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

3 files changed, 50 insertions, 0 deletions

diff --git a/src/gsm/gsm23003.c b/src/gsm/gsm23003.c
index 4fdad48a..1d9cefe6 100644
--- a/src/gsm/gsm23003.c
+++ b/src/gsm/gsm23003.c
@@ -31,6 +31,7 @@
 
 #include <osmocom/gsm/gsm23003.h>
 #include <osmocom/gsm/protocol/gsm_23_003.h>
+#include <osmocom/core/utils.h>
 
 static bool is_n_digits(const char *str, int min_digits, int max_digits)
 {
@@ -71,6 +72,23 @@ bool osmo_msisdn_str_valid(const char *msisdn)
 	return is_n_digits(msisdn, 1, 15);
 }
 
+/*! Determine whether the given IMEI is valid according to 3GPP TS 23.003,
+ * Section 6.2.1. It consists of 14 digits, the 15th check digit is not
+ * intended for digital transmission.
+ * \param imei  IMEI digits in ASCII string representation.
+ * \param with_15th_digit  when true, expect the 15th digit to be present and
+ *        verify it.
+ * \returns true when the IMEI is valid, false for invalid characters or number
+ *          of digits.
+ */
+bool osmo_imei_str_valid(const char *imei, bool with_15th_digit)
+{
+	if (with_15th_digit)
+		return is_n_digits(imei, 15, 15) && osmo_luhn(imei, 14) == imei[14];
+	else
+		return is_n_digits(imei, 14, 14);
+}
+
 /*! Return MCC string as standardized 3-digit with leading zeros.
  * \param[in] mcc  MCC value.
  * \returns string in static buffer.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index a9037870..331c3f0f 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -551,6 +551,7 @@ osmo_oap_decode;
 
 osmo_imsi_str_valid;
 osmo_msisdn_str_valid;
+osmo_imei_str_valid;
 
 osmo_mncc_stringify;
 osmo_mncc_names;
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';
+}
+
 /*! @} */