(Lianja) 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.

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

loCrypt = createobject("CkCrypt2")

loCrypt.EncodingMode = "hex"
loCrypt.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.
lcHashStr = loCrypt.HashFileENC("hamlet.xml")

loDsa = createobject("CkDsa")

// 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.

lcPemPrivateKey = loDsa.LoadText("dsa_priv.pem")
llSuccess = loDsa.FromPem(lcPemPrivateKey)
if (llSuccess <> .T.) then
    ? loDsa.LastErrorText
    release loCrypt
    release loDsa
    return
endif

// You may optionally verify the key to ensure that it is a valid
// DSA key.
llSuccess = loDsa.VerifyKey()
if (llSuccess <> .T.) then
    ? loDsa.LastErrorText
    release loCrypt
    release loDsa
    return
endif

// Load the hash to be signed into the DSA object:
llSuccess = loDsa.SetEncodedHash("hex",lcHashStr)
if (llSuccess <> .T.) then
    ? loDsa.LastErrorText
    release loCrypt
    release loDsa
    return
endif

// Now that the DSA object contains both the private key and hash,
// it is ready to create the signature:
llSuccess = loDsa.SignHash()
if (llSuccess <> .T.) then
    ? loDsa.LastErrorText
    release loCrypt
    release loDsa
    return
endif

// 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.)
lcHexSig = loDsa.GetEncodedSignature("hex")
? "Signature:"
? lcHexSig

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

loDsa2 = createobject("CkDsa")

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

lcPemPublicKey = loDsa2.LoadText("dsa_pub.pem")
llSuccess = loDsa2.FromPublicPem(lcPemPublicKey)
if (llSuccess <> .T.) then
    ? loDsa2.LastErrorText
    release loCrypt
    release loDsa
    release loDsa2
    return
endif

// Load the hash to be verified against the signature.
llSuccess = loDsa2.SetEncodedHash("hex",lcHashStr)
if (llSuccess <> .T.) then
    ? loDsa2.LastErrorText
    release loCrypt
    release loDsa
    release loDsa2
    return
endif

// Load the signature:
llSuccess = loDsa2.SetEncodedSignature("hex",lcHexSig)
if (llSuccess <> .T.) then
    ? loDsa2.LastErrorText
    release loCrypt
    release loDsa
    release loDsa2
    return
endif

// Verify:
llSuccess = loDsa2.Verify()
if (llSuccess <> .T.) then
    ? loDsa2.LastErrorText
else
    ? "DSA Signature Verified!"
endif



release loCrypt
release loDsa
release loDsa2