Add support for multiple same-type signatures with key ID parsing

boot/bootutil/CMakeLists.txt

@@ -1,5 +1,5 @@
 #------------------------------------------------------------------------------
-# Copyright (c) 2020-2023, Arm Limited. All rights reserved.
+# Copyright (c) 2020-2025, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -17,6 +17,9 @@ target_include_directories(bootutil
 target_sources(bootutil
     PRIVATE
         src/boot_record.c
+        src/bootutil_find_key.c
+        src/bootutil_img_hash.c
+        src/bootutil_img_security_cnt.c
         src/bootutil_misc.c
         src/bootutil_public.c
         src/caps.c
@@ -33,6 +36,7 @@ target_sources(bootutil
         src/swap_scratch.c
         src/tlv.c
 )
+
 if(CONFIG_BOOT_RAM_LOAD)
     target_sources(bootutil
         PRIVATE

boot/bootutil/include/bootutil/crypto/ecdsa.h

@@ -394,11 +394,6 @@ static inline void bootutil_ecdsa_init(bootutil_ecdsa_context *ctx)
     ctx->required_algorithm = 0;
 
 #else /* !MCUBOOT_BUILTIN_KEY */
-    /* The incoming key ID is equal to the image index. The key ID value must be
-     * shifted (by one in this case) because zero is reserved (PSA_KEY_ID_NULL)
-     * and considered invalid.
-     */
-    ctx->key_id++; /* Make sure it is not equal to 0. */
 #if defined(MCUBOOT_SIGN_EC256)
     ctx->curve_byte_count = 32;
     ctx->required_algorithm = PSA_ALG_SHA_256;

boot/bootutil/include/bootutil/image.h

@@ -98,6 +98,7 @@ extern "C" {
  */
 #define IMAGE_TLV_KEYHASH           0x01    /* hash of the public key */
 #define IMAGE_TLV_PUBKEY            0x02    /* public key */
+#define IMAGE_TLV_KEYID             0x03    /* Key ID */
 #define IMAGE_TLV_SHA256            0x10    /* SHA256 of image hdr and body */
 #define IMAGE_TLV_SHA384            0x11    /* SHA384 of image hdr and body */
 #define IMAGE_TLV_SHA512            0x12    /* SHA512 of image hdr and body */
@@ -235,6 +236,23 @@ int32_t bootutil_get_img_security_cnt(struct boot_loader_state *state, int slot,
                                       const struct flash_area *fap,
                                       uint32_t *img_security_cnt);
 
+#if defined(MCUBOOT_BUILTIN_KEY)
+int bootutil_find_key(uint8_t image_index, uint8_t *key_id_buf, uint8_t key_id_buf_len);
+#elif defined(MCUBOOT_HW_KEY)
+int bootutil_find_key(uint8_t image_index, uint8_t *key, uint16_t key_len);
+#else
+int bootutil_find_key(uint8_t image_index, uint8_t *keyhash, uint8_t keyhash_len);
+#endif
+
+int bootutil_img_hash(struct boot_loader_state *state,
+                     struct image_header *hdr, const struct flash_area *fap,
+                     uint8_t *tmp_buf, uint32_t tmp_buf_sz, uint8_t *hash_result,
+                     uint8_t *seed, int seed_len
+#if defined(MCUBOOT_SWAP_USING_OFFSET) && defined(MCUBOOT_SERIAL_RECOVERY)
+                     , uint32_t start_offset
+#endif
+);
+
 #ifdef __cplusplus
 }
 #endif

boot/bootutil/include/bootutil/sign_key.h

@@ -27,6 +27,12 @@
 
 /* mcuboot_config.h is needed for MCUBOOT_HW_KEY to work */
 #include "mcuboot_config/mcuboot_config.h"
+#ifdef MCUBOOT_IMAGE_MULTI_SIG_SUPPORT
+#include <stdbool.h>
+#endif /* MCUBOOT_IMAGE_MULTI_SIG_SUPPORT */
+#ifdef MCUBOOT_BUILTIN_KEY
+#include "bootutil/fault_injection_hardening.h"
+#endif /* MCUBOOT_BUILTIN_KEY */
 
 #ifdef __cplusplus
 extern "C" {
@@ -39,6 +45,17 @@ struct bootutil_key {
 };
 
 extern const struct bootutil_key bootutil_keys[];
+#ifdef MCUBOOT_BUILTIN_KEY
+/**
+ * Verify that the specified key ID is valid for authenticating the given image.
+ *
+ * @param[in]  image_index   Index of the image to be verified.
+ * @param[in]  key_id        Identifier of the key to be verified against the image.
+ *
+ * @return                   0 if the key ID is valid for the image; nonzero on failure.
+ */
+fih_ret boot_verify_key_id_for_image(uint8_t image_index, uint32_t key_id);
+#endif /* MCUBOOT_BUILTIN_KEY */
 #else
 struct bootutil_key {
     uint8_t *key;
@@ -51,17 +68,41 @@ extern struct bootutil_key bootutil_keys[];
  * Retrieve the hash of the corresponding public key for image authentication.
  *
  * @param[in]      image_index      Index of the image to be authenticated.
+ * @param[in]      key_index        Index of the key to be used.
  * @param[out]     public_key_hash  Buffer to store the key-hash in.
  * @param[in,out]  key_hash_size    As input the size of the buffer. As output
  *                                  the actual key-hash length.
  *
  * @return                          0 on success; nonzero on failure.
  */
 int boot_retrieve_public_key_hash(uint8_t image_index,
+                                  uint8_t key_index,
                                   uint8_t *public_key_hash,
                                   size_t *key_hash_size);
+
 #endif /* !MCUBOOT_HW_KEY */
 
+#ifdef MCUBOOT_IMAGE_MULTI_SIG_SUPPORT
+/**
+ * @brief Checks the key policy for signature verification.
+ *
+ * Determines whether a given key might or must be used to sign an image,
+ * based on the validity of the signature and the key index. Updates the
+ * provided output parameters to reflect the policy.
+ *
+ * @param valid_sig            Indicates if the signature is valid.
+ * @param key                  The key index to check.
+ * @param[out] key_might_sign  Set to true if the key might be used to sign.
+ * @param[out] key_must_sign   Set to true if the key must be used to sign.
+ * @param[out] key_must_sign_count  Set to the number of keys that must sign.
+ *
+ * @return 0 on success, or a negative error code on failure.
+ */
+int boot_plat_check_key_policy(bool valid_sig, uint32_t key,
+                               bool *key_might_sign, bool *key_must_sign,
+                               uint8_t *key_must_sign_count);
+#endif /* MCUBOOT_IMAGE_MULTI_SIG_SUPPORT */
+
 extern const int bootutil_key_cnt;
 
 #ifdef __cplusplus

boot/bootutil/src/bootutil_find_key.c

@@ -0,0 +1,151 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2017-2019 Linaro LTD
+ * Copyright (c) 2016-2019 JUUL Labs
+ * Copyright (c) 2019-2025 Arm Limited
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * Original license:
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#include <stdint.h>
+
+#include "bootutil/crypto/sha.h"
+#include "bootutil/fault_injection_hardening.h"
+#include "bootutil/image.h"
+#include "bootutil/sign_key.h"
+#include "bootutil_priv.h"
+#include "mcuboot_config/mcuboot_config.h"
+#include "bootutil/bootutil_log.h"
+
+BOOT_LOG_MODULE_DECLARE(mcuboot);
+
+#if defined(MCUBOOT_SIGN_RSA)       || \
+    defined(MCUBOOT_SIGN_EC256)     || \
+    defined(MCUBOOT_SIGN_EC384)     || \
+    defined(MCUBOOT_SIGN_EC)        || \
+    defined(MCUBOOT_SIGN_ED25519)
+#define IMAGE_VALIDATION_EXPECTS_KEY
+#else
+    /* no signing, sha256 digest only */
+#endif
+
+#ifdef IMAGE_VALIDATION_EXPECTS_KEY
+#ifdef MCUBOOT_IMAGE_MULTI_SIG_SUPPORT
+#define NUM_OF_KEYS MCUBOOT_ROTPK_MAX_KEYS_PER_IMAGE
+#else
+#define NUM_OF_KEYS 1
+#endif /* MCUBOOT_IMAGE_MULTI_SIG_SUPPORT */
+
+#if defined(MCUBOOT_BUILTIN_KEY)
+int bootutil_find_key(uint8_t image_index, uint8_t *key_id_buf, uint8_t key_id_buf_len)
+{
+    uint32_t key_id;
+    FIH_DECLARE(fih_rc, FIH_FAILURE);
+
+    BOOT_LOG_DBG("bootutil_find_key: image_index %d", image_index);
+
+    /* Key id is passed */
+    assert(key_id_buf_len == sizeof(uint32_t));
+    memcpy(&key_id, key_id_buf, sizeof(key_id));
+
+    /* Check if key id is associated with the image */
+    FIH_CALL(boot_verify_key_id_for_image, fih_rc, image_index, key_id);
+    if (FIH_EQ(fih_rc, FIH_SUCCESS)) {
+        return (int32_t)key_id;
+    }
+
+    return -1;
+}
+
+#elif defined(MCUBOOT_HW_KEY)
+extern unsigned int pub_key_len;
+int bootutil_find_key(uint8_t image_index, uint8_t *key, uint16_t key_len)
+{
+    bootutil_sha_context sha_ctx;
+    uint8_t hash[IMAGE_HASH_SIZE];
+    uint8_t key_hash[IMAGE_HASH_SIZE];
+    size_t key_hash_size = sizeof(key_hash);
+    int rc;
+    uint8_t key_index;
+    FIH_DECLARE(fih_rc, FIH_FAILURE);
+
+    BOOT_LOG_DBG("bootutil_find_key: image_index %d", image_index);
+
+    bootutil_sha_init(&sha_ctx);
+    bootutil_sha_update(&sha_ctx, key, key_len);
+    bootutil_sha_finish(&sha_ctx, hash);
+    bootutil_sha_drop(&sha_ctx);
+
+    for(key_index = 0; key_index < NUM_OF_KEYS; key_index++) {
+        rc = boot_retrieve_public_key_hash(image_index, key_index, key_hash, &key_hash_size);
+        if (rc) {
+            return -1;
+        }
+
+        /* Adding hardening to avoid this potential attack:
+         *  - Image is signed with an arbitrary key and the corresponding public
+         *    key is added as a TLV field.
+         * - During public key validation (comparing against key-hash read from
+         *   HW) a fault is injected to accept the public key as valid one.
+         */
+        FIH_CALL(boot_fih_memequal, fih_rc, hash, key_hash, key_hash_size);
+        if (FIH_EQ(fih_rc, FIH_SUCCESS)) {
+            BOOT_LOG_INF("Key %d hash found for image %d", key_index, image_index);
+            bootutil_keys[0].key = key;
+            pub_key_len = key_len;
+            return 0;
+        }
+    }
+    BOOT_LOG_ERR("Key hash NOT found for image %d!", image_index);
+
+    return -1;
+}
+
+#else /* !defined MCUBOOT_BUILTIN_KEY && !defined MCUBOOT_HW_KEY */
+int bootutil_find_key(uint8_t image_index, uint8_t *keyhash, uint8_t keyhash_len)
+{
+    bootutil_sha_context sha_ctx;
+    int i;
+    uint8_t hash[IMAGE_HASH_SIZE];
+    const struct bootutil_key *key;
+    (void)image_index;
+
+    BOOT_LOG_DBG("bootutil_find_key");
+
+    if (keyhash_len > IMAGE_HASH_SIZE) {
+        return -1;
+    }
+
+    for (i = 0; i < bootutil_key_cnt; i++) {
+        key = &bootutil_keys[i];
+        bootutil_sha_init(&sha_ctx);
+        bootutil_sha_update(&sha_ctx, key->key, *key->len);
+        bootutil_sha_finish(&sha_ctx, hash);
+        bootutil_sha_drop(&sha_ctx);
+        if (!memcmp(hash, keyhash, keyhash_len)) {
+            return i;
+        }
+    }
+    return -1;
+}
+#endif
+#endif /* IMAGE_VALIDATION_EXPECTS_KEY */