一、序言

之前在《前后端RSA互相加解密、加签验签、密钥对生成》中提到过PKCS#1格式和PKCS#8格式密钥的区别以及如何生成密钥。实际有些场景中有可能也会涉及到前后端密钥格式不一致，这篇文章我们会讨论关于PKCS#1和PKCS#8格式密钥的互相转换。

这里我们会用到Bouncy Castle，它提供了各种加密算法的Java实现，其中Java相关的资料可以参考Bouncy Castle Github。

二、代码示例

1、Maven依赖

<dependency>
	 <groupId>org.bouncycastle</groupId>
	 <artifactId>bcpkix-jdk18on</artifactId>
	 <version>1.72</version>
</dependency>

2、工具类封装

package com.universe.crypto;

import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.DERBitString;
import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.asn1.pkcs.RSAPrivateKey;
import org.bouncycastle.asn1.pkcs.RSAPublicKey;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.util.io.pem.PemObject;
import org.bouncycastle.util.io.pem.PemReader;
import org.bouncycastle.util.io.pem.PemWriter;

import java.io.IOException;
import java.io.InputStreamReader;
import java.io.StringReader;
import java.io.StringWriter;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Security;
import java.security.spec.KeySpec;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.RSAPublicKeySpec;
import java.util.Base64;
import java.util.Base64.Decoder;
import java.util.Base64.Encoder;

/**
 * @author Nick Liu
 * @date 2023/2/2
 */
public class AsymmetricKeyUtils {

	private static final String ALGORITHM_NAME = "RSA";
	private static final Encoder BASE64_ENCODER = Base64.getEncoder();
	private static final Decoder BASE64_DECODER = Base64.getDecoder();

	static {
		// 必须创建Bouncy Castle提供者
		Security.addProvider(new BouncyCastleProvider());
	}

	/**
	 * 格式化密钥为标准Pem格式
	 * @param keyFormat 密钥格式，参考{@link KeyFormat}
	 * @param asymmetricKey 非对称密钥
	 * @return .pem格式密钥字符串
	 * @throws IOException
	 */
	public static String formatKeyAsPemString(KeyFormat keyFormat, String asymmetricKey) throws IOException {
		byte[] keyInBytes = BASE64_DECODER.decode(asymmetricKey);
		PemObject pemObject = new PemObject(keyFormat.getName(), keyInBytes);
		try (StringWriter stringWriter = new StringWriter()) {
			PemWriter pemWriter = new PemWriter(stringWriter);
			pemWriter.writeObject(pemObject);
			pemWriter.flush();
			return stringWriter.toString();
		}
	}

	/**
	 * 从标准Pem格式中提取密钥
	 * @param asymmetricKeyAsPem
	 * @return 无---BEGIN---和---END---前后缀的密钥字符串
	 * @throws IOException
	 */
	public static String extractKeyFromPemString(String asymmetricKeyAsPem) throws IOException {
		try (PemReader pemReader = new PemReader(new StringReader(asymmetricKeyAsPem))) {
			PemObject pemObject = pemReader.readPemObject();
			return BASE64_ENCODER.encodeToString(pemObject.getContent());
		}
	}

	/**
	 * 从文件中提取密钥
	 * @param pemFilePath
	 * @return 无---BEGIN---和---END---前后缀的密钥字符串
	 * @throws Exception
	 */
	public static String readKeyFromPath(String pemFilePath) throws Exception {
		try (PemReader pemReader = new PemReader(new InputStreamReader(Files.newInputStream(Paths.get(pemFilePath))))) {
			PemObject pemObject = pemReader.readPemObject();
			return BASE64_ENCODER.encodeToString(pemObject.getContent());
		}
	}

	/**
	 * 将PKCS1公钥转换为PKCS8公钥
	 * @param pubKeyInPKCS1 PKCS1形式公钥
	 * @return PKCS8形式公钥
	 * @throws Exception
	 */
	public static String transformPubKeyFromPkcs1ToPkcs8(String pubKeyInPKCS1) throws Exception {
		RSAPublicKey rsaPublicKey = RSAPublicKey.getInstance(BASE64_DECODER.decode(pubKeyInPKCS1));
		KeySpec keySpec = new RSAPublicKeySpec(rsaPublicKey.getModulus(), rsaPublicKey.getPublicExponent());

		KeyFactory keyFactory = KeyFactory.getInstance(ALGORITHM_NAME);
		PublicKey publicKey = keyFactory.generatePublic(keySpec);
		return BASE64_ENCODER.encodeToString(publicKey.getEncoded());
	}

	/**
	 * 将PKCS8公钥转换为PKCS1公钥
	 * @param pubKeyInPKCS8 PKCS8公钥
	 * @return PKCS1公钥
	 */
	public static String transformPubKeyFromPkcs8ToPkcs1(String pubKeyInPKCS8) {
		ASN1Sequence publicKeyASN1Object = ASN1Sequence.getInstance(BASE64_DECODER.decode(pubKeyInPKCS8));
		DERBitString derBitString = (DERBitString) publicKeyASN1Object.getObjectAt(1);
		return BASE64_ENCODER.encodeToString(derBitString.getBytes());
	}

	/**
	 * 将PKCS1私钥转换为PKCS8公钥
	 * @param privateKeyInPKCS1 PKCS1公钥
	 * @return PKCS8公钥
	 * @throws Exception
	 */
	public static String transformPrivateKeyFromPkcs1ToPkcs8(String privateKeyInPKCS1) throws Exception {
		KeySpec keySpec = new PKCS8EncodedKeySpec(BASE64_DECODER.decode(privateKeyInPKCS1));
		KeyFactory keyFactory = KeyFactory.getInstance(ALGORITHM_NAME);
		PrivateKey privateKey = keyFactory.generatePrivate(keySpec);
		return BASE64_ENCODER.encodeToString(privateKey.getEncoded());
	}

	/**
	 * 将PKCS1私钥转换为PKCS8私钥
	 * @param privateKeyInPKCS8 PKCS8私钥
	 * @return PKCS1私钥
	 * @throws Exception
	 */
	public static String transformPrivateKeyFromPkcs8ToPkcs1(String privateKeyInPKCS8) throws Exception {
		PrivateKeyInfo privateKeyInfo = PrivateKeyInfo.getInstance(BASE64_DECODER.decode(privateKeyInPKCS8));
		RSAPrivateKey rsaPrivateKey = RSAPrivateKey.getInstance(privateKeyInfo.parsePrivateKey());
		return BASE64_ENCODER.encodeToString(rsaPrivateKey.getEncoded());
	}

	public enum KeyFormat {
		/**
		 * PKCS1格式RSA私钥
		 */
		RSA_PRIVATE_KEY_PKCS1("RSA PRIVATE KEY"),
		/**
		 * PKCS8格式RSA私钥
		 */
		RSA_PRIVATE_KEY_PKCS8("PRIVATE KEY"),
		/**
		 * PKCS1格式RSA公钥
		 */
		RSA_PUBLIC_KEY_PKCS1("RSA PUBLIC KEY"),
		/**
		 * PKCS8格式RSA公钥
		 */
		RSA_PUBLIC_KEY_PKCS8("PUBLIC KEY");

		private String name;

		KeyFormat(String name) {
			this.name = name;
		}

		public String getName() {
			return name;
		}
	}

}

备注：必须要添加Bouncy Castle提供者，代码中Security.addProvider(new BouncyCastleProvider())展示的是动态添加，也可以静态添加，更多请参考 Bouncy Castle提供者安装。

三、测试用例

1、密钥文件

准备.pem两个文件，里面分别是PKCS#1格式公钥和PKCS#1格式私钥，内容如下：

publicKeyInPkcs1.pem

privateKeyInPkcs1.pem

2、公私钥PKCS1和PKCS8格式互相转换

public class Test {

	/**
	 * 密钥保存在用户目录下keys文件夹
	 */
	private static final String BASE_PATH = Paths.get(System.getProperty("user.home"), "keys").toString();
	private static final String PRIVATE_KEY_PKCS1 = Paths.get(BASE_PATH, "privateKeyInPkcs1.pem").toString();
	private static final String PUBLIC_KEY_PKCS1 = Paths.get(BASE_PATH, "publicKeyInPkcs1.pem").toString();

	public static void main(String[] args) throws Exception {
		String pubicKeyInPkcs1 = AsymmetricKeyUtils.readKeyFromPath(PUBLIC_KEY_PKCS1);
		System.out.println("读取到的PKCS1格式公钥为：\n" + pubicKeyInPkcs1);
		String publicKeyInPkcs8 = AsymmetricKeyUtils.transformPubKeyFromPkcs1ToPkcs8(pubicKeyInPkcs1);
		System.out.println("转换后的PKCS8格式公钥为：\n" + publicKeyInPkcs8);
		pubicKeyInPkcs1 = AsymmetricKeyUtils.transformPubKeyFromPkcs8ToPkcs1(publicKeyInPkcs8);
		System.out.println("转换后的PKCS1格式公钥为：\n" + pubicKeyInPkcs1);

		System.out.println();

		String privateKeyInPkcs1 = AsymmetricKeyUtils.readKeyFromPath(PRIVATE_KEY_PKCS1);
		System.out.println("读取到的PKCS1格式私钥为：\n" + privateKeyInPkcs1);
		String privateKeyInPkcs8 = AsymmetricKeyUtils.transformPrivateKeyFromPkcs1ToPkcs8(privateKeyInPkcs1);
		System.out.println("转换后的PKCS8格式私钥为：\n" + privateKeyInPkcs8);
		privateKeyInPkcs1 = AsymmetricKeyUtils.transformPrivateKeyFromPkcs8ToPkcs1(privateKeyInPkcs8);
		System.out.println("转换后的PKCS1格式私钥为：\n" + privateKeyInPkcs1);
	}
}

控制台输出如下：

读取到的PKCS1格式公钥为：
MIGJAoGBAK3e6LCQixjb1WXzx+JX0WRmoXrlVbx6Z+QEUl8RWr3Ytmrq/Dij/ct11rkBAHRZlkgYgtChHDLutqIpteqbJ9newm770LArLlJ+uHlFeJODLRkB+DQvDZP4yXaXvQpkqRxKIDJpFMzVg9xhcixChHKnzy1Kdi8ef2CGhqWNQaXZAgMBAAE=
转换后的PKCS8格式公钥为：
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCt3uiwkIsY29Vl88fiV9FkZqF65VW8emfkBFJfEVq92LZq6vw4o/3Ldda5AQB0WZZIGILQoRwy7raiKbXqmyfZ3sJu+9CwKy5Sfrh5RXiTgy0ZAfg0Lw2T+Ml2l70KZKkcSiAyaRTM1YPcYXIsQoRyp88tSnYvHn9ghoaljUGl2QIDAQAB
转换后的PKCS1格式公钥为：
MIGJAoGBAK3e6LCQixjb1WXzx+JX0WRmoXrlVbx6Z+QEUl8RWr3Ytmrq/Dij/ct11rkBAHRZlkgYgtChHDLutqIpteqbJ9newm770LArLlJ+uHlFeJODLRkB+DQvDZP4yXaXvQpkqRxKIDJpFMzVg9xhcixChHKnzy1Kdi8ef2CGhqWNQaXZAgMBAAE=

读取到的PKCS1格式私钥为：
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
转换后的PKCS8格式私钥为：
MIICdwIBADANBgkqhkiG9w0BAQEFAASCAmEwggJdAgEAAoGBAK3e6LCQixjb1WXzx+JX0WRmoXrlVbx6Z+QEUl8RWr3Ytmrq/Dij/ct11rkBAHRZlkgYgtChHDLutqIpteqbJ9newm770LArLlJ+uHlFeJODLRkB+DQvDZP4yXaXvQpkqRxKIDJpFMzVg9xhcixChHKnzy1Kdi8ef2CGhqWNQaXZAgMBAAECgYEAiIQqDWAgZu9MeZVUTyqGlR9kELt6etddNRITnuNgbQl4CTDhwpM4VYT7iBz6P2CL96WPKMInAvKdYWwAkukp+sJ3NvN3vjxde4XF8U6p3zDqWw7bw9uGrLhi9eM56AYkb35HJ9laFM/0mZT966ae0sWQQuvmbzRKqKJgija5jQECQQDTt6R63ZQDLZ3O/eMwPfBtwSA/H3un+RNxOrie1ZQDle4I8LdNvQZ73+Jma7V61lwO0+m1Aq93f4d8uaYVDIRhAkEA0jzHLU+d++pbyVFtv1MJCbkGa+YdcI40d00ALdRbaMn1xQP3+jwCI82/xleZOCxJsG8Bb12VR7fmOhZoEC+UeQJBALmdal2xv59dsdoUqsbTtuNyiwPsZSwp6cq+kbfnReIy40MBT6OfEn0uFr52td/g6UqyrVXd6doZe750wM/5bmECQCeLRQ+RxS6II2GXjmQJU07f3YRYETuFk/rIofr80YDBVBgNqgwIU4K2hZbiDTDSq72i84sl+Bk4TvaoFkTNHkkCQGbC4fjwrpDconFS9AOnDoGCHbMZrKjLXYF0NAzRlb4BuUyqpveLCo/4tagRLdKoApGYDpyWH/0NSq06frrXSCQ=
转换后的PKCS1格式私钥为：
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