如何将RSA PlblicKey XML转换为PEM格式？

将RSA PublicKey XML转换为PEM格式的步骤如下：

首先，需要将XML格式的PublicKey加载到程序中。可以使用Java语言中的javax.xml.crypto.KeySelector类来实现。具体代码如下：

import javax.xml.crypto.KeySelector;
import javax.xml.crypto.dom.DOMStructure;
import javax.xml.crypto.dsig.keyinfo.KeyInfo;
import javax.xml.crypto.dsig.keyinfo.KeyValue;
import javax.xml.crypto.dsig.keyinfo.X509Data;
import javax.xml.crypto.dsig.keyinfo.X509IssuerSerial;
import javax.xml.crypto.dsig.keyinfo.X509SKI;
import javax.xml.crypto.dsig.keyinfo.X509SubjectName;
import javax.xml.crypto.dsig.keyinfo.X509Certificate;
import javax.xml.crypto.dsig.XMLSignatureFactory;
import javax.xml.crypto.dsig.XMLValidateContext;
import javax.xml.crypto.dsig.dom.DOMValidateContext;
import javax.xml.crypto.dsig.Reference;
import javax.xml.crypto.dsig.SignatureMethod;
import javax.xml.crypto.dsig.SignedInfo;
import javax.xml.crypto.dsig.Transform;
import javax.xml.crypto.dsig.XMLSignature;
import javax.xml.crypto.dsig.XMLSignatureException;
import javax.xml.crypto.dsig.XMLSignatureFactory;
import javax.xml.crypto.dsig.XMLValidateContext;
import javax.xml.crypto.dsig.dom.DOMValidateContext;
import javax.xml.crypto.dsig.keyinfo.KeyInfo;
import javax.xml.crypto.dsig.keyinfo.KeyValue;
import javax.xml.crypto.dsig.keyinfo.X509Data;
import javax.xml.crypto.dsig.keyinfo.X509IssuerSerial;
import javax.xml.crypto.dsig.keyinfo.X509SKI;
import javax.xml.crypto.dsig.keyinfo.X509SubjectName;
import javax.xml.crypto.dsig.keyinfo.X509Certificate;
import javax.xml.crypto.dsig.spec.TransformParameterSpec;
import javax.xml.crypto.dsig.spec.XPathFilterParameterSpec;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.security.KeyFactory;
import java.security.PublicKey;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.spec.X509EncodedKeySpec;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class RSAKeyConverter {
    public static void main(String[] args) throws Exception {
        // 加载XML格式的PublicKey
        String xmlPublicKey = "<RSAKeyValue><Modulus>...</Modulus><Exponent>...</Exponent></RSAKeyValue>";
        PublicKey publicKey = loadPublicKeyFromXml(xmlPublicKey);

        // 转换为PEM格式
        String pemPublicKey = convertToPem(publicKey);
        System.out.println(pemPublicKey);
    }

    private static PublicKey loadPublicKeyFromXml(String xmlPublicKey) throws Exception {
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        dbf.setNamespaceAware(true);
        InputStream is = new ByteArrayInputStream(xmlPublicKey.getBytes());
        Document doc = dbf.newDocumentBuilder().parse(is);

        // 创建XMLSignatureFactory实例
        XMLSignatureFactory fac = XMLSignatureFactory.getInstance("DOM");

        // 创建KeySelector
        KeySelector selector = KeySelector.singletonKeySelector(publicKey);

        // 创建XMLValidateContext
        XMLValidateContext valContext = new DOMValidateContext(selector, doc);

        // 验证XMLSignature
        NodeList nl = doc.getElementsByTagNameNS(XMLSignature.XMLNS, "Signature");
        if (nl.getLength() == 0) {
            throw new Exception("Cannot find Signature element");
        }

        // 验证XMLSignature
        XMLSignature signature = fac.unmarshalXMLSignature(valContext);
        boolean coreValidity = signature.validate(valContext);

        if (coreValidity) {
            // 获取PublicKey
            KeyInfo keyInfo = signature.getKeyInfo();
            KeyValue keyValue = keyInfo.itemKeyValue(0);
            return keyValue.getPublicKey();
        } else {
            throw new Exception("Signature failed core validation");
        }
    }

    private static String convertToPem(PublicKey publicKey) throws Exception {
        // 获取PublicKey的字节数组
        byte[] publicKeyBytes = publicKey.getEncoded();

        // 创建X509EncodedKeySpec实例
        X509EncodedKeySpec spec = new X509EncodedKeySpec(publicKeyBytes);

        // 创建KeyFactory实例
        KeyFactory kf = KeyFactory.getInstance("RSA");

        // 生成PEM格式的公钥
        String pemPublicKey = "-----BEGIN PUBLIC KEY-----\n";
        pemPublicKey += Base64.getEncoder().encodeToString(spec.getEncoded()) + "\n";
        pemPublicKey += "-----END PUBLIC KEY-----";

        return pemPublicKey;
    }
}