B4X

DSA Signature Create and Verify

See more DSA Examples

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.

B4X

Dim success As Boolean = False

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

Dim crypt As ChilkatCrypt2
crypt.Initialize("crypt")

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.
Dim hashStr As String = crypt.HashFileENC("hamlet.xml")

Dim dsa As ChilkatDsa
dsa.Initialize

'  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.
Dim pemPrivateKey As String
pemPrivateKey = dsa.LoadText("dsa_priv.pem")
success = dsa.FromPem(pemPrivateKey)
If success <> True Then
    Log(dsa.LastErrorText)
    Return
End If


'  You may optionally verify the key to ensure that it is a valid
'  DSA key.
success = dsa.VerifyKey
If success <> True Then
    Log(dsa.LastErrorText)
    Return
End If


'  Load the hash to be signed into the DSA object:
success = dsa.SetEncodedHash("hex", hashStr)
If success <> True Then
    Log(dsa.LastErrorText)
    Return
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 <> True Then
    Log(dsa.LastErrorText)
    Return
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.)
Dim hexSig As String = dsa.GetEncodedSignature("hex")
Log("Signature:")
Log(hexSig)

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

Dim dsa2 As ChilkatDsa
dsa2.Initialize

'  Load the DSA public key to be used for verification:
Dim pemPublicKey As String
pemPublicKey = dsa2.LoadText("dsa_pub.pem")
success = dsa2.FromPublicPem(pemPublicKey)
If success <> True Then
    Log(dsa2.LastErrorText)
    Return
End If


'  Load the hash to be verified against the signature.
success = dsa2.SetEncodedHash("hex", hashStr)
If success <> True Then
    Log(dsa2.LastErrorText)
    Return
End If


'  Load the signature:
success = dsa2.SetEncodedSignature("hex", hexSig)
If success <> True Then
    Log(dsa2.LastErrorText)
    Return
End If


'  Verify:
success = dsa2.Verify
If success <> True Then
    Log(dsa2.LastErrorText)
Else
    Log("DSA Signature Verified!")
End If