I ran into an issue where I want to derive child-keys from the parent private key, but there is currently no way to do this with the current spec.
I propose adding a `window.nostr.getDerivedKey(key: string): string` method to the spec, which is a simple HMAC method using the private key and a user supplied key. HMAC has wide-spread support in the WebCrypto spec, and is easy to implement.
https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto
Here is some reference code for performing a simple HMAC operation using WebCrypto API:
```ts
async function hmac (
// Perform an HMAC signing operation.
key : Uint8Array,
data : Uint8Array,
fmt : string = 'SHA-256'
) : Promise<Uint8Array> {
const cryptoKey = await importKey(key, fmt)
return crypto.subtle
.sign('HMAC', cryptoKey, data)
.then((buffer) => new Uint8Array(buffer))
}
async function importKey (
// Create a CryptoKey from the
// supplied key and format string.
key : Uint8Array,
fmt : string = 'SHA-256'
) : Promise<CryptoKey> {
const config = { name: 'HMAC', hash: fmt }
return crypto.subtle.importKey(
'raw', key, config, false, ['sign', 'verify']
)
}
```
