linux-kernel - Re: [PATCH 19/22] crypto: inside-secure - add check for xts input length equal to zero

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Message-ID: <d4a471e6-34c9-c702-63d6-1f6a3cba0ebe@nxp.com>
Date:   Tue, 11 Aug 2020 18:28:39 +0300
From:   Horia Geantă <horia.geanta@....com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "Van Leeuwen, Pascal" <pvanleeuwen@...bus.com>,
        "Andrei Botila (OSS)" <andrei.botila@....nxp.com>,
        "David S. Miller" <davem@...emloft.net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-arm-kernel@...s.com" <linux-arm-kernel@...s.com>,
        Andrei Botila <andrei.botila@....com>,
        Antoine Tenart <antoine.tenart@...tlin.com>
Subject: Re: [PATCH 19/22] crypto: inside-secure - add check for xts input
 length equal to zero

On 8/10/2020 8:03 PM, Eric Biggers wrote:
> On Mon, Aug 10, 2020 at 05:33:39PM +0300, Horia Geantă wrote:
>> On 8/10/2020 4:45 PM, Herbert Xu wrote:
>>> On Mon, Aug 10, 2020 at 10:20:20AM +0000, Van Leeuwen, Pascal wrote:
>>>>
>>>> With all due respect, but this makes no sense.
>>>
>>> I agree.  This is a lot of churn for no gain.
>>>
>> I would say the gain is that all skcipher algorithms would behave the same
>> when input length equals zero - i.e. treat the request as a no-op.
>>
>> We can't say "no input" has any meaning to the other skcipher algorithms,
>> but the convention is to accept this case and just return 0.
>> I don't see why XTS has to be handled differently.
>>
> 
> CTS also rejects empty inputs.
> 
> The rule it follows is just that all input lengths >= blocksize are allowed.
> Input lengths < blocksize aren't allowed.
> 
Indeed, thanks.

What about, for example, CBC?
AFAICT cbc(aes) with input length = 0 is valid.

Same for CTR (with the note that blocksize = 1) and several other algorithms
mentioned in the cover letter.

What's the rule in these cases?

Thanks,
Horia