React Native

ECDSA Sign Data and Get Raw R and S Values

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Demonstrates getting the raw R and S value of an ECDSA signature. React Native

import { Asn, BinData, Crypt2, Ecc, PrivateKey, Prng, Xml } from '@chilkat/react-native'

function chilkatExample() {
  // This example requires the Chilkat API to have been previously unlocked.
  // See Global Unlock Sample for sample code.

  // To create an ECDSA signature, the data first needs to be hashed.  Then the hash
  // is signed.

  const crypt = new Crypt2();
  crypt.hashAlgorithm = 'SHA256';
  crypt.charset = 'utf-8';
  crypt.encodingMode = 'base64';

  // Hash a string.
  const hash1 = crypt.hashStringENC('The quick brown fox jumps over the lazy dog');
  console.log(`hash1 = ${hash1}`);

  // -----------------------------------------------------------
  // An ECDSA private key is used for signing.  The public key is for signature verification.
  // Load our ECC private key.
  // Our private key file contains this:

  // 	// -----BEGIN PRIVATE KEY-----
  // 	MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg3J8q/24D1sEKGdP9
  // 	72MGYElLGpw/a56Y3t6pfON3uhShRANCAATlSmoizyhAwoYZAOuFBATl07/1RR54
  // 	a1Dzfm16grxJe666AGKR+bSs24hk7TEpaeCTvT8YOOM3l+xKFg7zq6Q9
  // 	-----END PRIVATE KEY-----

  const privKey = new PrivateKey();
  try {
    privKey.loadPemFile('qa_data/ecc/secp256r1-key-pkcs8.pem');
  } catch {
    console.log(privKey.lastErrorText);
    return;
  }

  // Sign the hash..
  const prng = new Prng();
  const ecdsa = new Ecc();
  let ecdsaSigBase64: string;
  try {
    ecdsaSigBase64 = ecdsa.signHashENC(hash1, 'base64', privKey, prng);
  } catch {
    console.log(ecdsa.lastErrorText);
    return;
  }

  // The ECDSA signature is ASN.1 that contains a sequence of 2 large integers (r and s)
  // For example:
  // SEQUENCE (2 elem)
  //   INTEGER (255 bit) 792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9
  //   INTEGER (255 bit) 481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5

  console.log(`Base64 ECDSA signature = ${ecdsaSigBase64}`);

  // If the raw R and S values are needed, here's how to get them:
  const asn = new Asn();
  try {
    asn.loadEncoded(ecdsaSigBase64, 'base64');
  } catch {
    console.log(asn.lastErrorText);
    return;
  }

  // The R and X will be in hexidecimal in the XML.
  const xml = new Xml();
  xml.loadXml(asn.asnToXml());
  console.log(xml.getXml());

  // The XML looks like this:
  // <sequence>
  // <int>792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9</int>
  // <int>481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5</int>
  // </sequence>

  // Copy raw R and S hex values into a Chilkat BinData object.
  const bd = new BinData();
  const r = xml.getChildContent('int[0]');
  const s = xml.getChildContent('int[1]');
  bd.appendEncoded(r, 'hex');
  bd.appendEncoded(s, 'hex');

  console.log(`Number of bytes in bd: ${bd.numBytes}`);
}