lib: crc: Add OpenPGP CRC-24

Implementation is based on the section 6.1 of the RFC 4880.

Signed-off-by: Adrien Ricciardi <[email protected]>

include/zephyr/sys/crc.h

@@ -30,6 +30,14 @@ extern "C" {
  */
 #define CRC8_CCITT_INITIAL_VALUE 0xFF
 
+/* Initial value expected to be used at the beginning of the OpenPGP CRC-24 computation. */
+#define CRC24_PGP_INITIAL_VALUE 0x00B704CEU
+/*
+ * The CRC-24 value is stored on a 32-bit value, only the 3 least significant bytes
+ * are meaningful. Use the following mask to only keep the CRC-24 value.
+ */
+#define CRC24_FINAL_VALUE_MASK 0x00FFFFFFU
+
 /**
  * @defgroup checksum Checksum
  * @ingroup os_services
@@ -56,6 +64,7 @@ enum crc_type {
 	CRC16_ANSI,  /**< Use @ref crc16_ansi */
 	CRC16_CCITT, /**< Use @ref crc16_ccitt */
 	CRC16_ITU_T, /**< Use @ref crc16_itu_t */
+	CRC24_PGP,   /**< Use @ref crc24_pgp */
 	CRC32_C,     /**< Use @ref crc32_c */
 	CRC32_IEEE,  /**< Use @ref crc32_ieee */
 };
@@ -315,16 +324,39 @@ uint8_t crc4_ti(uint8_t seed, const uint8_t *src, size_t len);
 uint8_t crc4(const uint8_t *src, size_t len, uint8_t polynomial, uint8_t initial_value,
 	  bool reversed);
 
+/**
+ * @brief Generate an OpenPGP CRC-24 checksum as defined in RFC 4880 section 6.1.
+ *
+ * @param data A pointer to the data on which the CRC will be calculated.
+ * @param len Data length in bytes.
+ *
+ * @return The CRC-24 value.
+ */
+uint32_t crc24_pgp(const uint8_t *data, size_t len);
+
+/**
+ * @brief Update an OpenPGP CRC-24 checksum.
+ *
+ * @param crc The CRC-24 checksum that needs to be updated. The full 32-bit value of the CRC needs
+ *            to be used between calls, do not mask the value to keep only the last 24 bits.
+ * @param data A pointer to the data on which the CRC will be calculated.
+ * @param len  Data length in bytes.
+ *
+ * @return The CRC-24 value. When the last buffer of data has been processed, mask the value
+ *         with CRC24_FINAL_VALUE_MASK to keep only the meaningful 24 bits of the CRC result.
+ */
+uint32_t crc24_pgp_update(uint32_t crc, const uint8_t *data, size_t len);
+
 /**
  * @brief Compute a CRC checksum, in a generic way.
  *
  * This is a dispatch function that calls the individual CRC routine
  * determined by @p type.
  *
- * For 7, 8, and 16-bit CRCs, the relevant @p seed and @p poly values should
+ * For 7, 8, 16 and 24-bit CRCs, the relevant @p seed and @p poly values should
  * be passed in via the least-significant byte(s).
  *
- * Similarly, for 7, 8, and 16-bit CRCs, the relevant result is stored in the
+ * Similarly, for 7, 8, 16 and 24-bit CRCs, the relevant result is stored in the
  * least-significant byte(s) of the returned value.
  *
  * @param type CRC algorithm to use.
@@ -364,6 +396,13 @@ static inline uint32_t crc_by_type(enum crc_type type, const uint8_t *src, size_
 		return crc16_ccitt(seed, src, len);
 	case CRC16_ITU_T:
 		return crc16_itu_t(seed, src, len);
+	case CRC24_PGP: {
+		uint32_t crc = crc24_pgp_update(seed, src, len);
+
+		if (last)
+			crc &= CRC24_FINAL_VALUE_MASK;
+		return crc;
+	}
 	case CRC32_C:
 		return crc32_c(seed, src, len, first, last);
 	case CRC32_IEEE:

lib/crc/CMakeLists.txt

@@ -3,6 +3,7 @@
 zephyr_sources_ifdef(CONFIG_CRC
   crc32c_sw.c
   crc32_sw.c
+  crc24_sw.c
   crc16_sw.c
   crc8_sw.c
   crc7_sw.c

lib/crc/crc24_sw.c

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2024 BayLibre.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/sys/crc.h>
+
+#define CRC24_PGP_POLY 0x01864cfbU
+
+uint32_t crc24_pgp(const uint8_t *data, size_t len)
+{
+	return crc24_pgp_update(CRC24_PGP_INITIAL_VALUE, data, len) & CRC24_FINAL_VALUE_MASK;
+}
+
+/* CRC-24 implementation from the section 6.1 of the RFC 4880 */
+uint32_t crc24_pgp_update(uint32_t crc, const uint8_t *data, size_t len)
+{
+	int i;
+
+	while (len--) {
+		crc ^= (*data++) << 16;
+		for (i = 0; i < 8; i++) {
+			crc <<= 1;
+			if (crc & 0x01000000)
+				crc ^= CRC24_PGP_POLY;
+		}
+	}
+
+	return crc;
+}

lib/crc/crc_shell.c

@@ -28,6 +28,7 @@ static const char *const crc_types[] = {
 	[CRC16_ANSI] = "16_ansi",
 	[CRC16_CCITT] = "16_ccitt",
 	[CRC16_ITU_T] = "16_itu_t",
+	[CRC24_PGP] = "24_pgp",
 	[CRC32_C] = "32_c",
 	[CRC32_IEEE] = "32_ieee",
 };