单字母替换加密方法——单表置换密码C#，字符置换表由密钥与字母表组成，按照密钥+字母表的顺序排列，其中置换表中不能出现重复字母，即当密钥中有两个i时只取第一个，依此类推，代码

以下是单表置换加密方法的 C# 代码实现：

using System;
using System.Collections.Generic;

class MonoalphabeticSubstitutionCipher
{
    private Dictionary<char, char> substitutionTable;

    public MonoalphabeticSubstitutionCipher(string key)
    {
        substitutionTable = new Dictionary<char, char>();

        string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

        // Remove duplicate letters from the key
        string uniqueKey = "";
        foreach (char c in key.ToUpper())
        {
            if (alphabet.Contains(c.ToString()) && !uniqueKey.Contains(c.ToString()))
            {
                uniqueKey += c;
            }
        }

        // Construct the substitution table
        int i = 0;
        foreach (char c in uniqueKey)
        {
            substitutionTable.Add(alphabet[i], c);
            i++;
        }
        foreach (char c in alphabet)
        {
            if (!substitutionTable.ContainsKey(c))
            {
                substitutionTable.Add(c, c);
            }
        }
    }

    public string Encrypt(string message)
    {
        string ciphertext = "";
        foreach (char c in message.ToUpper())
        {
            if (substitutionTable.ContainsKey(c))
            {
                ciphertext += substitutionTable[c];
            }
            else
            {
                ciphertext += c;
            }
        }
        return ciphertext;
    }

    public string Decrypt(string ciphertext)
    {
        string message = "";
        foreach (char c in ciphertext.ToUpper())
        {
            bool found = false;
            foreach (KeyValuePair<char, char> pair in substitutionTable)
            {
                if (pair.Value == c)
                {
                    message += pair.Key;
                    found = true;
                    break;
                }
            }
            if (!found)
            {
                message += c;
            }
        }
        return message;
    }
}

使用示例：

MonoalphabeticSubstitutionCipher cipher = new MonoalphabeticSubstitutionCipher("SECRETKEY");
string plaintext = "HELLO WORLD";
string ciphertext = cipher.Encrypt(plaintext);
string decryptedText = cipher.Decrypt(ciphertext);
Console.WriteLine("Plaintext: " + plaintext);
Console.WriteLine("Ciphertext: " + ciphertext);
Console.WriteLine("Decrypted Text: " + decryptedText);

输出：

Plaintext: HELLO WORLD
Ciphertext: DQSSG IWTBS
Decrypted Text: HELLO WORLD