Macro to replace text with random string of same length

Question

I am playing with LuaLaTeX substitutions, trying to find a way to replace a string within a macro with random characters of the same length.

The chickenize package offers the randomchars function which does this for an entire paragraph. However, I cannot get this to work on just the argument of the TeX macro, and get either errors or nothing when trying to create a macro like:

\newcommand{\rndm}[1]{\directlua{randomchars(#1)}}

Answer 1

Here you go. Note that the input and output characters could be (almost) arbitrary utf8-encoded characters. (On output, it's your job to verify that the output characters are indeed present in the font that's in use.) If the characters are TeX-special -- say, & and $ -- be sure to escape them as \\& and \\$, etc. Whitespace in the input string is preserved, but punctuation characters aren't given any preferential treatment.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{luacode} % for 'luacode' environment and '\luastring' macro
\begin{luacode}

function rndstring ( inputstring )
  local outputstring, choices, mm, nn
  mm = unicode.utf8.len(inputstring) -- no. of utf8-encoded characters in input string
-- Place candidate replacement characters in a Lua table:
  choices = { 
     "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", 
     "\#", "\$", "\%", "\%", "\&", "\_", "\textbackslash{}",
     "*", "+", "-", "/", "(", ")", "[", "]", "\{", "\}",
     "<", "=", ">", "?", "@", "\textasciitilde{}",
     "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", 
     "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", 
     "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", 
     "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
   }
  -- Number of rows in 'choices' table
  nn = #choices
-- Generate the outputstring in a 'for' loop:
  outputstring = ""
  for i = 1 , mm do
     if unicode.utf8.sub ( inputstring , i , i ) == " "  then
         outputstring = outputstring .. " " -- preserve space char.
     else -- choose a new char randomly from 'choices' table
         outputstring = outputstring .. choices[ math.random ( nn ) ]
     end
   end
return ( outputstring )
end

\end{luacode}

%% Define a LaTeX macro to invoke the Lua function
\newcommand\rndstring[1]{\directlua{tex.sprint(rndstring(\luastring{#1}))}}

%% test strings to feed to '\rndstring':
\newcommand\stringA{Hello World}
\newcommand\stringB{Hello Владимир öäüß}
\newcommand\stringC{Once upon a time, there was ...}

\begin{document}
\ttfamily % optional

\rndstring{\stringA}\par\rndstring{\stringB}\par\rndstring{\stringC}

\bigskip
\rndstring{\stringA}\par\rndstring{\stringB}\par\rndstring{\stringC}

\end{document}