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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bool success;

//  Use Chilkat Crypt to hash the contents of a file.
Chilkat.Crypt2 crypt = new Chilkat.Crypt2();
success = crypt.UnlockComponent("Anything for 30-day trial.");
if (success != true) {
    MessageBox.Show(crypt.LastErrorText);
    return;
}

crypt.EncodingMode = "hex";
crypt.HashAlgorithm = "sha-1";

string 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");

Chilkat.Dsa dsa = new Chilkat.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) {
    MessageBox.Show(dsa.LastErrorText);
    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.
string pemPrivateKey;
pemPrivateKey = dsa.LoadText("dsa_priv.pem");
success = dsa.FromPem(pemPrivateKey);
if (success != true) {
    MessageBox.Show(dsa.LastErrorText);
    return;
}

//  You may optionally verify the key to ensure that it is a valid
//  DSA key.
success = dsa.VerifyKey();
if (success != true) {
    MessageBox.Show(dsa.LastErrorText);
    return;
}

//  Load the hash to be signed into the DSA object:
success = dsa.SetEncodedHash("hex",hashStr);
if (success != true) {
    MessageBox.Show(dsa.LastErrorText);
    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) {
    MessageBox.Show(dsa.LastErrorText);
    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.)
string hexSig;
hexSig = dsa.GetEncodedSignature("hex");
textBox1.Text += "Signature:" + "\r\n";
textBox1.Text += hexSig + "\r\n";

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

Chilkat.Dsa dsa2 = new Chilkat.Dsa();

//  Load the DSA public key to be used for verification:
string pemPublicKey;
pemPublicKey = dsa2.LoadText("dsa_pub.pem");
success = dsa2.FromPublicPem(pemPublicKey);
if (success != true) {
    MessageBox.Show(dsa2.LastErrorText);
    return;
}

//  Load the hash to be verified against the signature.
success = dsa2.SetEncodedHash("hex",hashStr);
if (success != true) {
    MessageBox.Show(dsa2.LastErrorText);
    return;
}

//  Load the signature:
success = dsa2.SetEncodedSignature("hex",hexSig);
if (success != true) {
    MessageBox.Show(dsa2.LastErrorText);
    return;
}

//  Verify:
success = dsa2.Verify();
if (success != true) {
    MessageBox.Show(dsa2.LastErrorText);
}
else {
    textBox1.Text += "DSA Signature Verified!" + "\r\n";
}