.c、aescrypt.c、aeskey.c、aestab.c。

测试源码如下：

#include <iostream>
#include <string>
#include <time.h>
	
#include "aes/aes.h"

typedef unsigned char  uint8;
typedef char           int8;
typedef unsigned short uint16;
typedef short          int16;
typedef unsigned int   uint32;
typedef int            int32;
	
typedef unsigned __int64 uint64;
typedef __int64          int64;
/******************************
For _LINUX
typedef long   long    int64;
typedef unsigned long  long  uint64;
******************************/
	
using namespace std;
	
//same as function rfc3686_inc
void ctr_inc(unsigned char ctr_buf[16])
{
    if(!(++(ctr_buf[15])))
        if(!(++(ctr_buf[14])))
            if(!(++(ctr_buf[13])))
                ++(ctr_buf[12]);
}
	
//same as function rfc3686_init
//4Bytes nounce+8Bytes iv+4Bytes counter
void ctr_init( unsigned char nonce[4], unsigned char iv[8], unsigned char ctr_buf[16])
{
    memcpy(ctr_buf, nonce, 4);
    memcpy(ctr_buf +  4, iv, 8);
    memset(ctr_buf + 12, 0, 4);
    ctr_inc(ctr_buf);
}
	
void print_hex(uint8* buf, uint64 len) {
    //Print results:
    for(int i=0;i<len;i++) {
        printf("%02X",buf[i]);
        if(15 == i%16)
            printf("\n");
    }
    printf("\n");
}
	
void main() {   
    uint8   key[]   = {0x10,0xa5,0x88,0x69,0xd7,0x4b,0xe5,0xa3,0x74,0xcf,   0x86,0x7c,0xfb,0x47,0x38,0x59};//AES::DEFAULT_KEYLENGTH
    uint8   buf[16];//tmp buffer
	
    uint8   msg[]   = "HelloWorld!23456";
    uint64  fsize=strlen((char*)msg);//message size
    uint8*  DataBuf=new uint8[1024];//Data Buffer


    //AES with CBC
    printf("AES with CBC\n");          
	     
    //Copy data
    memset(DataBuf,0,1024);
    strcpy((char*)DataBuf,(char*)msg);
    uint8* pDataBuf = DataBuf;//tmp pointer
    uint8   iv1[]   = {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,   0x00,0x00,0x00,0x00,0x00,0x00};
	
    uint64  iEncryptTimes = fsize/16+1;
    uint8   iPaddings     = 16-fsize%16;//Padding size
    uint64  newlen = fsize+iPaddings;//New length
     
    //memcpy(DataBuf,iv,16);//Save iv
    memset(pDataBuf+fsize, iPaddings,iPaddings);//Padding
    printf("input   =\n");
    print_hex(DataBuf,newlen);
    aes_encrypt_ctx en_ctx[1];//Init encrypt
	     
    //Encrypt
    for(uint64 i=0;i<iEncryptTimes;i++) {
        aes_encrypt_key128(key,en_ctx);
	        aes_cbc_encrypt(pDataBuf,buf,16,iv1,en_ctx);//iv has been changed, ctx has been changed!!!
        memcpy(pDataBuf,buf,16);
        pDataBuf += 16;
    }
	     
    printf("encrypt =\n");
    print_hex(DataBuf,newlen);
     
    //Decrypt
    pDataBuf = DataBuf;
    uint8   iv2[]       = {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,   0x00,0x00,0x00,0x00,0x00,0x00};
    uint8   buf3[256]={'\0'};
    aes_decrypt_ctx de_ctx[1];
    aes_decrypt_key128(key,de_ctx);
    aes_cbc_decrypt(pDataBuf,buf3,newlen,iv2,de_ctx);
     
    printf("decrypt =\n");
    print_hex(buf3,newlen);
     
    //================================

    printf("AES with CTR\n");          
    //Copy data
    memset(DataBuf,0,1024);
    strcpy((char*)DataBuf,(char*)msg);
    pDataBuf = DataBuf;//tmp pointer
    uint8   iv3[]   = {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,   0x00,0x00,0x00,0x00,0x00,0x00};

    iEncryptTimes = fsize/16;
    uint8   iRemain       = fsize%16;
    uint8   ctr_buf[AES_BLOCK_SIZE];
	     
    //Save iv(as ctrl buffer)
    //memcpy(DataBuf,iv,16);
    printf("input   =\n");
    print_hex(DataBuf,fsize);
	     
    //Init encrypt
    //aes_encrypt_ctx en_ctx[1];
	     
    //Encrypt
    for(i=0;i<iEncryptTimes;i++) {
        aes_encrypt_key128(key,en_ctx);
        ctr_init(iv3,iv3+4,ctr_buf);//we set iv as the nouce
        aes_ctr_encrypt(pDataBuf,buf,16,ctr_buf,ctr_inc,en_ctx);//iv has been changed, ctx has been changed!!!
        memcpy(pDataBuf,buf,16);
        pDataBuf += 16;
    }
     
    if(iRemain!=0) {//last times
        pDataBuf += i*16;
        aes_encrypt_key128(key,en_ctx);
        ctr_init(iv3,iv3+4,ctr_buf);//we set iv as the nouce
        aes_ctr_encrypt(pDataBuf,buf,iRemain,ctr_buf,ctr_inc,en_ctx);//iv has been changed, ctx has been changed!!!
        memcpy(pDataBuf,buf,iRemain);
    }
     
    printf("encrypt =\n");
    print_hex(DataBuf,fsize);
	     
    //Decrypt
    pDataBuf = DataBuf;
    uint8   iv4[]       = {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,   0x00,0x00,0x00,0x00,0x00,0x00};
    //uint8 buf3[256]={'\0'};
    //aes_decrypt_ctx de_ctx[1];
    //aes_decrypt_key128(key,de_ctx);
    aes_encrypt_key128(key,en_ctx);
    ctr_init(iv4,iv4+4,ctr_buf);//we set iv as the nouce
    aes_ctr_decrypt(pDataBuf,buf3,16,ctr_buf,ctr_inc,en_ctx);
    printf("decrypt =\n");
    print_hex(buf3,fsize);
     
    printf("\n");
}

输出如下（CBC未去掉Padding）：

• AES with CBC
• input   =
• 48656C6C6F576F726C64213233343536
• 10101010101010101010101010101010
• encrypt =
• F928E09884AA2BA8CC4B73C09304250C
• C9A0EEFF2295B5D83BEA0410001BD7C6
• decrypt =
• 48656C6C6F576F726C64213233343536
• 10101010101010101010101010101010
• AES with CTR
• input   =
• 48656C6C6F576F726C64213233343536
• encrypt =
• 1CA0978FE499969C769B6346D46B66F9
• decrypt =
• 48656C6C6F576F726C64213233343536

参考资料：

http://zh.wikipedia.org/wiki/高级加密标准

http://zh.wikipedia.org/zh/块密码的工作模式