DSA and ECDSA are U.S. federal standards for digital signatures, specified in FIPS PUB 186-4.
Their security relies on the discrete logarithm problem in a prime finite field (the original DSA, now deprecated) or in an elliptic curve field (ECDSA, faster and with smaller keys, to be used in new applications).
A sender can use a private key (loaded from a file) to sign a message:
>>> from Crypto.Hash import SHA256
>>> from Crypto.PublicKey import ECC
>>> from Crypto.Signature import DSS
>>>
>>> message = b'I give my permission to order #4355'
>>> key = ECC.import_key(open('privkey.der').read())
>>> h = SHA256.new(message)
>>> signer = DSS.new(key, 'fips-186-3')
>>> signature = signer.sign(h)The receiver can use the matching public key to verify authenticity of the received message:
>>> from Crypto.Hash import SHA256
>>> from Crypto.PublicKey import ECC
>>> from Crypto.Signature import DSS
>>>
>>> key = ECC.import_key(open('pubkey.der').read())
>>> h = SHA256.new(received_message)
>>> verifier = DSS.new(key, 'fips-186-3')
>>> try:
>>> verifier.verify(h, signature)
>>> print "The message is authentic."
>>> except ValueError:
>>> print "The message is not authentic."Crypto.Signature.DSS