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Hi I am right now want to do a make a small tool to generate some test vector for ecdsa. Openssl is quite good, for support secp256k1 bitcoin provides an excellent library.

However, it looks what I get from secp256k1 API only the compressed key. I find some post about how to convert the compressed key (Qx) to uncompressed key (Qx,Qy). But I just wondering if I can directly get the uncompressed key?

Thanks!

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Use secp256k1_ec_pubkey_parse and secp256k1_ec_pubkey_serialize:

secp256k1_context *ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
secp256k1_pubkey pubkey;

const unsigned char input[33] = {0x02,0x8c,0x28,0xa9,0x7b,0xf8,0x29,0x8b,0xc0,0xd2,0x3d,0x8c,0x74,0x94,0x52,0xa3,0x2e,0x69,0x4b,0x65,0xe3,0x0a,0x94,0x72,0xa3,0x95,0x4a,0xb3,0x0f,0xe5,0x32,0x4c,0xaa};

if (secp256k1_ec_pubkey_parse(ctx, &pubkey, input, 33) != 1) {
    puts("Public key not on curve");
    secp256k1_context_destroy(ctx);
    return 1;
}

unsigned char outpubkey[65];
size_t written_bytes = 65;
secp256k1_ec_pubkey_serialize(ctx, outpubkey, &written_bytes, &pubkey, SECP256K1_EC_UNCOMPRESSED);

printf("Uncompress public key: ");

for (int i = 0; i < 65; i++) {
    printf("%.2x", (unsigned int) outpubkey[i]);
}

puts("");

secp256k1_context_destroy(ctx);
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  • thanks a lot for the suggestion. I find the similar way, firstly use the secp256k1_ec_pubkey_create() to get the pub key, then using secp256k1_ec_pubkey_serialize to form the either compresss or uncompressed key. – LeonMSH Apr 27 '20 at 2:37
  • by the way, for secp256k1 library, in linker stage, what is the -l flag to be used--I find someone metioned to use -lsecp256k1 but it does not work for me? I can compile the program by using some CMake but it is too heavy and what I need is just a small tool--so I want to make a small make file, then I meet warning as keygen.cpp:(.text+0x558): undefined reference to `secp256k1_ecdsa_signature_serialize_compact' – LeonMSH Apr 27 '20 at 2:57
  • Did you run make install in the libsecp256k1 folder? @LeonMSH – MCCCS Apr 27 '20 at 7:36
  • oh yes, that solves my problem! thanks so much!! – LeonMSH Apr 28 '20 at 5:57

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