SECURITY: fix timing variability in backend/serial/u32/scalar.rs #661
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Similar security fix to #659, but for the 32-bit backend. See that PR for more information about the problem.
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Relevant compiler outputs (thanks to @tarcieri): Without fix. Notice the |
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@rozbb the godbolt links might be worth including in the commit message when you squash-and-merge |
tarcieri
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Jun 20, 2024
Replaces the security mitigation added in #659 and #661 for masking-related timing variability which used an inline `black_box` using the recently added `subtle::BlackBox` newtype (see dalek-cryptography/subtle#123) Internally `BlackBox` uses a volatile read by default (i.e. same strategy which was used before) or when the `core_hint_black_box` feature of `subtle` is enabled, it uses `core::hint::black_box` (whose documentation was recently updated to reflect the nuances of potential cryptographic use, see rust-lang/rust#126703) This PR goes ahead and uses `BlackBox` for both `mask` and `underflow_mask` where previously it was only used on `underflow_mask`. The general pattern of bitwise masking inside a loop seems worrisome for the optimizer potentially inserting branches in the future. Below are godbolt inspections of the generated assembly, which are free of the `jns` instructions originally spotted in #659/#661: - 32-bit (read_volatile): https://godbolt.org/z/TKo9fqza4 - 32-bit (hint::black_box): https://godbolt.org/z/caoMxYbET - 64-bit (read_volatile): https://godbolt.org/z/PM6zKjj1f - 64-bit (hint::black_box): https://godbolt.org/z/nseaPvdWv
tarcieri
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Jun 24, 2024
Replaces the security mitigation added in #659 and #661 for masking-related timing variability which used an inline `black_box` using the recently added `subtle::BlackBox` newtype (see dalek-cryptography/subtle#123) Internally `BlackBox` uses a volatile read by default (i.e. same strategy which was used before) or when the `core_hint_black_box` feature of `subtle` is enabled, it uses `core::hint::black_box` (whose documentation was recently updated to reflect the nuances of potential cryptographic use, see rust-lang/rust#126703) This PR goes ahead and uses `BlackBox` for both `mask` and `underflow_mask` where previously it was only used on `underflow_mask`. The general pattern of bitwise masking inside a loop seems worrisome for the optimizer potentially inserting branches in the future. Below are godbolt inspections of the generated assembly, which are free of the `jns` instructions originally spotted in #659/#661: - 32-bit (read_volatile): https://godbolt.org/z/TKo9fqza4 - 32-bit (hint::black_box): https://godbolt.org/z/caoMxYbET - 64-bit (read_volatile): https://godbolt.org/z/PM6zKjj1f - 64-bit (hint::black_box): https://godbolt.org/z/nseaPvdWv
rozbb
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Jun 24, 2024
Replaces the security mitigation added in #659 and #661 for masking-related timing variability which used an inline `black_box` using the recently added `subtle::BlackBox` newtype (see dalek-cryptography/subtle#123) Internally `BlackBox` uses a volatile read by default (i.e. same strategy which was used before) or when the `core_hint_black_box` feature of `subtle` is enabled, it uses `core::hint::black_box` (whose documentation was recently updated to reflect the nuances of potential cryptographic use, see rust-lang/rust#126703) This PR goes ahead and uses `BlackBox` for both `mask` and `underflow_mask` where previously it was only used on `underflow_mask`. The general pattern of bitwise masking inside a loop seems worrisome for the optimizer potentially inserting branches in the future. Below are godbolt inspections of the generated assembly, which are free of the `jns` instructions originally spotted in #659/#661: - 32-bit (read_volatile): https://godbolt.org/z/TKo9fqza4 - 32-bit (hint::black_box): https://godbolt.org/z/caoMxYbET - 64-bit (read_volatile): https://godbolt.org/z/PM6zKjj1f - 64-bit (hint::black_box): https://godbolt.org/z/nseaPvdWv
tarcieri
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Jun 25, 2024
Alternative to #659/#661 and #662 which leverages `subtle::Choice` and `subtle::ConditionallySelectable` as the optimization barriers. Really the previous masking was there to conditionally add the scalar field modulus on underflow, so instead of that, we can conditionally select zero or the modulus using a `Choice` constructed from the underflow bit.
jmwample
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Jun 26, 2024
…ek-cryptography#661) Similar security fix to dalek-cryptography#659, but for the 32-bit backend. See that PR for more information about the problem. Relevant compiler outputs (thanks to @tarcieri): Without fix https://godbolt.org/z/zvaWxzvqv Notice the `jns` ("jump if not sign") instruction on line 106. With fix https://godbolt.org/z/jc9j7eb8E
jmwample
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Jun 26, 2024
…y#662) Replaces the security mitigation added in dalek-cryptography#659 and dalek-cryptography#661 for masking-related timing variability which used an inline `black_box` using the recently added `subtle::BlackBox` newtype (see dalek-cryptography/subtle#123) Internally `BlackBox` uses a volatile read by default (i.e. same strategy which was used before) or when the `core_hint_black_box` feature of `subtle` is enabled, it uses `core::hint::black_box` (whose documentation was recently updated to reflect the nuances of potential cryptographic use, see rust-lang/rust#126703) This PR goes ahead and uses `BlackBox` for both `mask` and `underflow_mask` where previously it was only used on `underflow_mask`. The general pattern of bitwise masking inside a loop seems worrisome for the optimizer potentially inserting branches in the future. Below are godbolt inspections of the generated assembly, which are free of the `jns` instructions originally spotted in dalek-cryptography#659/dalek-cryptography#661: - 32-bit (read_volatile): https://godbolt.org/z/TKo9fqza4 - 32-bit (hint::black_box): https://godbolt.org/z/caoMxYbET - 64-bit (read_volatile): https://godbolt.org/z/PM6zKjj1f - 64-bit (hint::black_box): https://godbolt.org/z/nseaPvdWv
yihau
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to anza-xyz/curve25519-dalek
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Jun 27, 2024
…ek-cryptography#661) Similar security fix to dalek-cryptography#659, but for the 32-bit backend. See that PR for more information about the problem. Relevant compiler outputs (thanks to @tarcieri): Without fix https://godbolt.org/z/zvaWxzvqv Notice the `jns` ("jump if not sign") instruction on line 106. With fix https://godbolt.org/z/jc9j7eb8E
yihau
pushed a commit
to anza-xyz/curve25519-dalek
that referenced
this pull request
Jun 27, 2024
…ek-cryptography#661) Similar security fix to dalek-cryptography#659, but for the 32-bit backend. See that PR for more information about the problem. Relevant compiler outputs (thanks to @tarcieri): Without fix https://godbolt.org/z/zvaWxzvqv Notice the `jns` ("jump if not sign") instruction on line 106. With fix https://godbolt.org/z/jc9j7eb8E
tarcieri
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Jul 17, 2024
Alternative to #659/#661 and #662 which leverages `subtle::Choice` and `subtle::ConditionallySelectable` as the optimization barriers. Really the previous masking was there to conditionally add the scalar field modulus on underflow, so instead of that, we can conditionally select zero or the modulus using a `Choice` constructed from the underflow bit.
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Similar security fix to #659, but for the 32-bit backend. See that PR for more information about the problem.