Re: Signing ascii text

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

...
> 
> 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.

How about two signatures, verbatim and space-collapsed.

That way if the latter was valid but the former was not, you would
know that spacing was altered but other info remained valid.

sdw

I don't think space-collapsed signatures make any sense; they're only
going to mess up

If you're going to do any canonicalization, do either exactly what PEM
does, or canonicalize tabs and trailing spaces out of the message at
posting time.  Then do the same canonicalization on the receive end
before the signature is verified.

If a message transfer path is still "lossy", stop using the unencoded
body and just live with radix64 encoding.

				- Bill