Proposal: Separate key export functionality

[Gozala]

I've been working on updating this implementation to align it with libp2p/js-libp2p-interfaces#100 and I am noticing that export function on private keys introduces unnecessary dependencies as in export functionality is is not key specific. So I would suggest factoring that functionality out into separate interface e.g.

interface KeyExporter<Format extends string = string> {
   format: Format
   export(key:PrivateKey, password?:Uint8Array): Uint8Array
}

This would make remove indirection and instead of doing e.g.

key.export('my secret', 'pkcs-8')
key.export('my secret', 'libp2p-key')

One would be a able to do following instead:

pkcs8.export(key, 'my secret')
libp2pExporter.export(key, 'my secret')

We did a similar change in js-multiformats for base encoding and hashing where instead of passing identifiers you pass references and it simplified things greatly.

[Gozala]

Also if we really want to we could still keep export method on PrivateKey but we could just swap format identifier with KeyExporter as described above so that same example could be rewritten as follows:

key.export('my secret', pkcs8)
key.export('my secret', libp2pKey)

[Gozala]

I see now that pkcs8 uses rsa specific node-forge calls

js-libp2p-crypto/src/keys/rsa-class.js

Lines 120 to 128 in 117b520

	const privateKey = forge.pki.privateKeyFromAsn1(asn1)
	const options = {
	algorithm: 'aes256',
	count: 10000,
	saltSize: 128 / 8,
	prfAlgorithm: 'sha512'
	}
	return forge.pki.encryptRsaPrivateKey(privateKey, password, options)

Which may explain why export was made key specific. Yet I still think my argument still stands and we could even get compile time compatibility errors when user attempts to export incompatible key

[BigLep]

2021-11-12 discussion: to pull this off, we need an "awesome endeavor" to pull all the crypto stuff apart so can have reduced bundle size.