I have spreadsheets that contain a mix of text and numbers and I need every cell fully protected when the files leave my machine. The solution must apply the RSA algorithm and it has to work in both directions: I should be able to feed the tool an ordinary .xlsx and receive an encrypted file, then point the tool at that encrypted file—using the correct private key—and get back the original sheet exactly as it was. I am flexible about how you build it: an Excel add-in, a lightweight desktop utility, or a command-line script are all fine as long as they use RSA under the hood and run smoothly on Windows. What matters is reliability, zero data loss for formulae, formatting and Unicode text, and a straightforward way for me to choose or generate key pairs. Acceptance criteria • Encrypts any Excel file (text + numerical data) with RSA and produces a file that cannot be opened without the matching key. • Decrypts that file back to an identical spreadsheet when the correct key is supplied. • Includes clear setup and usage instructions plus all source code. Let me know the language and libraries you prefer and how quickly you can deliver a working prototype.