v2: Implement 'pki' app powered by Smallstep for localhost certificates (#3125)

* pki: Initial commit of PKI app (WIP) (see #2502 and #3021)

* pki: Ability to use root/intermediates, and sign with root

* pki: Fix benign misnamings left over from copy+paste

* pki: Only install root if not already trusted

* Make HTTPS port the default; all names use auto-HTTPS; bug fixes

* Fix build - what happened to our CI tests??

* Fix go.mod

6 files changed, 844 insertions, 0 deletions

diff --git a/modules/caddypki/ca.go b/modules/caddypki/ca.go
new file mode 100644
index 0000000..f15883e
--- /dev/null
+++ b/modules/caddypki/ca.go
@@ -0,0 +1,334 @@
+// 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 caddypki
+
+import (
+	"crypto/x509"
+	"encoding/json"
+	"fmt"
+	"path"
+	"sync"
+	"time"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/certmagic"
+	"go.uber.org/zap"
+)
+
+// CA describes a certificate authority, which consists of
+// root/signing certificates and various settings pertaining
+// to the issuance of certificates and trusting them.
+type CA struct {
+	// The user-facing name of the certificate authority.
+	Name string `json:"name,omitempty"`
+
+	// The name to put in the CommonName field of the
+	// root certificate.
+	RootCommonName string `json:"root_common_name,omitempty"`
+
+	// The name to put in the CommonName field of the
+	// intermediate certificates.
+	IntermediateCommonName string `json:"intermediate_common_name,omitempty"`
+
+	// Whether Caddy will attempt to install the CA's root
+	// into the system trust store, as well as into Java
+	// and Mozilla Firefox trust stores. Default: true.
+	InstallTrust *bool `json:"install_trust,omitempty"`
+
+	Root         *KeyPair `json:"root,omitempty"`
+	Intermediate *KeyPair `json:"intermediate,omitempty"`
+
+	// Optionally configure a separate storage module associated with this
+	// issuer, instead of using Caddy's global/default-configured storage.
+	// This can be useful if you want to keep your signing keys in a
+	// separate location from your leaf certificates.
+	StorageRaw json.RawMessage `json:"storage,omitempty" caddy:"namespace=caddy.storage inline_key=module"`
+
+	id          string
+	storage     certmagic.Storage
+	root, inter *x509.Certificate
+	interKey    interface{} // TODO: should we just store these as crypto.Signer?
+	mu          *sync.RWMutex
+
+	rootCertPath string // mainly used for logging purposes if trusting
+	log          *zap.Logger
+}
+
+// Provision sets up the CA.
+func (ca *CA) Provision(ctx caddy.Context, id string, log *zap.Logger) error {
+	ca.mu = new(sync.RWMutex)
+	ca.log = log.Named("ca." + id)
+
+	if id == "" {
+		return fmt.Errorf("CA ID is required (use 'local' for the default CA)")
+	}
+	ca.mu.Lock()
+	ca.id = id
+	ca.mu.Unlock()
+
+	if ca.StorageRaw != nil {
+		val, err := ctx.LoadModule(ca, "StorageRaw")
+		if err != nil {
+			return fmt.Errorf("loading storage module: %v", err)
+		}
+		cmStorage, err := val.(caddy.StorageConverter).CertMagicStorage()
+		if err != nil {
+			return fmt.Errorf("creating storage configuration: %v", err)
+		}
+		ca.storage = cmStorage
+	}
+	if ca.storage == nil {
+		ca.storage = ctx.Storage()
+	}
+
+	if ca.Name == "" {
+		ca.Name = defaultCAName
+	}
+	if ca.RootCommonName == "" {
+		ca.RootCommonName = defaultRootCommonName
+	}
+	if ca.IntermediateCommonName == "" {
+		ca.IntermediateCommonName = defaultIntermediateCommonName
+	}
+
+	// load the certs and key that will be used for signing
+	var rootCert, interCert *x509.Certificate
+	var rootKey, interKey interface{}
+	var err error
+	if ca.Root != nil {
+		if ca.Root.Format == "" || ca.Root.Format == "pem_file" {
+			ca.rootCertPath = ca.Root.Certificate
+		}
+		rootCert, rootKey, err = ca.Root.Load()
+	} else {
+		ca.rootCertPath = "storage:" + ca.storageKeyRootCert()
+		rootCert, rootKey, err = ca.loadOrGenRoot()
+	}
+	if err != nil {
+		return err
+	}
+	if ca.Intermediate != nil {
+		interCert, interKey, err = ca.Intermediate.Load()
+	} else {
+		interCert, interKey, err = ca.loadOrGenIntermediate(rootCert, rootKey)
+	}
+	if err != nil {
+		return err
+	}
+
+	ca.mu.Lock()
+	ca.root, ca.inter, ca.interKey = rootCert, interCert, interKey
+	ca.mu.Unlock()
+
+	return nil
+}
+
+// ID returns the CA's ID, as given by the user in the config.
+func (ca CA) ID() string {
+	return ca.id
+}
+
+// RootCertificate returns the CA's root certificate (public key).
+func (ca CA) RootCertificate() *x509.Certificate {
+	ca.mu.RLock()
+	defer ca.mu.RUnlock()
+	return ca.root
+}
+
+// RootKey returns the CA's root private key. Since the root key is
+// not cached in memory long-term, it needs to be loaded from storage,
+// which could yield an error.
+func (ca CA) RootKey() (interface{}, error) {
+	_, rootKey, err := ca.loadOrGenRoot()
+	return rootKey, err
+}
+
+// IntermediateCertificate returns the CA's intermediate
+// certificate (public key).
+func (ca CA) IntermediateCertificate() *x509.Certificate {
+	ca.mu.RLock()
+	defer ca.mu.RUnlock()
+	return ca.inter
+}
+
+// IntermediateKey returns the CA's intermediate private key.
+func (ca CA) IntermediateKey() interface{} {
+	ca.mu.RLock()
+	defer ca.mu.RUnlock()
+	return ca.interKey
+}
+
+func (ca CA) loadOrGenRoot() (rootCert *x509.Certificate, rootKey interface{}, err error) {
+	rootCertPEM, err := ca.storage.Load(ca.storageKeyRootCert())
+	if err != nil {
+		if _, ok := err.(certmagic.ErrNotExist); !ok {
+			return nil, nil, fmt.Errorf("loading root cert: %v", err)
+		}
+
+		// TODO: should we require that all or none of the assets are required before overwriting anything?
+		rootCert, rootKey, err = ca.genRoot()
+		if err != nil {
+			return nil, nil, fmt.Errorf("generating root: %v", err)
+		}
+	}
+
+	if rootCert == nil {
+		rootCert, err = pemDecodeSingleCert(rootCertPEM)
+		if err != nil {
+			return nil, nil, fmt.Errorf("parsing root certificate PEM: %v", err)
+		}
+	}
+	if rootKey == nil {
+		rootKeyPEM, err := ca.storage.Load(ca.storageKeyRootKey())
+		if err != nil {
+			return nil, nil, fmt.Errorf("loading root key: %v", err)
+		}
+		rootKey, err = pemDecodePrivateKey(rootKeyPEM)
+		if err != nil {
+			return nil, nil, fmt.Errorf("decoding root key: %v", err)
+		}
+	}
+
+	return rootCert, rootKey, nil
+}
+
+func (ca CA) genRoot() (rootCert *x509.Certificate, rootKey interface{}, err error) {
+	repl := ca.newReplacer()
+
+	rootCert, rootKey, err = generateRoot(repl.ReplaceAll(ca.RootCommonName, ""))
+	if err != nil {
+		return nil, nil, fmt.Errorf("generating CA root: %v", err)
+	}
+	rootCertPEM, err := pemEncodeCert(rootCert.Raw)
+	if err != nil {
+		return nil, nil, fmt.Errorf("encoding root certificate: %v", err)
+	}
+	err = ca.storage.Store(ca.storageKeyRootCert(), rootCertPEM)
+	if err != nil {
+		return nil, nil, fmt.Errorf("saving root certificate: %v", err)
+	}
+	rootKeyPEM, err := pemEncodePrivateKey(rootKey)
+	if err != nil {
+		return nil, nil, fmt.Errorf("encoding root key: %v", err)
+	}
+	err = ca.storage.Store(ca.storageKeyRootKey(), rootKeyPEM)
+	if err != nil {
+		return nil, nil, fmt.Errorf("saving root key: %v", err)
+	}
+
+	return rootCert, rootKey, nil
+}
+
+func (ca CA) loadOrGenIntermediate(rootCert *x509.Certificate, rootKey interface{}) (interCert *x509.Certificate, interKey interface{}, err error) {
+	interCertPEM, err := ca.storage.Load(ca.storageKeyIntermediateCert())
+	if err != nil {
+		if _, ok := err.(certmagic.ErrNotExist); !ok {
+			return nil, nil, fmt.Errorf("loading intermediate cert: %v", err)
+		}
+
+		// TODO: should we require that all or none of the assets are required before overwriting anything?
+		interCert, interKey, err = ca.genIntermediate(rootCert, rootKey)
+		if err != nil {
+			return nil, nil, fmt.Errorf("generating new intermediate cert: %v", err)
+		}
+	}
+
+	if interCert == nil {
+		interCert, err = pemDecodeSingleCert(interCertPEM)
+		if err != nil {
+			return nil, nil, fmt.Errorf("decoding intermediate certificate PEM: %v", err)
+		}
+	}
+
+	if interKey == nil {
+		interKeyPEM, err := ca.storage.Load(ca.storageKeyIntermediateKey())
+		if err != nil {
+			return nil, nil, fmt.Errorf("loading intermediate key: %v", err)
+		}
+		interKey, err = pemDecodePrivateKey(interKeyPEM)
+		if err != nil {
+			return nil, nil, fmt.Errorf("decoding intermediate key: %v", err)
+		}
+	}
+
+	return interCert, interKey, nil
+}
+
+func (ca CA) genIntermediate(rootCert *x509.Certificate, rootKey interface{}) (interCert *x509.Certificate, interKey interface{}, err error) {
+	repl := ca.newReplacer()
+
+	rootKeyPEM, err := ca.storage.Load(ca.storageKeyRootKey())
+	if err != nil {
+		return nil, nil, fmt.Errorf("loading root key to sign new intermediate: %v", err)
+	}
+	rootKey, err = pemDecodePrivateKey(rootKeyPEM)
+	if err != nil {
+		return nil, nil, fmt.Errorf("decoding root key: %v", err)
+	}
+	interCert, interKey, err = generateIntermediate(repl.ReplaceAll(ca.IntermediateCommonName, ""), rootCert, rootKey)
+	if err != nil {
+		return nil, nil, fmt.Errorf("generating CA intermediate: %v", err)
+	}
+	interCertPEM, err := pemEncodeCert(interCert.Raw)
+	if err != nil {
+		return nil, nil, fmt.Errorf("encoding intermediate certificate: %v", err)
+	}
+	err = ca.storage.Store(ca.storageKeyIntermediateCert(), interCertPEM)
+	if err != nil {
+		return nil, nil, fmt.Errorf("saving intermediate certificate: %v", err)
+	}
+	interKeyPEM, err := pemEncodePrivateKey(interKey)
+	if err != nil {
+		return nil, nil, fmt.Errorf("encoding intermediate key: %v", err)
+	}
+	err = ca.storage.Store(ca.storageKeyIntermediateKey(), interKeyPEM)
+	if err != nil {
+		return nil, nil, fmt.Errorf("saving intermediate key: %v", err)
+	}
+
+	return interCert, interKey, nil
+}
+
+func (ca CA) storageKeyCAPrefix() string {
+	return path.Join("pki", "authorities", certmagic.StorageKeys.Safe(ca.id))
+}
+func (ca CA) storageKeyRootCert() string {
+	return path.Join(ca.storageKeyCAPrefix(), "root.crt")
+}
+func (ca CA) storageKeyRootKey() string {
+	return path.Join(ca.storageKeyCAPrefix(), "root.key")
+}
+func (ca CA) storageKeyIntermediateCert() string {
+	return path.Join(ca.storageKeyCAPrefix(), "intermediate.crt")
+}
+func (ca CA) storageKeyIntermediateKey() string {
+	return path.Join(ca.storageKeyCAPrefix(), "intermediate.key")
+}
+
+func (ca CA) newReplacer() *caddy.Replacer {
+	repl := caddy.NewReplacer()
+	repl.Set("pki.ca.name", ca.Name)
+	return repl
+}
+
+const (
+	defaultCAID                   = "local"
+	defaultCAName                 = "Caddy Local Authority"
+	defaultRootCommonName         = "{pki.ca.name} - {time.now.year} ECC Root"
+	defaultIntermediateCommonName = "{pki.ca.name} - ECC Intermediate"
+
+	defaultRootLifetime         = 24 * time.Hour * 30 * 12 * 10
+	defaultIntermediateLifetime = 24 * time.Hour * 7
+)
diff --git a/modules/caddypki/certificates.go b/modules/caddypki/certificates.go
new file mode 100644
index 0000000..a55c165
--- /dev/null
+++ b/modules/caddypki/certificates.go
@@ -0,0 +1,50 @@
+// 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 caddypki
+
+import (
+	"crypto/x509"
+	"time"
+
+	"github.com/smallstep/cli/crypto/x509util"
+)
+
+func generateRoot(commonName string) (rootCrt *x509.Certificate, privateKey interface{}, err error) {
+	rootProfile, err := x509util.NewRootProfile(commonName)
+	if err != nil {
+		return
+	}
+	rootProfile.Subject().NotAfter = time.Now().Add(defaultRootLifetime) // TODO: make configurable
+	return newCert(rootProfile)
+}
+
+func generateIntermediate(commonName string, rootCrt *x509.Certificate, rootKey interface{}) (cert *x509.Certificate, privateKey interface{}, err error) {
+	interProfile, err := x509util.NewIntermediateProfile(commonName, rootCrt, rootKey)
+	if err != nil {
+		return
+	}
+	interProfile.Subject().NotAfter = time.Now().Add(defaultIntermediateLifetime) // TODO: make configurable
+	return newCert(interProfile)
+}
+
+func newCert(profile x509util.Profile) (cert *x509.Certificate, privateKey interface{}, err error) {
+	certBytes, err := profile.CreateCertificate()
+	if err != nil {
+		return
+	}
+	privateKey = profile.SubjectPrivateKey()
+	cert, err = x509.ParseCertificate(certBytes)
+	return
+}
diff --git a/modules/caddypki/command.go b/modules/caddypki/command.go
new file mode 100644
index 0000000..9276fcb
--- /dev/null
+++ b/modules/caddypki/command.go
@@ -0,0 +1,89 @@
+// 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 caddypki
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/caddyserver/caddy/v2"
+	caddycmd "github.com/caddyserver/caddy/v2/cmd"
+	"github.com/smallstep/truststore"
+)
+
+func init() {
+	caddycmd.RegisterCommand(caddycmd.Command{
+		Name:  "untrust",
+		Func:  cmdUntrust,
+		Usage: "[--ca <id> | --cert <path>]",
+		Short: "Untrusts a locally-trusted CA certificate",
+		Long: `
+Untrusts a root certificate from the local trust store(s). Intended
+for development environments only.
+
+This command uninstalls trust; it does not necessarily delete the
+root certificate from trust stores entirely. Thus, repeatedly
+trusting and untrusting new certificates can fill up trust databases.
+
+This command does not delete or modify certificate files.
+
+Specify which certificate to untrust either by the ID of its CA with
+the --ca flag, or the direct path to the certificate file with the
+--cert flag. If the --ca flag is used, only the default storage paths
+are assumed (i.e. using --ca flag with custom storage backends or file
+paths will not work).
+
+If no flags are specified, --ca=local is assumed.`,
+		Flags: func() *flag.FlagSet {
+			fs := flag.NewFlagSet("untrust", flag.ExitOnError)
+			fs.String("ca", "", "The ID of the CA to untrust")
+			fs.String("cert", "", "The path to the CA certificate to untrust")
+			return fs
+		}(),
+	})
+}
+
+func cmdUntrust(fs caddycmd.Flags) (int, error) {
+	ca := fs.String("ca")
+	cert := fs.String("cert")
+
+	if ca != "" && cert != "" {
+		return caddy.ExitCodeFailedStartup, fmt.Errorf("conflicting command line arguments")
+	}
+	if ca == "" && cert == "" {
+		ca = defaultCAID
+	}
+	if ca != "" {
+		cert = filepath.Join(caddy.AppDataDir(), "pki", "authorities", ca, "root.crt")
+	}
+
+	// sanity check, make sure cert file exists first
+	_, err := os.Stat(cert)
+	if err != nil {
+		return caddy.ExitCodeFailedStartup, fmt.Errorf("accessing certificate file: %v", err)
+	}
+
+	err = truststore.UninstallFile(cert,
+		truststore.WithDebug(),
+		truststore.WithFirefox(),
+		truststore.WithJava())
+	if err != nil {
+		return caddy.ExitCodeFailedStartup, err
+	}
+
+	return caddy.ExitCodeSuccess, nil
+}
diff --git a/modules/caddypki/crypto.go b/modules/caddypki/crypto.go
new file mode 100644
index 0000000..e701c40
--- /dev/null
+++ b/modules/caddypki/crypto.go
@@ -0,0 +1,155 @@
+// 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 caddypki
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"io/ioutil"
+	"strings"
+)
+
+func pemDecodeSingleCert(pemDER []byte) (*x509.Certificate, error) {
+	pemBlock, remaining := pem.Decode(pemDER)
+	if pemBlock == nil {
+		return nil, fmt.Errorf("no PEM block found")
+	}
+	if len(remaining) > 0 {
+		return nil, fmt.Errorf("input contained more than a single PEM block")
+	}
+	if pemBlock.Type != "CERTIFICATE" {
+		return nil, fmt.Errorf("expected PEM block type to be CERTIFICATE, but got '%s'", pemBlock.Type)
+	}
+	return x509.ParseCertificate(pemBlock.Bytes)
+}
+
+func pemEncodeCert(der []byte) ([]byte, error) {
+	return pemEncode("CERTIFICATE", der)
+}
+
+// pemEncodePrivateKey marshals a EC or RSA private key into a PEM-encoded array of bytes.
+// TODO: this is the same thing as in certmagic. Should we reuse that code somehow? It's unexported.
+func pemEncodePrivateKey(key crypto.PrivateKey) ([]byte, error) {
+	var pemType string
+	var keyBytes []byte
+	switch key := key.(type) {
+	case *ecdsa.PrivateKey:
+		var err error
+		pemType = "EC"
+		keyBytes, err = x509.MarshalECPrivateKey(key)
+		if err != nil {
+			return nil, err
+		}
+	case *rsa.PrivateKey:
+		pemType = "RSA"
+		keyBytes = x509.MarshalPKCS1PrivateKey(key)
+	case *ed25519.PrivateKey:
+		var err error
+		pemType = "ED25519"
+		keyBytes, err = x509.MarshalPKCS8PrivateKey(key)
+		if err != nil {
+			return nil, err
+		}
+	default:
+		return nil, fmt.Errorf("unsupported key type: %T", key)
+	}
+	return pemEncode(pemType+" PRIVATE KEY", keyBytes)
+}
+
+// pemDecodePrivateKey loads a PEM-encoded ECC/RSA private key from an array of bytes.
+// Borrowed from Go standard library, to handle various private key and PEM block types.
+// https://github.com/golang/go/blob/693748e9fa385f1e2c3b91ca9acbb6c0ad2d133d/src/crypto/tls/tls.go#L291-L308
+// https://github.com/golang/go/blob/693748e9fa385f1e2c3b91ca9acbb6c0ad2d133d/src/crypto/tls/tls.go#L238)
+// TODO: this is the same thing as in certmagic. Should we reuse that code somehow? It's unexported.
+func pemDecodePrivateKey(keyPEMBytes []byte) (crypto.PrivateKey, error) {
+	keyBlockDER, _ := pem.Decode(keyPEMBytes)
+
+	if keyBlockDER.Type != "PRIVATE KEY" && !strings.HasSuffix(keyBlockDER.Type, " PRIVATE KEY") {
+		return nil, fmt.Errorf("unknown PEM header %q", keyBlockDER.Type)
+	}
+
+	if key, err := x509.ParsePKCS1PrivateKey(keyBlockDER.Bytes); err == nil {
+		return key, nil
+	}
+
+	if key, err := x509.ParsePKCS8PrivateKey(keyBlockDER.Bytes); err == nil {
+		switch key := key.(type) {
+		case *rsa.PrivateKey, *ecdsa.PrivateKey, ed25519.PrivateKey:
+			return key, nil
+		default:
+			return nil, fmt.Errorf("found unknown private key type in PKCS#8 wrapping: %T", key)
+		}
+	}
+
+	if key, err := x509.ParseECPrivateKey(keyBlockDER.Bytes); err == nil {
+		return key, nil
+	}
+
+	return nil, fmt.Errorf("unknown private key type")
+}
+
+func pemEncode(blockType string, b []byte) ([]byte, error) {
+	var buf bytes.Buffer
+	err := pem.Encode(&buf, &pem.Block{Type: blockType, Bytes: b})
+	return buf.Bytes(), err
+}
+
+func trusted(cert *x509.Certificate) bool {
+	chains, err := cert.Verify(x509.VerifyOptions{})
+	return len(chains) > 0 && err == nil
+}
+
+// KeyPair represents a public-private key pair, where the
+// public key is also called a certificate.
+type KeyPair struct {
+	Certificate string `json:"certificate,omitempty"`
+	PrivateKey  string `json:"private_key,omitempty"`
+	Format      string `json:"format,omitempty"`
+}
+
+// Load loads the certificate and key.
+func (kp KeyPair) Load() (*x509.Certificate, interface{}, error) {
+	switch kp.Format {
+	case "", "pem_file":
+		certData, err := ioutil.ReadFile(kp.Certificate)
+		if err != nil {
+			return nil, nil, err
+		}
+		keyData, err := ioutil.ReadFile(kp.PrivateKey)
+		if err != nil {
+			return nil, nil, err
+		}
+
+		cert, err := pemDecodeSingleCert(certData)
+		if err != nil {
+			return nil, nil, err
+		}
+		key, err := pemDecodePrivateKey(keyData)
+		if err != nil {
+			return nil, nil, err
+		}
+
+		return cert, key, nil
+
+	default:
+		return nil, nil, fmt.Errorf("unsupported format: %s", kp.Format)
+	}
+}
diff --git a/modules/caddypki/maintain.go b/modules/caddypki/maintain.go
new file mode 100644
index 0000000..2fce0d9
--- /dev/null
+++ b/modules/caddypki/maintain.go
@@ -0,0 +1,99 @@
+// 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 caddypki
+
+import (
+	"crypto/x509"
+	"fmt"
+	"time"
+
+	"go.uber.org/zap"
+)
+
+func (p *PKI) maintenance() {
+	ticker := time.NewTicker(10 * time.Minute) // TODO: make configurable
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ticker.C:
+			p.renewCerts()
+		case <-p.ctx.Done():
+			return
+		}
+	}
+}
+
+func (p *PKI) renewCerts() {
+	for _, ca := range p.CAs {
+		err := p.renewCertsForCA(ca)
+		if err != nil {
+			p.log.Error("renewing intermediate certificates",
+				zap.Error(err),
+				zap.String("ca", ca.id))
+		}
+	}
+}
+
+func (p *PKI) renewCertsForCA(ca *CA) error {
+	ca.mu.Lock()
+	defer ca.mu.Unlock()
+
+	log := p.log.With(zap.String("ca", ca.id))
+
+	// only maintain the root if it's not manually provided in the config
+	if ca.Root == nil {
+		if needsRenewal(ca.root) {
+			// TODO: implement root renewal (use same key)
+			log.Warn("root certificate expiring soon (FIXME: ROOT RENEWAL NOT YET IMPLEMENTED)",
+				zap.Duration("time_remaining", time.Until(ca.inter.NotAfter)),
+			)
+		}
+	}
+
+	// only maintain the intermediate if it's not manually provided in the config
+	if ca.Intermediate == nil {
+		if needsRenewal(ca.inter) {
+			log.Info("intermediate expires soon; renewing",
+				zap.Duration("time_remaining", time.Until(ca.inter.NotAfter)),
+			)
+
+			rootCert, rootKey, err := ca.loadOrGenRoot()
+			if err != nil {
+				return fmt.Errorf("loading root key: %v", err)
+			}
+			interCert, interKey, err := ca.genIntermediate(rootCert, rootKey)
+			if err != nil {
+				return fmt.Errorf("generating new certificate: %v", err)
+			}
+			ca.inter, ca.interKey = interCert, interKey
+
+			log.Info("renewed intermediate",
+				zap.Time("new_expiration", ca.inter.NotAfter),
+			)
+		}
+	}
+
+	return nil
+}
+
+func needsRenewal(cert *x509.Certificate) bool {
+	lifetime := cert.NotAfter.Sub(cert.NotBefore)
+	renewalWindow := time.Duration(float64(lifetime) * renewalWindowRatio)
+	renewalWindowStart := cert.NotAfter.Add(-renewalWindow)
+	return time.Now().After(renewalWindowStart)
+}
+
+const renewalWindowRatio = 0.2 // TODO: make configurable
diff --git a/modules/caddypki/pki.go b/modules/caddypki/pki.go
new file mode 100644
index 0000000..1b10a8e
--- /dev/null
+++ b/modules/caddypki/pki.go
@@ -0,0 +1,117 @@
+// 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 caddypki
+
+import (
+	"fmt"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/smallstep/truststore"
+	"go.uber.org/zap"
+)
+
+func init() {
+	caddy.RegisterModule(PKI{})
+}
+
+// PKI provides Public Key Infrastructure facilities for Caddy.
+type PKI struct {
+	// The CAs to manage. Each CA is keyed by an ID that is used
+	// to uniquely identify it from other CAs. The default CA ID
+	// is "local".
+	CAs map[string]*CA `json:"certificate_authorities,omitempty"`
+
+	ctx caddy.Context
+	log *zap.Logger
+}
+
+// CaddyModule returns the Caddy module information.
+func (PKI) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "pki",
+		New: func() caddy.Module { return new(PKI) },
+	}
+}
+
+// Provision sets up the configuration for the PKI app.
+func (p *PKI) Provision(ctx caddy.Context) error {
+	p.ctx = ctx
+	p.log = ctx.Logger(p)
+
+	// if this app is initialized at all, ensure there's
+	// at least a default CA that can be used
+	if len(p.CAs) == 0 {
+		p.CAs = map[string]*CA{defaultCAID: new(CA)}
+	}
+
+	for caID, ca := range p.CAs {
+		err := ca.Provision(ctx, caID, p.log)
+		if err != nil {
+			return fmt.Errorf("provisioning CA '%s': %v", caID, err)
+		}
+	}
+
+	return nil
+}
+
+// Start starts the PKI app.
+func (p *PKI) Start() error {
+	// install roots to trust store, if not disabled
+	for _, ca := range p.CAs {
+		if ca.InstallTrust != nil && !*ca.InstallTrust {
+			ca.log.Warn("root certificate trust store installation disabled; clients will show warnings without intervention",
+				zap.String("path", ca.rootCertPath))
+			continue
+		}
+
+		// avoid password prompt if already trusted
+		if trusted(ca.root) {
+			ca.log.Info("root certificate is already trusted by system",
+				zap.String("path", ca.rootCertPath))
+			continue
+		}
+
+		ca.log.Warn("trusting root certificate (you might be prompted for password)",
+			zap.String("path", ca.rootCertPath))
+
+		err := truststore.Install(ca.root,
+			truststore.WithDebug(),
+			truststore.WithFirefox(),
+			truststore.WithJava(),
+		)
+		if err != nil {
+			return fmt.Errorf("adding root certificate to trust store: %v", err)
+		}
+	}
+
+	// see if root/intermediates need renewal...
+	p.renewCerts()
+
+	// ...and keep them renewed
+	go p.maintenance()
+
+	return nil
+}
+
+// Stop stops the PKI app.
+func (p *PKI) Stop() error {
+	return nil
+}
+
+// Interface guards
+var (
+	_ caddy.Provisioner = (*PKI)(nil)
+	_ caddy.App         = (*PKI)(nil)
+)