PQC: Classic McEliece #3883

FAlbertDev · 2024-01-11T12:04:01Z

Pull Request Dependencies

FFI: Loading of raw FrodoKEM keys & FIX: "insufficient buffer handling" in FFI's decapsulate #4373 - for the generic FFI/Python tests

Description

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's mce module. See the Classic McEliece homepage, for a brief comparison.

TODO Tracker

coveralls · 2024-01-11T12:37:18Z

coverage: 91.304% (+0.2%) from 91.14%
when pulling 083e153 on Rohde-Schwarz:pqc/classic_mceliece
into e24dd8a on randombit:master.

reneme

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.

src/build-data/oids.txt

src/lib/pubkey/classic_mceliece/cmce_types.h

src/lib/pubkey/classic_mceliece/cmce_matrix.cpp

src/lib/pubkey/classic_mceliece/cmce.cpp

src/lib/pubkey/classic_mceliece/cmce_encaps.cpp

src/lib/utils/strong_type.h

reneme

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?

src/lib/pubkey/classic_mceliece/cmce_field_ordering.cpp

reneme

More comments. I didn't look at cmce_parameter_set.*, cmce_parameters_* and cmce_poly.*, yet.

src/lib/pubkey/classic_mceliece/cmce_keys_internal.cpp

src/lib/utils/strong_type.h

src/lib/pubkey/classic_mceliece/cmce_matrix.cpp

src/lib/pubkey/classic_mceliece/cmce.cpp

reneme · 2024-01-26T10:16:03Z

src/lib/pubkey/classic_mceliece/cmce_gf.cpp

+ // TODO: Only for test instances. Remove on final PR
+ size_t m = Classic_McEliece_GF::log_q_from_mod(mod);
+
+ 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);
+ }
+
+ return GF_Elem(static_cast<uint16_t>(x));


Don't forget to remove (and probably replace by some exception?)

src/lib/pubkey/classic_mceliece/cmce_matrix.cpp

reneme · 2024-01-29T16:11:55Z

src/lib/pubkey/classic_mceliece/cmce_matrix.cpp

+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);
+
+ 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();
+ }
+
+ BOTAN_ASSERT_NOMSG(pk_slicer.empty());
+ return s.as<Code_Word>();
+}


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 new bitvector and copies the content into a newly allocated buffer,

the c'tor of bitvector copies the passed-in buffer (particularly pk_current_bytes

.as<> produces a copy of the bitvector with a new type

For (3): This could already be the right type using e.subvector<Code_Word>(), no?

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.

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.

reneme

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! 😃

reneme · 2024-01-30T07:34:29Z

src/lib/pubkey/classic_mceliece/cmce_parameter_set.h

+ /// 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


TODO: remove before merging

src/lib/pubkey/classic_mceliece/cmce_parameter_set.h

reneme · 2024-01-30T07:39:16Z

src/lib/pubkey/classic_mceliece/cmce_parameters.h

+/**
+ * @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;
+}


Re-evaluate before merging this.

src/lib/pubkey/classic_mceliece/cmce_parameters.h

src/lib/pubkey/classic_mceliece/cmce_poly.cpp

src/lib/pubkey/classic_mceliece/info.txt

src/lib/utils/bit_ops.h

src/lib/pubkey/classic_mceliece/cmce_field_ordering.cpp

FAlbertDev · 2024-02-05T14:48:10Z

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.

aewag · 2024-02-10T22:22:15Z

src/lib/utils/bitvector.h

+ constexpr bitref& operator^=(bool other) noexcept { return assign(this->is_set() ^ other); }
+
+ private:
+ constexpr bitref& assign(bool bit) noexcept { return (bit) ? set() : unset(); }


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 [ ... ]

Thanks for your analysis and your report! I applied your fix using Botan's constant-time helper class (7a0fe59)

atreiber94 · 2024-02-29T13:26:53Z

src/lib/utils/bitvector.h

@@ -1,6 +1,8 @@
 /*
 * An abstraction for an arbitrarily large bitvector that can


@reneme the code still contains some artefacts for varying the underlying data type that we'd ideally want to get rid of.

FAlbertDev · 2024-07-23T14:15:09Z

src/lib/pubkey/classic_mceliece/cmce_matrix.cpp

+CT::Mask<uint64_t> bit_at_mask(uint64_t val, size_t pos) {
+ return CT::Mask<uint64_t>::expand((val >> pos) & 1);
+}


Using this method in the respective context implies a side channel in clang 17. If I change it to

CT::Mask<uint64_t> bit_at_mask(uint64_t val, size_t pos) { return CT::Mask<uint64_t>::expand((uint64_t(1) << pos) & val); }

It is fine. It seems that clang somehow sees that the input to expand can only be 1 or 0. I could not find the exact source of the problem (since I'm bad with assembly), but I thought the value barrier should prevent such cases. Maybe some other parts in the calling context cause this to happen, though. I cannot (yet) reproduce this case in Godbolt either, so it seems to be a very special case...

Update: In this and similar cases, clang-17 seems to use a bt instruction, which valgrind does not process correctly (see here). In the alternative version, no such instruction is generated. Note that both versions DO NOT induce any side channels, according to our assembly analysis.

- 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>

Co-Authored-By: Jack Lloyd <jack@randombit.net>

Co-Authored-By: Fabian Albert <fabian.albert@rohde-schwarz.com>

This allows extracting subvectors of the bitvector as unsigned integral bit masks.

... introduced in randombit#4170

Use CT::poison/unpoison for CMCE. Simultaneously, fixes some constant-time issues in the CMCE implementation that may leak some information about the pivots (for the semi-systematic matrix form). Co-Authored-By: René Meusel <rene.meusel@rohde-schwarz.com>

reneme · 2024-10-18T07:09:55Z

Rebased and resolved several conflicts after all other PQC algorithms got merged.

reneme added this to the Botan 3.4.0 milestone Jan 11, 2024

FAlbertDev force-pushed the pqc/classic_mceliece branch 4 times, most recently from f73829f to e505390 Compare January 19, 2024 14:53

atreiber94 force-pushed the pqc/classic_mceliece branch 2 times, most recently from 04b9314 to 190261d Compare January 19, 2024 15:59

reneme self-requested a review January 22, 2024 09:33

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reneme reviewed Jan 25, 2024

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github-advanced-security bot found potential problems Feb 2, 2024

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src/lib/pubkey/classic_mceliece/cmce_field_ordering.cpp Fixed Show resolved Hide resolved

FAlbertDev marked this pull request as ready for review February 5, 2024 14:49

aewag reviewed Feb 10, 2024

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This was referenced Feb 14, 2024

Classic McEliece Cryptodoc sehlen-bsi/botan-docs#186

Open

Classic McEliece Testspecification sehlen-bsi/botan-docs#187

Open

atreiber94 reviewed Feb 29, 2024

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FAlbertDev force-pushed the pqc/classic_mceliece branch from 5f0efcc to 54395c2 Compare March 8, 2024 13:24

reneme force-pushed the pqc/classic_mceliece branch from e0dc6cc to 60bd9fb Compare July 12, 2024 15:19

reneme mentioned this pull request Jul 16, 2024

Botan: add version 3.5.0 conan-io/conan-center-index#24638

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reneme force-pushed the pqc/classic_mceliece branch 3 times, most recently from e07e615 to b679113 Compare July 23, 2024 15:17

FAlbertDev mentioned this pull request Jul 24, 2024

Add expand_bit method to CT::Mask #4254

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randombit self-requested a review August 3, 2024 01:15

reneme mentioned this pull request Oct 14, 2024

FFI: Loading of raw FrodoKEM keys & FIX: "insufficient buffer handling" in FFI's decapsulate #4373

Merged

reneme force-pushed the pqc/classic_mceliece branch 3 times, most recently from f88fe6b to 5012217 Compare October 15, 2024 06:52

reneme modified the milestones: Botan 3.6.0, Botan 3.7.0 Oct 17, 2024

FAlbertDev and others added 13 commits October 18, 2024 08:42

Utility functions for Classic McEliece

2558f68

- constant time conditional swap with mask - floor_log2 Co-Authored-By: Amos Treiber <amos.treiber@rohde-schwarz.com>

bitvector<> with basic functionality

d1b70b4

Classic McEliece implementation

b3ad495

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>

Review comments on bitvector<>

6afafe6

Co-Authored-By: Jack Lloyd <jack@randombit.net>

add ct_reverse_bits<>

5ed7e6a

Co-Authored-By: Fabian Albert <fabian.albert@rohde-schwarz.com>

add bitvector<>::subvector<uintX_t>()

c032a7a

This allows extracting subvectors of the bitvector as unsigned integral bit masks.

Refactor: use nwe Strong<> unwrapping helpers

8cdfd40

... introduced in randombit#4170

Use Valgrind for CMCE

82b27e8

Use CT::poison/unpoison for CMCE. Simultaneously, fixes some constant-time issues in the CMCE implementation that may leak some information about the pivots (for the semi-systematic matrix form). Co-Authored-By: René Meusel <rene.meusel@rohde-schwarz.com>

remove superfluous Botan_Tests:: namespace

f78f277

bitvector<>::subvector_replace()

5948dd7

Use subvector_replace() in CMCE

cfc5094

Fix clang17 side-channel

15759b0

Integrate C. McEliece into FFI and Python bindings

083e153

reneme force-pushed the pqc/classic_mceliece branch from 5012217 to 083e153 Compare October 18, 2024 07:09

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PQC: Classic McEliece #3883

PQC: Classic McEliece #3883

FAlbertDev commented Jan 11, 2024 •

edited by reneme

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reneme commented Oct 18, 2024

		@@ -1,6 +1,8 @@
		/*
		* An abstraction for an arbitrarily large bitvector that can

PQC: Classic McEliece #3883

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PQC: Classic McEliece #3883

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FAlbertDev commented Jan 11, 2024 • edited by reneme Loading

Pull Request Dependencies

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TODO Tracker

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