View source for EVP Asymmetric Encryption and Decryption of an Envelope

==Opening and Envelope==
An envelope is opened using the EVP_Open* set of functions in the following steps:
* Initialise the context
* Initialise the open operation, providing the symmetric cipher that has been used, along with the private key to decrypt the session key with
* Provide the message to be decrypted and decrypt using the session key
* Complete the decryption operation

See the following code for an example:
<pre>
int envelope_open(EVP_PKEY *priv_key, unsigned char *ciphertext, int ciphertext_len,
	unsigned char *encrypted_key, int encrypted_key_len, 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. The asymmetric private key is
	 * provided and priv_key, whilst the encrypted session key is held in
	 * encrypted_key */
	if(1 != EVP_OpenInit(ctx, EVP_aes_256_cbc(), encrypted_key,
		encrypted_key_len, iv, priv_key))
		handleErrors();

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

	/* Clean up */
	EVP_CIPHER_CTX_free(ctx);

	return plaintext_len;
}
</pre>