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.

 Chilkat Python Module Downloads for Windows, Linux, and MAC OS X

import sys
import chilkat



#  Use Chilkat Crypt to hash the contents of a file.
crypt = chilkat.CkCrypt2()
success = crypt.UnlockComponent("Anything for 30-day trial.")
if (success != True):
    print crypt.lastErrorText()
    sys.exit()

crypt.put_EncodingMode("hex")
crypt.put_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.
hashStr = crypt.hashFileENC("hamlet.xml")

dsa = chilkat.CkDsa()

#  The Chilkat Crypt and Chilkat DSA components are separate
#  products.  To license both, it's least expensive to purchase
#  the "Chilkat Bundle" which provides licenses to all the
#  Chilkat components.
success = dsa.UnlockComponent("Anything for 30-day trial")
if (success != True):
    print dsa.lastErrorText()
    sys.exit()

#  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.

pemPrivateKey = dsa.loadText("dsa_priv.pem")
success = dsa.FromPem(pemPrivateKey)
if (success != True):
    print dsa.lastErrorText()
    sys.exit()

#  You may optionally verify the key to ensure that it is a valid
#  DSA key.
success = dsa.VerifyKey()
if (success != True):
    print dsa.lastErrorText()
    sys.exit()

#  Load the hash to be signed into the DSA object:
success = dsa.SetEncodedHash("hex",hashStr)
if (success != True):
    print dsa.lastErrorText()
    sys.exit()

#  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):
    print dsa.lastErrorText()
    sys.exit()

#  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.)

hexSig = dsa.getEncodedSignature("hex")
print "Signature:"
print hexSig

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

dsa2 = chilkat.CkDsa()

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

pemPublicKey = dsa2.loadText("dsa_pub.pem")
success = dsa2.FromPublicPem(pemPublicKey)
if (success != True):
    print dsa2.lastErrorText()
    sys.exit()

#  Load the hash to be verified against the signature.
success = dsa2.SetEncodedHash("hex",hashStr)
if (success != True):
    print dsa2.lastErrorText()
    sys.exit()

#  Load the signature:
success = dsa2.SetEncodedSignature("hex",hexSig)
if (success != True):
    print dsa2.lastErrorText()
    sys.exit()

#  Verify:
success = dsa2.Verify()
if (success != True):
    print dsa2.lastErrorText()
else:
    print "DSA Signature Verified!"