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背景：服务端使用Java开发的，接口需要客户端传一个RSA公钥，手机端使用Flutter开发的，然后就选择使用 pointycastle 生成密钥，很不巧，dart版本不像Java一样有个可以直接获取编码过的公钥信息的方法：publicKey.getEncoded() ，dart 版本的 RSAPublicKey 只有 modulus 和 publicExponent，需要自行编码。

大概研究了一下，数据是使用ASN1，于是看了下Java编码前的数据结构是什么样子的（当然，也可以看相关的技术文档，我一时半会儿没找到，也懒得找），项目需要添加下面的依赖：

<dependency><groupId>org.bouncycastle</groupId><artifactId>bcprov-jdk18on</artifactId><version>1.78.1</version>
</dependency>

使用下面的Java代码，打印出来，结构一目了然： 

import org.bouncycastle.asn1.*;import java.io.IOException;
import java.security.*;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;public class Main {public static void main(String[] args) throws NoSuchAlgorithmException {KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");keyPairGenerator.initialize(2048);KeyPair keyPair = keyPairGenerator.generateKeyPair();RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();RSAPrivateKey privateKey = (RSAPrivateKey) keyPair.getPrivate();System.out.printf("publicKey Algorithm: %s, format: %s\n", publicKey.getAlgorithm(), publicKey.getFormat());System.out.println("PublicKey Info:");System.out.println("\tModulus: " + publicKey.getModulus());System.out.println("\tPublic Exponent: " + publicKey.getPublicExponent());parseASN1(publicKey.getEncoded(), 0);System.out.println();System.out.printf("privateKey Algorithm: %s, format: %s\n", privateKey.getAlgorithm(), privateKey.getFormat());System.out.println("PrivateKey Info:");System.out.println("\tModulus: " + privateKey.getModulus());System.out.println("\tPrivate Exponent: " + privateKey.getPrivateExponent());parseASN1(privateKey.getEncoded(), 0);}private static void parseASN1(byte[] data, int indent) {try (ASN1InputStream asn1InputStream = new ASN1InputStream(data)) {ASN1Primitive primitive;while ((primitive = asn1InputStream.readObject()) != null) {parseASN1Encodable(primitive, indent);}} catch (IOException e) {throw new RuntimeException(e);}}private static void parseASN1Encodable(ASN1Encodable primitive, int indent) {printIndent(indent);if (primitive instanceof ASN1Sequence) {System.out.println("ASN1Sequence:");((ASN1Sequence) primitive).forEach(e -> parseASN1Encodable(e, indent + 1));} else if (primitive instanceof ASN1BitString) {System.out.println("ASN1BitString");parseASN1(((ASN1BitString) primitive).getOctets(), indent + 1);} else if (primitive instanceof ASN1OctetString) {System.out.println("ASN1OctetString");parseASN1(((ASN1OctetString) primitive).getOctets(), indent + 1);} else if (primitive instanceof ASN1Integer) {ASN1Integer asn1Integer = (ASN1Integer) primitive;System.out.printf("ASN1Integer: %s\n", asn1Integer.getValue());} else if (primitive instanceof ASN1ObjectIdentifier) {ASN1ObjectIdentifier asn1ObjectIdentifier = (ASN1ObjectIdentifier) primitive;System.out.printf("OID: %s\n", asn1ObjectIdentifier.getId());} else if (primitive instanceof DERNull){System.out.println("DERNull");} else {System.out.println("primitive Class: " + primitive.getClass());System.out.println("primitive: " + primitive);}}private static void printIndent(int tabCount) {for (int i = 0; i < tabCount; i++) {System.out.print('\t');}}
}

dart版本对公钥进行编码的代码写在另一篇文章了：Flutter RSA公钥转PEM​​​​​​​