Vカツのセーブデータを相互にごにょごにょ。Key/IVは自分で調べてね。

VKatsuCrypto.cs

using System;
using System.IO;
using System.Security.Cryptography;

namespace vkatsu {
    public class VKCrypto
    {
        public static RijndaelManaged Rijndael { get; private set; }

        public int MaxBufferSize { get; set; } = 4*1024;
        public bool IsDebug { get; set; } = false;

        private byte[] _bk = null;
        private byte[] _bv = null;

        static VKCrypto() {
            Rijndael = new RijndaelManaged();
            Rijndael.Padding = PaddingMode.PKCS7;
            Rijndael.Mode = CipherMode.CBC;
            Rijndael.KeySize = 256;
            // 現状 .Net coreだと AES128しか実装されてないので例外が出る
            Rijndael.BlockSize = 256;
        }

        public VKCrypto(string k, string v) {
            _bk = System.Text.Encoding.UTF8.GetBytes(k);
            _bv = System.Text.Encoding.UTF8.GetBytes(v);
            if ((_bk == null) || (_bv == null)) {
                throw new InvalidProgramException();
            } else if (IsDebug) {
                Console.WriteLine("key: {0}", k);
                Console.WriteLine("iv : {0}", v);
            }
        }

        public void decode(string srcPath, string dstPath) {
            try {
                using (var src = new CryptoStream(new FileStream(srcPath, FileMode.Open, FileAccess.Read), Rijndael.CreateDecryptor(_bk, _bv), CryptoStreamMode.Read)) {
                    using (var dst = new FileStream(dstPath, FileMode.Create, FileAccess.Write)) {
                        var buf = new byte[MaxBufferSize];
                        var len = 0;
                        while ((len = src.Read(buf, 0, buf.Length)) > 0) {
                            dst.Write(buf, 0, len);
                        }
                    }
                }
                if (IsDebug) {
                    Console.WriteLine("src: {0}", srcPath);
                    Console.WriteLine("dst: {0}", dstPath);
                }
            }
            catch (Exception e) {
                Console.WriteLine(e.Message);
            }
        }

        public void encode(string srcPath, string dstPath) {
            try {
                using (var src = new FileStream(srcPath, FileMode.Open, FileAccess.Read)) {
                    using (var dst = new CryptoStream(new FileStream(dstPath, FileMode.Create, FileAccess.Write), Rijndael.CreateEncryptor(_bk, _bv), CryptoStreamMode.Write)) {
                        var buf = new byte[MaxBufferSize];
                        var len = 0;
                        while ((len = src.Read(buf, 0, buf.Length)) > 0) {
                            dst.Write(buf, 0, len);
                        }
                    }
                }
                if (IsDebug) {
                    Console.WriteLine("src: {0}", srcPath);
                    Console.WriteLine("dst: {0}", dstPath);
                }
            }
            catch (Exception e) {
                Console.WriteLine(e.Message);
            }
        }
    }
}