使用Openssl的AES加密算法

 

原文链接: http://blog.csdn.net/yasi_xi/article/details/13997337

Openssl是很常见的C接口的库，个人觉得易用。以下是AES加密的使用备忘。如果你有一定的密码学基础，那么就很好理解。代码是从网上弄下来的（原始地址已经忘记了），然后在尝试的过程中改了一点东西。其它的cbc、cfb、ecb加密方式的用法都是类似的，只是函数名有点区别，就不一一列举了。

【yasi】IV: Initialization Vector，即初始化向量

一、接口简介

 

//设置加密密钥，使用字符缓冲区
int AES_set_encrypt_key(
        const unsigned char *userKey,
        const int bits,
        AES_KEY *key);

//设置解密密钥，同样适用字符缓冲区
int AES_set_decrypt_key(
        const unsigned char *userKey,
        const int bits,
        AES_KEY *key);

//加解密的接口，通过最后的enc来区分是加密还是解密操作
//每次执行AES_cbc_encrypt后，iv（向量）会被更新，
//所以需要自己保存它。
void AES_cbc_encrypt(
        const unsigned char *in,
        unsigned char *out,
        const unsigned long length,
        const AES_KEY *key,
        unsigned char *ivec,
        const int enc);

?

 

二、一个简单的Makefile

【yasi】针对CentOS环境，做了修改

 

LNK_OPT = -g -L/usr/lib64/ -lssl

all:
        rm -f codec
        g++ -g aes_codec.cpp -o codec $(LNK_OPT)

clean:
        rm -f codec

三、示例代码

【yasi】对源链接的代码做了点修改：随机明码 改 静态明码

 

#include <stdio.h>
#include <string.h>
#include <openssl/aes.h>
#include <openssl/rand.h>

/* file testaes.cpp */

static void hexdump(
                FILE *f,
                const char *title,
                const unsigned char *s,
                int l)
{
    int n = 0;

    fprintf(f, "%s", title);
    for (; n < l; ++n) {
        if ((n % 16) == 0) {
                fprintf(f, "\n%04x", n);
        }
        fprintf(f, " %02x", s[n]);
    }

    fprintf(f, "\n");
}

int main(int argc, char **argv)
{
        //128bits key.
        unsigned char   rkey[16];
        //Internal key.
        AES_KEY         key;

        //Testdata.
        // [yasi] Make static content instead of random text
        unsigned char   plaintext[AES_BLOCK_SIZE * 4] =
        {
                'a', 'b', 'c', 'd', 'e', 'f', 'g', 'i', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'i',
                '0', '1', '2', '3', '4', '5', '6', '7', '0', '1', '2', '3', '4', '5', '6', '7',
                'a', 'b', 'c', 'd', 'e', 'f', 'g', 'i', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'i',
                '0', '1', '2', '3', '4', '5', '6', '7', '0', '1', '2', '3', '4', '5', '6', '7'
        };
        unsigned char   ciphertext[AES_BLOCK_SIZE * 4];
        unsigned char   checktext[AES_BLOCK_SIZE * 4];

        //Init vector.
        unsigned char   iv[AES_BLOCK_SIZE * 4];
        //Save vector.
        unsigned char   saved_iv[AES_BLOCK_SIZE * 4];

        int nr_of_bits = 0;
        int nr_of_bytes = 0;

        //Zeror buffer.
        memset(ciphertext, 0, sizeof ciphertext);
        memset(checktext, 0, sizeof checktext);

        //Generate random
        RAND_pseudo_bytes(rkey, sizeof rkey);
        RAND_pseudo_bytes(saved_iv, sizeof saved_iv);

        hexdump(stdout, "== rkey ==",
                        rkey,
                        sizeof(rkey));
        hexdump(stdout, "== iv ==",
                        saved_iv,
                        sizeof(saved_iv));
        printf("\n");

        hexdump(stdout, "== plaintext ==",
                        plaintext,
                        sizeof(plaintext));
        printf("\n");

        //Entrypt
        memcpy(iv, saved_iv, sizeof(iv));
        nr_of_bits = 8 * sizeof(rkey);
        AES_set_encrypt_key(rkey, nr_of_bits, &key);
        nr_of_bytes = sizeof(plaintext);
        AES_cbc_encrypt(plaintext,
                        ciphertext,
                        nr_of_bytes,
                        &key,
                        iv,
                        AES_ENCRYPT);

        hexdump(stdout, "== ciphertext ==",
                        ciphertext,
                        sizeof(ciphertext));
        printf("\n");
        // [yasi] iv is changed in encryption
        hexdump(stdout, "== iv changed ==",
                        iv,
                        sizeof(iv));
        printf("\n");

        //Decrypt
        memcpy(iv, saved_iv, sizeof(iv));       // [yasi] without this line, decrypt will fail because iv is changed in encryption
        nr_of_bits = 8 * sizeof(rkey);
        AES_set_decrypt_key(rkey, nr_of_bits, &key);
        nr_of_bytes = sizeof(ciphertext);

        AES_cbc_encrypt(ciphertext,
                        checktext,
                        nr_of_bytes,
                        &key, iv,
                        AES_DECRYPT);
        hexdump(stdout, "== checktext ==",
                        checktext,
                        sizeof(checktext));
        printf("\n");

        return 0;
}

 

【yasi 运行结果】

[root@ampcommons02 aes-codec]# ./codec
== rkey ==
0000 81 ac 9b 38 1c 02 c5 c8 1d 7c a0 3f 87 be f2 c6
== iv ==
0000 34 a2 f1 f5 f3 93 76 32 cd 77 ad fb c8 82 f2 1b
0010 f3 cc 51 f6 35 f3 49 8d 44 97 8c 2c 89 50 0d d7
0020 68 21 d7 2f 0a 90 29 c1 dd c9 39 bc 7c 4f 18 2f
0030 04 cc 42 5e 84 8e fe a9 c5 49 00 9f 30 55 94 c0

== plaintext ==
0000 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0010 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37
0020 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0030 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37

== ciphertext ==
0000 c4 63 72 29 21 28 7b a2 27 24 4a e4 bb 95 1a d1
0010 b8 13 0e 77 0c 8a a4 09 2f ca 85 43 41 b5 5b d5
0020 a3 60 92 58 5b dd 45 0c e2 62 af f9 43 81 d7 06
0030 41 8e 85 28 3e eb 72 b5 ee 84 8c 27 7e 67 20 f6

== iv changed ==
0000 41 8e 85 28 3e eb 72 b5 ee 84 8c 27 7e 67 20 f6
0010 f3 cc 51 f6 35 f3 49 8d 44 97 8c 2c 89 50 0d d7
0020 68 21 d7 2f 0a 90 29 c1 dd c9 39 bc 7c 4f 18 2f
0030 04 cc 42 5e 84 8e fe a9 c5 49 00 9f 30 55 94 c0

== checktext ==
0000 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0010 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37
0020 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0030 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37

可见，encryption之后，IV立即被修改了。所以，为了能正确decrypt，在decrypt时，必须使用先前encrypt时的IV，即代码第98行。

 

【第二次运行的结果】

[root@ampcommons02 aes-codec]# ./codec
== rkey ==
0000 29 68 75 4d a5 9e 83 9a ed f8 ec bc 2e b8 09 7e
== iv ==
0000 b8 21 09 de 8f 58 6e be 73 be a7 10 fb 91 87 65
0010 65 9c d7 0e 4c 88 d2 65 ae de 0b 49 40 c7 75 df
0020 19 69 53 0b 11 5d ac e7 08 f6 ae df 16 66 e0 13
0030 75 41 f7 bb be 56 a1 dd a7 3e fb 4e 5d 9e e4 a2

== plaintext ==
0000 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0010 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37
0020 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0030 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37

== ciphertext ==
0000 5e 85 9c e8 65 d6 3b f9 03 9a a0 b5 78 bd f6 d4
0010 11 70 94 c1 c3 78 9a 1d 12 9a 84 48 3a 70 88 13
0020 d6 b5 bf c6 e8 e1 76 dc 3c b9 b0 4e b9 bb c4 74
0030 35 3d ac fc 29 e3 a0 64 d7 76 ab 76 c7 af dd 39

== iv changed ==
0000 35 3d ac fc 29 e3 a0 64 d7 76 ab 76 c7 af dd 39
0010 65 9c d7 0e 4c 88 d2 65 ae de 0b 49 40 c7 75 df
0020 19 69 53 0b 11 5d ac e7 08 f6 ae df 16 66 e0 13
0030 75 41 f7 bb be 56 a1 dd a7 3e fb 4e 5d 9e e4 a2

== checktext ==
0000 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0010 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37
0020 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0030 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37

可见，两次运行，虽然encrypt的明码是一样的，但encrypt出来的结果是不同的。随机明码 改 静态明码 的目的就是为了比较两次encrypt的结果。即，相同的明码用不同的IV做encrypt，结果是不同的。

 

【屏蔽第98行重新编译后的执行结果】

[root@ampcommons02 aes-codec]# ./codec
== rkey ==
0000 69 ef eb 49 25 5a c2 5e 0d 77 8a cb e6 fe ad 1d
== iv ==
0000 8e 05 8c 50 2f 69 9d fb 64 3e cd e6 2d 38 26 1c
0010 6e 21 00 e6 32 3f c6 ca 93 8b c1 e3 47 9a bd 81
0020 d7 e5 0b 63 dc f8 9d 1f 13 49 35 25 70 4e 64 c2
0030 ea 0c d0 78 e7 6c 65 64 41 4d db 2b 50 4d b4 06

== plaintext ==
0000 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0010 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37
0020 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0030 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37

== ciphertext ==
0000 a4 ed ba 4b 9f e9 74 bd 6d f6 03 76 79 9f 17 4f
0010 0c cd f3 b8 da 69 44 81 c9 f2 8b 03 83 0d 9d 77
0020 12 48 ea 46 3f eb 58 fd 48 c5 cc 2d 74 6c 99 4f
0030 93 bd 0d 06 f3 55 40 11 cb e7 d4 29 3b 8f 15 76

== iv changed ==
0000 93 bd 0d 06 f3 55 40 11 cb e7 d4 29 3b 8f 15 76
0010 6e 21 00 e6 32 3f c6 ca 93 8b c1 e3 47 9a bd 81
0020 d7 e5 0b 63 dc f8 9d 1f 13 49 35 25 70 4e 64 c2
0030 ea 0c d0 78 e7 6c 65 64 41 4d db 2b 50 4d b4 06

== checktext ==
0000 7c da e2 32 b9 5a ba 83 ce bb 7a ab 73 d1 54 03
0010 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37
0020 61 62 63 64 65 66 67 69 61 62 63 64 65 66 67 69
0030 30 31 32 33 34 35 36 37 30 31 32 33 34 35 36 37

可见，decrypt时使用和encrypt时不同的IV，不能正确decrypt出先前的明码。

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

【yasi】Openssl官方wiki，《EVP Symmetric Encryption and Decryption》中，有个的简单例子，整理如下（Linux下运行）：(代码下载)

algo_aes.h

#ifndef ALGO_AES_H
#define ALGO_AES_H

int encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *key,
  unsigned char *iv, unsigned char *ciphertext);

int decrypt(unsigned char *ciphertext, int ciphertext_len, unsigned char *key,
  unsigned char *iv, unsigned char *plaintext);

#endif

algo_aes.c

#include <stdlib.h>
#include <stdio.h>
#include "algo_aes.h"
#include <openssl/evp.h>

void handleErrors(void)
{
  ERR_print_errors_fp(stderr);
  abort();
}

int encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *key,
  unsigned char *iv, unsigned char *ciphertext)
{
  EVP_CIPHER_CTX *ctx;

  int len;

  int ciphertext_len;

  /* Create and initialise the context */
  if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors();

  /* Initialise the encryption operation. IMPORTANT - ensure you use a key
   * and IV size appropriate for your cipher
   * In this example we are using 256 bit AES (i.e. a 256 bit key). The
   * IV size for *most* modes is the same as the block size. For AES this
   * is 128 bits */
  if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv))
    handleErrors();

  /* Provide the message to be encrypted, and obtain the encrypted output.
   * EVP_EncryptUpdate can be called multiple times if necessary
   */
  if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
    handleErrors();
  ciphertext_len = len;

  /* Finalise the encryption. Further ciphertext bytes may be written at
   * this stage.
   */
  if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) handleErrors();
  ciphertext_len += len;

  /* Clean up */
  EVP_CIPHER_CTX_free(ctx);

  return ciphertext_len;
}

int decrypt(unsigned char *ciphertext, int ciphertext_len, unsigned char *key,
  unsigned char *iv, unsigned char *plaintext)
{
  EVP_CIPHER_CTX *ctx;

  int len;

  int plaintext_len;

  /* Create and initialise the context */
  if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors();

  /* Initialise the decryption operation. IMPORTANT - ensure you use a key
   * and IV size appropriate for your cipher
   * In this example we are using 256 bit AES (i.e. a 256 bit key). The
   * IV size for *most* modes is the same as the block size. For AES this
   * is 128 bits */
  if(1 != EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv))
    handleErrors();

  /* Provide the message to be decrypted, and obtain the plaintext output.
   * EVP_DecryptUpdate can be called multiple times if necessary
   */
  if(1 != EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len))
    handleErrors();
  plaintext_len = len;

  /* Finalise the decryption. Further plaintext bytes may be written at
   * this stage.
   */
  if(1 != EVP_DecryptFinal_ex(ctx, plaintext + len, &len)) handleErrors();
  plaintext_len += len;

  /* Clean up */
  EVP_CIPHER_CTX_free(ctx);

  return plaintext_len;
}

main.c

#include "algo_aes.h"
#include <stdio.h>
#include <string.h>
//#include <openssl/evp.h>

int main(int arc, char *argv[])
{
  /* Set up the key and iv. Do I need to say to not hard code these in a
   * real application? :-)
   */

  /* A 256 bit key */
  unsigned char *key = "01234567890123456789012345678901";

  /* A 128 bit IV */
  unsigned char *iv = "01234567890123456";

  /* Message to be encrypted */
  unsigned char *plaintext =
    "The quick brown fox jumps over the lazy dog1234";

  /* Buffer for ciphertext. Ensure the buffer is long enough for the
   * ciphertext which may be longer than the plaintext, dependant on the
   * algorithm and mode
   */
  unsigned char ciphertext[64];

  /* Buffer for the decrypted text */
  unsigned char decryptedtext[64];

  int decryptedtext_len, ciphertext_len;

  /* Initialise the library */
/*  ERR_load_crypto_strings();
  OpenSSL_add_all_algorithms();
  OPENSSL_config(NULL);*/

  printf("Plaintext is:\n%s~\n", plaintext);

  /* Encrypt the plaintext */
  ciphertext_len = encrypt(plaintext, strlen(plaintext), key, iv,
    ciphertext);

  /* Do something useful with the ciphertext here */
  printf("Ciphertext is %d bytes long:\n", ciphertext_len);
  BIO_dump_fp(stdout, ciphertext, ciphertext_len);

  /* Decrypt the ciphertext */
  decryptedtext_len = decrypt(ciphertext, ciphertext_len, key, iv,
    decryptedtext);

  /* Add a NULL terminator. We are expecting printable text */
  decryptedtext[decryptedtext_len] = '\0';

  /* Show the decrypted text */
  printf("Decrypted text is:\n");
  printf("%s~\n", decryptedtext);

  /* Clean up */
  EVP_cleanup();
  ERR_free_strings();

  return 0;
}

Mekefile：

OBJ_DIR = ./obj
BIN_DIR = ./bin
SRC_DIR = ./
OBJS = \
    $(OBJ_DIR)/algo_aes.o \
    $(OBJ_DIR)/main.o
TARGET = $(BIN_DIR)/main
INC_OPT = -I./
LNK_OPT = -lssl

$(TARGET) : clean chkobjdir chkbindir $(OBJS)
    gcc -g -o $@ $(OBJS) $(LNK_OPT)

$(OBJ_DIR)/%.o : $(SRC_DIR)/%.c
    gcc -g $(INC_OPT) -c -o $@ $<

chkobjdir :
    @if test ! -d $(OBJ_DIR) ; \
    then \
        mkdir $(OBJ_DIR) ; \
    fi

chkbindir :
    @if test ! -d $(BIN_DIR) ; \
    then \
        mkdir $(BIN_DIR) ; \
    fi

clean :
    -rm -f $(TARGET)
    -rm -rf $(OBJ_DIR)

运行结果：

Plaintext is:
The quick brown fox jumps over the lazy dog1234~
Ciphertext is 48 bytes long:
0000 - e0 6f 63 a7 11 e8 b7 aa-9f 94 40 10 7d 46 80 a1   .oc.......@.}F..
0010 - 17 99 43 80 ea 31 d2 a2-99 b9 53 02 d4 39 b9 70   ..C..1....S..9.p
0020 - e9 29 d5 a8 03 bd 71 31-b8 c3 6f 3d 39 3a 3d 3d   .)....q1..o=9:==
Decrypted text is:
The quick brown fox jumps over the lazy dog1234~

注意：（参考）

AES算法的块（block）的长度固定为16字节。假设一个字符串在AES加密前的长度为cleanLen，加密后的长度为cipherLen，则二者有下面的关系，其中的“/”是整除。

cipherLen = (clearLen/16 + 1) * 16

比如：（注意第二行，即使48刚好能被16整除，也要额外追加一个16字节的块）

clearLen	cipherLen
47	48
48	64
49	64

可见，对于AES算法：

1）加密后的长度>=加密前的长度

2）解密后的长度<=解密前的长度

这对于写代码时的指导意义在于：

1）假如我们要做AES加密的字符串为“The quick brown fox jumps over the lazy dog”，它的长度（不带/0）为43字节，则我们可以计算出加密后的串的长度为 (43 / 16 + 1) * 16 = 48 字节，于是就可以申明加密后的数组如下

unsigned char ciphertextp[48];

2）假如我们知道AES加密后的串的长度为64字节，那么就可以申明解密后的数组如下

unsigned char decryptedtext[64];