Group Key Agreement using OpenSSL

[Waleed1031]

Hi. I am working on Group Key Agreement using openssl. I am a bit weak in programming. So, needed a help. The issue is that i am deriving shared secret using following code.

`/* Generate shared secret /
EVP_PKEY_CTX ctx;
unsigned char* skey;
size_t skeylen;
ctx = EVP_PKEY_CTX_new(pkey, NULL);

	if (!ctx) {
		/* Error */
		printf("CTX is empty");
	}

	if (EVP_PKEY_derive_init(ctx) <= 0) {
		/* Error */
		printf("EVP derive initialization failed\n");
	}

	if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0) {
		/* Error */
		printf("EVP derive set peer failed\n");
	}

	/* Determine buffer length */
	if (EVP_PKEY_derive(ctx, NULL, &skeylen) <= 0) {
		/* Error */
		printf("EVP derive failed\n");
	}
	skey = OPENSSL_malloc(skeylen);

	if (!skey) {
		/* Malloc failure */
		printf("OpenSSL Malloc failed");
	}

	if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0) {
		/* Error */
		printf("Shared key derivation failed");
	}
	printf("\nShared secret:\n");

	for (size_t i = 0; i < skeylen; i++) {
		printf("%02x", skey[i]);
	}

`

Now, i want to save this skey generated to some EVP structure so that i can use it as a peerkey while deriving shared secret with another client's keyshare. It is in raw form. i need a snippet which saves it in an EVP structure and save it in a file inn PEM format so that it can be used afterwards. I tried EVP_write_PrivateKey or Pubkey functions but they extract the private or public key from the context. An assistance will be appreciated. For clarification, i am using X25519 curve and ECDH for key agreement.

Thank you in advance.

[Kazuki Yamaguchi]

Is this a question regarding OpenSSL's C API? This repository handles the openssl library for Ruby.

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