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The jose.4.j library is a robust and easy to use open source implementation of JSON Web Token (JWT) and the JOSE specification suite (JWS, JWE, and JWK). It is written in Java and relies solely on the JCA APIs for cryptography. Please see https://bitbucket.org/b_c/jose4j/wiki/Home for more info, examples, etc..
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/*
 * Copyright 2012-2017 Brian Campbell
 *
 * 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.
 */

package org.jose4j.jwe;

import org.jose4j.jca.ProviderContext;
import org.jose4j.jwa.AlgorithmAvailability;
import org.jose4j.jwa.AlgorithmInfo;
import org.jose4j.jwe.kdf.KdfUtil;
import org.jose4j.jwk.EcJwkGenerator;
import org.jose4j.jwk.EllipticCurveJsonWebKey;
import org.jose4j.jwk.JsonWebKey;
import org.jose4j.jwk.PublicJsonWebKey;
import org.jose4j.jwx.HeaderParameterNames;
import org.jose4j.jwx.Headers;
import org.jose4j.jwx.KeyValidationSupport;
import org.jose4j.keys.EcKeyUtil;
import org.jose4j.keys.EllipticCurves;
import org.jose4j.keys.KeyPersuasion;
import org.jose4j.lang.ByteUtil;
import org.jose4j.lang.InvalidKeyException;
import org.jose4j.lang.JoseException;
import org.jose4j.lang.UncheckedJoseException;

import javax.crypto.KeyAgreement;
import javax.crypto.spec.SecretKeySpec;
import java.math.BigInteger;
import java.security.Key;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.interfaces.ECPrivateKey;
import java.security.interfaces.ECPublicKey;
import java.security.spec.ECFieldFp;
import java.security.spec.ECParameterSpec;
import java.security.spec.ECPoint;
import java.security.spec.EllipticCurve;

/**
 */
public class EcdhKeyAgreementAlgorithm extends AlgorithmInfo implements KeyManagementAlgorithm
{
    String algorithmIdHeaderName = HeaderParameterNames.ENCRYPTION_METHOD;

    public EcdhKeyAgreementAlgorithm()
    {
        setAlgorithmIdentifier(KeyManagementAlgorithmIdentifiers.ECDH_ES);
        setJavaAlgorithm("ECDH");
        setKeyType(EllipticCurveJsonWebKey.KEY_TYPE);
        setKeyPersuasion(KeyPersuasion.ASYMMETRIC);
    }

    public EcdhKeyAgreementAlgorithm(String algorithmIdHeaderName)
    {
        this();
        this.algorithmIdHeaderName = algorithmIdHeaderName;
    }

    public ContentEncryptionKeys manageForEncrypt(Key managementKey, ContentEncryptionKeyDescriptor cekDesc, Headers headers, byte[] cekOverride, ProviderContext providerContext) throws JoseException
    {
        KeyValidationSupport.cekNotAllowed(cekOverride, getAlgorithmIdentifier());
        ECPublicKey receiversKey = (ECPublicKey) managementKey;
        String keyPairGeneratorProvider = providerContext.getGeneralProviderContext().getKeyPairGeneratorProvider();
        SecureRandom secureRandom = providerContext.getSecureRandom();
        EllipticCurveJsonWebKey ephemeralJwk = EcJwkGenerator.generateJwk(receiversKey.getParams(), keyPairGeneratorProvider, secureRandom);
        return manageForEncrypt(managementKey, cekDesc, headers, ephemeralJwk, providerContext);
    }

    ContentEncryptionKeys manageForEncrypt(Key managementKey, ContentEncryptionKeyDescriptor cekDesc, Headers headers, PublicJsonWebKey ephemeralJwk, ProviderContext providerContext) throws JoseException
    {
        headers.setJwkHeaderValue(HeaderParameterNames.EPHEMERAL_PUBLIC_KEY, ephemeralJwk);
        byte[] z = generateEcdhSecret(ephemeralJwk.getPrivateKey(), (PublicKey) managementKey, providerContext);
        byte[] derivedKey = kdf(cekDesc, headers, z, providerContext);
        return new ContentEncryptionKeys(derivedKey, null);
    }

    public Key manageForDecrypt(Key managementKey, byte[] encryptedKey, ContentEncryptionKeyDescriptor cekDesc, Headers headers,  ProviderContext providerContext) throws JoseException
    {
        String keyFactoryProvider = providerContext.getGeneralProviderContext().getKeyFactoryProvider();
        JsonWebKey ephemeralJwk = headers.getPublicJwkHeaderValue(HeaderParameterNames.EPHEMERAL_PUBLIC_KEY, keyFactoryProvider);
        ECPublicKey ephemeralPublicKey = (ECPublicKey) ephemeralJwk.getKey();
        ECPrivateKey privateKey = (ECPrivateKey) managementKey;
        checkPointIsOnCurve(ephemeralPublicKey, privateKey);
        byte[] z = generateEcdhSecret((ECPrivateKey) managementKey, ephemeralPublicKey, providerContext);
        byte[] derivedKey = kdf(cekDesc, headers, z, providerContext);
        String cekAlg = cekDesc.getContentEncryptionKeyAlgorithm();
        return new SecretKeySpec(derivedKey, cekAlg);
    }

    private void checkPointIsOnCurve(ECPublicKey ephemeralPublicKey, ECPrivateKey privateKey) throws JoseException
    {
        // to prevent 'Invalid Curve Attack': for NIST curves, check whether public key is on the private key's curve.
        // from https://www.cs.bris.ac.uk/Research/CryptographySecurity/RWC/2017/nguyen.quan.pdf

        // there appear to be similar checks in the JVM starting with 1.8.0_51 but
        // doing it here explicitly seems prudent

        // (y^2) mod p = (x^3 + ax + b) mod p
        // thanks to Antonio Sanso for guidance on how to do this check
        ECParameterSpec ecParameterSpec = privateKey.getParams();
        EllipticCurve curve = ecParameterSpec.getCurve();
        ECPoint point = ephemeralPublicKey.getW();
        BigInteger x = point.getAffineX();
        BigInteger y = point.getAffineY();
        BigInteger a = curve.getA();
        BigInteger b = curve.getB();
        BigInteger p = ((ECFieldFp) curve.getField()).getP();
        BigInteger leftSide = (y.pow(2)).mod(p);
        BigInteger rightSide = (x.pow(3).add(a.multiply(x)).add(b)).mod(p);
        boolean onCurve = leftSide.equals(rightSide);
        if (!onCurve)
        {
            throw new JoseException(HeaderParameterNames.EPHEMERAL_PUBLIC_KEY + " is invalid for " + EllipticCurves.getName(curve));
        }
    }

    private byte[] kdf(ContentEncryptionKeyDescriptor cekDesc, Headers headers, byte[] z, ProviderContext providerContext)
    {
        String messageDigestProvider = providerContext.getGeneralProviderContext().getMessageDigestProvider();
        KdfUtil kdf = new KdfUtil(messageDigestProvider);
        int keydatalen = ByteUtil.bitLength(cekDesc.getContentEncryptionKeyByteLength());
        /*
           AlgorithmID  In the Direct Key Agreement case, this is set to the
          octets of the UTF-8 representation of the "enc" header parameter
          value.  In the Key Agreement with Key Wrapping case, this is set
          to the octets of the UTF-8 representation of the "alg" header
          parameter value.*/
        String algorithmID = headers.getStringHeaderValue(algorithmIdHeaderName);
        String partyUInfo = headers.getStringHeaderValue(HeaderParameterNames.AGREEMENT_PARTY_U_INFO);
        String partyVInfo = headers.getStringHeaderValue(HeaderParameterNames.AGREEMENT_PARTY_V_INFO);
        return kdf.kdf(z, keydatalen, algorithmID, partyUInfo, partyVInfo);
    }


    private KeyAgreement getKeyAgreement(String provider) throws JoseException
    {
        String javaAlgorithm = getJavaAlgorithm();
        try
        {
            return provider == null ? KeyAgreement.getInstance(javaAlgorithm) : KeyAgreement.getInstance(javaAlgorithm, provider);
        }
        catch (NoSuchAlgorithmException e)
        {
            throw new UncheckedJoseException("No " + javaAlgorithm + " KeyAgreement available.", e);
        }
        catch (NoSuchProviderException e)
        {
            throw new JoseException("Cannot get "+javaAlgorithm+ " KeyAgreement with provider " + provider, e);
        }
    }

    private byte[] generateEcdhSecret(PrivateKey privateKey, PublicKey publicKey, ProviderContext providerContext) throws JoseException
    {
        String keyAgreementProvider = providerContext.getSuppliedKeyProviderContext().getKeyAgreementProvider();
        KeyAgreement keyAgreement = getKeyAgreement(keyAgreementProvider);

        try
        {
            keyAgreement.init(privateKey);
            keyAgreement.doPhase(publicKey, true);
        }
        catch (java.security.InvalidKeyException e)
        {
            throw new InvalidKeyException("Invalid Key for " + getJavaAlgorithm() + " key agreement." ,e);
        }

        return keyAgreement.generateSecret();
    }

    @Override
    public void validateEncryptionKey(Key managementKey, ContentEncryptionAlgorithm contentEncryptionAlg) throws InvalidKeyException
    {
        KeyValidationSupport.castKey(managementKey, ECPublicKey.class);
    }

    @Override
    public void validateDecryptionKey(Key managementKey, ContentEncryptionAlgorithm contentEncryptionAlg) throws InvalidKeyException
    {
        KeyValidationSupport.castKey(managementKey, ECPrivateKey.class);
    }

    @Override
    public boolean isAvailable()
    {
        EcKeyUtil ecKeyUtil = new EcKeyUtil();
        return ecKeyUtil.isAvailable() && AlgorithmAvailability.isAvailable("KeyAgreement", getJavaAlgorithm());
    }
}