'Extend the base signing behavior by using an SSH agent to sign the
data, if one is available.
@type publicKey: L{Key}
@type signData: L{bytes}'

def signData(self, publicKey, signData):

    if (not self.usedFiles):
        return self.keyAgent.signData(publicKey.blob(), signData)
    else:
        return userauth.SSHUserAuthClient.signData(self, publicKey, signData)
