Opaque Digital Signature Embedding

Download Chilkat Crypt ActiveX

Create an opaque digital signature that embeds the original file data along with the signature. Then verify and extract the original data.

    Dim crypt As New ChilkatCrypt2
    crypt.UnlockComponent "test"

    ' Select a digital certificate for signing.
    Dim StoreCreator As ChilkatCreateCS
    Set StoreCreator = New ChilkatCreateCS
    
    ' Create a certificate store object representing the registry-based
    ' current user certificate store.
    Dim certStore As ChilkatCertStore
    Set certStore = StoreCreator.OpenCurrentUserStore()
    
    ' Find a certificate matching an email address.
    Dim cert As ChilkatCert
    Set cert = certStore.FindCertBySubjectE("test@chilkatsoft.com")
    If (cert Is Nothing) Then
        MsgBox "Certificate not found!"
        Exit Sub
    End If
        
    ' Use this certificate.
    crypt.SetSigningCert cert
    
    Dim fileData As Variant
    Dim opaqueSig As Variant
    
    fileData = crypt.ReadFile("sample.pdf")
    If IsNull(fileData) Then
        MsgBox crypt.LastErrorText
        Exit Sub
    End If
    
    opaqueSig = crypt.OpaqueSignBytes(fileData)
    If IsNull(opaqueSig) Then
        MsgBox crypt.LastErrorText
        Exit Sub
    End If
    
    ' opaqueSig.dat contains both the file data and the signature.
    success = crypt.WriteFile("opaqueSig.dat", opaqueSig)
    If (success = 0) Then
        MsgBox crypt.LastErrorText
    End If
    
    MsgBox "OK!"
    
    ' Now verify it and retrieve the original file from opaqueSig.dat
    Dim opaqueSig2 As Variant
    
    opaqueSig2 = crypt.ReadFile("opaqueSig.dat")
    If IsNull(opaqueSig2) Then
        MsgBox crypt.LastErrorText
        Exit Sub
    End If
    
    Dim originalData As Variant
    originalData = crypt.OpaqueVerifyBytes(opaqueSig2)
    If IsNull(originalData) Then
        MsgBox "Invalid signature, the file may have been altered."
    Else
        crypt.WriteFile "sampleOut.pdf", originalData
        MsgBox "Signature is valid, the file is unchanged."
    End If