Unable to set IV for aes gcm using openssl

Question

I am trying to use AES GCM encryption mechanism provided by OpenSSL in C++ and using example on this link as reference: https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption

However, following statement gives me error:

/* Set IV length if default 12 bytes (96 bits) is not appropriate */
if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
    handleErrors();

The error that I get is:

error: ‘EVP_CTRL_GCM_SET_IVLEN’ was not declared in this scope".

I do not understand, why I cannot set IVLEN to 16 bytes? I do not want to use the default value of 12 bytes. Any pointers will be great.

I apologize for not being clear in my question. I was getting following error: "error: ‘EVP_CTRL_GCM_SET_IVLEN’ was not declared in this scope" — Riddhi Rathod, Aug 23 '16 at 13:38

score 1 · Answer 1 · edited Apr 25 '18 at 19:11

I resolved the error. Actually, in the example code, the order for initializing encryption operation and setting IV length is as follows:

* Initialise the encryption operation. */
if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL))
    handleErrors();

/* Set IV length if default 12 bytes (96 bits) is not appropriate */
if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
    handleErrors();

I had reverse order for these statements i.e. setting IV length first and then initializing encryption operation. I thought, these were independent steps and the order didn't matter. But, maybe, the interface needs to know, which encryption mechanism it has use before setting any parameters.

Unable to set IV for aes gcm using openssl

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