[openssl-users] GCM

[Matt Caswell]

On 09/01/15 19:42, Amir Reda wrote:
> dear all
> i'm trying to use AES-GCM model for encryption i use a sample code for that
> 
> and my problem is   
> 
>  ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
> 
> ret all the time is 0 this means that
> 
> the plaintext is not trustworthy.
> 
> encryption function
> 

<snip>

>      //get the tag
>      EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, 16, tag);

You should check the return value of this function here.

You also haven't free'd up your ctx.


<snip>
>     //add the tag
>     if (!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, 16, tag))

This is wrong. You need to use EVP_CTRL_GCM_SET_TAG.

>      {
>          cout<<"success adding tag"<<endl;
>      }
>      else
>      {
>          cout<<"something wrong"<<endl;
>      }
>     //finalize the Decryption
>     int ret = 1;
>     ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
> 
>     cout<<" ret value is "<<ret<<endl;
> 
>     if (ret > 0)
>      {
>          cout<<"success final decryption"<<endl;
>          plaintext_len += len;
>          cout<<"palin text is "<<plaintext_len<<endl;
>          return plaintext_len;
> 
>      }
>      else
>      {
>          cout<<"decrypt fail"<<endl;
>          return -1;
>      }
>     return ret;

Again you need to clean up your ctx.


Matt