Re: Signing ascii text

[Eric Messick <eric@parallax.com>]

I wrote:

>It would canonicalize a file by
>turning all sequences of white space into a single space and trimming
>leading and trailing whitespace from the file before computing the
>hash.

mark@coombs.anu.edu.au resopnded:

> If the message contained a table of figures formatted and seperated with
> spaces then that method would destroy the readability of the table.

The important part here is that the collapsing of whitespace would
only affect the message digest, not the text as seen by the user.  Two
texts which read the same, but differ in whitespace, would have the
same signature.  If you recieved both files, you could see the
difference in spacing, yet the same signature would be valid for both
files.  The main vulnerability is that a message whose meaning is
partially encoded it its whitespace (like an ascii graphic, map, or
chart) could have its meaning altered, without affecting the validity
of the signature.  Clearly one would not want to use this signature
method on such texts.  It would be a good feature for the signature
algorithm to warn the user if it detects a pattern of whitespace that
might convey information.  I am not sure how to detect this reliably,
though.

-- eric messick
eric@toad.com