(Xojo Plugin) 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 Xojo Plugin Download Xojo Plugin for Windows, Linux, Mac OS X, and ARM, ARM64

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

Dim crypt As New Chilkat.Crypt2

crypt.EncodingMode = "hex"
crypt.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.
Dim hashStr As String
hashStr = crypt.HashFileENC("hamlet.xml")

Dim dsa As New Chilkat.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.
Dim pemPrivateKey As String
pemPrivateKey = dsa.LoadText("dsa_priv.pem")
Dim success As Boolean
success = dsa.FromPem(pemPrivateKey)
If (success <> True) Then
    System.DebugLog(dsa.LastErrorText)
    Return
End If

// You may optionally verify the key to ensure that it is a valid
// DSA key.
success = dsa.VerifyKey()
If (success <> True) Then
    System.DebugLog(dsa.LastErrorText)
    Return
End If

// Load the hash to be signed into the DSA object:
success = dsa.SetEncodedHash("hex",hashStr)
If (success <> True) Then
    System.DebugLog(dsa.LastErrorText)
    Return
End If

// 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) Then
    System.DebugLog(dsa.LastErrorText)
    Return
End If

// 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.)
Dim hexSig As String
hexSig = dsa.GetEncodedSignature("hex")
System.DebugLog("Signature:")
System.DebugLog(hexSig)

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

Dim dsa2 As New Chilkat.Dsa

// Load the DSA public key to be used for verification:
Dim pemPublicKey As String
pemPublicKey = dsa2.LoadText("dsa_pub.pem")
success = dsa2.FromPublicPem(pemPublicKey)
If (success <> True) Then
    System.DebugLog(dsa2.LastErrorText)
    Return
End If

// Load the hash to be verified against the signature.
success = dsa2.SetEncodedHash("hex",hashStr)
If (success <> True) Then
    System.DebugLog(dsa2.LastErrorText)
    Return
End If

// Load the signature:
success = dsa2.SetEncodedSignature("hex",hexSig)
If (success <> True) Then
    System.DebugLog(dsa2.LastErrorText)
    Return
End If

// Verify:
success = dsa2.Verify()
If (success <> True) Then
    System.DebugLog(dsa2.LastErrorText)
Else
    System.DebugLog("DSA Signature Verified!")
End If