Re: [go-nuts] Crypto/rsa : OAEP documentation can lead to misunderstanding regarding max message length, is it a bug?

[Nigel Tao]

On Wed, Sep 21, 2016 at 1:41 AM, AnomalRoil  wrote:
> So here am I: should I open an issue for a trivial sign mistake in a
> sentence in the documentation?

Opening an issue would have been fine. In this case, though, I just
sent the trivial change out for review:
https://go-review.googlesource.com/29496

Thanks for the note.

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[go-nuts] Crypto/rsa : OAEP documentation can lead to misunderstanding regarding max message length, is it a bug?

[AnomalRoil]

Hi there,

I was reading the Crypto/RSA documentation, since I'm using OAEP encryption 
for testing purposes.
So I read this  which ends 
with:

> The message must be no longer than the length of the public modulus less 
twice the hash length *plus *2.

Which I directly *translated *(maybe me being no native speaker is at fault 
there) into: 
> len(msg) <= len(public_modulus) - 2*len(hash) + 2
which I found puzzling since I thought it should have been -2, as I had 
checked the standard recently, so I double-checked the RFC 
 which says:

> RSAES-OAEP can operate on messages of length up to k - 2hLen - 2 octets, 
where hLen is the length of the output from the underlying hash function 
and k is the length in octets of the recipient's RSA modulus.

I guess it was intended to mean len(msg) <= len(public_modulus) - 
(2*len(hash) + 2), but those parens are not enough implied by such 
phrasing, in my opinion.
I've checked and the code is correct 
, so in the worse case a 
user may once end up getting an error after trying this "false-positive" 
border-case, if I may say so.

So here am I: *should I open an issue for a trivial sign mistake in a 
sentence in the documentation?*
I don't know what the usage is, I took a look at the guidelines for 
contributing and did not see any mention of such minor contribution.

I think it would be clearer written as follows (but maybe my usage of the 
comma is not correct in English?):
> The message must be no longer than the length of the public modulus less 
twice the hash length, less 2.
Or maybe it would even be better to write it in a more mathematical 
fashion? Like:
> The message must be no longer than the length of the public modulus - 2 * 
the hash length - 2. 

However, any opinion would be appreciated.
Best regards,
Y.

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