Kalshi signing in Java

kalshi-signing.java

import java.nio.charset.StandardCharsets;
import java.security.PrivateKey;
import java.security.Security;
import java.security.Signature;
import java.util.Base64;

import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.openssl.jcajce.JcaPEMKeyConverter;
import org.bouncycastle.openssl.PEMKeyPair;
import org.bouncycastle.openssl.PEMParser;

// include the bottom code in your classes as needed

if (Security.getProvider("BC") == null) {
  Security.addProvider(new BouncyCastleProvider());
}

PrivateKey loadPrivateKey(String filepath) {
  // required for converting PKCS1 private keys provided by kalshi to PKCS8 required by java
  var pemParser = new PEMParser(new FileReader(filepath, StandardCharsets.UTF_8));
  var converter = new JcaPEMKeyConverter().setProvider("BC");
  var object = pemParser.readObject();
  return converter.getKeyPair((PEMKeyPair) object).getPrivate();
}

String signTextWithPrivateKey(String text, PrivateKey privateKey) {
  var signer = Signature.getInstance("SHA256withRSAandMGF1", "BC"); // same as "SHA256withRSA/PSS"
  signer.initSign(privateKey);
  signer.update(text.getBytes(StandardCharsets.UTF_8));
  return Base64.getEncoder().encodeToString(signer.sign());
}