CODEWITHSUNDEEP
cesp32ecdsambedtls
Jeremy
Jeremy

Reputation: 1143

ECDSA signature verify very SLOW on ESP32 with mbedtls

We’re using mbedtls on an Espressif ESP32 module (32-bit Xtensa core, 160 MHz) to verify the signature of commands sent over Bluetooth. We use ECDSA signatures and a key pair generated with the secp256r1 curve. The signed commands are sent by a mobile app. We send the public key to the ESP32 during a registration process.

I use a SHA256 hash of the data, giving a hash size of 32 bytes. The signature is 70-73 bytes (DER format I think).

After setting up a context and loading the public key (which is all reasonably fast), I am calling mbedtls_ecdsa_read_signature to verify the signature (against the calculated hash and public key).

The mbedtls_ecdsa_read_signature call takes about 900 mS to 1 second to complete. This is problematic because it makes the device slow to respond to the commands. When choosing this signature verification system, I had indications from a different product team that the signature verify took about 200 mS on an STM32 running at a considerably lower clock speed.

I've compared debug and release builds but that didn't seem to make much difference.

Is this execution time normal / expected? Anything I can do to speed it up?

Here is the gist of my code (simplified, error checking removed):

mbedtls_ecdsa_context ecdsa_context;
mbedtls_ecdsa_init(&ecdsa_context);

mbedtls_ecp_keypair public_key;
mbedtls_ecp_keypair_init(&public_key);

mbedtls_ecp_group_init(&public_key->grp);
mbedtls_ecp_group_load(&public_key->grp, MBEDTLS_ECP_DP_SECP256R1);

mbedtls_ecp_point_init(&public_key->Q);

result = mbedtls_ecp_point_read_binary(
    &public_key->grp,
    &public_key->Q,
    public_key_data, // points to 65 bytes of public key data in uncompressed format
    public_key_data_size
);

int result;

result = mbedtls_ecdsa_read_signature(
    &ecdsa_context,
    message_hash,
    message_hash_size,
    signature,
    signature_size
);

// Check result...
// Free context etc...

-- EDIT --

I figured out that the build system was confused, so I wasn't actually doing a release build. The actual release build dropped the signature verify time to 620 mS, so that's better but still a bit slow.

I also tried increasing the CPU clock speed from 160 MHZ to 240 MHZ. That is a 50% increase, but it only reduced the time by ~20%, so I think it's limited by other factors... maybe flash speed?

Upvotes: 1

Views: 1419

Answers (1)

Jeremy
Jeremy

Reputation: 1143

For anyone else looking for a faster option for ECDH and ECDSA operations on an ESP32, we found that "micro-ecc" was MUCH faster than mbedtls. It was fast enough to make our command signing usable.

See: https://github.com/kmackay/micro-ecc

Upvotes: 2

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