Replace node-forge by native node crypto.

[forty]

This requires dropping support for node 8, which is probably fine since
node 8 is EoL since December 31, 2019.

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Description

Drop node-forge (which is a huge dep), use native nodejs crypto.

References

About node 8 support
https://blog.risingstack.com/update-nodejs-8-end-of-life-no-support/

Node Crypto
https://nodejs.org/api/crypto.html#crypto_crypto_publicencrypt_key_buffer
https://nodejs.org/api/crypto.html#crypto_crypto_privatedecrypt_privatekey_buffer

Testing

I updated first the encryption part, ran the tests (it was fine) then the decryption part (tests still fine).
This made sure the new encryption works well with the

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master

[forty]

Hello maintainers! Any interest in having this merged?

[forty]

This PR would also fix #93 :)

@gkwang @esarafianou are you still working on this project ?

[esarafianou]

Hi @forty, seems like we missed your previous messages. I'll have the PR reviewed within the week.

[panva]

Unless you're going to be re-using the KeyObject instance it's better to pass the key material down to the encrypt/decrypt to avoid C/JS context switching.

[panva]

Suggested change

	var private_key = crypto.createPrivateKey(options.key);
	var decrypted = crypto.privateDecrypt({ key: private_key, padding: padding}, key);
	var decrypted = crypto.privateDecrypt({ key: options.key, padding: padding}, key);

[panva]

Suggested change

	var rsa_pub = crypto.createPublicKey(options.rsa_pub);
	var encrypted = crypto.publicEncrypt({
	key: rsa_pub,
	padding: padding
	}, symmetricKeyBuffer);
	var encrypted = crypto.publicEncrypt({
	key: options.rsa_pub,
	padding: padding
	}, symmetricKeyBuffer);

[esarafianou]

@forty could you address @panva's feedback? I'll then approve, merge and create a release PR.

[forty]

Thanks for the comments @panva !

@esarafianou I've just updated the PR.

Thanks!