Avoid validation of known EC groups when decoding an explicit curve block

[randombit]

Instead first check if the group is a known group, either a group known at compile time or something that the application registered. Only if that fails, perform the usual validation.

This skips various expensive primality checks for the common case when the explicit curve is just an explicit encoding of a standard group. It also sets up for easily removing support for non-standard explicit groups; if lookup_from_params fails, instead of trying to validate the group just throw an exception.

[Copilot]

Pull request overview

Improves performance of decoding explicitly-encoded EC domain parameters by first attempting to recognize them as a known/registered group (skipping expensive primality checks in the common case), and adds regression tests for explicit-curve decoding behavior.

Changes:

• Add a parameter-based lookup path (lookup_from_params) to map explicit curve parameters to known EC groups before doing heavyweight validation.
• Add an EC group parameter matcher that accepts the encoded base point.
• Add new text-based tests and vectors for explicit EC curve decoding/validation.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/tests/test_ecc_explicit_params.cpp	Adds a text-based test that loads explicit-curve public keys and checks acceptance/rejection behavior.
src/tests/data/pubkey/ecc_explicit_curve.vec	Adds test vectors for valid and invalid explicit curve encodings across multiple groups.
src/lib/pubkey/ec_group/ec_inner_data.h	Declares a new params_match overload that matches the encoded base point.
src/lib/pubkey/ec_group/ec_inner_data.cpp	Implements the new encoded-base-point matcher and refactors existing matcher.
src/lib/pubkey/ec_group/ec_group.cpp	Adds lookup_from_params and uses it early in explicit group decoding to avoid expensive validation for known groups.

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[coveralls]

coverage: 90.07% (-0.007%) from 90.077%
when pulling b325234 on jack/ecc-params-setup
into 0d718b1 on master.

[reneme]

Great idea! Changes look good to me. We could reorder the parameter lookup a little to avoid unnecessary point serializations in unlikely failure cases. But given that the benefit of that would be reaped only in fail cases, I don't see that as more than a nit.

Some SEC1 encoder/decoder helper classes would probably be nice at this point, though.

[reneme]

Given the number of times we jotted down the encoding/decoding of this SEC1 encoding across the code base now, I think we should introduce a small helper along those lines:

class SEC1_Decoder {
  public:
    enum Label {
      Identity = 0x00;
      Compressed_Even = 0x02;
      Compressed_Odd = 0x03;
      Full = 0x04;
      // Hybrid?
    };

  public:
    static SEC1_Decoder from_bytes(const EC_Group& group, std::span<const uint8_t> bytes);
    static SEC1_Decoder from_bytes(size_t field_len, std::span<const uint8_t> bytes);
    
    Label label() const { /* ... */ }
    std::span<const uint8_t> x_bytes() const { /* ... */ }
    std::optional<std::span<const uint8_t>> y_bytes() cosnt { /* ... */ }

    EC_AffinePoint affine_point() const;

  private:
    std::span<const uint8_t> m_bytes;
};

... certainly a follow-up, though.

[reneme]

Oh, and the CI failure is relevant. I guess it tries to instantiate a standard curve in a build where there's no backend for them. Perhaps try deserializing in a Text_Based_Test::skip_this_test() first and return true iff it catches Not_Implemented.

Failure 1: ECC explicit curve validation Rejected valid explicit curve parameters with error Not implemented EC_Group this group is not supported in this build configuration (at /home/runner/work/botan/botan/source/src/tests/test_ecc_explicit_params.cpp:62)
  Failure 2: ECC explicit curve validation Rejected valid explicit curve parameters with error Not implemented EC_Group the group 1.3.36.3.3.2.8.1.1.7 is not supported in this build configuration (at /home/runner/work/botan/botan/source/src/tests/test_ecc_explicit_params.cpp:62)
  Failure 3: ECC explicit curve validation Rejection failure other than what was expected with error Not implemented EC_Group the group 1.3.36.3.3.2.8.1.1.7 is not supported in this build configuration (at /home/runner/work/botan/botan/source/src/tests/test_ecc_explicit_params.cpp:62)
  Note 1: ECC explicit curve validation Test # 2 failed Pubkey=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 Result=OK 
  Note 2: ECC explicit curve validation Test # 3 failed Pubkey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esult=OK 
  Note 3: ECC explicit curve validation Test # 4 failed Pubkey=308201333081EC06072A8648CE3D02013081E0020101302C06072A8648CE3D0101022100A9FB57DBA1EEA9BC3E660A909D838D726E3BF623D52620282013481D1F6E537930440420A9FB57DBA1EEA9BC3E660A909D838D726E3BF623D52620282013481D1F6E53740420662C61C430D84EA4FE66A7733D0B76B7BF93EBC4AF2F49256AE58101FEE92B04044104A3E8EB3CC1CFE7B7732213B23A656149AFA142C47AAFBC2B79A191562E1305F42D996C823439C56D7F7B22E14644417E69BCB6DE39D027001DABE8F35B25C9BE022100A9FB57DBA1EEA9BC3E660A909D838D718C397AA3B561A6F7901E0E82974856A7020101034200043DE15A95F34445551D72A71D079EC862E5B6C6DDF6735E89C1AAA4FE41E6086B29E7BBA60FE13B86189FA92BA16BEEE1B6C9910A3861F67AAAA1046FFEC0C2DB Result=p parameter is not a prime