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/tlsalpn01.go

@@ -0,0 +1,98 @@
+// 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 acmez
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/asn1"
+	"encoding/pem"
+	"math/big"
+	"time"
+
+	"github.com/mholt/acmez/acme"
+)
+
+// TLSALPN01ChallengeCert creates a certificate that can be used for
+// handshakes while solving the tls-alpn-01 challenge. See RFC 8737 §3.
+func TLSALPN01ChallengeCert(challenge acme.Challenge) (*tls.Certificate, error) {
+	keyAuthSum := sha256.Sum256([]byte(challenge.KeyAuthorization))
+	keyAuthSumASN1, err := asn1.Marshal(keyAuthSum[:sha256.Size])
+	if err != nil {
+		return nil, err
+	}
+
+	certKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		return nil, err
+	}
+	challengeKeyASN1, err := x509.MarshalECPrivateKey(certKey)
+	if err != nil {
+		return nil, err
+	}
+
+	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
+	serialNumber, err := rand.Int(rand.Reader, serialNumberLimit)
+	if err != nil {
+		return nil, err
+	}
+	template := x509.Certificate{
+		SerialNumber:          serialNumber,
+		Subject:               pkix.Name{CommonName: "ACME challenge"},
+		NotBefore:             time.Now(),
+		NotAfter:              time.Now().Add(24 * time.Hour * 365),
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		BasicConstraintsValid: true,
+		DNSNames:              []string{challenge.Identifier.Value},
+
+		// add key authentication digest as the acmeValidation-v1 extension
+		// (marked as critical such that it won't be used by non-ACME software).
+		// Reference: https://www.rfc-editor.org/rfc/rfc8737.html#section-3
+		ExtraExtensions: []pkix.Extension{
+			{
+				Id:       idPEACMEIdentifierV1,
+				Critical: true,
+				Value:    keyAuthSumASN1,
+			},
+		},
+	}
+	challengeCertDER, err := x509.CreateCertificate(rand.Reader, &template, &template, &certKey.PublicKey, certKey)
+	if err != nil {
+		return nil, err
+	}
+
+	challengeCertPEM := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: challengeCertDER})
+	challengeKeyPEM := pem.EncodeToMemory(&pem.Block{Type: "EC PRIVATE KEY", Bytes: challengeKeyASN1})
+
+	cert, err := tls.X509KeyPair(challengeCertPEM, challengeKeyPEM)
+	if err != nil {
+		return nil, err
+	}
+
+	return &cert, nil
+}
+
+// ACMETLS1Protocol is the ALPN value for the TLS-ALPN challenge
+// handshake. See RFC 8737 §6.2.
+const ACMETLS1Protocol = "acme-tls/1"
+
+// idPEACMEIdentifierV1 is the SMI Security for PKIX Certification Extension OID referencing the ACME extension.
+// See RFC 8737 §6.1. https://www.rfc-editor.org/rfc/rfc8737.html#section-6.1
+var idPEACMEIdentifierV1 = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 31}