ECC wrong public key is generated

RE: ECC wrong public key is generated

Maxim — Fri, 24 Feb 2017 09:47:15 GMT

Hi, i'm agree with you, even the type uint64_t is limited to 18446744073709551615. The easiest way is to calculate this is to use only hexadecimal but if you really want to use decimal you must divide the Hexadecimal result in 8 bytes parts (uint64_t) and print them apart. I've implemented this solution with hexadecimal in my projet and it works really well ! In addition you can check your result with an hexa to decimal converter like this link text and comparte the key generated from the ECDH website you given.

RE: ECC wrong public key is generated

Shiroyama — Fri, 24 Feb 2017 01:16:08 GMT

Hello, i face the same problem to calculate to public key. I use the website you given to calculate the key, but the input and output should be displayed in decimalism. How can i switch my key from hexadecimal to decimalism? The number is so large that my calculator not working. Could you tell me the method to calculate it?Thank you !

RE: ECC wrong public key is generated

Maxim — Mon, 14 Nov 2016 14:28:39 GMT

Yeah, it Works but it's a bit difficult !!! But i've found an easier version, i've reverse the private key AND the public key to obtain the same result from this site but we must compute key shared key with no inversion on the public and private key. I've use this function :

static void reverse(uint8_t* p_arr, uint32_t len) // inverse Big-Endian

{ uint32_t i;

for (i = 0; i < len/2 ; i++)
{
    uint8_t tmp = p_arr[i];
    p_arr[i] = p_arr[len - i - 1];
    p_arr[len - i - 1] = tmp;
}

}

thanks you very much !

RE: ECC wrong public key is generated

syntroniks — Mon, 14 Nov 2016 13:50:38 GMT

Hey Maxim, check out page 16 and reverse the byte order

RE: ECC wrong public key is generated

Maxim — Mon, 14 Nov 2016 09:07:30 GMT

I think the problem came from the definition of the function, in the file curve-specific.inc (in micro-ecc) the curve secp255r1 is defined by:

P= FF, FF, FF, FF, FF, FF, FF, FF FF, FF, FF, FF, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 00, 01, 00, 00, 00, FF, FF, FF, FF B= 51, 25, 63, FC, C2, CA, B9, F3 84, 9E, 17, A7, AD, FA, E6, BC, FF, FF, FF, FF, FF, FF, FF, FF, 00, 00, 00, 00, FF, FF, FF, FF G= 96, C2, 98, D8, 45, 39, A1, F4, A0, 33, EB, 2D, 81, 7D, 03, 77 ,F2, 40, A4, 63, E5, E6, BC, F8, 47, 42, 2C, E1, F2, D1, 17, 6B F5, 51, BF, 37, 68, 40, B6, CB, CE, 5E, 31, 6B, 57, 33, CE, 2B,16, 9E, 0F, 7C, 4A, EB, E7, 8E, 9B, 7F, 1A, FE, E2, 42, E3, 4F N= 4B, 60, D2, 27, 3E, 3C, CE, 3B, F6, B0, 53, CC, B0, 06, 1D, 65, BC, 86, 98, 76, 55, BD, EB, B3, E7, 93, 3A, AA, D8, 35, C6, 5A

But these parameters are different from the curve that i've implemeted in android defined here Page15

RE: ECC wrong public key is generated

FormerMember — Mon, 14 Nov 2016 08:10:36 GMT

A public key will not be a random number, it will be generated based on the private key, using a function that "doesn't have" an inverse function.

RE: ECC wrong public key is generated

Maxim — Fri, 11 Nov 2016 14:40:50 GMT

I've already try this method, when I reverse the private key the result is different of what I'm expecting. But I'm now sure that the calculation are correct in the uECC because if i'm generate 2 public keys from 2 different private keys , the calculation of the 2 shared key are the same. So the generated public keys are not random numbers. Thanks for your answer !

RE: ECC wrong public key is generated

FormerMember — Fri, 11 Nov 2016 12:31:03 GMT

It is also possible to use nrfutil to generate keys, here

The generated keys from nrfutil can be formatted directly in c-code/style.

RE: ECC wrong public key is generated

syntroniks — Thu, 10 Nov 2016 15:17:20 GMT

First: Do you have uECC_VLI_NATIVE_LITTLE_ENDIAN=1 defined for your ecc library generation?

Second: Try reversing your private key. (last byte becomes 0x11, first byte becomes 0xDF)