Add support for multiple same-type signatures with key ID parsing #2305

maulik-arm · 2025-05-14T09:49:46Z

This PR adds support for signing and verifying images with multiple signatures of the same type (e.g., multiple EC256 signatures), enhancing flexibility in secure boot scenarios. It also introduces Key ID TLV parsing to enable the bootloader to select the correct key from a set of built-in keys.

Motivation

Previously, MCUboot only allowed a single signature per image per signature type. This limited use cases where multiple stakeholders need to sign the same image or when fallback keys are required.
This PR removes that limitation by allowing multiple signatures of the same type.

Use Cases

Multi-party signing: e.g., 2 (potentially independent) parties can sign the same image, enabling chain-of-trust across organizational boundaries.
Key rotation or backup: include signatures from both the current and next key, or a recovery key.

Changes Included

1. bootutil: Parse key ID TLV for built-in keys

Adds support for parsing Key ID TLVs in the image when MCUBOOT_BUILTIN_KEY is enabled.
This enables selection of the correct built-in key for verification.

2. imgtool: Add support for multiple signatures and key ID TLVs

Enables signing an image with multiple keys of the same type.
Adds CLI support for passing multiple --key arguments.
Allows optional specification of a key ID per signature, which gets encoded in the TLV.
Updates imgtool test suite to verify both multiple signatures and key ID inclusion.

3. bootutil: Add support for verifying multiple same-type signatures

Updates signature verification logic to loop through all same-type signature TLVs.
When MCUBOOT_BUILTIN_KEY or MCUBOOT_HW_KEY is enabled, the key ID is used to select the appropriate key for verification.

Notes

Backwards compatible: Images with a single signature continue to work as before.
Only takes effect when config 'MCUBOOT_IMAGE_MULTI_SIG_SUPPORT' is enabled.
Designed to work with existing MCUboot signature verification flow with minimal disruption.

davidvincze

Please update signature the data member of Image class to signatures.

boot/bootutil/src/image_validate.c

scripts/imgtool/image.py

davidvincze · 2025-05-21T19:43:40Z

scripts/imgtool/main.py

@@ -450,7 +464,7 @@ def sign(key, public_key_format, align, version, pad_sig, header_size,
         dependencies, load_addr, hex_addr, erased_val, save_enctlv,
         security_counter, boot_record, custom_tlv, rom_fixed, max_align,
         clear, fix_sig, fix_sig_pubkey, sig_out, user_sha, is_pure,
-         vector_to_sign, non_bootable):
+         vector_to_sign, non_bootable, psa_key_ids):


It should be passed to the Image constructor in line 473.
Currently it's only initialized to None which is then used by the create method.

I have updated so that psa_key_ids is applied to the image using the set function in the new PS.

scripts/imgtool/main.py

boot/bootutil/include/bootutil/sign_key.h

boot/bootutil/src/image_multi_sig.c

When MCUBOOT_BUILTIN_KEY is enabled, the key id TLV entry is added to the image. Parse this entry while validating the image to identify the key used to sign the image. This enables future support for scenarios such as multiple built-in keys or multi-signature. Signed-off-by: Maulik Patel <[email protected]> Change-Id: Ibe26bc2b09e63350f4214719606a5aa4bc1be93c

This patch adds support for multiple signatures to single image. This is useful for scenarios where multiple keys are used to sign images, allowing for greater flexibility and security in the image verification process. The tool command line interface is extended to support multiple signatures. The imgtool test suite is updated to test the new functionality. Change-Id: I285b426671f6ad76472f0a2f8fb3a330f8882c3d Signed-off-by: Maulik Patel <[email protected]>

boot/bootutil/CMakeLists.txt

This commit adds functionality to the bootutil library to support multiple sign verfication of same type when 'MCUBOOT_BUILTIN_KEY' or 'MCUBOOT_HW_KEY' is enabled. The image_validate.c file is refactored such that: * bootutil_find_key() find the key is moved to a new file bootutil_find_key.c. * bootutil_image_hash() is moved to a new file bootutil_image_hash.c. * bootutil_img_security_cnt() is moved to a new file bootutil_img_security_cnt.c. This allows common validation code to be reused for multiple signatures. All code specific to multi sign is under the option 'MCUBOOT_IMAGE_MULTI_SIG_SUPPORT'. Furthermore, key id type is updated to uint32_t as per PSA crypto spec. Signed-off-by: Maulik Patel <[email protected]> Change-Id: I05c97ac385c5816c812c51feb010028df8412fe5

Since the key id concept in the PSA specific, rename the variables accordingly. Signed-off-by: Maulik Patel <[email protected]> Change-Id: I8a8a5ceba5554211f185cc4045a6081b6d407507

RcColes

LGTM

Until the PR mcu-tools/mcuboot#2305 is merged, apply the added patches to support multiple signatures in mcuboot. Signed-off-by: Maulik Patel <[email protected]> Change-Id: Id8e62b7f332611858508bd445fab99fd7c0259ab

By default MCUBOOT_IMAGE_MULTI_SIG_SUPPORT is set to OFF, maintaining the existing single-signature behavior. When enabled, RSE secure image is also signed with additional ROTPK key. And both the signaures are verified during boot. This patch works with changes to mcuboot for multi-signature support (PR: mcu-tools/mcuboot#2305) Signed-off-by: Maulik Patel <[email protected]> Change-Id: Ic72d1dcfa3f3ada6c4d275281122f6d919a2d8e1

davidvincze · 2025-06-19T08:42:09Z

boot/bootutil/src/bootutil_find_key.c

+    key_id = (((uint32_t)key_id_buf[0] << 24) |
+                      ((uint32_t)key_id_buf[1] << 16)  |
+                      ((uint32_t)key_id_buf[2] << 8)   |
+                      ((uint32_t)key_id_buf[3]));


As I saw the key ID is now added with the set endianness (not hardcoded big). In this case this reordering is not required anymore, right? Thanks!

davidvincze · 2025-06-19T09:28:51Z

boot/bootutil/src/bootutil_image_hash.c

@@ -0,0 +1,193 @@
+/*


Could you please rename the file to bootutil_img_hash.c?
This naming better aligns with other files, please update the CMake file(s) as well.

davidvincze · 2025-06-19T09:32:51Z

boot/bootutil/CMakeLists.txt

@@ -33,9 +35,24 @@ target_sources(bootutil
        src/swap_scratch.c
        src/tlv.c
 )
+
+if(MCUBOOT_HW_ROLLBACK_PROT)


MCUboot is configured through the mcuboot_config/mcuboot_config.h file, it has no effect on the CMake system. Please include all files unconditionally in the target_sources for bootutil.

davidvincze · 2025-06-19T09:36:55Z

boot/bootutil/CMakeLists.txt

+target_compile_definitions(bootutil
+    PRIVATE
+        $<$<BOOL:${MCUBOOT_IMAGE_MULTI_SIG_SUPPORT}>:MCUBOOT_IMAGE_MULTI_SIG_SUPPORT>
+        $<$<BOOL:${MCUBOOT_IMAGE_MULTI_SIG_SUPPORT}>:MCUBOOT_ROTPK_MAX_KEYS_PER_IMAGE=${MCUBOOT_ROTPK_MAX_KEYS_PER_IMAGE}>
+)


Please use the mcuboot_config.h here as well:

Update the samples/mcuboot_config/mcuboot_config.template.h file describing this new option,

Update the config file (which is part of the porting layer - provided by the "target OS"), for TF-M it should be: https://git.trustedfirmware.org/plugins/gitiles/TF-M/trusted-firmware-m.git/+/refs/heads/main/bl2/ext/mcuboot/include/mcuboot_config/mcuboot_config.h.in (which can be configured from CMake).

davidvincze · 2025-06-19T09:50:40Z

boot/bootutil/src/image_ecdsa.c

@@ -38,7 +38,7 @@
 #if !defined(MCUBOOT_BUILTIN_KEY)
 fih_ret
 bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, size_t slen,
-                    uint8_t key_id)
+                    uint32_t key_id)


Please also update: boot/bootutil/src/image_ed25519.c:141.

davidvincze · 2025-06-19T13:19:34Z

boot/bootutil/src/bootutil_image_hash.c

+#include "bootutil/crypto/sha.h"
+#include "bootutil/fault_injection_hardening.h"
+#include "bootutil/image.h"
+#include "bootutil/sign_key.h"


As I see this include is not needed.

Suggested change

#include "bootutil/sign_key.h"

davidvincze · 2025-06-19T13:24:50Z

boot/bootutil/src/bootutil_find_key.c

+ */
+
+#include <stdint.h>
+#include <flash_map_backend/flash_map_backend.h>


As I see this include is not needed.

Suggested change

#include <flash_map_backend/flash_map_backend.h>

davidvincze · 2025-06-19T13:54:42Z

boot/bootutil/src/image_validate.c

@@ -671,6 +389,15 @@ bootutil_img_validate(struct boot_loader_state *state,
 #ifndef MCUBOOT_SIGN_PURE
            FIH_CALL(bootutil_verify_sig, valid_signature, hash, sizeof(hash),
                                                           buf, len, key_id);
+#ifdef MCUBOOT_IMAGE_MULTI_SIG_SUPPORT
+            rc = boot_plat_check_key_policy((valid_signature == 0), key_id,


Please add its declaration to boot/bootutil/include/bootutil/sign_key.h with some documentation.

davidvincze · 2025-06-19T14:00:24Z

boot/bootutil/src/image_validate.c

+    if (FIH_NOT_EQ(key_must_sign, true) || FIH_NOT_EQ(key_might_sign, true)) {
+        FIH_RET(FIH_FAILURE);
+    } else {
+        FIH_RET(FIH_SUCCESS);


By returning here we are skipping the security_counter_valid check in line 478.

maulik-arm requested review from davidvincze and d3zd3z as code owners May 14, 2025 09:49

davidvincze reviewed May 21, 2025

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nordicjm mentioned this pull request May 22, 2025

Multi-key support with MCUBOOT_HW_KEYS #2311

Closed

davidvincze added area: core Affects core functionality area: imgtool enhancement and removed area: imgtool labels May 22, 2025

maulik-arm force-pushed the feat/multi-sig-same-type branch from 6ce6e96 to e0027a2 Compare May 23, 2025 11:53

RcColes suggested changes Jun 3, 2025

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maulik-arm force-pushed the feat/multi-sig-same-type branch from e0027a2 to 8301988 Compare June 8, 2025 21:22

maulik-arm added 2 commits June 9, 2025 11:08

maulik-arm force-pushed the feat/multi-sig-same-type branch from 8301988 to 9322025 Compare June 9, 2025 12:21

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maulik-arm added 2 commits June 10, 2025 11:25

imgtool: Rename key-ids to psa-key-ids

291cf66

Since the key id concept in the PSA specific, rename the variables accordingly. Signed-off-by: Maulik Patel <[email protected]> Change-Id: I8a8a5ceba5554211f185cc4045a6081b6d407507

maulik-arm force-pushed the feat/multi-sig-same-type branch from 9322025 to 291cf66 Compare June 10, 2025 10:56

RcColes approved these changes Jun 17, 2025

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davidvincze reviewed Jun 19, 2025

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Add support for multiple same-type signatures with key ID parsing #2305

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maulik-arm commented May 14, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Motivation

Use Cases

Changes Included

1. bootutil: Parse key ID TLV for built-in keys

2. imgtool: Add support for multiple signatures and key ID TLVs

3. bootutil: Add support for verifying multiple same-type signatures

Notes

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davidvincze Jun 19, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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maulik-arm commented May 14, 2025 •

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davidvincze Jun 19, 2025 •

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