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.

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]>

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. 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

davidvincze

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

davidvincze · 2025-05-21T13:56:30Z

boot/bootutil/src/image_validate.c

+#else
+/* For MCUBOOT_BUILTIN_KEY, key id is passed */
+#define EXPECTED_KEY_TLV     IMAGE_TLV_KEYID
+#define KEY_BUF_SIZE         sizeof(int)


Suggested change

#define KEY_BUF_SIZE sizeof(int)

#define KEY_BUF_SIZE sizeof(int32_t)

davidvincze · 2025-05-21T13:58:38Z

boot/bootutil/src/image_validate.c

+    return ((int32_t)key_id_buf[0] << 24) |
+            ((int32_t)key_id_buf[1] << 16) |
+            ((int32_t)key_id_buf[2] << 8)  |
+            ((int32_t)key_id_buf[3]);


davidvincze · 2025-05-21T14:43:20Z

scripts/imgtool/image.py

@@ -135,13 +136,19 @@ def add(self, kind, payload):
        """
        e = STRUCT_ENDIAN_DICT[self.endian]
        if isinstance(kind, int):
-            if not TLV_VENDOR_RES_MIN <= kind <= TLV_VENDOR_RES_MAX:
+            if kind in TLV_VALUES.values():
+                buf = struct.pack(e + 'I', kind)


The payload length is missing (should be it_type + it_len).

davidvincze · 2025-05-21T14:49:26Z

scripts/imgtool/image.py

+    def _add_key_id_tlv_to_unprotected(self, tlv, key_id: int):
+        """Add a key ID TLV into the *unprotected* TLV area."""
+        tag = TLV_VALUES['KEYID']
+        value = key_id.to_bytes(4, 'big')


why 'big-endian'?

davidvincze · 2025-05-21T19:21:09Z

scripts/imgtool/image.py

+        base_tlv_off = header_size + img_size
+        tlv_off = base_tlv_off


What's the reason behind it? Leftover code from development?

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.

davidvincze · 2025-05-21T19:46:31Z

scripts/imgtool/main.py

@@ -441,6 +453,8 @@ def convert(self, value, param, ctx):
              help='send to OUTFILE the payload or payload''s digest instead '
              'of complied image. These data can be used for external image '
              'signing')
+@click.option('--psa-key-ids', multiple=True, type=int, required=False,


I believe multiple=True is not needed if it takes a list of IDs.

davidvincze · 2025-05-21T19:50:27Z

scripts/imgtool/main.py

@@ -97,6 +97,15 @@ def save_signature(sigfile, sig):
        signature = base64.b64encode(sig)


save_signature can be deleted if unused

maulik-arm added 3 commits May 14, 2025 10:28

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

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

6ce6e96

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

davidvincze reviewed May 21, 2025

View reviewed changes

nordicjm mentioned this pull request May 22, 2025

Multi-key support with MCUBOOT_HW_KEYS #2311

Open

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

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	#define KEY_BUF_SIZE sizeof(int)
	#define KEY_BUF_SIZE sizeof(int32_t)

		@@ -97,6 +97,15 @@ def save_signature(sigfile, sig):
		signature = base64.b64encode(sig)

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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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maulik-arm commented May 14, 2025 •

edited

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