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package name.neuhalfen.projects.crypto.symmetric.keygeneration.impl.derivation;


import static java.util.Objects.requireNonNull;

import java.security.GeneralSecurityException;
import java.util.Arrays;
import javax.annotation.Nullable;
import name.neuhalfen.projects.crypto.internal.Preconditions;
import org.bouncycastle.crypto.DerivationParameters;
import org.bouncycastle.crypto.digests.SHA256Digest;
import org.bouncycastle.crypto.generators.HKDFBytesGenerator;
import org.bouncycastle.crypto.params.HKDFParameters;

/**
 * Implements HKDF with SHA256 as hashing algorithm. The master key ("IKM - input key material") is
 * stored as part of the instance.
 * s
 * 
 * Key  derivation
 * function  on Wikipedia. 
 *  RFC 5869 HMAC-based Extract-and-Expand Key
 * Derivation Function (HKDF) 
 *  NIST
 * Recommendation for Key Derivation through Extraction-then-Expansion, Special Publication
 * SP800-56C 
 *  BSI
 * TR-02102-1  Kryptographische Verfahren: Empfehlungen und Schlussellaengen
 * 
 */
public class HKDFSHA256 implements KeyDerivationFunction {

  private final byte[] masterKey;

  public HKDFSHA256(byte[] masterKey) {
    requireNonNull(masterKey, "masterKey must not be null");

    this.masterKey = Arrays.copyOf(masterKey, masterKey.length);
  }

  @Override
  public byte[] deriveKey(@Nullable byte[] salt, byte[] info, int desiredKeyLengthInBits)
      throws GeneralSecurityException {

    requireNonNull(info, "info must not be null");
    Preconditions.checkArgument(desiredKeyLengthInBits % 8 == 0,
        "desiredKeyLengthInBits must be multiple of 8 but is " + desiredKeyLengthInBits);

    final int desiredKeyLengthInBytes = desiredKeyLengthInBits / 8;

    final DerivationParameters derivationParameters = new HKDFParameters(masterKey, salt, info);

    final SHA256Digest digest = new SHA256Digest();
    final HKDFBytesGenerator hkdfGenerator = new HKDFBytesGenerator(digest);

    hkdfGenerator.init(derivationParameters);

    final byte[] hkdf = new byte[desiredKeyLengthInBytes];

    final int generatedKeyLength = hkdfGenerator.generateBytes(hkdf, 0, hkdf.length);

    if (generatedKeyLength != desiredKeyLengthInBytes) {
      throw new GeneralSecurityException(String
          .format("Failed to derive key. Expected %d bytes, generated %d ", desiredKeyLengthInBytes,
              generatedKeyLength));
    }
    return hkdf;
  }

}