/*
 * Copyright contributors to the AERE Network.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
 * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations under the License.
 *
 * SPDX-License-Identifier: Apache-2.0
 */
package org.hyperledger.besu.evm.precompile;

import java.util.Arrays;

import org.apache.tuweni.bytes.Bytes;
import org.apache.tuweni.bytes.Bytes32;
import org.bouncycastle.pqc.crypto.falcon.FalconParameters;
import org.bouncycastle.pqc.crypto.falcon.FalconPublicKeyParameters;
import org.bouncycastle.pqc.crypto.falcon.FalconSigner;

/**
 * Shared logic for the AERE Falcon-512 / Falcon-1024 verification precompiles.
 *
 * <p>Transcodes the NIST round-3 reference "signed message" (sm) blob into the encoding Bouncy
 * Castle's {@link FalconSigner#verifySignature(byte[], byte[])} expects and drives the audited
 * verifier. No cryptography is reimplemented here.
 *
 * <p>NIST sm layout: {@code sigLen(2, big-endian) || nonce(40) || message || esig} where
 * {@code esig = (0x20+logn) || compressedSig} and {@code sigLen == esig.length}. The Falcon public
 * key is {@code (0x00+logn) || packed_h}; Bouncy Castle wants only {@code packed_h} (header
 * stripped), and the signature it wants is {@code (0x30+logn) || nonce(40) || compressedSig}.
 */
final class AereFalconSupport {

  static final int NONCE_LEN = 40;

  private AereFalconSupport() {}

  static Bytes resultWord(final boolean valid) {
    return valid ? Bytes32.leftPad(Bytes.of((byte) 1)) : Bytes32.ZERO;
  }

  /**
   * Verify a NIST signed-message blob against a Falcon public key.
   *
   * @param params Bouncy Castle Falcon parameter set (falcon_512 / falcon_1024)
   * @param logn 9 for Falcon-512, 10 for Falcon-1024
   * @param pkFull the 897- (512) or 1793-byte (1024) public key including the leading header byte
   * @param sm the NIST signed-message blob
   * @return true iff the signature is valid for the embedded message under pkFull
   */
  static boolean verify(
      final FalconParameters params, final int logn, final byte[] pkFull, final byte[] sm) {
    try {
      if (pkFull.length < 2 || sm.length < 2 + NONCE_LEN + 2) {
        return false;
      }
      if ((pkFull[0] & 0xff) != logn) {
        return false;
      }
      final int sigLen = ((sm[0] & 0xff) << 8) | (sm[1] & 0xff);
      if (sigLen < 2 || 2 + NONCE_LEN + sigLen > sm.length) {
        return false;
      }
      final int msgLen = sm.length - 2 - NONCE_LEN - sigLen;
      if (msgLen < 0) {
        return false;
      }
      final byte[] esig = Arrays.copyOfRange(sm, sm.length - sigLen, sm.length);
      if ((esig[0] & 0xff) != (0x20 + logn)) {
        return false;
      }
      final byte[] H = Arrays.copyOfRange(pkFull, 1, pkFull.length);
      final byte[] nonce = Arrays.copyOfRange(sm, 2, 2 + NONCE_LEN);
      final byte[] message = Arrays.copyOfRange(sm, 2 + NONCE_LEN, 2 + NONCE_LEN + msgLen);

      final byte[] bcSig = new byte[1 + NONCE_LEN + (esig.length - 1)];
      bcSig[0] = (byte) (0x30 + logn);
      System.arraycopy(nonce, 0, bcSig, 1, NONCE_LEN);
      System.arraycopy(esig, 1, bcSig, 1 + NONCE_LEN, esig.length - 1);

      final FalconPublicKeyParameters pub = new FalconPublicKeyParameters(params, H);
      final FalconSigner signer = new FalconSigner();
      signer.init(false, pub);
      return signer.verifySignature(message, bcSig);
    } catch (final Throwable t) {
      return false;
    }
  }
}