(C++) ECDSA Sign and Verify

Demonstrates how to create an ECDSA signature on the SHA256 hash of some data, and then verify.

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#include <CkPrivateKey.h>
#include <CkBinData.h>
#include <CkCrypt2.h>
#include <CkEcc.h>
#include <CkPrng.h>
#include <CkAsn.h>
#include <CkXml.h>
#include <CkPublicKey.h>

void ChilkatSample(void)
    {
    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // First load an ECDSA private key to be used for signing.
    CkPrivateKey privKey;
    bool success = privKey.LoadEncryptedPemFile("qa_data/ecc/secp256r1-key-pkcs8-secret.pem","secret");
    if (success == false) {
        std::cout << privKey.lastErrorText() << "\r\n";
        return;
    }

    // Sign the SHA256 hash of some data.
    CkBinData bd;
    success = bd.LoadFile("qa_data/hamlet.xml");
    if (success == false) {
        std::cout << "Failed to load file to be hashed." << "\r\n";
        return;
    }

    CkCrypt2 crypt;
    crypt.put_HashAlgorithm("sha256");
    crypt.put_EncodingMode("base64");
    const char *hashStr = crypt.hashBdENC(bd);

    CkEcc ecdsa;
    CkPrng prng;
    // Returns ASN.1 signature as a base64 string.
    const char *sig = ecdsa.signHashENC(hashStr,"base64",privKey,prng);
    std::cout << "sig = " << sig << "\r\n";

    // The signature is in ASN.1 format (which may be described as the "encoded DSS signature").
    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
    //   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...

    // If you wish, you can get the r and s components of the signature like this:
    CkAsn asn;
    asn.LoadEncoded(sig,"base64");
    CkXml xml;
    xml.LoadXml(asn.asnToXml());

    std::cout << xml.getXml() << "\r\n";

    // We now have this:
    // <?xml version="1.0" encoding="utf-8"?>
    // <sequence>
    //     <int>6650D422D86BA4A228B5617604E59052591B9B2C32EF324C44D09EF67E5F0060</int>
    //     <int>0CFD9F6AC85042FC70F672C141BA6B2A4CAFBB906C3D907BCCC1BED62B28326F</int>
    // </sequence>

    // Get the "r" and "s" as hex strings
    const char *r = xml.getChildContentByIndex(0);
    const char *s = xml.getChildContentByIndex(1);

    std::cout << "r = " << r << "\r\n";
    std::cout << "s = " << s << "\r\n";

    // --------------------------------------------------------------------
    // Now verify against the hash of the original data.

    // Get the corresponding public key.
    CkPublicKey pubKey;
    success = pubKey.LoadFromFile("qa_data/ecc/secp256r1-pub.pem");
    if (success == false) {
        std::cout << pubKey.lastErrorText() << "\r\n";
        return;
    }

    // We already have the SHA256 hash of the original data (hashStr) so no need to re-do it..
    CkEcc ecc2;
    int result = ecc2.VerifyHashENC(hashStr,sig,"base64",pubKey);
    if (result != 1) {
        std::cout << ecc2.lastErrorText() << "\r\n";
        return;
    }

    std::cout << "Verified!" << "\r\n";

    // Note: If we have only r,s and wish to reconstruct the ASN.1 signature, we do it like this:
    CkXml xml2;
    xml2.put_Tag("sequence");
    xml2.NewChild2("int",r);
    xml2.NewChild2("int",s);

    CkAsn asn2;
    asn2.LoadAsnXml(xml2.getXml());
    const char *encodedSig = asn2.getEncodedDer("base64");
    std::cout << "encoded DSS signature: " << encodedSig << "\r\n";

    // You can go to https://lapo.it/asn1js/  and copy/paste the base64 encodedSig into the online tool, then press the "decode" button.
    // You will see the ASN.1 such as this:

    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
    //   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...
    }