Re: [Bitcoin-development] BIP 38 NFC normalisation issue

[Brooks Boyd <boydb@midnightdesign.ws>]

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I was part of adding in that test vector, and I think it's a good test
vector since it is an extreme edge-case of the current definition: If the
BIP38 proposal allows any password that can be in UTF-8, NFC normalized
form, those characters cover the various edge cases (combining characters,
null character, astral range) that if your implementation doesn't handle,
then it can't really be said to be "BIP38-compatible/compliant", right?

The "passphrase" in the test vector is NOT in NFC form; that's the point.
Whatever implementation gets designed has to assume the input is not
already NFC-normalized and needs to handle/sanitize that input before
further processing. To test your implementation for compliance, you should
not be inputting the NFC-normalized bytestring as the password input, you
should be entering the original passphrase as the test. My original pull
request for this change (https://github.com/bitcoin/bips/pull/29) shows a
Python and a NodeJS way to input that test vector password as intended.

Some input devices may already handle the input as NFC, which is great, but
per the BIP38 proposal, that shouldn't be assumed, so various
implementations are cross-compatible. If one implementation assumes the
input is already NFC, they may encode/decode the password incorrectly, and
lock a user out of their wallet. Android allows different user keyboards to
be used, so I'm guessing there's one somewhere that allows manual entry of
unicode codepoints that could be used to enter a null character, and with
the next version of iOS, Apple devices will also get custom keyboard
options, too, so even if the default Apple keyboard does NFC-form properly,
other developers' keyboards may not. So while it is an extreme edge case,
that is not very likely to be used as a "real password" by any user, that's
what test vectors are for: to test for the edge case that you might not
have expected and handled in your implementation.

Brooks


On Tue, Jul 15, 2014 at 8:07 AM, Eric Winer <enwiner@gmail.com> wrote:

> I don't know for sure if the test vector is correct NFC form.  But for
> what it's worth, the Pile of Poo character is pretty easily accessible on
> the iPhone and Android keyboards, and in this string it's already in NFC
> form (f09f92a9 in the test result).  I've certainly seen it in usernames
> around the internet, and wouldn't be surprised to see it in passphrases
> entered on smartphones, especially if the author of a BIP38-compatible app
> includes a (possibly ill-advised) suggestion to have your passphrase
> "include special characters".
>
> I haven't seen the NULL character on any smartphone keyboards, though - I
> assume the iOS and Android developers had the foresight to know how much
> havoc that would wreak on systems assuming null-terminated strings.  It
> seems unlikely that NULL would be in a real-world passphrase entered by a
> sane user.
>
>
> On Tue, Jul 15, 2014 at 8:03 AM, Mike Hearn <mike@plan99.net> wrote:
>
>> [+cc aaron]
>>
>> We recently added an implementation of BIP 38 (password protected private
>> keys) to bitcoinj. It came to my attention that the third test vector may
>> be broken. It gives a hex version of what the NFC normalised version of the
>> input string should be, but this does not match the results of the Java
>> unicode normaliser, and in fact I can't even get Python to print the names
>> of the characters past the embedded null. I'm curious where this normalised
>> version came from.
>>
>> Given that "pile of poo" is not a character I think any sane user would
>> put into a passphrase, I question the value of this test vector. NFC form
>> is intended to collapse things like umlaut control characters onto their
>> prior code point, but here we're feeding the algorithm what is basically
>> garbage so I'm not totally surprised that different implementations appear
>> to disagree on the outcome.
>>
>> Proposed action: we remove this test vector as it does not represent any
>> real world usage of the spec, or if we desperately need to verify NFC
>> normalisation I suggest using a different, more realistic test string, like
>> Zürich, or something written in Thai.
>>
>>
>>
>> Test 3:
>>
>>    - Passphrase ϓ␀𐐀💩 (\u03D2\u0301\u0000\U00010400\U0001F4A9; GREEK
>>    UPSILON WITH HOOK <http://codepoints.net/U+03D2>, COMBINING ACUTE
>>    ACCENT <http://codepoints.net/U+0301>, NULL
>>    <http://codepoints.net/U+0000>, DESERET CAPITAL LETTER LONG I
>>    <http://codepoints.net/U+10400>, PILE OF POO
>>    <http://codepoints.net/U+1F4A9>)
>>    - Encrypted key:
>>    6PRW5o9FLp4gJDDVqJQKJFTpMvdsSGJxMYHtHaQBF3ooa8mwD69bapcDQn
>>    - Bitcoin Address: 16ktGzmfrurhbhi6JGqsMWf7TyqK9HNAeF
>>    - Unencrypted private key (WIF):
>>    5Jajm8eQ22H3pGWLEVCXyvND8dQZhiQhoLJNKjYXk9roUFTMSZ4
>>    - *Note:* The non-standard UTF-8 characters in this passphrase should
>>    be NFC normalized to result in a passphrase of0xcf9300f0909080f09f92a9 before
>>    further processing
>>
>>
>>
>>
>>
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