> On Apr 28, 2023, at 12:09 PM, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Fri, 28 Apr 2023 at 13:59, Chuck Lever III <chuck.lever@xxxxxxxxxx> wrote:
>>
>>
>>
>>> On Apr 28, 2023, at 5:57 AM, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>>>
>>> On Fri, 28 Apr 2023 at 10:44, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> Scott Mayhew <smayhew@xxxxxxxxxx> wrote:
>>>>>
>>>>> diff --git a/arch/arm64/crypto/aes-modes.S b/arch/arm64/crypto/aes-modes.S
>>>>> index 0e834a2c062c..477605fad76b 100644
>>>>> --- a/arch/arm64/crypto/aes-modes.S
>>>>> +++ b/arch/arm64/crypto/aes-modes.S
>>>>> @@ -268,6 +268,7 @@ AES_FUNC_START(aes_cbc_cts_encrypt)
>>>>> add x4, x0, x4
>>>>> st1 {v0.16b}, [x4] /* overlapping stores */
>>>>> st1 {v1.16b}, [x0]
>>>>> + st1 {v1.16b}, [x5]
>>>>> ret
>>>>> AES_FUNC_END(aes_cbc_cts_encrypt)
>>>>>
>>>>> But I don't know if that change is at all correct! (I've never even
>>>>> looked at arm64 asm before). If someone who's knowledgeable about this
>>>>> code could chime in, I'd appreciate it.
>>>>
>>>> Ard, could you please take a look at this?
>>>>
>>>
>>> The issue seems to be that the caller expects iv_out to have been
>>> populated even when doing ciphertext stealing.
>>>
>>> This is meaningless, because the output IV can only be used to chain
>>> additional encrypted data if the split is at a block boundary. The
>>> ciphertext stealing fundamentally terminates the encryption, and
>>> produces a block of ciphertext that is shorter than the block size, so
>>> what the output IV should be is actually unspecified.
>>>
>>> IOW, test cases having plain/ciphertext vectors whose size is not a
>>> multiple of the cipher block size should not specify an expected value
>>> for the output IV.
>>
>> The test cases are extracted from RFC 3962 Appendix B. The
>> standard clearly expects there to be a non-zero next IV for
>> plaintext sizes that are not block-aligned.
>>
>
> OK, so this is the Kerberos V specific spec on how to use AES in CBC
> mode, which appears to describe how to chain multiple CBC encryptions
> together.
>
> CBC-CTS itself does not define this: the IV is an input only, and the
> reason we happen to return the IV is because a single CBC operation
> may be broken up into several ones, which can only be done on block
> boundaries. This is why CBC-CTS itself passes all its tests: a single
> CBC-CTS encryption only performs ciphertext stealing at the very end,
> and the next IV is never used in that case. (This is why the CTS mode
> tests in crypto/testmgr.h don't have iv_out vectors)
>
> Note that RFC3962 defines that the penultimate block of CBC-CTS
> ciphertext is used as the next IV. CBC returns the last ciphertext
> block as the output IV. It is a happy coincidence that the generic CTS
> template encapsulates CBC in a way where its output IV ends up in the
> right place.
>
>> Also, these test cases pass on other hardware platforms.
>>
>
> Fair enough.
>
> I am not opposed to fixing this, but given that it is the RFC3962 spec
> that defines that the next IV is the penultimate full block of the
> previous CBC-CTS ciphertext, we might consider doing the memcpy() in
> the Kerberos code not in the CBC-CTS implementations.
Interesting thought. I'm all about proper layering, so I think this
is worth considering. Can you send an RFC patch?
David, any opinion about this for crypto/krb5?
> (The 32-bit ARM
> code will be broken in the same way, and potentially other
> implementations as well)
--
Chuck Lever
