diff --git a/osbase/pki/certificate.go b/osbase/pki/certificate.go
new file mode 100644
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+++ b/osbase/pki/certificate.go
@@ -0,0 +1,413 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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 pki implements an x509 PKI (Public Key Infrastructure) system backed
+// on etcd.
+package pki
+
+import (
+	"bytes"
+	"context"
+	"crypto/ed25519"
+	"crypto/rand"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/pem"
+	"fmt"
+	"net"
+
+	clientv3 "go.etcd.io/etcd/client/v3"
+
+	"source.monogon.dev/osbase/fileargs"
+)
+
+// Namespace represents some path in etcd where certificate/CA data will be
+// stored. Creating a namespace via Namespaced then permits the consumer of
+// this library to start creating certificates within this namespace.
+type Namespace struct {
+	prefix string
+}
+
+// Namespaced creates a namespace for storing certificate data in etcd at a
+// given 'path' prefix.
+func Namespaced(prefix string) Namespace {
+	return Namespace{
+		prefix: prefix,
+	}
+}
+
+type CertificateMode int
+
+const (
+	// CertificateManaged is a certificate whose key material is fully managed by
+	// the Certificate code. When set, PublicKey and PrivateKey must not be set by
+	// the user, and instead will be populated by the Ensure call. Name must be set,
+	// and will be used to store this Certificate and its keys within etcd. After
+	// the initial generation during Ensure, other Certificates with the same Name
+	// will be retrieved (including key material) from etcd.
+	CertificateManaged CertificateMode = iota
+
+	// CertificateExternal is a certificate whose key material is not managed by
+	// Certificate or stored in etcd, but the X509 certificate itself is. PublicKey
+	// must be set while PrivateKey must not be set. Name must be set, and will be
+	// used to store the emitted X509 certificate in etcd on Ensure. After the
+	// initial generation during Ensure, other Certificates with the same Name will
+	// be retrieved (without key material) from etcd.
+	CertificateExternal
+
+	// CertificateEphemeral is a certificate whose data (X509 certificate and
+	// possibly key material) is generated on demand each time Ensure is called.
+	// Nothing is stored in etcd or loaded from etcd. PrivateKey or PublicKey can be
+	// set, if both are nil then a new keypair will be generated. Name is ignored.
+	CertificateEphemeral
+)
+
+// Certificate is the promise of a Certificate being available to the caller.
+// In this case, Certificate refers to a pair of x509 certificate and
+// corresponding private key.  Certificates can be stored in etcd, and their
+// issuers might also be store on etcd. As such, this type's methods contain
+// references to an etcd KV client.
+type Certificate struct {
+	Namespace *Namespace
+
+	// Issuer is the Issuer that will generate this certificate if one doesn't
+	// yet exist or etcd, or the requested certificate is ephemeral (not to be
+	// stored on etcd).
+	Issuer Issuer
+	// Name is a unique key for storing the certificate in etcd (if the requested
+	// certificate is not ephemeral).
+	Name string
+	// Template is an x509 certificate definition that will be used to generate
+	// the certificate when issuing it.
+	Template x509.Certificate
+
+	// Mode in which this Certificate will operate. This influences the behaviour of
+	// the Ensure call.
+	Mode CertificateMode
+
+	// PrivateKey is the private key for this Certificate. It should never be set by
+	// the user, and instead will be populated by the Ensure call for Managed
+	// Certificates.
+	PrivateKey ed25519.PrivateKey
+
+	// PublicKey is the public key for this Certificate. It should only be set by
+	// the user for External or Ephemeral certificates, and will be populated by the
+	// next Ensure call if missing.
+	PublicKey ed25519.PublicKey
+}
+
+func (n *Namespace) etcdPath(f string, args ...interface{}) string {
+	return n.prefix + fmt.Sprintf(f, args...)
+}
+
+// Client makes a Kubernetes PKI-compatible client certificate template.
+// Directly derived from Kubernetes PKI requirements documented at
+//   https://kubernetes.io/docs/setup/best-practices/certificates/#configure-certificates-manually
+func Client(identity string, groups []string) x509.Certificate {
+	return x509.Certificate{
+		Subject: pkix.Name{
+			CommonName:   identity,
+			Organization: groups,
+		},
+		KeyUsage:    x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
+		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
+	}
+}
+
+// Server makes a Kubernetes PKI-compatible server certificate template.
+func Server(dnsNames []string, ips []net.IP) x509.Certificate {
+	return x509.Certificate{
+		Subject:     pkix.Name{},
+		KeyUsage:    x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
+		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		DNSNames:    dnsNames,
+		IPAddresses: ips,
+	}
+}
+
+// CA makes a Certificate that can sign other certificates.
+func CA(cn string) x509.Certificate {
+	return x509.Certificate{
+		Subject: pkix.Name{
+			CommonName: cn,
+		},
+		IsCA:        true,
+		KeyUsage:    x509.KeyUsageCertSign | x509.KeyUsageCRLSign | x509.KeyUsageDigitalSignature,
+		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth, x509.ExtKeyUsageOCSPSigning},
+	}
+}
+
+// ensure returns a DER-encoded x509 certificate and internally encoded bare
+// ed25519 key for a given Certificate, in memory (if ephemeral), loading it
+// from etcd, or creating and saving it on etcd if needed.
+// This function is safe to call in parallel from multiple etcd clients
+// (including across machines), but it will error in case a concurrent
+// certificate generation happens. These errors are, however, safe to retry -
+// as long as all the certificate creators (ie., Metropolis nodes) run the same
+// version of this code.
+func (c *Certificate) ensure(ctx context.Context, kv clientv3.KV) (cert []byte, err error) {
+	// Ensure key is available.
+	if err := c.ensureKey(ctx, kv); err != nil {
+		return nil, err
+	}
+
+	switch c.Mode {
+	case CertificateEphemeral:
+		// TODO(q3k): cache internally?
+		cert, err = c.Issuer.Issue(ctx, c, kv)
+		if err != nil {
+			return nil, fmt.Errorf("failed to issue: %w", err)
+		}
+		return cert, nil
+	case CertificateManaged, CertificateExternal:
+	default:
+		return nil, fmt.Errorf("invalid certificate mode %v", c.Mode)
+	}
+
+	if c.Name == "" {
+		if c.Mode == CertificateExternal {
+			return nil, fmt.Errorf("external certificate must have name set")
+		} else {
+			return nil, fmt.Errorf("managed certificate must have name set")
+		}
+	}
+
+	certPath := c.Namespace.etcdPath("issued/%s-cert.der", c.Name)
+
+	// Try loading certificate from etcd.
+	certRes, err := kv.Get(ctx, certPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get certificate from etcd: %w", err)
+	}
+
+	if len(certRes.Kvs) == 1 {
+		certBytes := certRes.Kvs[0].Value
+		cert, err := x509.ParseCertificate(certBytes)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse certificate retrieved from etcd: %w", err)
+		}
+		pk, ok := cert.PublicKey.(ed25519.PublicKey)
+		if !ok {
+			return nil, fmt.Errorf("unexpected non-ed25519 certificate found in etcd")
+		}
+		if !bytes.Equal(pk, c.PublicKey) {
+			return nil, fmt.Errorf("certificate stored in etcd emitted for different public key")
+		}
+		// TODO(q3k): ensure issuer and template haven't changed
+		return certBytes, nil
+	}
+
+	// No certificate found - issue one and save to etcd.
+	cert, err = c.Issuer.Issue(ctx, c, kv)
+	if err != nil {
+		return nil, fmt.Errorf("failed to issue: %w", err)
+	}
+
+	res, err := kv.Txn(ctx).
+		If(
+			clientv3.Compare(clientv3.CreateRevision(certPath), "=", 0),
+		).
+		Then(
+			clientv3.OpPut(certPath, string(cert)),
+		).Commit()
+	if err != nil {
+		err = fmt.Errorf("failed to write newly issued certificate: %w", err)
+	} else if !res.Succeeded {
+		err = fmt.Errorf("certificate issuance transaction failed: concurrent write")
+	}
+
+	return
+}
+
+// ensureKey retrieves or creates PublicKey as needed based on the Certificate
+// Mode. For Managed Certificates and Ephemeral Certificates with no PrivateKey
+// it will also populate PrivateKay.
+func (c *Certificate) ensureKey(ctx context.Context, kv clientv3.KV) error {
+	// If we have a public key then we're all set.
+	if c.PublicKey != nil {
+		return nil
+	}
+
+	// For ephemeral keys, we just generate them.
+	// For external keys, we can't do anything - not having the keys set means
+	// a programming error.
+
+	switch c.Mode {
+	case CertificateEphemeral:
+		pub, priv, err := ed25519.GenerateKey(rand.Reader)
+		if err != nil {
+			return fmt.Errorf("when generating ephemeral key: %w", err)
+		}
+		c.PublicKey = pub
+		c.PrivateKey = priv
+		return nil
+	case CertificateExternal:
+		if c.PrivateKey != nil {
+			// We prohibit having PrivateKey set in External Certificates to simplify the
+			// different logic paths this library implements. Being able to assume External
+			// == PublicKey only makes things easier elsewhere.
+			return fmt.Errorf("external certificate must not have PrivateKey set")
+		}
+		return fmt.Errorf("external certificate must have PublicKey set")
+	case CertificateManaged:
+	default:
+		return fmt.Errorf("invalid certificate mode %v", c.Mode)
+	}
+
+	// For managed keys, synchronize with etcd.
+	if c.Name == "" {
+		return fmt.Errorf("managed certificate must have Name set")
+	}
+
+	// First, try loading.
+	privPath := c.Namespace.etcdPath("keys/%s-privkey.bin", c.Name)
+	privRes, err := kv.Get(ctx, privPath)
+	if err != nil {
+		return fmt.Errorf("failed to get private key from etcd: %w", err)
+	}
+	if len(privRes.Kvs) == 1 {
+		privBytes := privRes.Kvs[0].Value
+		if len(privBytes) != ed25519.PrivateKeySize {
+			return fmt.Errorf("stored private key has invalid size")
+		}
+		c.PrivateKey = privBytes
+		c.PublicKey = c.PrivateKey.Public().(ed25519.PublicKey)
+		return nil
+	}
+
+	// No key in etcd? Generate and save.
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		return fmt.Errorf("while generating keypair: %w", err)
+	}
+
+	res, err := kv.Txn(ctx).
+		If(
+			clientv3.Compare(clientv3.CreateRevision(privPath), "=", 0),
+		).
+		Then(
+			clientv3.OpPut(privPath, string(priv)),
+		).Commit()
+	if err != nil {
+		return fmt.Errorf("failed to write newly generated keypair: %w", err)
+	} else if !res.Succeeded {
+		return fmt.Errorf("key generation transaction failed: concurrent write")
+	}
+
+	crlPath := c.crlPath()
+	emptyCRL, err := c.makeCRL(ctx, kv, nil)
+	if err != nil {
+		return fmt.Errorf("failed to generate empty CRL: %w", err)
+	}
+
+	// Also attempt to emit an empty CRL if one doesn't exist yet.
+	_, err = kv.Txn(ctx).
+		If(
+			clientv3.Compare(clientv3.CreateRevision(crlPath), "=", 0),
+		).
+		Then(
+			clientv3.OpPut(crlPath, string(emptyCRL)),
+		).Commit()
+	if err != nil {
+		return fmt.Errorf("failed to upsert empty CRL")
+	}
+
+	c.PrivateKey = priv
+	c.PublicKey = pub
+	return nil
+}
+
+// Ensure returns an x509 DER-encoded (but not PEM-encoded) certificate for a
+// given Certificate.
+//
+// If the Certificate is ephemeral, each call to Ensure will cause a new
+// certificate to be generated. Otherwise, it will be retrieved from etcd, or
+// generated and stored there if needed.
+func (c *Certificate) Ensure(ctx context.Context, kv clientv3.KV) (cert []byte, err error) {
+	return c.ensure(ctx, kv)
+}
+
+func (c *Certificate) PrivateKeyX509() ([]byte, error) {
+	if c.PrivateKey == nil {
+		return nil, fmt.Errorf("certificate has no private key")
+	}
+	key, err := x509.MarshalPKCS8PrivateKey(c.PrivateKey)
+	if err != nil {
+		return nil, fmt.Errorf("could not marshal private key (data corruption?): %w", err)
+	}
+	return key, nil
+}
+
+// FilesystemCertificate is a fileargs.FileArgs wrapper which will contain PEM
+// encoded certificate material when Mounted. This construct is useful when
+// dealing with services that want to access etcd-backed certificates as files
+// available locally.
+// Paths to the available files are considered opaque and should not be leaked
+// outside of the struct. Further restrictions on access to these files might
+// be imposed in the future.
+type FilesystemCertificate struct {
+	*fileargs.FileArgs
+	// CACertPath is the full path at which the CA certificate is available.
+	// Read only.
+	CACertPath string
+	// CertPath is the full path at which the certificate is available. Read
+	// only.
+	CertPath string
+	// KeyPath is the full path at which the private key is available, or an empty
+	// string if the Certificate was created without a private key. Read only.
+	KeyPath string
+}
+
+// Mount returns a locally mounted FilesystemCertificate for this Certificate,
+// which allows services to access this Certificate via local filesystem
+// access.
+// The embeded fileargs.FileArgs can also be used to add additional file-backed
+// data under the same mount by calling ArgPath.
+// The returned FilesystemCertificate must be Closed in order to prevent a
+// system mount leak.
+func (c *Certificate) Mount(ctx context.Context, kv clientv3.KV) (*FilesystemCertificate, error) {
+	fa, err := fileargs.New()
+	if err != nil {
+		return nil, fmt.Errorf("when creating fileargs mount: %w", err)
+	}
+	fs := &FilesystemCertificate{FileArgs: fa}
+
+	cert, err := c.Ensure(ctx, kv)
+	if err != nil {
+		return nil, fmt.Errorf("when issuing certificate: %w", err)
+	}
+
+	cacert, err := c.Issuer.CACertificate(ctx, kv)
+	if err != nil {
+		return nil, fmt.Errorf("when getting issuer CA: %w", err)
+	}
+	// cacert will be null if this is a self-signed certificate.
+	if cacert == nil {
+		cacert = cert
+	}
+
+	fs.CACertPath = fs.ArgPath("ca.crt", pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: cacert}))
+	fs.CertPath = fs.ArgPath("tls.crt", pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: cert}))
+	if c.PrivateKey != nil {
+		key, err := c.PrivateKeyX509()
+		if err != nil {
+			return nil, err
+		}
+		fs.KeyPath = fs.ArgPath("tls.key", pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: key}))
+	}
+
+	return fs, nil
+}