src/com/android/remoteprovisioner/X509Utils.java

/**
 * Copyright (C) 2021 The Android Open Source Project
 *
 * 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 com.android.remoteprovisioner;

import java.io.ByteArrayInputStream;
import java.math.BigInteger;
import java.security.cert.Certificate;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.interfaces.ECPublicKey;
import java.util.ArrayList;

/**
 * Provides convenience methods for parsing certificates and extracting information.
 */
public class X509Utils {

    /**
     * Takes a byte array composed of DER encoded certificates and returns the X.509 certificates
     * contained within as an X509Certificate array.
     */
    public static X509Certificate[] formatX509Certs(byte[] certStream)
            throws CertificateException {
        CertificateFactory fact = CertificateFactory.getInstance("X.509");
        ByteArrayInputStream in = new ByteArrayInputStream(certStream);
        ArrayList<Certificate> certs = new ArrayList<Certificate>(fact.generateCertificates(in));
        return certs.toArray(new X509Certificate[certs.size()]);
    }

    /**
     * Extracts an ECDSA-P256 key from a certificate and formats it so that it can be used to match
     * the certificate chain to the proper key when passed into the keystore database.
     */
    public static byte[] getAndFormatRawPublicKey(X509Certificate cert) {
        ECPublicKey key = (ECPublicKey) cert.getPublicKey();
        // Remote key provisioning internally supports the default, uncompressed public key
        // format for ECDSA. This defines the format as (s | x | y), where s is the byte
        // indicating if the key is compressed or not, and x and y make up the EC point.
        // However, the key as stored in a COSE_Key object is just the two points. As such,
        // the raw public key is stored in the database as (x | y), so the compression byte
        // should be dropped here. Leading 0's must be preserved.
        //
        // s: 1 byte, x: 32 bytes, y: 32 bytes
        BigInteger xCoord = key.getW().getAffineX();
        BigInteger yCoord = key.getW().getAffineY();
        byte[] formattedKey = new byte[64];
        byte[] xBytes = xCoord.toByteArray();
        // BigInteger returns the value as two's complement big endian byte encoding. This means
        // that a positive, 32-byte value with a leading 1 bit will be converted to a byte array of
        // length 33 in order to include a leading 0 bit.
        if (xBytes.length == 33) {
            System.arraycopy(xBytes, 1 /* offset */, formattedKey, 0 /* offset */, 32);
        } else {
            System.arraycopy(xBytes, 0 /* offset */,
                             formattedKey, 32 - xBytes.length, xBytes.length);
        }
        byte[] yBytes = yCoord.toByteArray();
        if (yBytes.length == 33) {
            System.arraycopy(yBytes, 1 /* offset */, formattedKey, 32 /* offset */, 32);
        } else {
            System.arraycopy(yBytes, 0 /* offset */,
                             formattedKey, 64 - yBytes.length, yBytes.length);
        }
        return formattedKey;
    }
}