. NET encryption and decryption--symmetric encryption

One, the idea

Symmetric encryption contains a thing known as a key, before the message is sent using a key to encrypt the message, get ciphertext and send, the receiver receives the ciphertext, using the same key to decrypt, to obtain the original message.

PS : The process of encrypting a message with a key, which is done by a cryptographic algorithm, is usually public.

Two, the process of symmetric encryption

1 , the sender and receiver hold the same key and are strictly confidential

2 , the sender encrypts the message using the key, and then sends the message

3 , the receiving party decrypts the message with the same key after it receives the message

PS: in this process, third parties may intercept messages, but get the knowledge of a bunch of garbled

Three, Demo

Using system;using system.collections.generic;using system.linq;using system.text;using System.Security.Cryptography;            Note introducing this namespace using system.io;namespace symmetric encryption and decryption {class Program {static void Main (string[] args) {  String key = "Secret key";  Key string plaintext = "Hello,world";  PlainText string encryptedtext = Symmetriccryptohelper.encrypt (plaintext, key);            Encryption Console.WriteLine (ENCRYPTEDTEXT);  String cleartext = Symmetriccryptohelper.decrypt (Encryptedtext, key);            Decryption Console.WriteLine (cleartext);        Console.readkey (); }}///<summary>///symmetric encryption Help class///</summary>//<remarks>editor:v-liuhch createtime:2015 /5/15 22:05:41</remarks> public class Symmetriccryptohelper {//symmetric cryptographic algorithm provider private ICRYPTOTRANSF        ORM encryptor;//Cipher Object Private ICryptoTransform decryptor;//decryption Object Private Const int buffersize = 1024; <summary>//Initializes a new instance of the <see cref= "Symmetriccryptohelper"/> class. </summary>//<param name= "Algorithmname" >name of the algorithm.</param>//<para M name= "key" >the key.</param>//<remarks>Editor:v-liuhch</remarks> public SYMMETRICCR  Yptohelper (String algorithmname, byte[] key) {//symmetricalgorithm is the base class for the symmetric algorithm SymmetricAlgorithm            Provider = Symmetricalgorithm.create (Algorithmname); Provider. Key = key;//Specifies the key, typically 128-bit or 196-bit PROVIDER.IV = new byte[] {0x12,0x34,0x56,0x78,0x90,0xab,0xcd,0xef};//initializa tion vector, initialization vectors, avoids the related part of the text after the encryption is also a repetition problem, usually 64-bit encryptor = provider. CreateEncryptor ();//Create an Encryption object decryptor = provider. CreateDecryptor ();//Create decryption Object} public Symmetriccryptohelper (byte[] key): This ("TripleDES", key) {}/ /encryption algorithm//<summary>//encrypts the SPECIFied clear text. </summary>//<param name= "cleartext" >the clear text.</param>//<returns>syst Em. string.</returns>//<remarks>editor:v-liuhch CREATETIME:2015/5/17 16:56:15</remarks> Pub Lic string Encrypt (String cleartext) {//create clear Text stream byte[] Clearbuffer = Encoding.UTF8.GetBytes (            cleartext);            MemoryStream clearStream = new MemoryStream (clearbuffer);            Creates an empty ciphertext stream MemoryStream Encryptedstream = new MemoryStream (); /* Encryption decryption involves two streams, one is clear flow, one is a cipher stream so there must be a mediator, the clear flow into the ciphertext stream, or the ciphertext flow into the clear stream; * The broker performing this operation in. NET is a stream type called Cryptostre AM; * * Constructor parameters when encrypting: * 1,stream: Ciphertext stream (at this time the ciphertext stream does not contain data, just an empty stream); * 2,icry            Ptotransform: The created cipher, which is responsible for cryptographic computations, * 3, enumeration: Write, writes the plaintext stream flowing through CryptoStream to the ciphertext stream, and finally obtains the encrypted data from the ciphertext stream */ CryptoStream cryptostream = new CryptoStream (Encryptedstream, Encryptor, CryptoStreamMode.Write);            Writes the plaintext stream to buffer//writes data in buffer to CryptoStream int bytesread = 0;            byte[] buffer = new Byte[buffersize];                do {bytesread = clearstream.read (buffer, 0, buffersize);                            Cryptostream.write (buffer, 0, bytesread);            } while (bytesread>0);            Cryptostream.flushfinalblock ();//clear buffer//Get encrypted text after buffer = Encryptedstream.toarray ();            String encryptedtext = convert.tobase64string (buffer);                return encryptedtext; }///<summary>//decryption algorithm//</summary>//<param name= "en Cryptedtext ">the encrypted text.</param>//<returns>System.String.</returns>//< Remarks>editor:v-liuhch createtime:2015/5/17 16:56:22</remarks> public string Decrypt (string encryptedtext )        {           byte[] Encryptedbuffer = convert.frombase64string (encryptedtext);            Stream Encryptedstream = new MemoryStream (encryptedbuffer);            MemoryStream clearStream = new MemoryStream ();              /* When decrypting the constructor parameter: * 1,stream: Ciphertext stream (at this time the ciphertext stream contains data); * 2,icryptotransform: The decryption device created, responsible for the decryption calculation, * 3, enumeration: Write, the ciphertext stream of data read out into the clear stream, and then converted to clear text, the original format */CryptoStream cryptostream = new Cry            Ptostream (Encryptedstream, Decryptor, CryptoStreamMode.Read);            int bytesread = 0;            byte[] buffer = new Byte[buffersize];                do {bytesread = cryptostream.read (buffer, 0, buffersize);            Clearstream.write (buffer, 0, bytesread);            } while (bytesread>0);            Buffer = Clearstream.getbuffer ();            String cleartext = Encoding.UTF8.GetString (buffer, 0, (int) clearstream.length);                        return cleartext; }//<summAry>//Encrypts the specified clear text. </summary>//<param name= "cleartext" >the clear text.</param>//<param name= "key ">the key.</param>//<returns>System.String.</returns>//<remarks>editor:v-li            UHCH createtime:2015/5/17 16:56:40</remarks> public static string Encrypt (String cleartext, string key) {            byte[] KeyData = new BYTE[16];            byte[] SourceData = Encoding.Default.GetBytes (key);            int copybytes = 16;                            if (sourcedata.length<16) {copybytes = Sourcedata.length;            } array.copy (SourceData, KeyData, copybytes);            Symmetriccryptohelper helper = new Symmetriccryptohelper (keyData); Return helper.        Encrypt (cleartext);        }///<summary>//Decrypts the specified encrypted text. </summary>//<param Name= "Encryptedtext" >the encrypted text.</param>//<param name= "key" >the key.</param> <returns>System.String.</returns>//<remarks>editor:v-liuhch CREATETIME:2015/5/17 16:56:4  4</remarks> public static string Decrypt (string encryptedtext, String key) {byte[] KeyData            = new BYTE[16];            byte[] SourceData = Encoding.Default.GetBytes (key);            int copybytes = 16;            if (sourcedata.length<16) {copybytes = Sourcedata.length;            } array.copy (SourceData, KeyData, copybytes);            Symmetriccryptohelper helper = new Symmetriccryptohelper (keyData); Return helper.        Decrypt (Encryptedtext); }        }}

Four, hidden problems

1 , both the sender and the receiver need to hold the key and ensure that the key is not compromised.

2 , if a third party illegally obtains the key, after tampering with the message, it is re-encrypted to the receiver, and the receiver cannot discern. Since there is no way to tell if a message has been tampered with, it is not possible to determine who sent the message, which is not sufficient for completeness and authentication.

. NET encryption and decryption-symmetric encryption