public static void enableAuth(final AbstractHttpClient client, final Keychain keychain, final KeyId keyId) {
    if (client == null) {
        throw new NullPointerException("client");
    }

    if (keychain == null) {
        throw new NullPointerException("keychain");
    }

    client.getAuthSchemes().register(Constants.SCHEME, new AuthSchemeFactory() {
        public AuthScheme newInstance(HttpParams params) {
            return new Http4SignatureAuthScheme();
        }
    });

    Signer signer = new Signer(keychain, keyId);
    client.getCredentialsProvider().setCredentials(AuthScope.ANY, new SignerCredentials(signer));
    client.getParams().setParameter(AuthPNames.TARGET_AUTH_PREF,
                                    Arrays.asList(Constants.SCHEME));

    HttpClientParams.setAuthenticating(client.getParams(), true);
}