(Excel) ECDSA Sign and Verify

Demonstrates how to create an ECDSA signature on the SHA256 hash of some data, and then verify.

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

' First load an ECDSA private key to be used for signing.
Dim privKey As Chilkat.PrivateKey
Set privKey = Chilkat.NewPrivateKey

success = privKey.LoadEncryptedPemFile("qa_data/ecc/secp256r1-key-pkcs8-secret.pem","secret")
If (success = False) Then
    Debug.Print privKey.LastErrorText
    Exit Sub
End If

' Sign the SHA256 hash of some data.
Dim bd As Chilkat.BinData
Set bd = Chilkat.NewBinData
success = bd.LoadFile("qa_data/hamlet.xml")
If (success = False) Then
    Debug.Print "Failed to load file to be hashed."
    Exit Sub
End If

Dim crypt As Chilkat.Crypt2
Set crypt = Chilkat.NewCrypt2
crypt.HashAlgorithm = "sha256"
crypt.EncodingMode = "base64"

hashStr = crypt.HashBdENC(bd)

Dim ecdsa As Chilkat.Ecc
Set ecdsa = Chilkat.NewEcc
Dim prng As Chilkat.Prng
Set prng = Chilkat.NewPrng
' Returns ASN.1 signature as a base64 string.

sig = ecdsa.SignHashENC(hashStr,"base64",privKey,prng)
Debug.Print "sig = "; sig

' The signature is in ASN.1 format (which may be described as the "encoded DSS signature").
' SEQUENCE (2 elem)
'   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
'   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...

' If you wish, you can get the r and s components of the signature like this:
Dim asn As Chilkat.Asn
Set asn = Chilkat.NewAsn
Dim success As Boolean
success = asn.LoadEncoded(sig,"base64")
Dim xml As Chilkat.Xml
Set xml = Chilkat.NewXml
success = xml.LoadXml(asn.AsnToXml())

Debug.Print xml.GetXml()

' We now have this:
' <?xml version="1.0" encoding="utf-8"?>
' <sequence>
'     <int>6650D422D86BA4A228B5617604E59052591B9B2C32EF324C44D09EF67E5F0060</int>
'     <int>0CFD9F6AC85042FC70F672C141BA6B2A4CAFBB906C3D907BCCC1BED62B28326F</int>
' </sequence>

' Get the "r" and "s" as hex strings

r = xml.GetChildContentByIndex(0)

s = xml.GetChildContentByIndex(1)

Debug.Print "r = "; r
Debug.Print "s = "; s

' --------------------------------------------------------------------
' Now verify against the hash of the original data.

' Get the corresponding public key.
Dim pubKey As Chilkat.PublicKey
Set pubKey = Chilkat.NewPublicKey
success = pubKey.LoadFromFile("qa_data/ecc/secp256r1-pub.pem")
If (success = False) Then
    Debug.Print pubKey.LastErrorText
    Exit Sub
End If

' We already have the SHA256 hash of the original data (hashStr) so no need to re-do it..
Dim ecc2 As Chilkat.Ecc
Set ecc2 = Chilkat.NewEcc

result = ecc2.VerifyHashENC(hashStr,sig,"base64",pubKey)
If (result <> 1) Then
    Debug.Print ecc2.LastErrorText
    Exit Sub
End If

Debug.Print "Verified!"

' Note: If we have only r,s and wish to reconstruct the ASN.1 signature, we do it like this:
Dim xml2 As Chilkat.Xml
Set xml2 = Chilkat.NewXml
xml2.Tag = "sequence"
xml2.NewChild2 "int",r
xml2.NewChild2 "int",s

Dim asn2 As Chilkat.Asn
Set asn2 = Chilkat.NewAsn
success = asn2.LoadAsnXml(xml2.GetXml())

encodedSig = asn2.GetEncodedDer("base64")
Debug.Print "encoded DSS signature: "; encodedSig

' You can go to https://lapo.it/asn1js/  and copy/paste the base64 encodedSig into the online tool, then press the "decode" button.
' You will see the ASN.1 such as this:

' SEQUENCE (2 elem)
'   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
'   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...