Rust crypto mini warts

[kegsay]

Amalgamated issue so I don't forget about it.

• Buffer is used liberally assuming it exists when it practice it may not (in browsers without polyfills). ../matrix-js-sdk/src/rust-crypto/rust-crypto.ts:1073: return Buffer.from(backupKeys.decryptionKey.toBase64(), "base64"); - Solution: use the util class which handles this for us encodeBase64.
• Type definitions on export interface CreateSecretStorageOpts imply all args are optional but createSecretStorageKey is not, it is required.
• Rust crypto calls keyFromPassphrase https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/crypto/key_passphrase.ts#L55 which does a global.Olm check and fails if it isn't found, meaning rust crypto relies on old crypto..

[kegsay]

• https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/secret-storage.ts#L183 lists getSecretStorageKey as an optional callback. It isn't, and is required to be implemented at the app layer else random bits of code fail.

[kegsay]

• cacheSecretStorageKey should probably live on SecretStorageCallbacks as it is the pair to getSecretStorageKey which is in SecretStorageCallbacks. Currently the cache function sits in CryptoCallbacks which extends SecretStorageCallbacks.

[kegsay]

Keys won't be backed up for worst case 10 seconds after realising they need to be done. Worse still, this is randomised, causing flakey tests.

matrix-js-sdk/src/rust-crypto/backup.ts

Line 319 in 49624d5

// wait between 0 and `maxDelay` seconds, to avoid backup

This should be configurable. One of the playwright tests also needs to sleep(10s) as a result of this.