我正在使用OpenSSL的c库生成椭圆曲线Diffie-Hellman（ECDH）密钥对，按照此处的第一个代码示例 here。它在实际公钥交换方面稍作掩盖：
peerkey = get_peerkey(pkey);
pkey变量和返回值都是EVP *类型。 pkey包含之前生成的公钥、私钥和参数，而返回值仅包含对等方的公钥。因此有三个问题:
pkey中提取仅用于发送到对等方的公钥？pKey中提取私钥和参数以便在密钥交换后存储它们？EVP_PKEY结构？我看到了OpenSSL函数EVP_PKEY_print_public()、EVP_PKEY_print_private()和EVP_PKEY_print_params()，但这些函数用于生成人类可读输出。我没有找到任何将人类可读的公钥转换回EVP_PKEY结构的等效方法。
// We assume the public and private keys have been already generated.
// EVP_PKEY* keyPair...
// Get the serialized public key length.
size_t serializedPublicKeyLen = 0;
if (EVP_PKEY_get_octet_string_param(keyPair, OSSL_PKEY_PARAM_PUB_KEY,
NULL, 0, &serializedPublicKeyLen) != 1) {
return;
}
// Allocate memory for the serialized public key.
unsigned char* serializedPublicKey = (unsigned char*)OPENSSL_malloc(serializedPublicKeyLen);
if (serializedPublicKey == NULL) {
return;
}
// Get the serialized public key.
if (EVP_PKEY_get_octet_string_param(keyPair, OSSL_PKEY_PARAM_PUB_KEY,
serializedPublicKey, serializedPublicKeyLen, &serializedPublicKeyLen) != 1) {
return;
}
// Deallocate the memory when you finish using the serialized public key.
OPENSSL_free(serializedPublicKey);
// A parameter build for the public key.
OSSL_PARAM_BLD* paramBuild = OSSL_PARAM_BLD_new();
if (paramBuild == NULL) {
return;
}
// This is just an example. Set the curve
// you used to generate the public and private keys.
const char curveName[] = "secp384r1";
// Set the curve name to the parameter build.
if (OSSL_PARAM_BLD_push_utf8_string(paramBuild,
OSSL_PKEY_PARAM_GROUP_NAME, curveName, 0) != 1) {
OSSL_PARAM_BLD_free(paramBuild);
return;
}
// Set the serialized public key.
if (OSSL_PARAM_BLD_push_octet_string(paramBuild, OSSL_PKEY_PARAM_PUB_KEY,
serializedPublicKey, serializedPublicKeyLen) != 1) {
OSSL_PARAM_BLD_free(paramBuild);
return;
}
// Convert the OSSL_PARAM_BLD to an OSSL_PARAM.
OSSL_PARAM* params = OSSL_PARAM_BLD_to_param(paramBuild);
if (params == NULL) {
OSSL_PARAM_BLD_free(paramBuild);
return;
}
// Create a EVP_PKEY context.
EVP_PKEY_CTX* publicKeyCtx = EVP_PKEY_CTX_new_from_name(NULL, "EC", NULL);
if (publicKeyCtx == NULL) {
OSSL_PARAM_BLD_free(paramBuild);
OSSL_PARAM_free(params);
return;
}
// Initialize the EVP_PKEY context.
if (EVP_PKEY_fromdata_init(publicKeyCtx) <= 0) {
OSSL_PARAM_BLD_free(paramBuild);
OSSL_PARAM_free(params);
EVP_PKEY_CTX_free(publicKeyCtx);
return;
}
// Create the peer public key object.
EVP_PKEY* publicKey = NULL;
if (EVP_PKEY_fromdata(publicKeyCtx, &publicKey,
EVP_PKEY_PUBLIC_KEY, params) <= 0) {
OSSL_PARAM_BLD_free(paramBuild);
OSSL_PARAM_free(params);
EVP_PKEY_CTX_free(publicKeyCtx);
return;
}
// Free auxiliary things...
OSSL_PARAM_BLD_free(paramBuild);
OSSL_PARAM_free(params);
EVP_PKEY_CTX_free(publicKeyCtx);
// Now you can use publicKey for EVP_PKEY_derive_set_peer.
// Call EVP_PKEY_free when you finish using it.
BIGNUM* privateKey = NULL;
EVP_PKEY_get_bn_param(keyPair, OSSL_PKEY_PARAM_PRIV_KEY, &privateKey);
openssl / evp.h 具有 i2d_PublicKey（）和 d2i_PublicKey（）函数，分别用于将公钥转换为二进制表示形式以及从二进制表示形式转换回公钥（私钥也有相应的函数-请参见：https://www.openssl.org/docs/manmaster/man3/d2i_PublicKey.html）。vector<unsigned char> ecdhPubkeyData(EVP_PKEY *key)
{
int len = i2d_PublicKey(key, 0); // with 0 as second arg it gives length
vector<unsigned char> ret(len);
unsigned char *ptr = ret.data();
len = i2d_PublicKey(key, &ptr);
return ret;
}
// Make sure you free the returned pointer when you are done with it
EVP_PKEY *ecdhPubkeyFromData(vector <unsigned char> const &pubkeyData)
{ // You do need to put in in an existing EVP_PKEY that is assigned
// an EC_KEY, because it needs to know what curve you use
EC_KEY *ec_key = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
EVP_PKEY *ret = EVP_PKEY_new();
EVP_PKEY_assign_EC_KEY(ret, ec_key);
unsigned char const *ptr = pubkeyData.data();
d2i_PublicKey(EVP_PKEY_EC, &ret, &ptr, pubkeyData.size());
return ret;
}
// PS: In a real example you want to check if any of these functions
// return NULL or some error code
我正在使用C++向量来包含二进制数据，但当然你也可以使用C风格的数组 :-)
我绝对不是OpenSSL专家，所以如果我在这个实现中做错了什么，请告诉我 :-p