diff --git a/src/gsm/auth_xor_2g.c b/src/gsm/auth_xor_2g.c
new file mode 100644
index 0000000..9efe5cc
--- /dev/null
+++ b/src/gsm/auth_xor_2g.c
@@ -0,0 +1,79 @@
+/*! \file auth_xor.c
+ * GSM XOR-2G algorithm as specified in Annex 4 (A.4.1.2) of 3GPP TS 51.010-1.
+ * This is implemented by typical GSM MS tester */
+
+/*
+ * (C) 2023 by Harald Welte <laforge@gnumonks.org>
+ *
+ * All Rights Reserved
+ *
+ * Author: Daniel Willmann <dwillmann@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ */
+
+#include <string.h>
+#include <stdint.h>
+#include <errno.h>
+
+#include <osmocom/crypt/auth.h>
+
+/*! \addtogroup auth
+ *  @{
+ */
+
+static void xor(uint8_t *out, const uint8_t *a, const uint8_t *b, size_t len)
+{
+	size_t i;
+
+	for (i = 0; i < len; i++)
+		out[i] = a[i] ^ b[i];
+}
+
+/* GSM XOR-2G algorithm as specified in Annex 4 (A.4.1.2) of 3GPP TS 51.010-1. */
+static int xor2g_gen_vec(struct osmo_auth_vector *vec,
+		       struct osmo_sub_auth_data *aud,
+		       const uint8_t *_rand)
+{
+	uint8_t res1[16];
+
+	if (aud->type != OSMO_AUTH_TYPE_GSM)
+		return -ENOTSUP;
+
+	/* Step 1: XOR to the challenge RAND, a predefined number Ki, having the same bit length (128 bits) as
+	 * RAND. */
+	xor(res1, aud->u.gsm.ki, _rand, sizeof(res1));
+
+	/* Step 2: The most significant 32 bits of RES1 form SRES. */
+	memcpy(vec->sres, res1, 4);
+	/* The next 64 bits of RES1 form Kc */
+	memcpy(vec->kc, res1+4, 8);
+
+	vec->auth_types = OSMO_AUTH_TYPE_GSM;
+	return 0;
+}
+
+static struct osmo_auth_impl xor2g_alg = {
+	.algo = OSMO_AUTH_ALG_XOR_2G,
+	.name = "XOR-2G (libosmogsm built-in)",
+	.priority = 1000,
+	.gen_vec = &xor2g_gen_vec,
+};
+
+static __attribute__((constructor)) void on_dso_load_xor(void)
+{
+	osmo_auth_register(&xor2g_alg);
+}
+
+/*! @} */