Challenge.spec.key

The ACME challenge key for this challenge For HTTP01 challenges, this is the value that must be responded with to complete the HTTP01 challenge in the format: `<private key JWK thumbprint>.<key from acme server for challenge>`. For DNS01 challenges, this is the base64 encoded SHA256 sum of the `<private key JWK thumbprint>.<key from acme server for challenge>` text that must be set as the TXT record content.