AES CBC和CTR加解密实例

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2012

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（相同输入，相同输出，容易明文攻击），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：http://gladman.plushost.co.uk/oldsite/AES/index.php，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 _LINUXtypedef long   long    int64;typedef unsigned long  long  uint64;******************************/using namespace std;//same as function rfc3686_incvoid 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 countervoid 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

如您对AES/DEX加密解密有兴趣，建议阅读：Java DES加密和解密，Java AES加密和解密两篇文章。