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PQC: Classic McEliece #3883
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PQC: Classic McEliece #3883
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This mostly looks at cmce_decaps.cpp
, cmce_encaps.cpp
and cmce.cpp
, noting many minor code style things and a few suggestions for alternative code structuring. Not looking into the Classic McEliece specifics at all here.
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First pass on cmce_field_orderings.cpp
. I'm somewhat concerned that this isn't very efficient. But it may well be "good enough". Should we profile that?
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More comments. I didn't look at cmce_parameter_set.*
, cmce_parameters_*
and cmce_poly.*
, yet.
// TODO: Only for test instances. Remove on final PR | ||
size_t m = Classic_McEliece_GF::log_q_from_mod(mod); | ||
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for(int i = static_cast<int>(m) - 2; i >= 0; --i) { | ||
x ^= CT::Mask<uint32_t>::expand((uint32_t(1) << (i + m)) & x) | ||
.if_set_return(static_cast<uint32_t>(mod.get()) << i); | ||
} | ||
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return GF_Elem(static_cast<uint16_t>(x)); |
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Don't forget to remove (and probably replace by some exception?)
Code_Word Classic_McEliece_Matrix::mul(const Classic_McEliece_Parameters& params, const Error_Vector& e) const { | ||
auto s = e.subvector(0, params.pk_no_rows()); | ||
auto e_T = e.subvector(params.pk_no_rows()); | ||
auto pk_slicer = BufferSlicer(m_mat_bytes); | ||
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for(size_t i = 0; i < params.pk_no_rows(); ++i) { | ||
auto pk_current_bytes = pk_slicer.take(params.pk_row_size_bytes()); | ||
auto row = secure_bitvector(pk_current_bytes, params.n() - params.pk_no_rows()); | ||
row &= e_T; | ||
s.at(i) = s.at(i) ^ row.has_odd_hamming_weight(); | ||
} | ||
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BOTAN_ASSERT_NOMSG(pk_slicer.empty()); | ||
return s.as<Code_Word>(); | ||
} |
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This produces quite a few copies and allocations. If that's in the implementation's hot path, we should probably want to have another look. Here's which (I believe cause allocations/copies):
.subvector()
always creates a newbitvector
and copies the content into a newly allocated buffer,- the c'tor of
bitvector
copies the passed-in buffer (particularlypk_current_bytes
.as<>
produces a copy of thebitvector
with a new type
For (3): This could already be the right type using e.subvector<Code_Word>()
, no?
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Some context: This function is called once per encapsulation. Also, e
and s
are pretty small, while m_mat_bytes
is gigantic. Therefore, 1) and 3) are not critical; I'll apply your suggestion for 3) anyway. I don't know how we can prevent 2) without having something like a bitvector view or dropping the bitvector altogether.
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I think I'll have a timeboxed look into a bitvector that doesn't own its underlying storage. Perhaps that isn't too hard to achieve, especially when we can limit it to byte-aligned subvectors.
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Done with a first pass on the implementation. Don't get put off by the number of comments. Most are just C++ style nits and smaller programming suggestions.
That's really good work! 😃
/// Reduced instances for side channel analysis (Self-created test instance with | ||
/// m=8, n=128, t=8, f(z)=z^8+z^7+z^2+z+1, F(y)=y^8+y^4+y^3+y^2+1) | ||
/// Minimal instance without semi-systematic matrix creation and no plaintext confirmation | ||
test, | ||
/// Minimal instance with semi-systematic matrix creation and no plaintext confirmation | ||
testf, | ||
/// Minimal instance without semi-systematic matrix creation and with plaintext confirmation | ||
testpc, | ||
/// Minimal instance with semi-systematic matrix creation and with plaintext confirmation | ||
testpcf |
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TODO: remove before merging
/** | ||
* @returns ceil(n/d) | ||
* TODO: Remove once LMS is merged | ||
*/ | ||
constexpr size_t ceil_div(size_t n, size_t d) { | ||
return (n + d - 1) / d; | ||
} |
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Re-evaluate before merging this.
Thanks a lot for your extensive review, @reneme! I addressed your suggestions and am optimistic that this PR is ready to drop its Draft status 🎉. Note that some suggestions that depend on other PRs are still open, which are not critical, though. Also, note that an extensive side-channel analysis is still in progress. |
src/lib/utils/bitvector.h
Outdated
constexpr bitref& operator^=(bool other) noexcept { return assign(this->is_set() ^ other); } | ||
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private: | ||
constexpr bitref& assign(bool bit) noexcept { return (bit) ? set() : unset(); } |
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As discussed, DATA identified this line as a leakage during our SCA review.
Due to the ?
operator, a control-flow difference is observed based on the boolean input bit
variable.
The assign()
routine is used within the push_back()
routine, which in turn is used within the decode()
routine. This may allow an adversary to observe the error vector e
and, hence recover the shared secret.
We suggest to perform both the set()
and unset()
functions with the input as a mask, for example:
private:
constexpr bitref& assign(bool bit) noexcept {
const block_type assign_mask = 0 - static_cast<block_type>(bit);
this->m_block |= (this->m_mask & assign_mask);
this->m_block &= ~(this->m_mask & ~assign_mask);
return *this;
}
In our case, this results in the following instructions - without any conditional branch based on the input:
[ ... ]
const block_type assign_mask = 0 - static_cast<block_type>(bit);
41d397: 41 f7 dc neg %r12d
[ ... ]
this->m_block |= (this->m_mask & assign_mask);
41d3ab: 44 89 e1 mov %r12d,%ecx
41d3ae: 21 c1 and %eax,%ecx
this->m_block &= ~(this->m_mask & ~assign_mask);
41d3b0: f7 d0 not %eax
this->m_block |= (this->m_mask & assign_mask);
41d3b2: 0a 0a or (%rdx),%cl
this->m_block &= ~(this->m_mask & ~assign_mask);
41d3b4: 44 09 e0 or %r12d,%eax
41d3b7: 21 c8 and %ecx,%eax
[ ... ]
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Thanks for your analysis and your report! I applied your fix using Botan's constant-time helper class (7a0fe59)
@@ -1,6 +1,8 @@ | |||
/* | |||
* An abstraction for an arbitrarily large bitvector that can |
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@reneme the code still contains some artefacts for varying the underlying data type that we'd ideally want to get rid of.
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Have not done a full review yet, leaving some initial comments
src/lib/utils/bit_ops.h
Outdated
x = (x + (x >> 4)) & 0xF0F0F0F0F0F0F0F; | ||
return (x * 0x101010101010101) >> 56; | ||
} else { | ||
static_assert(!std::unsigned_integral<T>, "T is not a suitable unsigned integer value"); |
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I don’t understand this static_assert
- shouldn’t the concept check on T
already prevent this? It feels like this is intended to catch the case where T > 64 bits but that doesn’t necesarily apply if the compiler considers a 128 bit integer type to be unsigned_integral
(maybe C++20 prohibits this, idk)
Perhaps instead starting the function with
static_assert(sizeof(T) <= 8, “T is not …”)
which anyway seems like a more clear statement of requirement?
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I think you are right. @reneme?
} | ||
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size_t Classic_McEliece_PublicKey::key_length() const { | ||
return m_public->matrix().bytes().size(); |
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a) Is it really necessary to encode the matrix (with memory allocation etc) just to determine the key length
b) Is the byte size of the matrix really the best value to return here? Returning the integer encoding of {n}{t} would be just as meaningul (ie, an arbitrary integer that goes upwards as the key gets stronger) while allowing some plausible method of mapping it back to a parameter set.
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a) The matrix object stores the matrix bytes as a member. The method bytes
only returns a const reference to these bytes. No new memory is allocated here.
b) Yeah, I got confused by the meaning of the method key length
. I assumed it was the size of the public key in bytes without looking into the description. Thanks for mentioning this.
I think the most sensible value is
Thanks very much for your initial review 🚀 I'll look into it. |
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I rebased to master (which removes the --no-stdout flag commit). |
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Yeah.
IMHO, running |
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Rebased to master + commit with some left-over suggestions. |
I applied this and decreased the total time for CMCE long tests from 81 sec to 17 sec. |
Thanks!
I do agree, but then we should move away from running such tests for every push (like we actually do in the "sanitizer" and "coverage" builds, currently). If we wanted to have such long-running tests, we should outsource them to a nightly, to keep CI times manageable for interactive use cases. |
@FAlbertDev This needs another rebase. Also, with #3985 merged, it'll need to implement the new method |
- constant time conditional swap with mask - floor_log2 Co-Authored-By: Amos Treiber <amos.treiber@rohde-schwarz.com>
This is an implementation of the Classic McEliece KEM according to the NIST Round 4 submission and the ISO draft 20230419. Co-Authored-By: Amos Treiber <amos.treiber@rohde-schwarz.com>
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Done! @reneme Do you still want to reduce the test times further for the |
This PR relates to the Classic McEliece KEM as specified in this ISO draft. It also contains the instances defined in the NIST Round 4 submission. The test cases were generated using the NIST submissions reference implementation. Note that Classic McEliece (module
cmce
) is not the original McEliece Algorithm that is implemented in Botan'smce
module. See the Classic McEliece homepage, for a brief comparison.TODO Tracker
Use uint8_t as bitvector return type(see: PQC: Classic McEliece #3883 (comment))