Compute EC_KEY starting from X or Y coordinate only
Luca Di Mauro
luca.dimauro at cnit.it
Fri Oct 25 09:43:04 UTC 2019
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.
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
> 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
>> 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?
>> 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
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