From e5fea5a37032f4e080911733658ab13fd6767d84 Mon Sep 17 00:00:00 2001
From: Jef Driesen <jefdriesen@users.sourceforge.net>
Date: Thu, 5 Feb 2009 10:05:15 +0000
Subject: [PATCH] Added a checksum function for the Oceanic VT Pro version
 string.

---
 src/checksum.c      | 14 ++++++++++++++
 src/checksum.h      |  3 +++
 src/oceanic_vtpro.c | 18 ++++++++++++++----
 3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/src/checksum.c b/src/checksum.c
index 759294d..aa06edd 100644
--- a/src/checksum.c
+++ b/src/checksum.c
@@ -21,6 +21,20 @@
 
 #include "checksum.h"
 
+
+unsigned char
+checksum_add_uint4 (const unsigned char data[], unsigned int size, unsigned char init)
+{
+	unsigned char crc = init;
+	for (unsigned int i = 0; i < size; ++i) {
+		crc += (data[i] & 0xF0) >> 4;
+		crc += (data[i] & 0x0F);
+	}
+
+	return crc;
+}
+
+
 unsigned char
 checksum_add_uint8 (const unsigned char data[], unsigned int size, unsigned char init)
 {
diff --git a/src/checksum.h b/src/checksum.h
index 576fbe7..f078ef8 100644
--- a/src/checksum.h
+++ b/src/checksum.h
@@ -26,6 +26,9 @@
 extern "C" {
 #endif /* __cplusplus */
 
+unsigned char
+checksum_add_uint4 (const unsigned char data[], unsigned int size, unsigned char init);
+
 unsigned char
 checksum_add_uint8 (const unsigned char data[], unsigned int size, unsigned char init);
 
diff --git a/src/oceanic_vtpro.c b/src/oceanic_vtpro.c
index 1a72b9e..2b5addd 100644
--- a/src/oceanic_vtpro.c
+++ b/src/oceanic_vtpro.c
@@ -396,8 +396,13 @@ oceanic_vtpro_device_version (device_t *abstract, unsigned char data[], unsigned
 	if (rc != DEVICE_STATUS_SUCCESS)
 		return rc;
 
-	// FIXME: The answer seems to contain a checksum byte,
-	// but the correct checksum function is not known.
+	// Verify the checksum of the answer.
+	unsigned char crc = ans[OCEANIC_VTPRO_PACKET_SIZE / 2];
+	unsigned char ccrc = checksum_add_uint4 (ans, OCEANIC_VTPRO_PACKET_SIZE / 2, 0x00);
+	if (crc != ccrc) {
+		WARNING ("Unexpected answer CRC.");
+		return DEVICE_STATUS_PROTOCOL;
+	}
 
 #ifndef NDEBUG
 	ans[OCEANIC_VTPRO_PACKET_SIZE / 2] = 0;
@@ -414,8 +419,13 @@ oceanic_vtpro_device_version (device_t *abstract, unsigned char data[], unsigned
 		if (rc != DEVICE_STATUS_SUCCESS)
 			return rc;
 
-		// FIXME: The answer seems to contain a checksum byte,
-		// but the correct checksum function is not known.
+		// Verify the checksum of the answer.
+		unsigned char crc = answer[OCEANIC_VTPRO_PACKET_SIZE / 2];
+		unsigned char ccrc = checksum_add_uint4 (answer, OCEANIC_VTPRO_PACKET_SIZE / 2, 0x00);
+		if (crc != ccrc) {
+			WARNING ("Unexpected answer CRC.");
+			return DEVICE_STATUS_PROTOCOL;
+		}
 
 		// Verify the last byte of the answer.
 		if (answer[OCEANIC_VTPRO_PACKET_SIZE / 2 + 1] != 0x51) {