Censor by Blank Characters Loaded from Font

Question

The censor package censors text but keeps space tokens and periods to keep the uncensored document parts similar in position to those in uncensored document. But it doesn't match uncensored output because ligatures and hyphenation affect the box construction and subsequently the paragraph breaking into lines. It is not the case that the censored text is removed from the document and the document appears otherwise identical. The example below shows this. Is it is possible to load a null font that has all the same characters and parameters for them visible to TeX, but simply doesn't draw them, and to use this font inside a macro (or to add to a font such null characters and use them only inside a macro)? Then TeX would run effectively unaware of the censoring and guarantee match of uncensored content in the censored document and the content in the uncensored document.

EDIT: censor replaces with black boxes, but for the purpose of censoring it is sufficient to have blank (white space). Then one doesn't have to worry about making censor bars overreach the box height, width, depth to prevent information leak, like in accented characters. The censor can be indicated by zero width, height, and depth boxes which contain rules, if necessary. See related posts:

• How to replace a large block of text by an empty block of the same size?. The accepted answer uses \phantom, which only replaces an hbox with blank space. It has the same problem as \blackout from \censor for hyphenation and ligatures.
• Generate a “blank” font using the metrics of another font. See egreg's answer that a solution would keep the character data, defeating the purpose for the easier and insecure color change censoring. It would be required to change the characters to a null equivalent, but if the null equivalent has the same font metrics and those are kept in output source the censored information would be easily decoded.
• Multilines, multipages \phantom analog macro. Has the same stated purpose of not changing layout based on censored content. LuaTeX solution is to replace at shipout all horizontal and vertical boxes with boxes of the same dimension containing nothing.

\documentclass{article}
\usepackage{censor}
\begin{document}
\begin{minipage}[t]{.5\textwidth}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus
elit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur
dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer
id, vulputate a, magna. Donec vehicula augue eu neque. Pellentesque
habitant morbi tristique senectus et netus et malesuada fames ac
turpis egestas. Mauris ut leo. Cras viverra metus rhoncus sem. Nulla
et lectus vestibulum urna fringilla ultrices. Phasellus eu tellus
sit amet tortor gravida placerat. Integer sapien est, iaculis in,
pretium quis, viverra ac, nunc. Praesent eget sem vel leo ultrices
bibendum. Aenean faucibus. Morbi dolor nulla, malesuada eu, pulvinar
at, mollis ac, nulla. Curabitur auctor semper nulla.  Donec varius orci
eget risus. Duis nibh mi, congue eu, accumsan eleifend, sagittis quis,
diam. Duis eget orci sit amet orci dignissim rutrum.
\end{minipage}
\begin{minipage}[t]{.5\textwidth}
\blackout{Lorem} ipsum \blackout{dolor} sit \blackout{amet},
consectetuer \blackout{adipiscing} elit. \blackout{Ut} purus
\blackout{elit}, vestibulum \blackout{ut}, \blackout{placerat}
ac, \blackout{adipiscing} vitae, \blackout{felis}. Curabitur
\blackout{dictum} gravida \blackout{mauris}. Nam \blackout{arcu}
libero, \blackout{nonummy} eget, \blackout{consectetuer}
id, \blackout{vulputate} a, \blackout{magna}. Donec
\blackout{vehicula} augue \blackout{eu} neque. \blackout{Pellentesque}
habitant \blackout{morbi} tristique \blackout{senectus} et
\blackout{netus} et \blackout{malesuada} fames \blackout{ac} turpis
\blackout{egestas}. Mauris \blackout{ut} leo. \blackout{Cras}
viverra \blackout{metus} rhoncus \blackout{sem}. Nulla \blackout{et}
lectus \blackout{vestibulum} urna \blackout{fringilla} ultrices.
\blackout{Phasellus} eu \blackout{tellus} sit \blackout{amet} tortor
\blackout{gravida} placerat. \blackout{Integer} sapien \blackout{est},
iaculis \blackout{in}, pretium \blackout{quis}, viverra \blackout{ac},
nunc. \blackout{Praesent} eget \blackout{sem} vel \blackout{leo}
ultrices \blackout{bibendum}. Aenean \blackout{faucibus}. Morbi
\blackout{dolor} nulla, \blackout{malesuada} eu, \blackout{pulvinar}
at, \blackout{mollis} ac, \blackout{nulla}. Curabitur \blackout{auctor}
semper \blackout{nulla}.  Donec \blackout{varius} orci \blackout{eget}
risus. \blackout{Duis} nibh \blackout{mi}, congue \blackout{eu},
accumsan \blackout{eleifend}, sagittis \blackout{quis}, diam.
\blackout{Duis} eget \blackout{orci} sit \blackout{amet} orci
\blackout{dignissim} rutrum.
\end{minipage}
\newpage
\begin{minipage}[t]{.5\textwidth}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus
elit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur
dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer
id, vulputate a, magna. Donec vehicula augue eu neque. Pellentesque
habitant morbi tristique senectus et netus et malesuada fames ac
turpis egestas. Mauris ut leo. Cras viverra metus rhoncus sem. Nulla
et lectus vestibulum urna fringilla ultrices. Phasellus eu tellus
sit amet tortor gravida placerat. Integer sapien est, iaculis in,
pretium quis, viverra ac, nunc. Praesent eget sem vel leo ultrices
bibendum. Aenean faucibus. Morbi dolor nulla, malesuada eu, pulvinar
at, mollis ac, nulla. Curabitur auctor semper nulla.  Donec varius orci
eget risus. Duis nibh mi, congue eu, accumsan eleifend, sagittis quis,
diam. Duis eget orci sit amet orci dignissim rutrum.
\end{minipage}
\begin{minipage}[t]{.5\textwidth}
\blackout{Lorem ipsum} dolor sit \blackout{amet, consectetuer}
adipiscing elit. \blackout{Ut purus} elit, vestibulum \blackout{ut,
placerat} ac, adipiscing \blackout{vitae, felis.} Curabitur dictum
\blackout{gravida mauris.} Nam arcu \blackout{libero, nonummy} eget,
consectetuer \blackout{id, vulputate} a, magna. \blackout{Donec
vehicula} augue eu \blackout{neque. Pellentesque} habitant morbi
\blackout{tristique senectus} et netus \blackout{et malesuada}
fames ac \blackout{turpis egestas.} Mauris ut \blackout{leo. Cras}
viverra metus \blackout{rhoncus sem.} Nulla et \blackout{lectus
vestibulum} urna fringilla \blackout{ultrices. Phasellus} eu tellus
\blackout{sit amet} tortor gravida \blackout{placerat. Integer}
sapien est, \blackout{iaculis in,} pretium quis, \blackout{viverra
ac,} nunc. Praesent \blackout{eget sem} vel leo \blackout{ultrices
bibendum.} Aenean faucibus. \blackout{Morbi dolor} nulla, malesuada
\blackout{eu, pulvinar} at, mollis \blackout{ac, nulla.} Curabitur
auctor \blackout{semper nulla.}  Donec \blackout{varius orci} eget
risus. \blackout{Duis nibh} mi, congue \blackout{eu, accumsan} eleifend,
sagittis \blackout{quis, diam.} Duis eget \blackout{orci sit} amet orci
\blackout{dignissim rutrum.}
\end{minipage}
\newpage
\begin{minipage}[t]{.5\textwidth}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus
elit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur
dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer
id, vulputate a, magna. Donec vehicula augue eu neque. Pellentesque
habitant morbi tristique senectus et netus et malesuada fames ac
turpis egestas. Mauris ut leo. Cras viverra metus rhoncus sem. Nulla
et lectus vestibulum urna fringilla ultrices. Phasellus eu tellus
sit amet tortor gravida placerat. Integer sapien est, iaculis in,
pretium quis, viverra ac, nunc. Praesent eget sem vel leo ultrices
bibendum. Aenean faucibus. Morbi dolor nulla, malesuada eu, pulvinar
at, mollis ac, nulla. Curabitur auctor semper nulla.  Donec varius orci
eget risus. Duis nibh mi, congue eu, accumsan eleifend, sagittis quis,
diam. Duis eget orci sit amet orci dignissim rutrum.
\end{minipage}
\begin{minipage}[t]{.5\textwidth}
\blackout{Lorem ipsum dolor} sit amet, consectetuer \blackout{adipiscing
elit. Ut} purus elit, vestibulum \blackout{ut, placerat ac,} adipiscing
vitae, felis. \blackout{Curabitur dictum gravida} mauris. Nam arcu
\blackout{libero, nonummy eget,} consectetuer id, vulputate \blackout{a,
magna. Donec} vehicula augue eu \blackout{neque. Pellentesque habitant}
morbi tristique senectus \blackout{et netus et} malesuada fames ac
\blackout{turpis egestas. Mauris} ut leo. Cras \blackout{viverra metus
rhoncus} sem. Nulla et \blackout{lectus vestibulum urna} fringilla
ultrices. Phasellus \blackout{eu tellus sit} amet tortor gravida
\blackout{placerat. Integer sapien} est, iaculis in, \blackout{pretium
quis, viverra} ac, nunc. Praesent \blackout{eget sem vel} leo ultrices
bibendum. \blackout{Aenean faucibus. Morbi} dolor nulla, malesuada
\blackout{eu, pulvinar at,} mollis ac, nulla. \blackout{Curabitur auctor
semper} nulla.  Donec \blackout{varius orci eget} risus. Duis nibh
\blackout{mi, congue eu,} accumsan eleifend, sagittis \blackout{quis,
diam. Duis} eget orci sit \blackout{amet orci dignissim} rutrum.
\end{minipage}
\newpage
\begin{minipage}[t]{.5\textwidth}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus
elit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur
dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer
id, vulputate a, magna. Donec vehicula augue eu neque. Pellentesque
habitant morbi tristique senectus et netus et malesuada fames ac
turpis egestas. Mauris ut leo. Cras viverra metus rhoncus sem. Nulla
et lectus vestibulum urna fringilla ultrices. Phasellus eu tellus
sit amet tortor gravida placerat. Integer sapien est, iaculis in,
pretium quis, viverra ac, nunc. Praesent eget sem vel leo ultrices
bibendum. Aenean faucibus. Morbi dolor nulla, malesuada eu, pulvinar
at, mollis ac, nulla. Curabitur auctor semper nulla.  Donec varius orci
eget risus. Duis nibh mi, congue eu, accumsan eleifend, sagittis quis,
diam. Duis eget orci sit amet orci dignissim rutrum.
\end{minipage}
\begin{minipage}[t]{.5\textwidth}
\blackout{Lorem ipsum dolor sit} amet, consectetuer adipiscing elit.
\blackout{Ut purus elit, vestibulum} ut, placerat ac, adipiscing
\blackout{vitae, felis. Curabitur dictum} gravida mauris. Nam
arcu \blackout{libero, nonummy eget, consectetuer} id, vulputate
a, magna. \blackout{Donec vehicula augue eu} neque. Pellentesque
habitant morbi \blackout{tristique senectus et netus} et malesuada
fames ac \blackout{turpis egestas. Mauris ut} leo. Cras viverra
metus \blackout{rhoncus sem. Nulla et} lectus vestibulum urna
fringilla \blackout{ultrices. Phasellus eu tellus} sit amet tortor
gravida \blackout{placerat. Integer sapien est,} iaculis in, pretium
quis, \blackout{viverra ac, nunc. Praesent} eget sem vel leo
\blackout{ultrices bibendum. Aenean faucibus.} Morbi dolor nulla,
malesuada \blackout{eu, pulvinar at, mollis} ac, nulla. Curabitur auctor
\blackout{semper nulla.  Donec} varius orci eget risus. \blackout{Duis
nibh mi, congue} eu, accumsan eleifend, sagittis \blackout{quis,
diam. Duis eget} orci sit amet orci \blackout{dignissim rutrum.}
\end{minipage}
\end{document}

Answer 1

Could a custom Open Type font feature help?

Because of glue's stretchiness, though, lines and paragraph become de-linked in terms of shape. Judicious choice (and/or adjustment) of glyph will be needed.

Proof of concept: only lowercase letters and a few punctuation marks are substituted here; hyphens (which are characters, too) can also be substituted (see linked question).

Random glyph as "black square":

The black square, from another font:

Scaled down a bit:

MWE

\documentclass{article}
\usepackage{fontspec}
\usepackage{lipsum}
\newcommand\anycxchar{uniEBA3}
\newcommand\anycxcharb{uni2580}
\newcommand\anycxcharc{circlecopyrt}
\directlua {
luaotfload.add_colorscheme("cxscheme",
{
["FF200080"] = {"\anycxchar"},
})
}
\directlua {
luaotfload.add_colorscheme("cyscheme",
{
["FF20A090"] = {"\anycxcharb"},
})
}
\directlua {
luaotfload.add_colorscheme("czscheme",
{
["FF0000"] = {"T_h"},
})
}
\directlua{
fonts.handlers.otf.addfeature {
    name = "anycx",
    type = "substitution",
    data = {
        ["a"] = "\anycxchar",
        ["b"] = "\anycxchar",
        ["c"] = "\anycxchar",
        ["d"] = "\anycxchar",
        ["e"] = "\anycxchar",
        ["f"] = "\anycxchar",
        ["g"] = "\anycxchar",
        ["h"] = "\anycxchar",
        ["i"] = "\anycxchar",
        ["j"] = "\anycxchar",
        ["k"] = "\anycxchar",
        ["l"] = "\anycxchar",
        ["m"] = "\anycxchar",
        ["n"] = "\anycxchar",
        ["o"] = "\anycxchar",
        ["p"] = "\anycxchar",
        ["q"] = "\anycxchar",
        ["r"] = "\anycxchar",
        ["s"] = "\anycxchar",
        ["t"] = "\anycxchar",
        ["u"] = "\anycxchar",
        ["v"] = "\anycxchar",
        ["w"] = "\anycxchar",
        ["x"] = "\anycxchar",
        ["y"] = "\anycxchar",
        ["z"] = "\anycxchar",
        ["0"] = "\anycxchar",
        ["1"] = "\anycxchar",
        ["2"] = "\anycxchar",
        ["3"] = "\anycxchar",
        ["4"] = "\anycxchar",
        ["5"] = "\anycxchar",
        ["6"] = "\anycxchar",
        ["7"] = "\anycxchar",
        ["8"] = "\anycxchar",
        ["9"] = "\anycxchar",
        ["r"] = "\anycxchar",
        [","] = "\anycxchar",
        ["."] = "\anycxchar",
        ["/"] = "\anycxchar",
        ["<"] = "\anycxchar",
        [">"] = "\anycxchar",
        ["?"] = "\anycxchar",
        }
    }
}
\directlua{
fonts.handlers.otf.addfeature {
    name = "anycy",
    type = "substitution",
    data = {
        ["hyphen"] = "\anycxcharb",
        ["a"] = "\anycxcharb",
        ["b"] = "\anycxcharb",
        ["c"] = "\anycxcharb",
        ["d"] = "\anycxcharb",
        ["e"] = "\anycxcharb",
        ["f"] = "\anycxcharb",
        ["g"] = "\anycxcharb",
        ["h"] = "\anycxcharb",
        ["i"] = "\anycxcharb",
        ["j"] = "\anycxcharb",
        ["k"] = "\anycxcharb",
        ["l"] = "\anycxcharb",
        ["m"] = "\anycxcharb",
        ["n"] = "\anycxcharb",
        ["o"] = "\anycxcharb",
        ["p"] = "\anycxcharb",
        ["q"] = "\anycxcharb",
        ["r"] = "\anycxcharb",
        ["s"] = "\anycxcharb",
        ["t"] = "\anycxcharb",
        ["u"] = "\anycxcharb",
        ["v"] = "\anycxcharb",
        ["w"] = "\anycxcharb",
        ["x"] = "\anycxcharb",
        ["y"] = "\anycxcharb",
        ["z"] = "\anycxcharb",
        ["0"] = "\anycxcharb",
        ["1"] = "\anycxcharb",
        ["2"] = "\anycxcharb",
        ["3"] = "\anycxcharb",
        ["4"] = "\anycxcharb",
        ["5"] = "\anycxcharb",
        ["6"] = "\anycxcharb",
        ["7"] = "\anycxcharb",
        ["8"] = "\anycxcharb",
        ["9"] = "\anycxcharb",
        [32] = "\anycxcharb",
        ["r"] = "\anycxcharb",
        [","] = "\anycxcharb",
        ["."] = "\anycxcharb",
        ["/"] = "\anycxcharb",
        ["<"] = "\anycxcharb",
        [">"] = "\anycxcharb",
        ["?"] = "\anycxcharb",
        }
    }
}
\directlua{
fonts.handlers.otf.addfeature {
    name = "anycz2",
    type = "substitution",
    data = {
        ["T_h"] = "\anycxcharc",
        }
}}
\directlua{
fonts.handlers.otf.addfeature {
    name = "anycz",
    type = "substitution",
    data = {
        ["hyphen"] = "\anycxcharc",
        ["a"] = "\anycxcharc",
        ["b"] = "\anycxcharc",
        ["c"] = "\anycxcharc",
        ["d"] = "\anycxcharc",
        ["e"] = "\anycxcharc",
        ["f"] = "\anycxcharc",
        ["g"] = "\anycxcharc",
        ["h"] = "\anycxcharc",
        ["i"] = "\anycxcharc",
        ["j"] = "\anycxcharc",
        ["k"] = "\anycxcharc",
        ["l"] = "\anycxcharc",
        ["m"] = "\anycxcharc",
        ["n"] = "\anycxcharc",
        ["o"] = "\anycxcharc",
        ["p"] = "\anycxcharc",
        ["q"] = "\anycxcharc",
        ["r"] = "\anycxcharc",
        ["s"] = "\anycxcharc",
        ["t"] = "\anycxcharc",
        ["u"] = "\anycxcharc",
        ["v"] = "\anycxcharc",
        ["w"] = "\anycxcharc",
        ["x"] = "\anycxcharc",
        ["y"] = "\anycxcharc",
        ["z"] = "\anycxcharc",
        ["0"] = "\anycxcharc",
        ["1"] = "\anycxcharc",
        ["2"] = "\anycxcharc",
        ["3"] = "\anycxcharc",
        ["4"] = "\anycxcharc",
        ["5"] = "\anycxcharc",
        ["6"] = "\anycxcharc",
        ["7"] = "\anycxcharc",
        ["8"] = "\anycxcharc",
        ["9"] = "\anycxcharc",
        [32] = "\anycxcharc",
        [","] = "\anycxcharc",
        ["."] = "\anycxcharc",
        ["/"] = "\anycxcharc",
        ["<"] = "\anycxcharc",
        [">"] = "\anycxcharc",
        ["?"] = "\anycxcharc",
        }
    }
}
\newfontfamily\fany{Clara}[
RawFeature=+anycx,
Colour=cxscheme,
]
\newfontfamily\fancy{FiraMath}[
RawFeature=+anycy,
Colour=cyscheme,
Scale=0.75,
]
\newfontfamily\fancz{AntykwaTorunska}[
RawFeature=+anycz,
]
\newfontfamily\fancza{AntykwaTorunska}[
Ligatures=TeX,
Colour=czscheme,
]
\newfontfamily\fanczb{AntykwaTorunska}[
RawFeature=+anycz;+anycz2,
]
\newcommand\blackout[1]{{\fancy#1}}
\begin{document}
abcde fghij klmnop qrstu wxyz 1234567890 ,./<>?
\fany
abcde fghij klmnop qrstu wxyz 1234567890 ,./<>?
\lipsum
\newpage
\fancy
abcde fghij klmnop qrstu wxyz 1234567890 ,./<>?
\lipsum
\newpage
\normalfont
Q: What is the question to $1+1=1$?
A: {\fancz The cat sat on the mat.}
\fancza ``Th'' is a ligature: \fanczb The cat sat on the mat.
\newpage
\normalfont
\begin{minipage}[t]{.5\textwidth}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus
elit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur
dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer
id, vulputate a, magna. Donec vehicula augue eu neque. Pellentesque
habitant morbi tristique senectus et netus et malesuada fames ac
turpis egestas. Mauris ut leo. Cras viverra metus rhoncus sem. Nulla
et lectus vestibulum urna fringilla ultrices. Phasellus eu tellus
sit amet tortor gravida placerat. Integer sapien est, iaculis in,
pretium quis, viverra ac, nunc. Praesent eget sem vel leo ultrices
bibendum. Aenean faucibus. Morbi dolor nulla, malesuada eu, pulvinar
at, mollis ac, nulla. Curabitur auctor semper nulla.  Donec varius orci
eget risus. Duis nibh mi, congue eu, accumsan eleifend, sagittis quis,
diam. Duis eget orci sit amet orci dignissim rutrum.
\end{minipage}
\begin{minipage}[t]{.5\textwidth}
\blackout{Lorem} ipsum \blackout{dolor} sit \blackout{amet},
consectetuer \blackout{adipiscing} elit. \blackout{Ut} purus
\blackout{elit}, vestibulum \blackout{ut}, \blackout{placerat}
ac, \blackout{adipiscing} vitae, \blackout{felis}. Curabitur
\blackout{dictum} gravida \blackout{mauris}. Nam \blackout{arcu}
libero, \blackout{nonummy} eget, \blackout{consectetuer}
id, \blackout{vulputate} a, \blackout{magna}. Donec
\blackout{vehicula} augue \blackout{eu} neque. \blackout{Pellentesque}
habitant \blackout{morbi} tristique \blackout{senectus} et
\blackout{netus} et \blackout{malesuada} fames \blackout{ac} turpis
\blackout{egestas}. Mauris \blackout{ut} leo. \blackout{Cras}
viverra \blackout{metus} rhoncus \blackout{sem}. Nulla \blackout{et}
lectus \blackout{vestibulum} urna \blackout{fringilla} ultrices.
\blackout{Phasellus} eu \blackout{tellus} sit \blackout{amet} tortor
\blackout{gravida} placerat. \blackout{Integer} sapien \blackout{est},
iaculis \blackout{in}, pretium \blackout{quis}, viverra \blackout{ac},
nunc. \blackout{Praesent} eget \blackout{sem} vel \blackout{leo}
ultrices \blackout{bibendum}. Aenean \blackout{faucibus}. Morbi
\blackout{dolor} nulla, \blackout{malesuada} eu, \blackout{pulvinar}
at, \blackout{mollis} ac, \blackout{nulla}. Curabitur \blackout{auctor}
semper \blackout{nulla}.  Donec \blackout{varius} orci \blackout{eget}
risus. \blackout{Duis} nibh \blackout{mi}, congue \blackout{eu},
accumsan \blackout{eleifend}, sagittis \blackout{quis}, diam.
\blackout{Duis} eget \blackout{orci} sit \blackout{amet} orci
\blackout{dignissim} rutrum.
\end{minipage}
\end{document}

See also: Hiding informations

Answer 2

Assuming that you have a font that has an associated tfm file, I could see a route to doing this using virtual fonts.

The basic strategy would be to take the original tfm and run tftopl on the file to get a pl file. This is a human-readable version of the TeX font metrics.

Copy that file to, e.g., xcmr10.vpl (or whatever you want to name your blackout font).

The vpl format is an extended version of the pl format, documented in the vptovf source code (texdoc vptovf to see the details). The short version is that we want to take each character description that looks like:

(CHARACTER O 13
   (CHARWD R 0.583336)
   (CHARHT R 0.694445)
   (CHARIC R 0.077779)
   (COMMENT
      (LIG C i O 16)
      (LIG C l O 17)
      (KRN O 47 R 0.077779)
      (KRN O 77 R 0.077779)
      (KRN O 41 R 0.077779)
      (KRN O 51 R 0.077779)
      (KRN O 135 R 0.077779)
      )
   )

and modify it to add a MAP instruction before the COMMENT (which may or may not be present) which will set a rule. We don't need to worry about font mapping since all we'll do is set rules. The transformed character would look like

(CHARACTER O 13
   (CHARWD R 0.583336)
   (CHARHT R 0.694445)
   (CHARIC R 0.077779)
   (MAP
      (SETRULE 0.683332 0.583336)
   )
   (COMMENT
      (LIG C i O 16)
      (LIG C l O 17)
      (KRN O 47 R 0.077779)
      (KRN O 77 R 0.077779)
      (KRN O 41 R 0.077779)
      (KRN O 51 R 0.077779)
      (KRN O 135 R 0.077779)
      )
   )

The SETRULE command takes height and width in terms of design size. I used the width of the character and picked the height of a capital letter for the height. You could put a MOVEDOWN before the rule and make it taller to simulate covering descenders but that's probably not necessary for your purposes.

I would not recommend doing this by hand. You will want to write a program to parse the VPL file and add the MAP automatically. Perhaps someone here eager to help will write such a program.

The final step would be to run vptovf on the final vpl file and place the resulting vf and tfm files in appropriate locations for your TeX install. Then you can use the resulting fonts like any other TeX font and get nice black boxes for your censored text. Note that we're keeping all the kerns and ligatures in the original font so there will be little gotchas when there are positive kerns (so for my example character from cmr10, it might be best to add an extra 0.077779 onto the width of the rule to keep those from being apparent.

You could also go ahead and add the original font as a MAPFONT into the VPL file you generate and set any characters you don't want to censor (perhaps you only censor letters and numerals but leave punctuation printing).