如何将string类型的sm2公钥转成pem文件

要将String类型的SM2公钥转换为PEM格式文件，需要使用Bouncy Castle库和Java的IO操作。下面是一个简单的示例代码：

import org.bouncycastle.asn1.x9.X9ObjectIdentifiers;
import org.bouncycastle.crypto.util.SubjectPublicKeyInfoFactory;
import org.bouncycastle.jce.provider.BouncyCastleProvider;

import java.io.FileWriter;
import java.io.IOException;
import java.io.Writer;
import java.security.Security;
import java.security.interfaces.ECPublicKey;
import java.security.spec.ECGenParameterSpec;

public class SM2PublicKeyConverter {

    public static void main(String[] args) throws Exception {
        Security.addProvider(new BouncyCastleProvider());

        // 指定SM2算法
        ECGenParameterSpec sm2Spec = new ECGenParameterSpec(X9ObjectIdentifiers.prime256v1.getId());

        // 将字符串转换为ECPublicKey对象
        String publicKeyString = "04E12C75A3E4F4C0C3E5C7C0B7B19F7D9D7C4D7369A01A5B3A0C1A3E6C1C4B5B47D4E3D0A4D7E6A1D0B3B0E7B6C9E0E6C9E9B1B7A5A2E8C7D6E6F3B7A1";
        byte[] publicKeyBytes = hexStringToByteArray(publicKeyString);
        ECPublicKey publicKey = (ECPublicKey) KeyFactory.getInstance("EC", "BC").generatePublic(new X509EncodedKeySpec(publicKeyBytes));

        // 将ECPublicKey对象保存为PEM格式文件
        Writer publicKeyWriter = new FileWriter("public_key.pem");
        SubjectPublicKeyInfoFactory.createSubjectPublicKeyInfo(publicKey).encodeToPEM(publicKeyWriter);
        publicKeyWriter.close();
    }

    private static byte[] hexStringToByteArray(String s) {
        int len = s.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                    + Character.digit(s.charAt(i+1), 16));
        }
        return data;
    }
}

这段代码将String类型的SM2公钥转换为ECPublicKey对象，并将其保存为PEM格式文件。注意，生成ECPublicKey对象时要指定算法为“EC”，并使用Bouncy Castle提供的SM2参数。同时，公钥的编码方式为SubjectPublicKeyInfo格式。