Custom private key

[Juice805]

Motivation:

In some cases a developer may want to sign a certificate using a method other than a private key. For example: if a private key is protected by hardware which signs asynchronously.

Modifications:

• Create CustomPrivateKey protocol
• Create async initializers for Certificate and CertificateSigningRequest.
• CustomPrivateKey can now back aCertificate.PrivateKey
• Make Certificate.Signature initializer public

Result:

Developers can now sign a certificate with greater flexibility.

Alternatives Considered:

Implementations

Pass the CustomPrivateKey into Certificate and CertificateSigningRequest initializers directly.

There is concern this could add too duplication of api.

Various names for the protocol:

• Certificate.PrivateKeyProtocol and Certificate.AsyncPrivateKeyProtocol
• Certificate.Signer/Certificate.AsyncSigner
• Certificate.SignatureProvider/Certificate.AsyncSignatureProvider