java算法中使用的ASN编码格式

前言

java中的算法实现，不论是生成密钥、生成签名都会对结果进行编码，而编码方式则是ASN.1格式

java.security.Key

这个接口是java中算法密钥接口，定义了所有算法密钥的最顶层设计。

其中getFormat()接口返回这个密钥的编码方式。

getEncoded()接口返回编码后的密钥值。

其中密钥的编码格式是什么？其注释中也有相关说明

/**
* <LI>An Encoded Form
 *
 * <P>This is an external encoded form for the key used when a standard
 * representation of the key is needed outside the Java Virtual Machine,
 * as when transmitting the key to some other party. The key
 * is encoded according to a standard format (such as
 * X.509 {@code SubjectPublicKeyInfo} or PKCS#8), and
 * is returned using the {@link #getEncoded() getEncoded} method.
 * Note: The syntax of the ASN.1 type {@code SubjectPublicKeyInfo}
 * is defined as follows:
 *
* <pre>
 * SubjectPublicKeyInfo ::= SEQUENCE {
 *   algorithm AlgorithmIdentifier,
 *   subjectPublicKey BIT STRING }
 *
 * AlgorithmIdentifier ::= SEQUENCE {
 *   algorithm OBJECT IDENTIFIER,
 *   parameters ANY DEFINED BY algorithm OPTIONAL }
 * </pre>
 *
 * For more information, see
 * <a href="http://www.ietf.org/rfc/rfc3280.txt">RFC 3280:
 * Internet X.509 Public Key Infrastructure Certificate and CRL Profile</a>.
 *
 * <LI>A Format
 *
 * <P>This is the name of the format of the encoded key. It is returned
 * by the {@link #getFormat() getFormat} method.
 *
 * </UL>
 **/

由上面注释可知，其中publicKey的格式是按照ASN.1格式去编码，并且有SubjectPublicKeyInfo结构体来承载publicKey的信息。

java rsa算法解析编码后的公私玥

public static void main(String[] args) throws Exception{
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
        keyPairGenerator.initialize(1024);
        KeyPair keyPair = keyPairGenerator.generateKeyPair();

        /**
         * 解析编码后的publicKey
         */
        PublicKey publicKey = keyPair.getPublic();

        byte[] b = publicKey.getEncoded();
        SubjectPublicKeyInfo publicKeyInfo = SubjectPublicKeyInfo.getInstance(b);
        AlgorithmIdentifier algorithmIdentifier = publicKeyInfo.getAlgorithm();
        DERBitString derBitString = publicKeyInfo.getPublicKeyData();
        byte[] bit = derBitString.getBytes();
        System.out.println(HexUtil.byteToHex(bit));

        ASN1Sequence seq = ASN1Sequence.getInstance(bit);
        byte[] exponent = ASN1Integer.getInstance(seq.getObjectAt(0)).getValue().toByteArray();
        byte[] modulus = ASN1Integer.getInstance(seq.getObjectAt(1)).getValue().toByteArray();

        System.out.println(HexUtil.byteToHex(exponent));
        System.out.println(HexUtil.byteToHex(modulus));


        /**
         * 解析编码后的privateKey
         */
        PrivateKey privateKey = keyPair.getPrivate();
        byte[] bb = privateKey.getEncoded();

        ASN1Sequence seq1 = ASN1Sequence.getInstance(bb);
        ASN1OctetString derOctetString = ASN1OctetString.getInstance(seq1.getObjectAt(2));
        byte[] bbb = derOctetString.getOctets();

        ASN1Sequence seq11 = ASN1Sequence.getInstance(bbb);
        byte[] pri_exponent = 	    ASN1Integer.getInstance(seq11.getObjectAt(1)).getValue().toByteArray();
        byte[] pri_modulus = ASN1Integer.getInstance(seq11.getObjectAt(3)).getValue().toByteArray();

        System.out.println(HexUtil.byteToHex(pri_exponent));
        System.out.println(HexUtil.byteToHex(pri_modulus));

    }

java ecc算法解析编码后的签名值

import com.sankuai.cx.etcp.code.util.HexUtil;
import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.DERSequence;

import java.io.IOException;
import java.math.BigInteger;
import java.util.Arrays;

/**
 * @Author:wangchao
 * @Date: 2019/8/8 11:17 AM
 * @Description:
 **/
public class ASNUtil {

    /**
     * BC的SM3withSM2验签需要的rs是asn1格式的，这个方法将直接拼接r||s的字节数组转化成asn1格式
     * @param sign in plain byte array
     * @return rs result in asn1 format
     */
    public static byte[] rsPlainByteArrayToAsn1(byte[] sign,int rsLen){
        if(sign.length != rsLen * 2) {
            throw new RuntimeException("err rs. ");
        }
        BigInteger r = new BigInteger(1, Arrays.copyOfRange(sign, 0, rsLen));
        BigInteger s = new BigInteger(1, Arrays.copyOfRange(sign, rsLen, rsLen * 2));
        ASN1EncodableVector v = new ASN1EncodableVector();
        v.add(new ASN1Integer(r));
        v.add(new ASN1Integer(s));
        try {
            return new DERSequence(v).getEncoded("DER");
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * BC的SM3withSM2签名得到的结果的rs是asn1格式的，这个方法转化成直接拼接r||s
     * @param rsDer rs in asn1 format
     * @return sign result in plain byte array
     */
    public static byte[] rsAsn1ToPlainByteArray(byte[] rsDer,int rsLen){
        ASN1Sequence seq = ASN1Sequence.getInstance(rsDer);
        byte[] r = bigIntToFixexLengthBytes(ASN1Integer.getInstance(seq.getObjectAt(0)).getValue(),rsLen);
        byte[] s = bigIntToFixexLengthBytes(ASN1Integer.getInstance(seq.getObjectAt(1)).getValue(),rsLen);
        byte[] result = new byte[rsLen * 2];
        System.arraycopy(r, 0, result, 0, r.length);
        System.arraycopy(s, 0, result, rsLen, s.length);
        return result;
    }

    public static byte[] bigIntToFixexLengthBytes(BigInteger rOrS,int FixedLength){
        // for sm2p256v1, n is 00fffffffeffffffffffffffffffffffff7203df6b21c6052b53bbf40939d54123,
        // r and s are the result of mod n, so they should be less than n and have length<=32
        byte[] rs = rOrS.toByteArray();
        if(rs.length == FixedLength) {
            return rs;
        }
        else if(rs.length == FixedLength + 1 && rs[0] == 0){
            return Arrays.copyOfRange(rs, 1, FixedLength + 1);
        }
        else if(rs.length < FixedLength) {
            byte[] result = new byte[FixedLength];
            Arrays.fill(result, (byte)0);
            System.arraycopy(rs, 0, result, FixedLength - rs.length, rs.length);
            return result;
        } else {
            throw new RuntimeException("err rs: " + HexUtil.byteToHex(rs));
        }
    }
}

注意：上面两段代码示例，均依赖 bouncycastle的实现

<dependency>
  <groupId>org.bouncycastle</groupId>
  <artifactId>bcmail-jdk15on</artifactId>
  <version>1.59</version>
</dependency>

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