--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/atsc.c	Sun Feb 02 08:45:56 2014 +0000
@@ -0,0 +1,58 @@
+/*
+ * It is known that some GSM devices have undocumented AT commands for
+ * changing the IMEI.  There is no standard syntax for such an AT command
+ * (by the "proper rules" one is not supposed to exist at all), and instead
+ * there seem to be several different ad hoc syntaxes.  This source file,
+ * found on a Chinese site, implements one of these numerous ad hoc
+ * IMEI-changing AT commands:
+ *
+ * ftp://ftp.ifctf.org/pub/GSM/TI_src/ati_sc.c
+ *
+ * Notice that this particular incarnation of the "set IMEI" AT command
+ * is called AT@SC; there just happens to be an identically-named AT@SC
+ * command on Openmoko's GSM modems.  Might it perchance be the same
+ * IMEI changing command?
+ *
+ * This program constructs what should be a valid input to the decoding
+ * logic in the ati_sc.c source above, for the purpose of testing whether
+ * or not such a command would indeed effect an IMEI change on a GTA02 modem.
+ */
+
+#include <stdio.h>
+#include <ctype.h>
+#include <string.h>
+#include <strings.h>
+#include <stdlib.h>
+
+static char hexdigits[] = "0123456789abcdef";
+
+main(argc, argv)
+	char **argv;
+{
+	char hexout[16];
+	unsigned n1, n2, cksum;
+	int i, c;
+
+	if (argc != 2) {
+usage:		fprintf(stderr, "usage: %s 15-IMEI-digits\n", argv[0]);
+		exit(1);
+	}
+	if (strlen(argv[1]) != 15)
+		goto usage;
+	n1 = n2 = 0;
+	for (i = 0; i < 15; i++) {
+		c = argv[1][i];
+		if (!isdigit(c))
+			goto usage;
+		c -= '0';
+		hexout[i] = hexdigits[c ^ 5];
+		if (i < 7)
+			n1 = n1 * 10 + c;
+		else
+			n2 = n2 * 10 + c;
+	}
+	hexout[15] = '\0';
+	cksum = (n1 + n2) % 1973;
+	printf("AT@SC=%s%04u\n", hexout, cksum);
+	exit(0);
+}