Hey Matt,
I'm using openssl-1.0.2j to do this.
After a lot of debugging and poking around, I realized that my initial thoughts were not quite correct. Somewhere in trying to come up with an example, I noticed that the problem actually appears to be the buffer I am passing in..
If I use a char array, it will fail to derive the shared secret, if I use a char* it will succeed. I'm not quite sure what is going on there, but I have some code below that shows the issue:
static int deriveSharedSecret( EVP_PKEY *pkey, EVP_PKEY *peerkey,
char *sharedSecretHex ){
/*
* Generalized function to derive shared secret and return the hex format of it.
*/
unsigned char sharedSecret[ 4096 ] = {0};
size_t sharedSecretLen = 0;
// Now derive the Shared Secret
EVP_PKEY_CTX *ctx;
ctx = EVP_PKEY_CTX_new(pkey, NULL);
if (!ctx){
fprintf( stderr, "Failed to make EVP_PKEY ctx\n" );
ERR_load_crypto_strings();
ERR_print_errors(BIO_new_fp(stderr, BIO_NOCLOSE));
return 0;
}
if (EVP_PKEY_derive_init(ctx) <= 0){
fprintf( stderr, "Failed to init EVP_PKEY ctx\n" );
ERR_load_crypto_strings();
ERR_print_errors(BIO_new_fp(stderr, BIO_NOCLOSE));
return 0;
}
if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0) {
fprintf( stderr, "Failed to set EVP_PKEY peer\n" );
ERR_load_crypto_strings();
ERR_print_errors(BIO_new_fp(stderr, BIO_NOCLOSE));
return 0;
}
int secretLen = 2048;
unsigned char *skey;
/* Create the buffer */
if(NULL == (skey = OPENSSL_malloc(secretLen))){
fprintf( stderr, "Failed to malloc buffer for secret\n" );
return;
}
/* Derive the shared secret */
if(1 != (EVP_PKEY_derive(ctx, skey, &secretLen))){
fprintf( stderr, "Failed to derive secret and place into buffer\n" );
return;
} else {
fprintf( stderr, "Found the darn secret!\n" );
}
if (EVP_PKEY_derive(ctx, sharedSecret, &sharedSecretLen) <= 0){
fprintf( stderr, "Failed to derive shared secret\n" );
ERR_load_crypto_strings();
ERR_print_errors(BIO_new_fp(stderr, BIO_NOCLOSE));
return 0;
} else {
fprintf( stderr, "FOUND IT!!!!!\n" );
}
bin2hex( sharedSecret, sharedSecretHex, sharedSecretLen );
return 1;
}
On Fri, Mar 10, 2017 at 1:44 PM, Matt Caswell <matt@xxxxxxxxxxx> wrote:
Which version of OpenSSL are you using?
On 10/03/17 20:58, Ethan Rahn wrote:
> Hello Openssl-users,
>
> I'm trying to write some code that derives the shared secret for 2
> elliptic curve keys ( i.e. does ECDH )
>
> I am doing the following to load up both the local and remote EC key (
> code shown for local side ):
>
> EC_KEY* localEC = EC_KEY_new_by_curve_name( curveName );
> EC_KEY_set_private_key( localEC, privateKeyLocal )
> EC_KEY_set_public_key_affine_coordinates( localEC, publicXCoordLocal,
> publicYCoordLocal )
>
> I check the return values for all of these, as well as EC_KEY_check_key
> at the end. Everything returns non-zero, so I assume that it is good to
> go. I then do the following to turn the EC_KEY into an EVP_PKEY for ECDH:
>
> pkey = EVP_PKEY_new();
> EVP_PKEY_set1_EC_KEY( *pkey, localEC );
>
> The same is done for the remote EC, except that the private key is not
> loaded up.
>
> Now this is where things get weird.
>
> I run code pretty similar to the example given here ( starting from
> EVP_PKEY_CTX_new() since I already have the pkey and peerkey. (
> https://wiki.openssl.org/index.php/Elliptic_Curve_ ) andDiffie_Hellman
> it fails on the call to EVP_PKEY_derive()without an error message. I
> tried running into under gdb() and it gets to ecdh_check() before it's
> unable to fill in the ecdh_data structure, i.e. it returns it as NULL.
>
> If I use the example code to generate the local EVP_PKEY with a random
> set of points on the correct curve, then run the following line, the key
> derivation will work with the parameters I read in:
> ( in this example, pkey is as in the example code, i.e. generated
> randomly. pkey2 is the one I made via EVP_PKEY_set1_EC_KEY )
>
> EVP_PKEY_set1_EC_KEY( pkey, EVP_PKEY_get1_EC_KEY( pkey2 ) );
>
> It would appear that there is something that EVP_PKEY_set1_EC_KEY is not
> setting, or perhaps that I need to add, but I'm unclear what that would
> be. Does anyone on this list have any ideas?
Can you provide a simple reproducer of the problem?
Matt
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