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.

Download Chilkat DSA ActiveX

Download Chilkat Crypt ActiveX

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'  Use Chilkat Crypt to hash the contents of a file.
set crypt = Server.CreateObject("Chilkat.Crypt2")
success = crypt.UnlockComponent("Anything for 30-day trial.")
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(crypt.LastErrorText) & "</pre>"

End If

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.
hashStr = crypt.HashFileENC("hamlet.xml")

set dsa = Server.CreateObject("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 <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa.LastErrorText) & "</pre>"

End If

'  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.

pemPrivateKey = dsa.LoadText("dsa_priv.pem")
success = dsa.FromPem(pemPrivateKey)
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa.LastErrorText) & "</pre>"

End If

'  You may optionally verify the key to ensure that it is a valid
'  DSA key.
success = dsa.VerifyKey()
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa.LastErrorText) & "</pre>"

End If

'  Load the hash to be signed into the DSA object:
success = dsa.SetEncodedHash("hex",hashStr)
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa.LastErrorText) & "</pre>"

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 <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa.LastErrorText) & "</pre>"

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.)

hexSig = dsa.GetEncodedSignature("hex")
Response.Write "<pre>" & Server.HTMLEncode( "Signature:") & "</pre>"
Response.Write "<pre>" & Server.HTMLEncode( hexSig) & "</pre>"

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

set dsa2 = Server.CreateObject("Chilkat.Dsa")

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

pemPublicKey = dsa2.LoadText("dsa_pub.pem")
success = dsa2.FromPublicPem(pemPublicKey)
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa2.LastErrorText) & "</pre>"

End If

'  Load the hash to be verified against the signature.
success = dsa2.SetEncodedHash("hex",hashStr)
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa2.LastErrorText) & "</pre>"

End If

'  Load the signature:
success = dsa2.SetEncodedSignature("hex",hexSig)
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa2.LastErrorText) & "</pre>"

End If

'  Verify:
success = dsa2.Verify()
If (success <> 1) Then
    Response.Write "<pre>" & Server.HTMLEncode(dsa2.LastErrorText) & "</pre>"
Else
    Response.Write "<pre>" & Server.HTMLEncode( "DSA Signature Verified!") & "</pre>"
End If


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