     protected function generateAuthHeader($data)
    {
        $key    = $this->getSigningKey();
        $digest = $this->getOption('signing_key_digest');
 
        if ( !$key )
        {
            throw ClientException( 'No signing key to sign request' );
        } else if ( !isset($this->digests[$digest]) )
        {
            throw new ClientException( 'Digest %s is invalid. Available digests are %s',
                $digest, implode(',', array_keys( $this->digests ) ) );
        }
 
        $signature  = hash_hmac($this->digests[ $digest ], $data, base64_decode($key), true);
 
        return sprintf('%s %s:%s', $digest, $this->getApiToken(), base64_encode($signature));
    }

	protected function generateAuthHeader($data)
	{
	$key = $this->getSigningKey();
	$digest = $this->getOption('signing_key_digest');

	if ( !$key )
	{
	throw ClientException( 'No signing key to sign request' );
	} else if ( !isset($this->digests[$digest]) )
	{
	throw new ClientException( 'Digest %s is invalid. Available digests are %s',
	$digest, implode(',', array_keys( $this->digests ) ) );
	}

	$signature = hash_hmac($this->digests[ $digest ], $data, base64_decode($key), true);

	return sprintf('%s %s:%s', $digest, $this->getApiToken(), base64_encode($signature));
	}