diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index f412e3e..02bfe4c 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -195,6 +195,9 @@
 };
 
 /* Deprectated functions */
+/* Limit encoding and decoding to use no more than this amount of buffer bytes */
+#define GSM_7BIT_LEGACY_MAX_BUFFER_SIZE  0x10000
+
 int gsm_7bit_decode(char *decoded, const uint8_t *user_data, uint8_t length) OSMO_DEPRECATED("Use gsm_7bit_decode_n() instead");
 int gsm_7bit_decode_ussd(char *decoded, const uint8_t *user_data, uint8_t length) OSMO_DEPRECATED("Use gsm_7bit_decode_n_ussd() instead");
 int gsm_7bit_encode(uint8_t *result, const char *data) OSMO_DEPRECATED("Use gsm_7bit_encode_n() instead");
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index 5241c91..198ec69 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -273,7 +273,7 @@
 {
 	int y = 0;
 	int o;
-	int max_septets = n * 8 / 7;
+	size_t max_septets = n * 8 / 7;
 
 	/* prepare for the worst case, every character expanding to two bytes */
 	uint8_t *rdata = calloc(strlen(data) * 2, sizeof(uint8_t));
@@ -683,7 +683,8 @@
 
 int gsm_7bit_decode(char *text, const uint8_t *user_data, uint8_t septet_l)
 {
-	gsm_7bit_decode_n(text, SIZE_MAX, user_data, septet_l);
+	gsm_7bit_decode_n(text, GSM_7BIT_LEGACY_MAX_BUFFER_SIZE,
+			  user_data, septet_l);
 
 	/* Mimic the original behaviour. */
 	return septet_l;
@@ -691,21 +692,25 @@
 
 int gsm_7bit_decode_ussd(char *text, const uint8_t *user_data, uint8_t length)
 {
-	return gsm_7bit_decode_n_ussd(text, SIZE_MAX, user_data, length);
+	return gsm_7bit_decode_n_ussd(text, GSM_7BIT_LEGACY_MAX_BUFFER_SIZE,
+				      user_data, length);
 }
 
 int gsm_7bit_encode(uint8_t *result, const char *data)
 {
 	int out;
-	return gsm_7bit_encode_n(result, SIZE_MAX, data, &out);
+	return gsm_7bit_encode_n(result, GSM_7BIT_LEGACY_MAX_BUFFER_SIZE,
+				 data, &out);
 }
 
 int gsm_7bit_encode_ussd(uint8_t *result, const char *data, int *octets)
 {
-	return gsm_7bit_encode_n_ussd(result, SIZE_MAX, data, octets);
+	return gsm_7bit_encode_n_ussd(result, GSM_7BIT_LEGACY_MAX_BUFFER_SIZE,
+				      data, octets);
 }
 
 int gsm_7bit_encode_oct(uint8_t *result, const char *data, int *octets)
 {
-	return gsm_7bit_encode_n(result, SIZE_MAX, data, octets);
+	return gsm_7bit_encode_n(result, GSM_7BIT_LEGACY_MAX_BUFFER_SIZE,
+				 data, octets);
 }
diff --git a/tests/sms/sms_test.c b/tests/sms/sms_test.c
index 2c9d8d8..755b321 100644
--- a/tests/sms/sms_test.c
+++ b/tests/sms/sms_test.c
@@ -280,6 +280,17 @@
 
 	/* test 7-bit encoding */
 	for (i = 0; i < ARRAY_SIZE(test_encode); ++i) {
+		/* Test legacy function (return value only) */
+		septet_length = gsm_7bit_encode(coded,
+						(const char *) test_encode[i].input);
+		printf("Legacy encode case %d: "
+		       "septet length %d (expected %d)\n"
+		       , i
+		       , septet_length, test_encode[i].expected_septet_length
+		      );
+		OSMO_ASSERT (septet_length == test_encode[i].expected_septet_length);
+
+		/* Test new function */
 		memset(coded, 0x42, sizeof(coded));
 		septet_length = gsm_7bit_encode_n(coded, sizeof(coded),
 			       			  (const char *) test_encode[i].input,
@@ -296,6 +307,7 @@
 		OSMO_ASSERT (octets_written == test_encode[i].expected_octet_length);
 		OSMO_ASSERT (octets_written == computed_octet_length);
 		OSMO_ASSERT (memcmp(coded, test_encode[i].expected, octets_written) == 0);
+		OSMO_ASSERT (septet_length == test_encode[i].expected_septet_length);
 
 		/* check buffer limiting */
 		memset(coded, 0xaa, sizeof(coded));
@@ -357,6 +369,16 @@
 
 	/* test 7-bit decoding */
 	for (i = 0; i < ARRAY_SIZE(test_decode); ++i) {
+		/* Test legacy function (return value only) */
+		if (!test_decode[i].ud_hdr_ind) {
+			nchars = gsm_7bit_decode(result, test_decode[i].input,
+						 test_decode[i].expected_septet_length);
+			printf("Legacy decode case %d: "
+			       "return value %d (expected %d)\n",
+			       i, nchars, test_decode[i].expected_septet_length);
+		}
+
+		/* Test new function */
 		memset(result, 0x42, sizeof(result));
 		nchars = gsm_7bit_decode_n_hdr(result, sizeof(result), test_decode[i].input,
 				test_decode[i].expected_septet_length, test_decode[i].ud_hdr_ind);
diff --git a/tests/sms/sms_test.ok b/tests/sms/sms_test.ok
index a71567d..fa536ea 100644
--- a/tests/sms/sms_test.ok
+++ b/tests/sms/sms_test.ok
@@ -1,11 +1,19 @@
 SMS testing
+Legacy encode case 0: septet length 9 (expected 9)
 Encode case 0: Octet length 8 (expected 8, computed 8), septet length 9 (expected 9)
+Legacy encode case 1: septet length 41 (expected 41)
 Encode case 1: Octet length 36 (expected 36, computed 36), septet length 41 (expected 41)
+Legacy encode case 2: septet length 39 (expected 39)
 Encode case 2: Octet length 35 (expected 35, computed 35), septet length 39 (expected 39)
+Legacy encode case 3: septet length 40 (expected 40)
 Encode case 3: Octet length 35 (expected 35, computed 35), septet length 40 (expected 40)
+Legacy decode case 0: return value 9 (expected 9)
 Decode case 0: return value 9 (expected 9)
+Legacy decode case 1: return value 41 (expected 41)
 Decode case 1: return value 40 (expected 40)
+Legacy decode case 2: return value 39 (expected 39)
 Decode case 2: return value 31 (expected 31)
+Legacy decode case 3: return value 40 (expected 40)
 Decode case 3: return value 32 (expected 32)
 Decode case 4: return value 153 (expected 153)
 Decode case 5: return value 40 (expected 40)