How to get R and S values from ECDSA signature using PHP - php

Question

I am working on Signature of data using ECDSA from Starkbank library and I can get the base64 format of the signature value MEUCIALlD6Xsd0Xdj7XTrD2gP4Q3PlssTxLOCUi6R8FbXMlbAiEAmW8HLiBnhaBBPzIL64FGzFYzUwF1HfX+a8ep5/NpI0k= and the Der value is 0E ���wEݏ�Ӭ=�?�7>[,O� H�G�[\�[!�o. g��A?2�F�V3Su��kǩ��i#I
but I want to know the R and S values with the length as well, how can I achieve it?
my PHP code is:
<?php
require_once "src/ellipticcurve.php";
#privateKey from PEM string
$privateKey = EllipticCurve\PrivateKey::fromPem("
-----BEGIN EC PRIVATE KEY-----
MIGEAgEAMBAGByqGSM49AgEGBSuBBAAKBG0wawIBAQQgPUAdAJuELXoxEumrKUPd
yHLP9bITTV+yOSw5q1H8W/2hRANCAASW6MSUA/wJRcj0AljN0tnpMBp5ISqTp8j/
rY7C2BXCXyy03V/lP7jn0LSgJvykVyNRPXfA4zjpFRaOUNWUBNuU
-----END EC PRIVATE KEY-----
");
$message = "j4+wDOaRLRQn7oweoCbob1WDaqPRCTHzonn08b+dJr0";
$signature = EllipticCurve\Ecdsa::sign($message, $privateKey);
# Generate Signature in base64. This result can be sent to Stark Bank in header as Digital-Signature parameter
$base64 = $signature->toBase64();
$der = $signature->toDer();
echo "\n" . $der;
echo "\n" . $base64;
$publicKeyPem = EllipticCurve\Utils\File::read("publicKey.pem");
$publicKey = EllipticCurve\PublicKey::fromPem($publicKeyPem);
# To double check if message matches the signature
//$publicKey = $privateKey->publicKey();
echo "\n" . EllipticCurve\Ecdsa::verify($message, $signature, $publicKey);
?>
the ECDSA config which I am working on:
digest_alg = "sha256",
private_key_bits = 2048,
private_key_type = OPENSSL_KEYTYPE_EC,
curve_name = secp256k1,

Answer 1

You Can Use SOP/ASN1 To get the Sequence from DER.
Try https://lapo.it/asn1js to find the Positions from R and S. Use bin2hex($signature->toDer()) to get Hex-Encoded ASN.1
It depends on the Type of your $signature->toDer().
Maybe your Hexstring is already Sequence, then following two ANS.1-Integers (R/S).
Then use for e.g.
$seq = \Sop\ASN1\Type\UnspecifiedType::fromDER($signature->toDer())->asSequence();
$seq->at(0)->asInteger()...
$seq->at(1)->asInteger()...