From 5f17123f3574629c8211ceed46674958db9eea17 Mon Sep 17 00:00:00 2001
From: "Christian W. Zuckschwerdt" <christian@zuckschwerdt.org>
Date: Sun, 15 Dec 2024 15:36:39 +0100
Subject: [PATCH] minor: Add lfsr_digest8_reverse

---
 include/bit_util.h | 19 ++++++++++++++-----
 src/bit_util.c     | 35 +++++++++++++++++++++++++++++++----
 2 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/include/bit_util.h b/include/bit_util.h
index 550967c14..518f76f0d 100644
--- a/include/bit_util.h
+++ b/include/bit_util.h
@@ -144,20 +144,29 @@ uint16_t crc16lsb(uint8_t const message[], unsigned nBytes, uint16_t polynomial,
 /// @return CRC value
 uint16_t crc16(uint8_t const message[], unsigned nBytes, uint16_t polynomial, uint16_t init);
 
-/// Digest-8 by "LFSR-based Toeplitz hash".
+/// Digest-8 by "LFSR-based Toeplitz hash", bits MSB to LSB.
 ///
 /// @param message bytes of message data
 /// @param bytes number of bytes to digest
-/// @param gen key stream generator, needs to includes the MSB if the LFSR is rolling
+/// @param gen key stream generator, needs to includes the MSB for ROR if the LFSR is rolling
 /// @param key initial key
 /// @return digest value
 uint8_t lfsr_digest8(uint8_t const message[], unsigned bytes, uint8_t gen, uint8_t key);
 
-/// Digest-8 by "LFSR-based Toeplitz hash", byte reflect, bit reflect.
+/// Digest-8 by "LFSR-based Toeplitz hash", byte reversed, bits MSB to LSB.
 ///
-/// @param message bytes of message data
+/// @param message bytes of message data, read in reverse
 /// @param bytes number of bytes to digest
-/// @param gen key stream generator, needs to includes the MSB if the LFSR is rolling
+/// @param gen key stream generator, needs to includes the MSB for ROR if the LFSR is rolling
+/// @param key initial key
+/// @return digest value
+uint8_t lfsr_digest8_reverse(uint8_t const message[], int bytes, uint8_t gen, uint8_t key);
+
+/// Digest-8 by "LFSR-based Toeplitz hash", byte reversed, bit reflect (LSB to MSB).
+///
+/// @param message bytes of message data, read in reverse
+/// @param bytes number of bytes to digest
+/// @param gen key stream generator, needs to includes the LSB for ROL if the LFSR is rolling
 /// @param key initial key
 /// @return digest value
 uint8_t lfsr_digest8_reflect(uint8_t const message[], int bytes, uint8_t gen, uint8_t key);
diff --git a/src/bit_util.c b/src/bit_util.c
index fad3b4af3..a1b9f5eb9 100644
--- a/src/bit_util.c
+++ b/src/bit_util.c
@@ -303,10 +303,12 @@ uint16_t crc16(uint8_t const message[], unsigned nBytes, uint16_t polynomial, ui
 uint8_t lfsr_digest8(uint8_t const message[], unsigned bytes, uint8_t gen, uint8_t key)
 {
     uint8_t sum = 0;
+    // Process message from first byte to last byte
     for (unsigned k = 0; k < bytes; ++k) {
         uint8_t data = message[k];
+        // Process individual bits of each byte (MSB to LSB)
         for (int i = 7; i >= 0; --i) {
-            // fprintf(stderr, "key is %02x\n", key);
+            // fprintf(stderr, "key at %d.%d : %02x\n", k, i, key);
             // XOR key into sum if data bit is set
             if ((data >> i) & 1)
                 sum ^= key;
@@ -322,6 +324,31 @@ uint8_t lfsr_digest8(uint8_t const message[], unsigned bytes, uint8_t gen, uint8
     return sum;
 }
 
+uint8_t lfsr_digest8_reverse(uint8_t const *message, int bytes, uint8_t gen, uint8_t key)
+{
+    uint8_t sum = 0;
+    // Process message from last byte to first byte (reflected)
+    for (int k = bytes - 1; k >= 0; --k) {
+        uint8_t data = message[k];
+        // Process individual bits of each byte (MSB to LSB)
+        for (int i = 7; i >= 0; --i) {
+            // fprintf(stderr, "key at %d.%d : %02x\n", k, i, key);
+            // XOR key into sum if data bit is set
+            if ((data >> i) & 1) {
+                sum ^= key;
+            }
+
+            // roll the key right (actually the lsb is dropped here)
+            // and apply the gen (needs to include the dropped lsb as msb)
+            if (key & 1)
+                key = (key >> 1) ^ gen;
+            else
+                key = (key >> 1);
+        }
+    }
+    return sum;
+}
+
 uint8_t lfsr_digest8_reflect(uint8_t const message[], int bytes, uint8_t gen, uint8_t key)
 {
     uint8_t sum = 0;
@@ -330,14 +357,14 @@ uint8_t lfsr_digest8_reflect(uint8_t const message[], int bytes, uint8_t gen, ui
         uint8_t data = message[k];
         // Process individual bits of each byte (reflected)
         for (int i = 0; i < 8; ++i) {
-            // fprintf(stderr, "key is %02x\n", key);
+            // fprintf(stderr, "key at %d.%d : %02x\n", k, i, key);
             // XOR key into sum if data bit is set
             if ((data >> i) & 1) {
                 sum ^= key;
             }
 
-            // roll the key left (actually the lsb is dropped here)
-            // and apply the gen (needs to include the dropped lsb as msb)
+            // roll the key left (actually the msb is dropped here)
+            // and apply the gen (needs to include the dropped msb as lsb)
             if (key & 0x80)
                 key = (key << 1) ^ gen;
             else