DSA Signature Create and Verify

Shows how to create a DSA (DSS) signature for the contents of a file. The first step is to create an SHA-1 hash of the file contents. The hash is signed using the Digital Signature Algorithm and the signature bytes are retrieved as a hex-encoded string.

The 2nd part of the example loads the signature and verifies it against the hash.

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// Needs #include <CkCrypt2.h>
// Needs #include <CkDsa.h>

    CkString strOut;

    bool success;

    //  Use Chilkat Crypt to hash the contents of a file.
    CkCrypt2 crypt;
    success = crypt.UnlockComponent("Anything for 30-day trial.");
    if (success != true) {
        strOut.append(crypt.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    crypt.put_EncodingMode("hex");
    crypt.put_HashAlgorithm("sha-1");

    const char * hashStr;

    //  Return the SHA-1 hash of a file.  The file may be any size.
    //  The Chilkat Crypt component will stream the file when
    //  computing the hash, keeping the memory usage constant
    //  and reasonable.
    //  The 20-byte SHA-1 hash is returned as a hex-encoded string.
    hashStr = crypt.hashFileENC("hamlet.xml");

    CkDsa dsa;

    //  The Chilkat Crypt and Chilkat DSA components are separate
    //  products.  To license both, it's least expensive to purchase
    //  the "Chilkat Bundle" which provides licenses to all the
    //  Chilkat components.
    success = dsa.UnlockComponent("Anything for 30-day trial");
    if (success != true) {
        strOut.append(dsa.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  Load a DSA private key from a PEM file.  Chilkat DSA
    //  provides the ability to load and save DSA public and private
    //  keys from encrypted or non-encrypted PEM or DER.
    //  The LoadText method is for convenience only.  You may
    //  use any means to load the contents of a PEM file into
    //  a string.
    const char * pemPrivateKey;
    pemPrivateKey = dsa.loadText("dsa_priv.pem");
    success = dsa.FromPem(pemPrivateKey);
    if (success != true) {
        strOut.append(dsa.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  You may optionally verify the key to ensure that it is a valid
    //  DSA key.
    success = dsa.VerifyKey();
    if (success != true) {
        strOut.append(dsa.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  Load the hash to be signed into the DSA object:
    success = dsa.SetEncodedHash("hex",hashStr);
    if (success != true) {
        strOut.append(dsa.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  Now that the DSA object contains both the private key and hash,
    //  it is ready to create the signature:
    success = dsa.SignHash();
    if (success != true) {
        strOut.append(dsa.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  If SignHash is successful, the DSA object contains the
    //  signature.  It may be accessed as a hex or base64 encoded
    //  string.  (It is also possible to access directly in byte array form via
    //  the "Signature" property.)
    const char * hexSig;
    hexSig = dsa.getEncodedSignature("hex");
    strOut.append("Signature:");
    strOut.append("\r\n");
    strOut.append(hexSig);
    strOut.append("\r\n");

    //  -----------------------------------------------------------
    //  Step 2: Verify the DSA Signature
    //  -----------------------------------------------------------

    CkDsa dsa2;

    //  Load the DSA public key to be used for verification:
    const char * pemPublicKey;
    pemPublicKey = dsa2.loadText("dsa_pub.pem");
    success = dsa2.FromPublicPem(pemPublicKey);
    if (success != true) {
        strOut.append(dsa2.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  Load the hash to be verified against the signature.
    success = dsa2.SetEncodedHash("hex",hashStr);
    if (success != true) {
        strOut.append(dsa2.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  Load the signature:
    success = dsa2.SetEncodedSignature("hex",hexSig);
    if (success != true) {
        strOut.append(dsa2.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    //  Verify:
    success = dsa2.Verify();
    if (success != true) {
        strOut.append(dsa2.lastErrorText());
        strOut.append("\r\n");
    }
    else {
        strOut.append("DSA Signature Verified!");
        strOut.append("\r\n");
    }


    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());