ed25519,x25519: fix possible bad public key conversion

[MichaelMure]

Note

Medium Risk
Touches cryptographic key parsing: invalid or unexpected X.509 public keys now error instead of panicking or being accepted, which could affect callers relying on previous lax behavior.

Overview
Hardens X.509 public key decoding for ed25519 and x25519 by replacing unchecked type assertions with explicit type checks and clear errors.

Adds validation that the parsed key is the expected algorithm (and curve for X25519) and that the raw key material is the expected length before constructing the library PublicKey wrappers.

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[Copilot]

Pull request overview

This PR hardens X.509 DER public-key decoding for the ed25519 and x25519 key types by avoiding unsafe type assertions and returning a proper error instead of panicking when the parsed key is not of the expected type.

Changes:

• Replace direct type assertions with checked assertions (ok idiom) in X.509 DER public key parsing.
• Return descriptive errors when x509.ParsePKIXPublicKey yields an unexpected key type.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
crypto/x25519/public.go	Makes X.509 DER parsing robust to unexpected public key types by avoiding panics.
crypto/ed25519/public.go	Makes X.509 DER parsing robust to unexpected public key types by avoiding panics.

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[Copilot]

There’s no test exercising the new non-panicking error path when ParsePKIXPublicKey returns an unexpected type. Add a unit test that feeds a non-X25519 X.509 public key (e.g., P-256) into PublicKeyFromX509DER and asserts it returns an error (and does not panic), to prevent regressions.

[Copilot]

There’s no test covering the newly added error path when ParsePKIXPublicKey returns a non-ed25519 key. Add a unit test that passes an X.509 public key of a different algorithm into PublicKeyFromX509DER and asserts it returns the new error (and does not panic).

[cursor]

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