ECDH密钥交换的C程序
由于项目需要，使用openssl编写一段ECDH代码实现网络两端实体的密钥交换。
虽然对openssl不熟悉，但也开始做。
最先参照的是openssl官方wiki上的Elliptic Curve Diffie Hellman：http://wiki.openssl.org/index.php/Elliptic_Curve_Diffie_Hellman
直接使用文中的代码。不过其中有段代码需要替换：
/* Get the peer's public key, and provide the peer with our public key - * how this is done will be specific to your circumstances */ peerkey = get_peerkey(pkey);
我以为只要将这pkey和peerkey指向的密钥块通过网络交换一下就行了。
先不经过网络交换这两个密钥数据，在本地测试，完全通过。但是，一涉及网络交换，程序就崩溃，并爆出内存错误。
总以为是外围程序的问题，调试了好久好久……
直到后来，最后打印出pkey和peerkey指向的类型为EVP_PKEY的数据来看，才发现EVP_PKEY占存储量很小（32字节），这才发现奇怪。因为密钥块一般挺大的。
在openssl库安装路径include目录中找到evp.h，打开查看，发现：
struct evp_pkey_st
{
int type;
int save_type;
int references;
const EVP_PKEY_ASN1_METHOD *ameth;
ENGINE *engine;
union {
char *ptr;
#ifndef OPENSSL_NO_RSA
struct rsa_st *rsa; /* RSA */
#endif
#ifndef OPENSSL_NO_DSA
struct dsa_st *dsa; /* DSA */
#endif
#ifndef OPENSSL_NO_DH
struct dh_st *dh; /* DH */
#endif
#ifndef OPENSSL_NO_EC
struct ec_key_st *ec; /* ECC */
#endif
} pkey;
int save_parameters;
STACK_OF(X509_ATTRIBUTE) *attributes; /* [ 0 ] */
} /* EVP_PKEY */;
这下明白了，原来在网络上传输的我以为的“密钥块”原来是一堆指针！（而且是公私密钥对的指针。。。。）
于是得想办法解决。找到这个帖子：
How does one access the raw ECDH public key, private key and params inside OpenSSL's EVP_PKEY structure?
http://stackoverflow.com/questions/18155559/how-does-one-access-the-raw-ecdh-public-key-private-key-and-params-inside-opens
文中指出要进行公钥的序列化。
然后挨个搜索API的说明，花了不少时间。又是一阵子调试……
最后还是卡在反序列化过程中EC_KEY_set_public_key() to get an EC_KEY和EC_KEY to EVP_PKEY_set1_EC_KEY上了。
心想只好换一种方法了。调试途中在google搜索EC_POINT_point2oct关键字时发现了一个页面：
OpenSSL - User - ECDH http://openssl.6102.n7.nabble.com/ECDH-td22150.html
3楼Rick的回复给的代码比较好。采用并改进，调试之。
最终得到完整的ECDH代码：
#include <openssl/ssl.h>
#define ECDH_SIZE 33
void handleErrors()
{
printf("Error occurred.\n");
}
static void disp(const char *str, const void *pbuf, const int size)
{
int i=0;
if(str != NULL){
printf("%s:\n", str);
}
if(pbuf != NULL && size > 0){
for(i=0;i<size;i++)
printf("%02x ", *((unsigned char *)pbuf+i));
putchar('\n');
}
putchar('\n');
}
int main() {
/* alice */
EC_KEY *ecdh = EC_KEY_new();
EC_POINT *point = NULL;
EC_POINT *point2c;
EC_GROUP *group;
unsigned char pubkey[ECDH_SIZE];
unsigned char shared[ECDH_SIZE];
int len;
//Generate Public
ecdh = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);//NID_secp521r1
EC_KEY_generate_key(ecdh);
point = EC_KEY_get0_public_key(ecdh);
group = EC_KEY_get0_group(ecdh);
if(0 == (len = EC_POINT_point2oct(group, point, POINT_CONVERSION_COMPRESSED, pubkey, ECDH_SIZE, NULL))) handleErrors();
printf("len=%d\n",len);
/* bob */
EC_KEY *ecdh2 = EC_KEY_new();
EC_POINT *point2 = NULL;
EC_POINT *pointc;
EC_GROUP *group2;
unsigned char pubkey2[ECDH_SIZE];
unsigned char shared2[ECDH_SIZE];
int len2;
//Generate Public
ecdh2 = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);//NID_secp521r1
EC_KEY_generate_key(ecdh2);
point2 = EC_KEY_get0_public_key(ecdh2);
group2 = EC_KEY_get0_group(ecdh2);
if(0 == (len2 = EC_POINT_point2oct(group2, point2, POINT_CONVERSION_COMPRESSED, pubkey2, ECDH_SIZE, NULL))) handleErrors();
printf("len2=%d\n",len);
/* alice */
//ComputeKey
point2c = EC_POINT_new(group);
EC_POINT_oct2point(group, point2c, pubkey2, ECDH_SIZE, NULL);
if (0 != EC_POINT_cmp(group, point2, point2c, NULL)) handleErrors();
if(0 == (len = ECDH_compute_key(shared, ECDH_SIZE, point2c, ecdh, NULL))) handleErrors();
printf("len=%d\n",len);
disp("shared", shared, len);
/* bob */
//ComputeKey
pointc = EC_POINT_new(group2);
EC_POINT_oct2point(group2, pointc, pubkey, ECDH_SIZE, NULL);
if (0 != EC_POINT_cmp(group2, point, pointc, NULL)) handleErrors();
if(0 == (len2 = ECDH_compute_key(shared2, ECDH_SIZE, pointc, ecdh2, NULL))) handleErrors();
printf("len2=%d\n",len2);
disp("shared2", shared2, len2);
/* alice */
EC_POINT_free(pointc);
EC_KEY_free(ecdh);
/* bob */
EC_POINT_free(point2c);
EC_KEY_free(ecdh2);
printf("To the end\n");
return 0;
}
之后抽象、整理封装后的ECDH代码：
#include <openssl/ssl.h>
#define ECDH_SIZE 33
void handleErrors()
{
printf("Error occurred.\n");
}
static void disp(const char *str, const void *pbuf, const int size)
{
int i=0;
if(str != NULL){
printf("%s:\n", str);
}
if(pbuf != NULL && size > 0){
for(i=0;i<size;i++)
printf("%02x ", *((unsigned char *)pbuf+i));
putchar('\n');
}
putchar('\n');
}
static EC_KEY *genECDHtemppubkey(unsigned char *pubkey)
{
int len;
EC_KEY *ecdh = EC_KEY_new();
//Generate Public
ecdh = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);//NID_secp521r1
EC_KEY_generate_key(ecdh);
const EC_POINT *point = EC_KEY_get0_public_key(ecdh);
const EC_GROUP *group = EC_KEY_get0_group(ecdh);
//unsigned char *pubkey = malloc(ECDH_SIZE);
if(0 == (len = EC_POINT_point2oct(group, point, POINT_CONVERSION_COMPRESSED, pubkey, ECDH_SIZE, NULL))) handleErrors();
printf("len=%d\n",len);
//return pubkey;
return ecdh;
}
static unsigned char *genECDHsharedsecret(EC_KEY *ecdh, unsigned char *peerkey, size_t secret_len)
{
int len;
unsigned char *shared = malloc(ECDH_SIZE);
const EC_GROUP *group = EC_KEY_get0_group(ecdh);
//ComputeKey
EC_POINT *point_peer = EC_POINT_new(group);
EC_POINT_oct2point(group, point_peer, peerkey, ECDH_SIZE, NULL);
//if (0 != EC_POINT_cmp(group, point2, point2c, NULL)) handleErrors();
if(0 == (len = ECDH_compute_key(shared, secret_len, point_peer, ecdh, NULL))) handleErrors();
printf("len=%d\n",len);
disp("shared", shared, secret_len);
return shared;
}
int main() {
unsigned char *keydata = malloc(ECDH_SIZE);
unsigned char *keydata2 = malloc(ECDH_SIZE);
EC_KEY *ecdh = genECDHtemppubkey(keydata);
EC_KEY *ecdh2 = genECDHtemppubkey(keydata2);
unsigned char *ECDH_keydata = genECDHsharedsecret(ecdh2, keydata, ECDH_SIZE-1);
unsigned char *ECDH_keydata2 = genECDHsharedsecret(ecdh, keydata2, ECDH_SIZE-1);
printf("To the end\n");
free(keydata);
free(keydata2);
EC_KEY_free(ecdh);
EC_KEY_free(ecdh2);
free(ECDH_keydata);
free(ECDH_keydata2);
return 0;
}
调试时在“ECDH_SIZE-1”的问题上纠结了好一阵子，不小心把EC_POINT_oct2point中参数ECDH_SIZE换成值为ECDH_SIZE-1的secret_len了；导致ECDH_compute_key一直返回-1，怎么搜索都得不到解决。
最后还是耐心的一步步调试、对比前面参考的源码、调整变量数值，终于解决。编译通过，执行的非常好。
耐心真的是非常重要，失之毫厘差之千里。
参考资料：
ECC加密算法原理入门介绍
http://blog.csdn.net/sahusoft/article/details/6868016