Visual FoxPro

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.

Visual FoxPro

LOCAL lnSuccess
LOCAL loCrypt
LOCAL lcHashStr
LOCAL loDsa
LOCAL lcPemPrivateKey
LOCAL lcHexSig
LOCAL loDsa2
LOCAL lcPemPublicKey

lnSuccess = 0

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

loCrypt = CreateObject('Chilkat.Crypt2')

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('Chilkat.Dsa')

* 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")
lnSuccess = loDsa.FromPem(lcPemPrivateKey)
IF (lnSuccess <> 1) THEN
    ? loDsa.LastErrorText
    RELEASE loCrypt
    RELEASE loDsa
    CANCEL
ENDIF

* You may optionally verify the key to ensure that it is a valid
* DSA key.
lnSuccess = loDsa.VerifyKey()
IF (lnSuccess <> 1) THEN
    ? loDsa.LastErrorText
    RELEASE loCrypt
    RELEASE loDsa
    CANCEL
ENDIF

* Load the hash to be signed into the DSA object:
lnSuccess = loDsa.SetEncodedHash("hex",lcHashStr)
IF (lnSuccess <> 1) THEN
    ? loDsa.LastErrorText
    RELEASE loCrypt
    RELEASE loDsa
    CANCEL
ENDIF

* Now that the DSA object contains both the private key and hash,
* it is ready to create the signature:
lnSuccess = loDsa.SignHash()
IF (lnSuccess <> 1) THEN
    ? loDsa.LastErrorText
    RELEASE loCrypt
    RELEASE loDsa
    CANCEL
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('Chilkat.Dsa')

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

lcPemPublicKey = loDsa2.LoadText("dsa_pub.pem")
lnSuccess = loDsa2.FromPublicPem(lcPemPublicKey)
IF (lnSuccess <> 1) THEN
    ? loDsa2.LastErrorText
    RELEASE loCrypt
    RELEASE loDsa
    RELEASE loDsa2
    CANCEL
ENDIF

* Load the hash to be verified against the signature.
lnSuccess = loDsa2.SetEncodedHash("hex",lcHashStr)
IF (lnSuccess <> 1) THEN
    ? loDsa2.LastErrorText
    RELEASE loCrypt
    RELEASE loDsa
    RELEASE loDsa2
    CANCEL
ENDIF

* Load the signature:
lnSuccess = loDsa2.SetEncodedSignature("hex",lcHexSig)
IF (lnSuccess <> 1) THEN
    ? loDsa2.LastErrorText
    RELEASE loCrypt
    RELEASE loDsa
    RELEASE loDsa2
    CANCEL
ENDIF

* Verify:
lnSuccess = loDsa2.Verify()
IF (lnSuccess <> 1) THEN
    ? loDsa2.LastErrorText
ELSE
    ? "DSA Signature Verified!"
ENDIF

RELEASE loCrypt
RELEASE loDsa
RELEASE loDsa2