The semantics of EC_KEY_generate_key are to generate a new private and
public key pair. It does this regardless of whether or not a private or
public key was previously set in the EC_KEY structure. It *will* reuse
the existing BIGNUM/EC_POINT structures if they were previously set -
but it just overwrites them with the new private/public key.
The semantics of BN_rand_range(priv_key, order) are to find a random
number between 0 and order and store it in priv_key. It will just
overwrite priv_key with the new value - the floor is always 0.
If you want to set your own private key then you should set it using
EC_KEY_set_private_key. You then need to multiply the generator point by
the private key to calculate the public key using:
EC_POINT_mul
<http://openssl.sourcearchive.com/documentation/1.0.0d-2/crypto_2ec_2ec_8h_a5e5532068774ce6da8ca68ac627176a8.html#a5e5532068774ce6da8ca68ac627176a8>(group,
pub_key, priv_key, NULL, NULL, ctx)
Finally, set the public key using EC_KEY_set_public_key.
Hope that helps.
Matt
On 06/01/12 20:35, Bobby Beckmann wrote:
Hi,
I'm implementing a library that requires me to generate a public key given a
pre-determined private key. I'm creating an EC_KEY structure, and setting the
private key.
At first, I thought I could use EC_KEY_generate_key, since it doesn't overwrite
an existing private key
From just glancing at the code, my expectation would be that if I passed this
into EC_KEY_generate_key that it would use the existing private key and
calculate the public key, since it doesn't overwrite the private_key if it's
been set.
However, a few lines down, I noticed that it's calling: BN_rand_range(priv_key,
order), which then overwrites the priv_key with the random number from the
function. This guarantees that a new private/public key pair will be generated
every time EC_KEY_generate_key is called.
So, my questions are:
1) Is this correct? It seems to me that the BN_rand_range should only be called
if it's a new private_key. If not, then if you continuously call
EC_KEY_generate_key with the same EC_KEY, you'll end up narrowing the range for
the random number until priv_key == order. That doesn't seem right to me. It
seems that either it should grab the random number if it's a new BN or the
floor of the range should be 0.
2) Is there a different OpenSSL EC function that I should be using. I just
wrote my own function which skips the BN_rand_range and just does the point
calculation to get my public key. This seems to work fine.
Thanks,
Bobby
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