Add X25519 generateKey overload to SubtleCrypto

[kairosci]

Summary

This PR adds a specific overload for the X25519 key exchange algorithm to the SubtleCrypto.generateKey() method, following the same pattern as the existing Ed25519 overload.

Changes

Added the following overload to SubtleCrypto interface in inputfiles/overridingTypes.jsonc:

generateKey(algorithm: "X25519" | { name: "X25519" }, extractable: boolean, keyUsages: ReadonlyArray<"deriveBits" | "deriveKey">): Promise<CryptoKeyPair>;

Rationale

X25519 is used for key derivation (ECDH), so it uses deriveBits and deriveKey as valid keyUsages, unlike Ed25519 which uses sign and verify.

This allows TypeScript to correctly infer CryptoKeyPair as the return type when using X25519, eliminating the need for type assertions or instanceof checks:

// Before: keyPair is CryptoKey | CryptoKeyPair, requires type assertion
const keyPair = await crypto.subtle.generateKey({ name: "X25519" }, true, ["deriveKey"]);

// After: keyPair is CryptoKeyPair, publicKey is directly accessible
const keyPair = await crypto.subtle.generateKey({ name: "X25519" }, true, ["deriveKey"]);
const publicKey = keyPair.publicKey; // No error!

Browser Support

X25519 is supported by:

• Chrome/Edge (Blink): Yes, since version 133
• Firefox (Gecko): Yes, since version 135
• Safari (WebKit): Yes, since version 17.0

This meets the requirement of support by 2+ major browser engines.

References

• Fixes Add generateKey X25519 CryptoKeyPair TypeScript#62926
• MDN: SubtleCrypto.generateKey()

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @saschanaz - if they write a comment saying "LGTM" then it will be merged.

[kairosci]

@microsoft-github-policy-service agree

[saschanaz]

LGTM, thanks!

(#2050 is a bigger version of it but it's too big)

[github-actions]

Merging because @saschanaz is a code-owner of all the changes - thanks!