How to use ECDSA for signature verification?

How to use ECDSA for signature verification?

[Marko Mäkelä]

Hi all,

Yesterday, I successfully built the u-boot master branch with 
CONFIG_FIT_SIGNATURE=y and CONFIG_RSA=y and got the signature 
verification working with sha256,rsa2048.

Today, I wanted to try out CONFIG_ECDSA=y, but I am facing some trouble.  
I am generating the key and trying to add its public part to the device 
tree blob as with fdt_add_pubkey as follows:

openssl ecparam -name secp521r1 -genkey -noout -out dev-private.pem
openssl ec -in dev-private.pem -pubout -out dev.pem
cd u-boot
make ...
cp u-boot.dtb u-pubkey.dtb
tools/fdt_add_pubkey -a sha512,secp521r1 -n dev -k .. -r conf u-pubkey.dtb

The next step would be to run make EXT_DTB=u-pubkey.dtb ... so that the 
public key will be embedded and the signature verification will be 
enabled in the final u-boot.img.

Alas, the fdt_add_pubkey command would crash in SIGSEGV in alloc_ctx().  
I figured out that the hash algorithm is not being looked up and 
assigned. Maybe this is not necessary for the -a sha256,rsa2048 which I 
was successfully using previously. The following patch fixes the 
SIGSEGV:

diff --git a/tools/fdt_add_pubkey.c b/tools/fdt_add_pubkey.c
index 5582d7a8efe..4f7028cc15c 100644
--- a/tools/fdt_add_pubkey.c
+++ b/tools/fdt_add_pubkey.c
@@ -73,9 +73,10 @@ static void reset_info(struct image_sign_info *info)
  	info->keyname = keyname;
  	info->name = algo_name;
  	info->require_keys = require_keys;
+	info->checksum = image_get_checksum_algo(algo_name);
  	info->crypto = image_get_crypto_algo(algo_name);
  
-	if (!info->crypto) {
+	if (!info->checksum || !info->crypto) {
  		fprintf(stderr, "Unsupported signature algorithm '%s'\n",
  			algo_name);
  		exit(EXIT_FAILURE);

However, with the above patch applied, I am facing the next trouble:
tools/fdt_add_pubkey -a sha512,secp521r1 -n dev -k .. -r conf u-boot-key.dtb
Can not read key from '../dev.pem'
tools/fdt_add_pubkey: Cannot add public key to FIT blob: Unknown error -5

In GDB, I can see that read_key() in lib/ecdsa/ecdsa-libcrypto.c would 
invoke PEM_read_PrivateKey(), which I assume will fail, because 
../dev.pem only contains a public key. In the planned deployment, the 
private key would be private to a HSM or an external service. Moreover, 
I do not understand why fdt_add_pubkey would attempt to access a private 
key in the first place; that should only be needed for signing the 
fitImage. Both prepare_ctx() and ecdsa_add_verify_data() would return 
-5, which main() is reporting to stderr.

Has some way of using ECDSA based signature verification been tested?  
The reason why I would like to avoid RSA is that it is not thought to be 
quantum-secure, or secure against a cryptalalytic attack by a quantum 
computer.

With best regards,

	Marko Mäkelä

Re: How to use ECDSA for signature verification?

[Marko Mäkelä]

Hi all,

I am new to u-boot, please bear with me. I got CONFIG_FIT_SIGNATURE=y to 
work with the RSA algorithm, but not with ECDSA.

My two main questions are:

Is CONFIG_ECDSA_VERIFY only implemented for the two targets:
rom_api_ops in arch/arm/mach-stm32mp/ecdsa_romapi.c
cptra_ecdsa_ops in drivers/crypto/aspeed/cptra_ecdsa.c. 

Is it feasible to support something more modern than RSA signatures on a 
reasonably high-end target, such as ARMv8? Are there any suggestions or 
git commits that you would suggest as a reference?

Mon, Oct 13, 2025 at 09:36:50PM +0300, Marko Mäkelä wrote:
>Hi all,
>
>Yesterday, I successfully built the u-boot master branch with 
>CONFIG_FIT_SIGNATURE=y and CONFIG_RSA=y and got the signature 
>verification working with sha256,rsa2048.
>
>Today, I wanted to try out CONFIG_ECDSA=y, but I am facing some 
>trouble.  I am generating the key and trying to add its public part to 
>the device tree blob as with fdt_add_pubkey as follows:

I got a bit further with this, using the algorithm sha256,ecdsa256.

With a patch and some work-arounds, I think I understood the host-side 
workflow. However, the signature check on my target (TI Sitara am62x) is 
failing.

Unlike with RSA, the mkimage command expects the ECDSA private key in a 
file like dev.pem, not dev.key. I successfully created a private key and 
constructed a signed FIT image with the following commands:

openssl ecparam -name prime256v1 -genkey -noout -out dev.pem
mkimage -k . -f fitImage.its fitImage

I verified the presence of a signature by running the following command, 
which produced the signature in a "value" subnode:

dtc -I dtb fitImage|grep -A10 signature

For the algorithm sha256,ecdsa256 that I chose, the fdt_add_pubkey tool 
requires a patch to avoid SIGSEGV, which I am copying below from my 
previous message:

diff --git a/tools/fdt_add_pubkey.c b/tools/fdt_add_pubkey.c
index 5582d7a8efe..4f7028cc15c 100644
--- a/tools/fdt_add_pubkey.c
+++ b/tools/fdt_add_pubkey.c
@@ -73,9 +73,10 @@ static void reset_info(struct image_sign_info *info)
  	info->keyname = keyname;
  	info->name = algo_name;
  	info->require_keys = require_keys;
+	info->checksum = image_get_checksum_algo(algo_name);
  	info->crypto = image_get_crypto_algo(algo_name);
  
-	if (!info->crypto) {
+	if (!info->checksum || !info->crypto) {
  		fprintf(stderr, "Unsupported signature algorithm '%s'\n",
  			algo_name);
  		exit(EXIT_FAILURE);

Unlike with RSA, fdt_add_pubkey does not accept a public key:

fdt_add_pubkey: Cannot add public key to FIT blob: Unknown error -5

I am able to work around this by invoking the tool on a _private_ key 
dev.pem that the fitImage had been signed with.

I don't know if there is a cleaner way, but here's how I am embedding 
the public key to the image. I first build U-boot from the scratch, 
modify the generated u-boot.dtb, and finally rebuild to have the 
modified DTB included:

make clean
make -j$(nproc) CROSS_COMPILE=aarch64-linux-gnu- ...
cp u-boot.dtb u-boot-pubkey.dtb
fdt_add_pubkey -a sha256,ecdsa256 -n dev -k . -r conf u-boot-pubkey.dtb
fit_check_sign -f fitImage -k u-boot-pubkey.dtb
make EXT_DTB=u-boot-pubkey.dtb \
-j$(nproc) CROSS_COMPILE=aarch64-linux-gnu- ...

Even though fit_check_sign passes on the host system, the signature 
check would fail on the target system (TI AM62x) as follows:

8304957 bytes read in 346 ms (22.9 MiB/s)
## Executing script at 90000000
sha256,ecdsa256:dev-  error!
Unknown signature algorithm for '<NULL>' hash node in 'conf-1' config node
Failed to verify required signature 'dev'
Boot failed (err=1)

It turns out that for ECDSA signature verification, at least three 
configuration options will be needed:

CONFIG_FIT_SIGNATURE=y
CONFIG_ECDSA=y
CONFIG_ECDSA_VERIFY=y

Rebuilding with CONFIG_ECDSA_VERIFY=y changed the error message to the 
following:

sha256,ecdsa256:dev-  error!
Verification failed for '<NULL>' hash node in 'conf-1' config node
Failed to verify required signature 'dev'

I did not attach a debugger to the target, but I am rather sure that the 
verification fails because of the following:

static int ecdsa_verify_hash(struct udevice *dev,
			     const struct image_sign_info *info,
			     const void *hash, const void *sig, uint sig_len)
{
	const struct ecdsa_ops *ops = device_get_ops(dev);
	const struct checksum_algo *algo = info->checksum;
	struct ecdsa_public_key key;
	int sig_node, key_node, ret;

	if (!ops || !ops->verify)
		return -ENODEV;

I found only two definitions of ecdsa_ops, and it does not look like 
either one should be available on my target system.

The cosmetic error <NULL> in the error message occurs because the local 
variable "noffset" in fit_image_verify_sig() will only be valid if the 
for loop is executing "goto error". This bug is tricky to fix, because 
as far as I understand, we want to allow multiple signatures to be 
verified.

One more thing that I noticed is that the function fit_check_sign() has 
an unused parameter:

diff --git a/tools/fit_check_sign.c b/tools/fit_check_sign.c
index ab3266aff20..3d1e66f2b58 100644
--- a/tools/fit_check_sign.c
+++ b/tools/fit_check_sign.c
@@ -80,7 +80,7 @@ int main(int argc, char **argv)
  			return EXIT_FAILURE;
  	}
  	image_set_host_blob(key_blob);
-	ret = fit_check_sign(fit_blob, key_blob, config_name);
+	ret = fit_check_sign(fit_blob, NULL, config_name);
  	if (!ret) {
  		ret = EXIT_SUCCESS;
  		fprintf(stderr, "Signature check OK\n");

The above patch shows the only caller of the function. It also shows 
that the caller is invoking image_set_host_blob() on that parameter, 
which is why the check is able to work. Maybe that call should be made 
part of fit_check_sign() itself? Anyway, this code does not execute on 
the target.

With best regards,

	Marko Mäkelä

Re: How to use ECDSA for signature verification?

[Anshul Dalal]

Hello Marko,

On Sat Nov 8, 2025 at 10:54 PM IST, Marko Mäkelä wrote:
> Hi all,
>
> I am new to u-boot, please bear with me. I got CONFIG_FIT_SIGNATURE=y to 
> work with the RSA algorithm, but not with ECDSA.
>
> My two main questions are:
>
> Is CONFIG_ECDSA_VERIFY only implemented for the two targets:
> rom_api_ops in arch/arm/mach-stm32mp/ecdsa_romapi.c
> cptra_ecdsa_ops in drivers/crypto/aspeed/cptra_ecdsa.c. 
>

Yes, those two seem to be the only one's implementing UCLASS_ECDSA.

> Is it feasible to support something more modern than RSA signatures on a 
> reasonably high-end target, such as ARMv8? Are there any suggestions or 
> git commits that you would suggest as a reference?
>

Should be possible, you can look at the current implementaitons of RSA
and lib/ecdsa/ecdsa-libcrypto.c for reference.

> Mon, Oct 13, 2025 at 09:36:50PM +0300, Marko Mäkelä wrote:
>>Hi all,
>>
>>Yesterday, I successfully built the u-boot master branch with 
>>CONFIG_FIT_SIGNATURE=y and CONFIG_RSA=y and got the signature 
>>verification working with sha256,rsa2048.
>>
>>Today, I wanted to try out CONFIG_ECDSA=y, but I am facing some 
>>trouble.  I am generating the key and trying to add its public part to 
>>the device tree blob as with fdt_add_pubkey as follows:
>
> I got a bit further with this, using the algorithm sha256,ecdsa256.
>
> With a patch and some work-arounds, I think I understood the host-side 
> workflow. However, the signature check on my target (TI Sitara am62x) is 
> failing.
>
> Unlike with RSA, the mkimage command expects the ECDSA private key in a 
> file like dev.pem, not dev.key. I successfully created a private key and 
> constructed a signed FIT image with the following commands:
>
> openssl ecparam -name prime256v1 -genkey -noout -out dev.pem
> mkimage -k . -f fitImage.its fitImage
>
> I verified the presence of a signature by running the following command, 
> which produced the signature in a "value" subnode:
>
> dtc -I dtb fitImage|grep -A10 signature
>
> For the algorithm sha256,ecdsa256 that I chose, the fdt_add_pubkey tool 
> requires a patch to avoid SIGSEGV, which I am copying below from my 
> previous message:
>
> diff --git a/tools/fdt_add_pubkey.c b/tools/fdt_add_pubkey.c
> index 5582d7a8efe..4f7028cc15c 100644
> --- a/tools/fdt_add_pubkey.c
> +++ b/tools/fdt_add_pubkey.c
> @@ -73,9 +73,10 @@ static void reset_info(struct image_sign_info *info)
>   	info->keyname = keyname;
>   	info->name = algo_name;
>   	info->require_keys = require_keys;
> +	info->checksum = image_get_checksum_algo(algo_name);
>   	info->crypto = image_get_crypto_algo(algo_name);
>   
> -	if (!info->crypto) {
> +	if (!info->checksum || !info->crypto) {
>   		fprintf(stderr, "Unsupported signature algorithm '%s'\n",
>   			algo_name);
>   		exit(EXIT_FAILURE);
>
> Unlike with RSA, fdt_add_pubkey does not accept a public key:
>
> fdt_add_pubkey: Cannot add public key to FIT blob: Unknown error -5
>
> I am able to work around this by invoking the tool on a _private_ key 
> dev.pem that the fitImage had been signed with.
>
> I don't know if there is a cleaner way, but here's how I am embedding 
> the public key to the image. I first build U-boot from the scratch, 
> modify the generated u-boot.dtb, and finally rebuild to have the 
> modified DTB included:
>
> make clean
> make -j$(nproc) CROSS_COMPILE=aarch64-linux-gnu- ...
> cp u-boot.dtb u-boot-pubkey.dtb
> fdt_add_pubkey -a sha256,ecdsa256 -n dev -k . -r conf u-boot-pubkey.dtb
> fit_check_sign -f fitImage -k u-boot-pubkey.dtb
> make EXT_DTB=u-boot-pubkey.dtb \
> -j$(nproc) CROSS_COMPILE=aarch64-linux-gnu- ...
>
> Even though fit_check_sign passes on the host system, the signature 
> check would fail on the target system (TI AM62x) as follows:
>
> 8304957 bytes read in 346 ms (22.9 MiB/s)
> ## Executing script at 90000000
> sha256,ecdsa256:dev-  error!
> Unknown signature algorithm for '<NULL>' hash node in 'conf-1' config node
> Failed to verify required signature 'dev'
> Boot failed (err=1)
>
> It turns out that for ECDSA signature verification, at least three 
> configuration options will be needed:
>
> CONFIG_FIT_SIGNATURE=y
> CONFIG_ECDSA=y
> CONFIG_ECDSA_VERIFY=y
>
> Rebuilding with CONFIG_ECDSA_VERIFY=y changed the error message to the 
> following:
>
> sha256,ecdsa256:dev-  error!
> Verification failed for '<NULL>' hash node in 'conf-1' config node
> Failed to verify required signature 'dev'
>

This is probably due to U-Boot failing to find a driver with
UCLASS_ECDSA, you can verify by adding a "#define DEBUG" to the top of
lib/ecdsa/ecdsa-verify.c and check if the following error shows up:

	ECDSA: Could not find ECDSA implementation: -19

Regards,
Anshul

> I did not attach a debugger to the target, but I am rather sure that the 
> verification fails because of the following:
>
> static int ecdsa_verify_hash(struct udevice *dev,
> 			     const struct image_sign_info *info,
> 			     const void *hash, const void *sig, uint sig_len)
> {
> 	const struct ecdsa_ops *ops = device_get_ops(dev);
> 	const struct checksum_algo *algo = info->checksum;
> 	struct ecdsa_public_key key;
> 	int sig_node, key_node, ret;
>
> 	if (!ops || !ops->verify)
> 		return -ENODEV;
>
> I found only two definitions of ecdsa_ops, and it does not look like 
> either one should be available on my target system.
>
> The cosmetic error <NULL> in the error message occurs because the local 
> variable "noffset" in fit_image_verify_sig() will only be valid if the 
> for loop is executing "goto error". This bug is tricky to fix, because 
> as far as I understand, we want to allow multiple signatures to be 
> verified.
>
> One more thing that I noticed is that the function fit_check_sign() has 
> an unused parameter:
>
> diff --git a/tools/fit_check_sign.c b/tools/fit_check_sign.c
> index ab3266aff20..3d1e66f2b58 100644
> --- a/tools/fit_check_sign.c
> +++ b/tools/fit_check_sign.c
> @@ -80,7 +80,7 @@ int main(int argc, char **argv)
>   			return EXIT_FAILURE;
>   	}
>   	image_set_host_blob(key_blob);
> -	ret = fit_check_sign(fit_blob, key_blob, config_name);
> +	ret = fit_check_sign(fit_blob, NULL, config_name);
>   	if (!ret) {
>   		ret = EXIT_SUCCESS;
>   		fprintf(stderr, "Signature check OK\n");
>
> The above patch shows the only caller of the function. It also shows 
> that the caller is invoking image_set_host_blob() on that parameter, 
> which is why the check is able to work. Maybe that call should be made 
> part of fit_check_sign() itself? Anyway, this code does not execute on 
> the target.
>
> With best regards,
>
> 	Marko Mäkelä

Re: How to use ECDSA for signature verification?

[Marko Mäkelä]

Hello Anshul,

Tue, Nov 11, 2025 at 09:52:51AM +0530, Anshul Dalal wrote:
>Hello Marko,
>
>On Sat Nov 8, 2025 at 10:54 PM IST, Marko Mäkelä wrote:
>> Hi all,
>>
>> I am new to u-boot, please bear with me. I got CONFIG_FIT_SIGNATURE=y to
>> work with the RSA algorithm, but not with ECDSA.
>>
>> My two main questions are:
>>
>> Is CONFIG_ECDSA_VERIFY only implemented for the two targets:
>> rom_api_ops in arch/arm/mach-stm32mp/ecdsa_romapi.c
>> cptra_ecdsa_ops in drivers/crypto/aspeed/cptra_ecdsa.c.
>>
>
>Yes, those two seem to be the only one's implementing UCLASS_ECDSA.
>
>> Is it feasible to support something more modern than RSA signatures on a
>> reasonably high-end target, such as ARMv8? Are there any suggestions or
>> git commits that you would suggest as a reference?
>>
>
>Should be possible, you can look at the current implementaitons of RSA
>and lib/ecdsa/ecdsa-libcrypto.c for reference.

Thank you. I will look at that.

[snip]

>> Rebuilding with CONFIG_ECDSA_VERIFY=y changed the error message to 
>> the
>> following:
>>
>> sha256,ecdsa256:dev-  error!
>> Verification failed for '<NULL>' hash node in 'conf-1' config node
>> Failed to verify required signature 'dev'
>>
>
>This is probably due to U-Boot failing to find a driver with
>UCLASS_ECDSA, you can verify by adding a "#define DEBUG" to the top of
>lib/ecdsa/ecdsa-verify.c and check if the following error shows up:
>
>	ECDSA: Could not find ECDSA implementation: -19

Thank you for the tip. So, the #define DEBUG would enable the debug() 
statements. This indeed confirms my hypothesis:

## Executing script at 90000000
sha256,ecdsa256:devECDSA: Could not find ECDSA implementation: -19
-  error!
Verification failed for '<NULL>' hash node in 'conf-1' config node
Failed to verify required signature 'dev'
Boot failed (err=1)

I'm working on this on a hobby basis for now, and it may take some time 
before I will submit any patches for review.

Best regards,

	Marko

Re: How to use ECDSA for signature verification?

[Anshul Dalal]

On Tue Nov 11, 2025 at 9:26 PM IST, Marko Mäkelä wrote:
> Hello Anshul,
>
> Tue, Nov 11, 2025 at 09:52:51AM +0530, Anshul Dalal wrote:
>>Hello Marko,
>>
>>On Sat Nov 8, 2025 at 10:54 PM IST, Marko Mäkelä wrote:
>>> Hi all,
>>>
>>> I am new to u-boot, please bear with me. I got CONFIG_FIT_SIGNATURE=y to
>>> work with the RSA algorithm, but not with ECDSA.
>>>
>>> My two main questions are:
>>>
>>> Is CONFIG_ECDSA_VERIFY only implemented for the two targets:
>>> rom_api_ops in arch/arm/mach-stm32mp/ecdsa_romapi.c
>>> cptra_ecdsa_ops in drivers/crypto/aspeed/cptra_ecdsa.c.
>>>
>>
>>Yes, those two seem to be the only one's implementing UCLASS_ECDSA.
>>
>>> Is it feasible to support something more modern than RSA signatures on a
>>> reasonably high-end target, such as ARMv8? Are there any suggestions or
>>> git commits that you would suggest as a reference?
>>>
>>
>>Should be possible, you can look at the current implementaitons of RSA
>>and lib/ecdsa/ecdsa-libcrypto.c for reference.

FYI Phillippe Reynes has posted an RFC for the same[1], you can provide
feedback there if interested :)

[1]: https://lore.kernel.org/u-boot/20260202170307.217200-1-philippe.reynes@softathome.com/

>
> Thank you. I will look at that.
>
> [snip]
>
>>> Rebuilding with CONFIG_ECDSA_VERIFY=y changed the error message to 
>>> the
>>> following:
>>>
>>> sha256,ecdsa256:dev-  error!
>>> Verification failed for '<NULL>' hash node in 'conf-1' config node
>>> Failed to verify required signature 'dev'
>>>
>>
>>This is probably due to U-Boot failing to find a driver with
>>UCLASS_ECDSA, you can verify by adding a "#define DEBUG" to the top of
>>lib/ecdsa/ecdsa-verify.c and check if the following error shows up:
>>
>>	ECDSA: Could not find ECDSA implementation: -19
>
> Thank you for the tip. So, the #define DEBUG would enable the debug() 
> statements. This indeed confirms my hypothesis:
>
> ## Executing script at 90000000
> sha256,ecdsa256:devECDSA: Could not find ECDSA implementation: -19
> -  error!
> Verification failed for '<NULL>' hash node in 'conf-1' config node
> Failed to verify required signature 'dev'
> Boot failed (err=1)
>
> I'm working on this on a hobby basis for now, and it may take some time 
> before I will submit any patches for review.
>
> Best regards,
>
> 	Marko