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(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.

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Xojo Plugin for Windows, Linux, Mac OS X, and ARM

// 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


 

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