tls: Add pem_loader module

This migrates a feature that was previously reserved for enterprise
users, according to https://github.com/caddyserver/caddy/issues/2786.

The PEM loader allows you to embed PEM files (certificates and keys)
directly into your config, rather than requiring them to be stored on
potentially insecure storage, which adds attack vectors. This is useful
in automated settings where sensitive key material is stored only in
memory.

Note that if the config is persisted to disk, that added benefit may go
away, but there will still be the benefit of having lesser dependence on
external files.

modules/caddytls/pemloader.go

@@ -0,0 +1,65 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// 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 caddytls
+
+import (
+	"crypto/tls"
+	"fmt"
+
+	"github.com/caddyserver/caddy/v2"
+)
+
+func init() {
+	caddy.RegisterModule(PEMLoader{})
+}
+
+// PEMLoader loads certificates and their associated keys by
+// decoding their PEM blocks directly. This has the advantage
+// of not needing to store them on disk at all.
+type PEMLoader []CertKeyPEMPair
+
+// CaddyModule returns the Caddy module information.
+func (PEMLoader) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		Name: "tls.certificates.load_pem",
+		New:  func() caddy.Module { return PEMLoader{} },
+	}
+}
+
+// CertKeyPEMPair pairs certificate and key PEM blocks.
+type CertKeyPEMPair struct {
+	CertificatePEM string   `json:"certificate"`
+	KeyPEM         string   `json:"key"`
+	Tags           []string `json:"tags,omitempty"`
+}
+
+// LoadCertificates returns the certificates contained in pl.
+func (pl PEMLoader) LoadCertificates() ([]Certificate, error) {
+	var certs []Certificate
+	for i, pair := range pl {
+		cert, err := tls.X509KeyPair([]byte(pair.CertificatePEM), []byte(pair.KeyPEM))
+		if err != nil {
+			return nil, fmt.Errorf("PEM pair %d: %v", i, err)
+		}
+		certs = append(certs, Certificate{
+			Certificate: cert,
+			Tags:        pair.Tags,
+		})
+	}
+	return certs, nil
+}
+
+// Interface guard
+var _ CertificateLoader = (PEMLoader)(nil)