(Excel) DSA R,S Signature Values

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Creates a DSA signature. Gets r,s values from the signature. Re-creates the DSA signature ASN.1 from the r,s values. Then verifies the signature using the re-created ASN.1 DSA signature.

Download Excel Class Modules Chilkat Excel Class Modules

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

Dim crypt As Chilkat.Crypt2
Set crypt = Chilkat.NewCrypt2

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


hashStr = crypt.HashFileENC("qa_data/hamlet.xml")
Debug.Print "hash to sign: "; hashStr

Dim dsa As Chilkat.Dsa
Set dsa = Chilkat.NewDsa

pemPrivateKey = dsa.LoadText("qa_data/dsa/dsaPrivKey2.pem")

success = dsa.FromPem(pemPrivateKey)
If (success = False) Then
    Debug.Print dsa.LastErrorText
    Exit Sub
End If

' Load the hash to be signed into the DSA object:
success = dsa.SetEncodedHash("hex",hashStr)
If (success = False) Then
    Debug.Print dsa.LastErrorText
    Exit Sub
End If

' Sign the hash.
success = dsa.SignHash()
If (success = False) Then
    Debug.Print dsa.LastErrorText
    Exit Sub
End If

' Get the ASN.1 signature.

asnSig = dsa.GetEncodedSignature("base64")
Debug.Print "Signature: "; asnSig

' Examine the details of the ASN.1 signature.
' We want to get the r,s values as hex strings..
Dim asn As Chilkat.Asn
Set asn = Chilkat.NewAsn
success = asn.LoadEncoded(asnSig,"base64")
If (success = False) Then
    Debug.Print asn.LastErrorText
    Exit Sub
End If

' Get the ASN.1 as XML.
Dim xml As Chilkat.Xml
Set xml = Chilkat.NewXml
success = xml.LoadXml(asn.AsnToXml())
Debug.Print "Signature as XML: "
Debug.Print xml.GetXml()

' Sample XML shown here.
' The r and s values are the two hex strings in the XML.

' <?xml version="1.0" encoding="utf-8"?>
' <sequence>
'     <int>2C187F3AB6E47A66497B86CE97BB39E2133810F5</int>
'     <int>588E53D3F7B69636B48FD7175E99A3961BD7D775</int>
' </sequence>

' Pretend we're starting with r,s

r = "2C187F3AB6E47A66497B86CE97BB39E2133810F5"

s = "588E53D3F7B69636B48FD7175E99A3961BD7D775"

' Build the XML that will be converted to ASN.1
xml.Clear 
xml.Tag = "sequence"
xml.NewChild2 "int",r
xml.NewChild2 "int",s

' Convert the XML to ASN.1
success = asn.LoadAsnXml(xml.GetXml())

' Emit the signature as DER encoded ASN.1 (base64)
asnSig = asn.GetEncodedDer("base64")

' --------------------------------------------------------------------
' Verify the signature using the asnSig we built from the r,s values
' --------------------------------------------------------------------

Dim dsa2 As Chilkat.Dsa
Set dsa2 = Chilkat.NewDsa

' Load the DSA public key to be used for verification:

pemPublicKey = dsa2.LoadText("qa_data/dsa/dsaPubKey2.pem")
success = dsa2.FromPublicPem(pemPublicKey)
If (success = False) Then
    Debug.Print dsa2.LastErrorText
    Exit Sub
End If

' Load the hash to be verified.
success = dsa2.SetEncodedHash("hex",hashStr)
If (success = False) Then
    Debug.Print dsa2.LastErrorText
    Exit Sub
End If

' Load the ASN.1 signature:
success = dsa2.SetEncodedSignature("base64",asnSig)
If (success = False) Then
    Debug.Print dsa2.LastErrorText
    Exit Sub
End If

' Verify:
success = dsa2.Verify()
If (success = False) Then
    Debug.Print dsa2.LastErrorText
Else
    Debug.Print "DSA Signature Verified!"
End If