'Sign a piece of data with ElGamal.
:Parameter M: The piece of data to sign with ElGamal. It may
not be longer in bit size than *p-1*.
:Type M: byte string or long
:Parameter K: A secret number, chosen randomly in the closed
range *[1,p-2]* and such that *gcd(k,p-1)=1*.
:Type K: long (recommended) or byte string (not recommended)
:attention: selection of *K* is crucial for security. Generating a
random number larger than *p-1* and taking the modulus by *p-1* is
**not** secure, since smaller values will occur more frequently.
Generating a random number systematically smaller than *p-1*
(e.g. *floor((p-1)/8)* random bytes) is also **not** secure.
In general, it shall not be possible for an attacker to know
the value of any bit of K.
:attention: The number *K* shall not be reused for any other
operation and shall be discarded immediately.
:attention: M must be be a cryptographic hash, otherwise an
attacker may mount an existential forgery attack.
:Return: A tuple with 2 longs.'

def sign(self, M, K):

    return pubkey.sign(self, M, K)
