項(xiàng)目中是服務(wù)端持有私鑰，客戶端持有公鑰；
客戶端使用公鑰加密，并使用公鑰解密服務(wù)端 用私鑰加密的數(shù)據(jù)。

網(wǎng)上相關(guān)的資料實(shí)在再少，終于功夫不負(fù)有心人，被我找到了。附上代碼。

先引入包 ：
https://pub-web.flutter-io.cn/packages/encrypt
https://pub.dev/packages/pointycastle

encrypt: ^5.0.1
pointycastle: ^3.1.1

import 'dart:convert';
import 'package:encrypt/encrypt.dart';
import 'package:flutter/services.dart';
import 'package:pointycastle/api.dart';
import 'package:pointycastle/asymmetric/api.dart';
import 'package:pointycastle/asymmetric/pkcs1.dart';
import 'package:pointycastle/asymmetric/rsa.dart';
// import 'package:pointycastle/export.dart';

class EncryptUtil {

  // Rsa加密最大長(zhǎng)度(密鑰長(zhǎng)度/8-11)
  static const int MAX_ENCRYPT_BLOCK = 245;

  // Rsa解密最大長(zhǎng)度(密鑰長(zhǎng)度/8)
  static const int MAX_DECRYPT_BLOCK = 256;

  //公鑰分段加密
  static Future encodeString(String content) async {
    //加載公鑰字符串
    final publicPem = await rootBundle.loadString(這里寫密鑰路徑);
    //創(chuàng)建公鑰對(duì)象
    RSAPublicKey publicKey = RSAKeyParser().parse(publicPem) as RSAPublicKey;
    //創(chuàng)建加密器
    final encrypter = Encrypter(RSA(publicKey: publicKey));

    //分段加密
    // 原始字符串轉(zhuǎn)成字節(jié)數(shù)組
    List<int> sourceBytes = utf8.encode(content);
    //數(shù)據(jù)長(zhǎng)度
    int inputLength = sourceBytes.length;
    // 緩存數(shù)組
    List<int> cache = [];
    // 分段加密 步長(zhǎng)為MAX_ENCRYPT_BLOCK
    for(int i=0;i<inputLength;i+=MAX_ENCRYPT_BLOCK){
      //剩余長(zhǎng)度
      int endLen = inputLength-i;
      List<int> item;
      if(endLen > MAX_ENCRYPT_BLOCK){
        item = sourceBytes.sublist(i,i+MAX_ENCRYPT_BLOCK);
      }else {
        item = sourceBytes.sublist(i,i+endLen);
      }
      // 加密后對(duì)象轉(zhuǎn)換成數(shù)組存放到緩存
      cache.addAll(encrypter.encryptBytes(item).bytes);
    }
    return base64Encode(cache);
  }

  //公鑰分段解密
  static Future decodeString(String content) async{
    //加載公鑰字符串
    final publicPem = await rootBundle.loadString(這里寫密鑰路徑);
    //創(chuàng)建公鑰對(duì)象
    RSAPublicKey publicKey = RSAKeyParser().parse(publicPem) as RSAPublicKey;

    AsymmetricBlockCipher cipher = PKCS1Encoding(RSAEngine());
    cipher.init(false, PublicKeyParameter<RSAPublicKey>(publicKey));

    //分段解密
    //原始數(shù)據(jù)
    List<int> sourceBytes = base64Decode(content);
    //數(shù)據(jù)長(zhǎng)度
    int inputLength = sourceBytes.length;
    // 緩存數(shù)組
    List<int> cache = [];
    // 分段解密 步長(zhǎng)為MAX_DECRYPT_BLOCK
    for(var i=0;i<inputLength;i+=MAX_DECRYPT_BLOCK){
      //剩余長(zhǎng)度
      int endLen =inputLength - i;
      List<int> item;
      if(endLen > MAX_DECRYPT_BLOCK){
        item = sourceBytes.sublist(i,i+MAX_DECRYPT_BLOCK);
      }else {
        item = sourceBytes.sublist(i,i+endLen);
      }
      //解密后放到數(shù)組緩存
      cache.addAll(cipher.process(Uint8List.fromList(item)));
    }
    return utf8.decode(cache);


  }
}

參考資料：
https://stackoverflow.com/questions/61858252/flutter-how-to-decrypt-a-rsa-private-key-encrypted-string-if-we-have-rsa-publi
https://github.com/leocavalcante/encrypt/issues/180
http://m.itdecent.cn/p/0194b99a4b3d