Problem 79

Passcode derivation

A common security method used for online banking is to ask the user for three random characters from a passcode. For example, if the passcode was $531278$, they may ask for the $2$nd, $3$rd, and $5$th characters; the expected reply would be: $317$.

The text file, keylog.txt, contains fifty successful login attempts.

Given that the three characters are always asked for in order, analyse the file so as to determine the shortest possible secret passcode of unknown length.

密码推断

网上银行常用的一种密保手段是向用户询问密码中的随机三位字符。例如，如果密码是$531278$，询问第$2$、$3$、$5$位字符时，正确的回答应当是$317$。

在文本文件keylog.txt中包含了$50$次成功登陆的正确回答。

假设三个字符总是按顺序询问的，分析这个文本文件并求出最短的密码。