How to get this Enigma example work with user input?

Question

As in this question,

Assume that you have some text in parts of your LaTeX document that you only want people with a password to see. A reason could e.g. be a case with an guiding answer. You want to make the case public but the answer only to teachers.

I would like to have a link action in the PDF that, when clicked, will decrypt the content after asking for a password, using the Adobe Acrobat Javascript API. (Motivated by this question and answer: How to add print link action of specific pages?)

You can choose any source of encrypted material as a test ASCII content, but a minimum example is in the code below. In the answers to the question Encrypt and decrypt parts of text in a LaTeX source file, it is indicated that the problem of encrypting parts of an ASCII file is really an easy piece of the main task, which can be done by a good editor such as Emacs.

In the code below, which is based on this answer, the line \password"this.getField("myinput").value; tries to cast the user input to integer. This does not work. How can I make it work?

\documentclass{article}
\usepackage{fontspec}

% https://tex.stackexchange.com/a/275434/13173
\newcount\password
\newcount\crycount
\newcount\crycounta
\newcount\crycountb

\def\private#1{%
    \ifx#1\end
        \expandafter\end
    \else
        \crycount`#1
        % ↓↓↓ some math involving \crycount and \password
        \advance\crycount-\password
        \crycounta\crycount\crycountb\crycount\divide\crycounta16\multiply\crycounta16\advance\crycountb-\crycounta\divide\crycounta16
    \xdef\crychara{\ifcase\crycounta0\or1\or2\or3\or4\or5\or6\or7\or8\or9\or a\or b\or c\or d\or e\else f\fi}
    \xdef\crycharb{\ifcase\crycountb0\or1\or2\or3\or4\or5\or6\or7\or8\or9\or a\or b\or c\or d\or e\else f\fi}
        % ↑↑↑ some math involving \crycount and \password
        \xdef\crystring{\crystring\string^\string^\crychara\crycharb}
        \expandafter\private
    \fi
}

% https://tex.stackexchange.com/a/387474/13173
\usepackage{xparse,calc}
\usepackage{eforms

\begin{document}

 \medskip
 \textField[
    \Ff\FfMultiline
    \Ff\FfDoNotSpellCheck
           ]
  {myinput}{\dimexpr\linewidth-10ex}{5\baselineskip}


If key is set to be 2600, you can see the hint below:
\password"2600
\def\crystring{}
\begin{private}
    ♕♳♥☠♭♡♴♨♥♭♡♴♩♣♡♬☠♩♮♤♵♣♴♩♯♮☠♯♮☠☤♜♬♡♭♢♤♡☤☮
\end{private}

\newwrite\cryfile
\immediate\openout\cryfile=\jobname.cry
\immediate\write\cryfile{\crystring}
\immediate\closeout\cryfile
\input{\jobname.cry}

If key is not 2600, you see random characters:
\password"0
\def\crystring{}
\begin{private}
    ♕♳♥☠♭♡♴♨♥♭♡♴♩♣♡♬☠♩♮♤♵♣♴♩♯♮☠♯♮☠☤♜♬♡♭♢♤♡☤☮
\end{private}

\newwrite\cryfile
\immediate\openout\cryfile=\jobname.cry
\immediate\write\cryfile{\crystring}
\immediate\closeout\cryfile
\input{\jobname.cry}

% TODO how to pass the value as integer there?
\AACalculate{event.value=getField("myinput").value;}
If user input is set to be 7777, you can see the hint below:
\password"this.getField("myinput").value;
\def\crystring{}
\begin{private}
    ♕♳♥☠♭♡♴♨♥♭♡♴♩♣♡♬☠♩♮♤♵♣♴♩♯♮☠♯♮☠☤♜♬♡♭♢♤♡☤☮
\end{private}

\newwrite\cryfile
\immediate\openout\cryfile=\jobname.cry
\immediate\write\cryfile{\crystring}
\immediate\closeout\cryfile
\input{\jobname.cry}

\end{document}

Fig. 1 Output

TeXLive: 2017
OS: Debian 9.1
PDF viewer: Adobe Acrobat 9.x (acroread) installed as described here
Acrotex installation: here