Use caddy's certmagic library for extensible/robust ACME handling (#14177)

* use certmagic for more extensible/robust ACME cert handling

* accept TOS based on config option

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/mholt/acmez/acme/challenge.go

@@ -0,0 +1,133 @@
+// Copyright 2020 Matthew Holt
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package acme
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/base64"
+)
+
+// Challenge holds information about an ACME challenge.
+//
+// "An ACME challenge object represents a server's offer to validate a
+// client's possession of an identifier in a specific way.  Unlike the
+// other objects listed above, there is not a single standard structure
+// for a challenge object.  The contents of a challenge object depend on
+// the validation method being used.  The general structure of challenge
+// objects and an initial set of validation methods are described in
+// Section 8." §7.1.5
+type Challenge struct {
+	// "Challenge objects all contain the following basic fields..." §8
+
+	// type (required, string):  The type of challenge encoded in the
+	// object.
+	Type string `json:"type"`
+
+	// url (required, string):  The URL to which a response can be posted.
+	URL string `json:"url"`
+
+	// status (required, string):  The status of this challenge.  Possible
+	// values are "pending", "processing", "valid", and "invalid" (see
+	// Section 7.1.6).
+	Status string `json:"status"`
+
+	// validated (optional, string):  The time at which the server validated
+	// this challenge, encoded in the format specified in [RFC3339].
+	// This field is REQUIRED if the "status" field is "valid".
+	Validated string `json:"validated,omitempty"`
+
+	// error (optional, object):  Error that occurred while the server was
+	// validating the challenge, if any, structured as a problem document
+	// [RFC7807].  Multiple errors can be indicated by using subproblems
+	// Section 6.7.1.  A challenge object with an error MUST have status
+	// equal to "invalid".
+	Error *Problem `json:"error,omitempty"`
+
+	// "All additional fields are specified by the challenge type." §8
+	// (We also add our own for convenience.)
+
+	// "The token for a challenge is a string comprised entirely of
+	// characters in the URL-safe base64 alphabet." §8.1
+	//
+	// Used by the http-01, tls-alpn-01, and dns-01 challenges.
+	Token string `json:"token,omitempty"`
+
+	// A key authorization is a string that concatenates the token for the
+	// challenge with a key fingerprint, separated by a "." character (§8.1):
+	//
+	//     keyAuthorization = token || '.' || base64url(Thumbprint(accountKey))
+	//
+	// This client package automatically assembles and sets this value for you.
+	KeyAuthorization string `json:"keyAuthorization,omitempty"`
+
+	// We attach the identifier that this challenge is associated with, which
+	// may be useful information for solving a challenge. It is not part of the
+	// structure as defined by the spec but is added by us to provide enough
+	// information to solve the DNS-01 challenge.
+	Identifier Identifier `json:"identifier,omitempty"`
+}
+
+// HTTP01ResourcePath returns the URI path for solving the http-01 challenge.
+//
+// "The path at which the resource is provisioned is comprised of the
+// fixed prefix '/.well-known/acme-challenge/', followed by the 'token'
+// value in the challenge." §8.3
+func (c Challenge) HTTP01ResourcePath() string {
+	return "/.well-known/acme-challenge/" + c.Token
+}
+
+// DNS01TXTRecordName returns the name of the TXT record to create for
+// solving the dns-01 challenge.
+//
+// "The client constructs the validation domain name by prepending the
+// label '_acme-challenge' to the domain name being validated, then
+// provisions a TXT record with the digest value under that name." §8.4
+func (c Challenge) DNS01TXTRecordName() string {
+	return "_acme-challenge." + c.Identifier.Value
+}
+
+// DNS01KeyAuthorization encodes a key authorization value to be used
+// in a TXT record for the _acme-challenge DNS record.
+//
+// "A client fulfills this challenge by constructing a key authorization
+// from the 'token' value provided in the challenge and the client's
+// account key.  The client then computes the SHA-256 digest [FIPS180-4]
+// of the key authorization.
+//
+// The record provisioned to the DNS contains the base64url encoding of
+// this digest." §8.4
+func (c Challenge) DNS01KeyAuthorization() string {
+	h := sha256.Sum256([]byte(c.KeyAuthorization))
+	return base64.RawURLEncoding.EncodeToString(h[:])
+}
+
+// InitiateChallenge "indicates to the server that it is ready for the challenge
+// validation by sending an empty JSON body ('{}') carried in a POST request to
+// the challenge URL (not the authorization URL)." §7.5.1
+func (c *Client) InitiateChallenge(ctx context.Context, account Account, challenge Challenge) (Challenge, error) {
+	if err := c.provision(ctx); err != nil {
+		return Challenge{}, err
+	}
+	_, err := c.httpPostJWS(ctx, account.PrivateKey, account.Location, challenge.URL, struct{}{}, &challenge)
+	return challenge, err
+}
+
+// The standard or well-known ACME challenge types.
+const (
+	ChallengeTypeHTTP01    = "http-01"     // RFC 8555 §8.3
+	ChallengeTypeDNS01     = "dns-01"      // RFC 8555 §8.4
+	ChallengeTypeTLSALPN01 = "tls-alpn-01" // RFC 8737 §3
+)