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Repair EC x/y coordinates when importing JWK #585

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merged 8 commits into from
Jan 29, 2024

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julik
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@julik julik commented Jan 28, 2024

Description

Some encoders on some platform omit a single 0-byte at the start of either Y or X coordinate of the elliptic curve point. This leads to an encoding error when data is passed to OpenSSL BN. It is known to have happened to exported JWKs on a Java application and on a Flutter/Dart application (both iOS and Android). All that is needed to fix the problem is adding a leading 0-byte. We know the required byte is 0 because with any other byte the point is no longer on the curve - and OpenSSL will actually communicate this via another exception. The indication of a stripped byte will be the fact that the coordinates - once decoded into bytes - should always be an even bytesize. For example, with a P-521 curve, both x and y must be 66 bytes. With a P-256 curve, both x and y must be 32 and so on. The simplest way to check for this truncation is thus to check whether the number of bytes is odd, and restore the leading 0-byte if it is.

Closes #412 and (by extension) #542

Checklist

Before the PR can be merged be sure the following are checked:

  • There are tests for the fix or feature added/changed
  • A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md

@julik julik changed the title Zero-prefix EC coordinates when needed Repair EC x/y coordinates when importing JWK Jan 28, 2024
lib/jwt/jwk/ec.rb Outdated Show resolved Hide resolved
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anakinj commented Jan 29, 2024

You can ignore the failure on ruby head. Thats the missing base64 dependency. Gonna merge #582 soonish to address that.

@julik julik requested a review from anakinj January 29, 2024 10:02
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anakinj commented Jan 29, 2024

Really appreciated, thank you for your thorough investigations. I'll merge this even if the base64 dependency problem is making the CI unhappy.

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OpenSSL::PKey::EC::Point::Error: EC_POINT_bn2point: invalid encoding when importing a JWK
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