古典密码是密码学中的其中一个类型，其大部分加密方式都是利用替换式密码或移项式密码，有时则是两者的混合。其于历史中经常使用，但现代已经很少使用，大部分的已经不再使用了。一般而言，经典密码是基于一个拼音字母（像是 A-Z）、动手操作或是简单的设备。它们可能是一种简单的密码法，以致于不可信赖的地步，特别是有新技术被发展出来后。

现代的方法是用电脑或是其它数字科技，基于比特和字节上操作。许多经典密码被受尊重的人使用，像是尤利乌斯·凯撒和拿破仑，他们创造了一些常被人们使用的密码。许多密码起源于军事上，相同立场的人常使用来寄送秘密消息。经典的方法常攻击密码文，有时候甚至不知其密码系统，也可以使用工具，像是频率分析法。有些经典密码是使用先进的机器或是机电密码机器，像是恩尼格玛密码机。

A cipher is a means of concealing a message, where letters of the message are substituted or transposed for other letters, letter pairs, and sometimes for many letters. In cryptography, a classical cipher is a type of cipher that was used historically but now has fallen, for the most part, into disuse. In general, classical ciphers operate on an alphabet of letters (such as "A-Z"), and are implemented by hand or with simple mechanical devices. They are probably the most basic types of ciphers, which made them not very reliable, especially after new technology was developed. Modern schemes use computers or other digital technology, and operate on bits and bytes. Many classical ciphers were used by well-respected people, such as Julius Caesar and Napoleon, who created their own ciphers which were then popularly used. Many ciphers had their origins in the military and were used for transporting secret messages among people on the same side. Classical schemes are often susceptible to ciphertext-only attacks, sometimes even without knowledge of the system itself, using tools such as frequency analysis. Sometimes grouped with classical ciphers are more advanced mechanical or electro-mechanical cipher machines, such as the Enigma machine.