Compute EC_KEY starting from X or Y coordinate only

[Luca Di Mauro]

Thank you! I thought they were the same.

And given an x-only coordinate, how can I find the y coordinate? I  
don't find the relative functions on the documentation.

Luca

Billy Brumley <bbrumley at gmail.com> ha scritto:

> Don't do that. As I said, the library is trying to tell you that's not a
> point on the secp256k1 curve.
>
> Quickly browsing the standard, you are likely looking for the prime256v1
> curve.
>
> BBB
>
> On Fri, 25 Oct 2019, 9.28 Luca Di Mauro, <luca.dimauro at cnit.it> wrote:
>
>> I think it is correct because I extracted the hexadecimal string from
>> a packet contained in a pcap.
>>
>> This compressed point is created following the ETSI TS 103 097 v1.3.1
>> standard for secured communications in the vehicular communication
>> context
>> (
>> https://www.etsi.org/deliver/etsi_ts/103000_103099/103097/01.03.01_60/ts_103097v010301p.pdf
>> ).
>>
>> I notice that the point 'pubPoint' that I created is empty when I try
>> to call 'EC_POINT_set_compressed_coordinates_GFp' function. How can I
>> put a BIGNUM into an EC_POINT?
>>
>> Luca
>>
>> Billy Brumley <bbrumley at gmail.com> ha scritto:
>>
>> >>         EC_GROUP* group         = EC_GROUP_new_by_curve_name
>> >> (NID_secp256k1);
>> >
>> >> "c16b4ce0532f5dc9d09114fe121d3956ae84f9eb677a0d4bdac1d3af7a91950c";
>> >
>> > I don't believe there's a point on secp256k1 with that x-coordinate.
>> > If you check the failure reason for
>> > EC_POINT_set_compressed_coordinates_GFp in the debugger, that is
>> > probably what it is telling you.
>> >
>> > Where did this curve / x-coord pair come from?
>> >
>> > BBB
>>
>>
>>
>>