I wrote some “Secret Message Cyphers” i call it Vari_Cyph
They are Windows programs but run fine under Wine on Linux
They can only load 8 bit ASCII text files.
The *.bas file is the source code and can be compiled to either 32 or 64 bit.
Written in FreeBASIC for Windows.
If you know the Linux libs ( GTK , QT ) , you could probably compile under Linux using FreeBASIC for Linux.