[PATCH RFC 0/4] Reintroduce the sm2 algorithm

[Gu Bowen]

Hi,

On 7/3/2025 9:14 PM, Jason A. Donenfeld wrote:
> Hi,
> 
> On Mon, Jun 30, 2025 at 09:39:30PM +0800, Gu Bowen wrote:
>> To reintroduce the sm2 algorithm, the patch set did the following:
>>   - Reintroduce the mpi library based on libgcrypt.
>>   - Reintroduce ec implementation to MPI library.
>>   - Rework sm2 algorithm.
>>   - Support verification of X.509 certificates.
>>
>> Gu Bowen (4):
>>    Revert "Revert "lib/mpi: Extend the MPI library""
>>    Revert "Revert "lib/mpi: Introduce ec implementation to MPI library""
>>    crypto/sm2: Rework sm2 alg with sig_alg backend
>>    crypto/sm2: support SM2-with-SM3 verification of X.509 certificates
> 
> I am less than enthusiastic about this. Firstly, I'm kind of biased
> against the whole "national flag algorithms" thing. But I don't know how
> much weight that argument will have here. More importantly, however,
> implementing this atop MPI sounds very bad. The more MPI we can get rid
> of, the better.
> 
> Is MPI constant time? Usually the good way to implement EC algorithms
> like this is to very carefully work out constant time (and fast!) field
> arithmetic routines, verify their correctness, and then implement your
> ECC atop that. At this point, there's *lots* of work out there on doing
> fast verified ECC and a bunch of different frameworks for producing good
> implementations. There are also other implementations out there you
> could look at that people have presumably studied a lot. This is old
> news. (In 3 minutes of scrolling around, I noticed that
> count_leading_zeros() on a value is used as a loop index, for example.
> Maybe fine, maybe not, I dunno; this stuff requires analysis.)

Actually, I wasn't very familiar with MPI in the past. Previously, the 
implementation of sm2 was done through MPI, so I used it as well. 
Perhaps I could try using the ecc algorithm in the kernel.

> On the other hand, maybe you don't care because you only implement
> verification, not signing, so all info is public? If so, the fact that
> you don't care about CT should probably be made pretty visible. But
> either way, you should still be concerned with having an actually good &
> correct implementation of which you feel strongly about the correctness.
> 
> Secondly, the MPI stuff you're proposing here adds a 25519 and 448
> implementation, and support for weierstrauss, montgomery, and edwards,
> and... surely you don't need all of this for SM-2. Why add all this
> unused code? Presumably because you don't really understand or "own" all
> of the code that you're proposing to add. And that gives me a lot of
> hesitation, because somebody is going to have to maintain this, and if
> the person sending patches with it isn't fully on top of it, we're not
> off to a good start.
> 
> Lastly, just to nip in the bud the argument, "but weierstrauss is all
> the same, so why not just have one library to do all possible
> weierstrauss curves?" -- the fact that this series reintroduces the
> removed "generic EC library" indicates there's actually not another user
> of it, even before we get into questions of whether it's a good idea.

Thank you for your advice, it has been very beneficial for me as I just 
started participating in the community. I will try to implement the 
functionality with more robust code and only submit parts that I fully 
understand.

Best Regards，
Guber