Mật thư is a Chrome extension designed for converting selected text and numbers into corresponding alphabet letters using various encoding rules. This tool is particularly useful for enthusiasts of cryptography, decryption, or anyone needing quick text transformations. The extension provides an intuitive interface that enhances the user experience.

Key features include the ability to convert numbers to letters, showcasing results using the Caesar cipher with shifts ranging from 0 to 25. Additionally, users can convert selected text into different Caesar cipher strings, displaying a total of 26 unique encoded results. The straightforward usage instructions guide users through selecting text or numbers, accessing the context menu, and viewing results in a new window or notification, making it an efficient tool for quick text encoding tasks.