B4X

ECDSA Sign and Verify

See more ECC Examples

Demonstrates how to create an ECDSA signature on the SHA256 hash of some data, and then verify. B4X

Dim success As Boolean = False

'  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 ChilkatPrivateKey
privKey.Initialize("privKey")
success = privKey.LoadEncryptedPemFile("qa_data/ecc/secp256r1-key-pkcs8-secret.pem", "secret")
If success = False Then
    Log(privKey.LastErrorText)
    Return
End If


'  Sign the SHA256 hash of some data.
Dim bd As ChilkatBinData
bd.Initialize
success = bd.LoadFile("qa_data/hamlet.xml")
If success = False Then
    Log("Failed to load file to be hashed.")
    Return
End If


Dim crypt As ChilkatCrypt2
crypt.Initialize("crypt")
crypt.HashAlgorithm = "sha256"
crypt.EncodingMode = "base64"
Dim hashStr As String = crypt.HashBdENC(bd)

Dim ecdsa As ChilkatEcc
ecdsa.Initialize
Dim prng As ChilkatPrng
prng.Initialize
'  Returns ASN.1 signature as a base64 string.
Dim sig As String = ecdsa.SignHashENC(hashStr, "base64", privKey, prng)
Log("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 ChilkatAsn
asn.Initialize
asn.LoadEncoded(sig, "base64")
Dim xml As ChilkatXml
xml.Initialize
xml.LoadXml(asn.AsnToXml)

Log(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
Dim r As String = xml.GetChildContentByIndex(0)
Dim s As String = xml.GetChildContentByIndex(1)

Log("r = " & r)
Log("s = " & s)

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

'  Get the corresponding public key.
Dim pubKey As ChilkatPublicKey
pubKey.Initialize
success = pubKey.LoadFromFile("qa_data/ecc/secp256r1-pub.pem")
If success = False Then
    Log(pubKey.LastErrorText)
    Return
End If


'  We already have the SHA256 hash of the original data (hashStr) so no need to re-do it..
Dim ecc2 As ChilkatEcc
ecc2.Initialize
Dim result As Int = ecc2.VerifyHashENC(hashStr, sig, "base64", pubKey)
If result <> 1 Then
    Log(ecc2.LastErrorText)
    Return
End If


Log("Verified!")

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

Dim asn2 As ChilkatAsn
asn2.Initialize
asn2.LoadAsnXml(xml2.GetXml)
Dim encodedSig As String = asn2.GetEncodedDer("base64")
Log("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...