'Checks whether the credentials themselves are valid, now that we know
if the key matches the user.
@param pubKey: the key in the credentials (just to prevent it from
having to be calculated again)
@type pubKey: L{twisted.conch.ssh.keys.Key}
@param credentials: the credentials offered by the user
@type credentials: L{ISSHPrivateKey} provider
@raise UnauthorizedLogin: If the key signature is invalid or there
was any error verifying the signature.
@return: The user\'s username, if authentication was successful
@rtype: L{bytes}'

def _verifyKey(self, pubKey, credentials):

    try:
        if pubKey.verify(credentials.signature, credentials.sigData):
            return credentials.username
    except:
        log.err()
        raise UnauthorizedLogin('Error    while    verifying    key')
    raise UnauthorizedLogin('Key    signature    invalid.')
