(MFC) 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.

Chilkat C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

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

void ChilkatSample(void)
    {
    CkString strOut;

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkCrypt2 crypt;

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

    // 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.
    const char *hashStr = crypt.hashFileENC("hamlet.xml");

    CkDsa dsa;

    // 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 = 0;
    pemPrivateKey = dsa.loadText("dsa_priv.pem");
    bool 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 = 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 = 0;
    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());

    }