AES是一套对称密钥的密码术,目前已广泛使用,用于替代已经不够安全的DES算法。所谓对称密钥,就是说加密和解密用的是同一个密钥,消息的发送方和接收方在消息传递前需要享有这个密钥。和非对称密钥体系不同,这里的密钥是双方保密的,不会让任何第三方知道。

对称密钥加密法主要基于块加密,选取固定长度的密钥,去加密明文中固定长度的块,生成的密文块与明文块长度一样。显然密钥长度十分重要,块的长度也很重要。如果太短,则很容易枚举出所有的明文-密文映射;如果太长,性能则会急剧下降。AES中规定块长度为128 bit,而密钥长度可以选择128, 192或256 bit 。暴力破解密钥需要万亿年,这保证了AES的安全性。

AES的算法较为复杂,在此不细加阐述。下面是使用openssl进行AES加密和解密的示例程序:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
/*
* aes.cc
* - Show the usage of AES encryption/decryption
*/
 
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <openssl/aes.h>
 
int main(int argc, char** argv) {
    AES_KEY aes;
    unsigned char key[AES_BLOCK_SIZE];        // AES_BLOCK_SIZE = 16
    unsigned char iv[AES_BLOCK_SIZE];        // init vector
    unsigned char* input_string;
    unsigned char* encrypt_string;
    unsigned char* decrypt_string;
    unsigned int len;        // encrypt length (in multiple of AES_BLOCK_SIZE)
    unsigned int i;
 
    // check usage
    if (argc != 2) {
        fprintf(stderr, "%s <plain text>\n", argv[0]);
        exit(-1);
    }
 
    // set the encryption length
    len = 0;
    if ((strlen(argv[1]) + 1) % AES_BLOCK_SIZE == 0) {
        len = strlen(argv[1]) + 1;
    } else {
        len = ((strlen(argv[1]) + 1) / AES_BLOCK_SIZE + 1) * AES_BLOCK_SIZE;
    }
 
    // set the input string
    input_string = (unsigned char*)calloc(len, sizeof(unsigned char));
    if (input_string == NULL) {
        fprintf(stderr, "Unable to allocate memory for input_string\n");
        exit(-1);
    }
    strncpy((char*)input_string, argv[1], strlen(argv[1]));
 
    // Generate AES 128-bit key
    for (i=0; i<16; ++i) {
        key[i] = 32 + i;
    }
 
    // Set encryption key
    for (i=0; i<AES_BLOCK_SIZE; ++i) {
        iv[i] = 0;
    }
    if (AES_set_encrypt_key(key, 128, &aes) < 0) {
        fprintf(stderr, "Unable to set encryption key in AES\n");
        exit(-1);
    }
 
    // alloc encrypt_string
    encrypt_string = (unsigned char*)calloc(len, sizeof(unsigned char));    
    if (encrypt_string == NULL) {
        fprintf(stderr, "Unable to allocate memory for encrypt_string\n");
        exit(-1);
    }
 
    // encrypt (iv will change)
    AES_cbc_encrypt(input_string, encrypt_string, len, &aes, iv, AES_ENCRYPT);
 
    // alloc decrypt_string
    decrypt_string = (unsigned char*)calloc(len, sizeof(unsigned char));
    if (decrypt_string == NULL) {
        fprintf(stderr, "Unable to allocate memory for decrypt_string\n");
        exit(-1);
    }
 
    // Set decryption key
    for (i=0; i<AES_BLOCK_SIZE; ++i) {
        iv[i] = 0;
    }
    if (AES_set_decrypt_key(key, 128, &aes) < 0) {
        fprintf(stderr, "Unable to set decryption key in AES\n");
        exit(-1);
    }
 
    // decrypt
    AES_cbc_encrypt(encrypt_string, decrypt_string, len, &aes, iv, 
            AES_DECRYPT);
 
    // print
    printf("input_string = %s\n", input_string);
    printf("encrypted string = ");
    for (i=0; i<len; ++i) {
        printf("%x%x", (encrypt_string[i] >> 4) & 0xf, 
                encrypt_string[i] & 0xf);    
    }
    printf("\n");
    printf("decrypted string = %s\n", decrypt_string);
 
    return 0;
}

编译Makefile:

CC=g++
CFLAGS=-Wall -g -O2
LIBS=-lcrypto
 
all: aes
 
aes: aes.cc
    $(CC) $(CFLAGS) aes.cc -o [email protected] $(LIBS)
 
clean:
    @rm -f aes