From 894be2d9da60a8e0f7ffc0224e3f294ce49b70be Mon Sep 17 00:00:00 2001
From: Oliver Smith <osmith@sysmocom.de>
Date: Fri, 11 Jan 2019 13:13:37 +0100
Subject: gsm23003: add osmo_imei_str_valid()

Verify 14 digit and 15 digit IMEI strings. OsmoHLR will use the 14
digit version to check IMEIs before writing them to the DB.

Place the Luhn checksum code in a dedicated osmo_luhn() function, so
it can be used elsewhere.

Related: OS#2541
Change-Id: Id2d2a3a93b033bafc74c62e15297034bf4aafe61
---
 src/utils.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

(limited to 'src/utils.c')

diff --git a/src/utils.c b/src/utils.c
index 35d70ace..d1da4fa8 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -765,4 +765,35 @@ const char *osmo_str_toupper(const char *src)
 	return buf;
 }
 
+/*! Calculate the Luhn checksum (as used for IMEIs).
+ * \param[in] in  Input digits in ASCII string representation.
+ * \param[in] in_len  Count of digits to use for the input (14 for IMEI).
+ * \returns checksum char (e.g. '3'); negative on error
+ */
+const char osmo_luhn(const char* in, int in_len)
+{
+	int i, sum = 0;
+
+	/* All input must be numbers */
+	for (i = 0; i < in_len; i++) {
+		if (!isdigit(in[i]))
+			return -EINVAL;
+	}
+
+	/* Double every second digit and add it to sum */
+	for (i = in_len - 1; i >= 0; i -= 2) {
+		int dbl = (in[i] - '0') * 2;
+		if (dbl > 9)
+			dbl -= 9;
+		sum += dbl;
+	}
+
+	/* Add other digits to sum */
+	for (i = in_len - 2; i >= 0; i -= 2)
+		sum += in[i] - '0';
+
+	/* Final checksum */
+	return (sum * 9) % 10 + '0';
+}
+
 /*! @} */
-- 
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