ECDsa signature not a valid DER sequence

I am developing my own custom U2F for webauthn. It’s working with other demo websites but getting an error while registering with https://www.passwordless.dev/. The error I’m getting is “ECDsa signature not a valid DER sequence”. I’m using window crypto API for signing.

I have been trying to encode it properly that can be verified by OpenSSL properly but couldn’t get it done. Is it feasible to interoperate between windows crypto API and OpenSSL?

Signing code I’m using:

    public async sign(data: Uint8Array): Promise<any> {
        if (!this.privateKey) {
            throw new Error('no private key available for signing');
        }
        return window.crypto.subtle.sign(
            this.getKeyParams(),
            this.privateKey,
            data, //new TextEncoder().encode(data),
        );
    }

    private getKeyParams(): EcdsaParams {
        return { name: 'ECDSA', hash: coseEllipticCurveNames[ECDSA.ellipticCurveKeys[this.algorithm]] };
    }

    private async toCOSE(key: CryptoKey): Promise<Map<number, any>> {
  
        const exportedKey = await window.crypto.subtle.exportKey('jwk', key);
        const attData = new Map();
        attData.set(1, 2); // EC2 key type
        attData.set(3, this.algorithm);
        attData.set(-1, ECDSA.ellipticCurveKeys[this.algorithm]);
        attData.set(-2, base64ToByteArray(exportedKey.x, true));
        attData.set(-3, base64ToByteArray(exportedKey.y, true));
        return attData;
    }

[tarun14110]

Thanks for helping through. It’s working now. There was some issue the way I was returning the signature form ‘then’.

[tarun14110]

• I am developing a WebAuthn authenticator.
• I prepended r with an extra 0x00 because the first byte of r was >0x7F. I was following https://crypto.stackexchange.com/questions/57731/ecdsa-signature-rs-to-asn1-der-encoding-question