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Date: Sat, 7 Mar 2015 15:38:07 +0100
From: Thomas Pornin <pornin@...et.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] PHC output specifics

On Sat, Mar 07, 2015 at 09:04:05PM +1300, Peter Gutmann wrote:
> Another consideration is, how much of a problem is this in practice?

The problem arises in practice when:
 - the user chooses a password that contains a character (or glyph) which
   admits two or more representations as sequences of Unicode code points,
 - the user enters the password on two distinct systems,
 - and these systems don't agree on the representation used for the
   problematic character.

Therefore, the problem is not very common: most users will use a single
system (always the same browser on the same OS, for instance), and, even
when they use two distinct systems, most user interfaces follow NFC
normalization naturally (they represent "é" as U+00E9, not as U+0065
followed by U+0301). So I'd say that once you have said "UTF-8" you have
avoided most of the problems (unless one of the systems takes it upon
itself to add a BOM...).

UTF-8/UTF-16 confusions tend to be solved quite fast since they break
pure-ASCII passwords. Bugs that still lurk around are usually related to
non-Unicode encodings: one system uses UTF-8, another uses one of the
various monobyte codepages (e.g. Windows-1252); these bugs can remain
undetected for quite some time because it takes a non-American tester
(or an inordinately creative American tester) to find them.

Another point is that most users will type passwords "blindly" (the GUI
displays dots or stars, not the actual glyphs), so they will naturally
stick to characters that minimize typing errors -- they tend not to use
Alt+NNN combinations.

I have no idea how such things fare in practice for users with non-latin
scripts. Especially for scripts where glyphs tend to morph depending on
their situation in a word and their neighbours (Arabic, Hangul...).



I still think that implementers should be alerted of the issue. While
the encoding will be addressed elsewhere, the need for an unambiguous
deterministic encoding still arises from the password hashing function
itself, so it seems "natural" that the PHS specification says a word
about it.



Another point that is to be made is that while a PHS implementation is
usually written in C or assembly (since speed is its lifeforce), most
developers will use the function from another language, through
bindings. In these other languages, character strings are often "Unicode
aware" in some way (e.g. Java and C# strings are sequences of 16-bit
code units). The binding implementation will actually decide on the
encoding.

I think it would be part of the specification job to define API for some
non-C languages (Java, C#/.NET, Python, even Node.js and PHP), for
pretty much the same reasons that a standard C API is a good idea. but
this naturally implies that a "standard encoding" WILL have to be
decided upon.


>   Implementers should be aware of potential interoperability problems
>   due to character-representation issues and, if cross-platform
>   portability for a wide range of character types is an issue, use
>   appropriate encodings such as Unicode or UTF-8.

I cringe at the use of the word "Unicode" to describe an encoding. I
know that it is a widespread usage in most Windows-related
documentations, but it still is quite wrong. I'd suggest something like
that:

    Implementers should be aware of potential interoperability problems
    due to character-representation issues; they should define and
    follow unambiguous encoding rules that will cover all character
    types that will be encountered by the implementation in practice. It
    is RECOMMENDED that passwords are encoded in UTF-8, with NFC
    normalization and no BOM [REF-Unicode]. The security properties of
    PHS are maintained regardless of the used encoding, but using these
    encoding rules is likely to maximize interoperability.

where REF-Unicode points to a couple of places on www.unicode.org that
give all the needed information with the proper terminology. That way,
the developer is alerted about the issues AND learns the adequate
keywords for further research if he decides to deal with them in a clean
way (i.e. by not ignoring it).

In general, raw UTF-8 encoding of code points "as received" will be
enough to make issues rare enough to be ignored or handled manually.


	--Thomas Pornin

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