feat: 添加凯撒密码实现示例用于教学目的

实现凯撒加密和解密算法，包含用户交互界面

classical-encryption/caesar_cipher.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+"""
+凯撒加密算法示例
+一个教学用的凯撒密码实现
+"""
+
+def caesar_encrypt(plaintext, shift):
+    """凯撒加密函数"""
+    ciphertext = ""
+    
+    for char in plaintext:
+        if char.isalpha():
+            base = ord('A') if char.isupper() else ord('a')
+            new_char = chr((ord(char) - base + shift) % 26 + base)
+            ciphertext += new_char
+        else:
+            ciphertext += char
+    
+    return ciphertext
+
+def caesar_decrypt(ciphertext, shift):
+    """凯撒解密函数"""
+    return caesar_encrypt(ciphertext, -shift)
+
+def main():
+    """主函数，用户交互界面"""
+    print("凯撒密码示例")
+    
+    while True:
+        print("\n1.加密\n2.解密\n3.退出")
+        choice = input("选择: ").strip()
+        
+        if choice == "1":
+            text = input("明文: ")
+            try:
+                shift = int(input("移位值: "))
+                result = caesar_encrypt(text, shift)
+                print(f"密文: {result}")
+            except ValueError:
+                print("移位值必须是整数!")
+                
+        elif choice == "2":
+            text = input("密文: ")
+            try:
+                shift = int(input("移位值: "))
+                result = caesar_decrypt(text, shift)
+                print(f"明文: {result}")
+            except ValueError:
+                print("移位值必须是整数!")
+                
+        elif choice == "3":
+            print("再见!")
+            break
+        else:
+            print("无效选择")
+
+if __name__ == "__main__":
+    main()