Add initial implementation of an SSH CA using the JWK provisioner.

Fixes smallstep/ca-component#187

api/api.go

@@ -26,6 +26,7 @@ import (
 
 // Authority is the interface implemented by a CA authority.
 type Authority interface {
+	SSHAuthority
 	// NOTE: Authorize will be deprecated in future releases. Please use the
 	// context specific Authoirize[Sign|Revoke|etc.] methods.
 	Authorize(ott string) ([]provisioner.SignOption, error)
@@ -249,6 +250,8 @@ func (h *caHandler) Route(r Router) {
 	r.MethodFunc("GET", "/federation", h.Federation)
 	// For compatibility with old code:
 	r.MethodFunc("POST", "/re-sign", h.Renew)
+	// SSH CA
+	r.MethodFunc("POST", "/sign-ssh", h.SignSSH)
 }
 
 // Health is an HTTP handler that returns the status of the server.

api/ssh.go

@@ -0,0 +1,150 @@
+package api
+
+import (
+	"bytes"
+	"encoding/base64"
+	"encoding/json"
+	"net/http"
+
+	"github.com/pkg/errors"
+	"github.com/smallstep/certificates/authority/provisioner"
+	"golang.org/x/crypto/ssh"
+)
+
+// SSHAuthority is the interface implemented by a SSH CA authority.
+type SSHAuthority interface {
+	SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, signOpts ...provisioner.SignOption) (*ssh.Certificate, error)
+}
+
+// SignSSHRequest is the request body of an SSH certificate request.
+type SignSSHRequest struct {
+	PublicKey   []byte       `json:"publicKey"` //base64 encoded
+	OTT         string       `json:"ott"`
+	CertType    string       `json:"certType"`
+	Principals  []string     `json:"principals"`
+	ValidAfter  TimeDuration `json:"validAfter"`
+	ValidBefore TimeDuration `json:"validBefore"`
+}
+
+// SignSSHResponse is the response object that returns the SSH certificate.
+type SignSSHResponse struct {
+	Certificate SSHCertificate `json:"crt"`
+}
+
+// SSHCertificate represents the response SSH certificate.
+type SSHCertificate struct {
+	*ssh.Certificate
+}
+
+// MarshalJSON implements the json.Marshaler interface. The certificate is
+// quoted string using the PEM encoding.
+func (c SSHCertificate) MarshalJSON() ([]byte, error) {
+	if c.Certificate == nil {
+		return []byte("null"), nil
+	}
+	s := base64.StdEncoding.EncodeToString(c.Certificate.Marshal())
+	return []byte(`"` + s + `"`), nil
+}
+
+// UnmarshalJSON implements the json.Unmarshaler interface. The certificate is
+// expected to be a quoted string using the PEM encoding.
+func (c *SSHCertificate) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return errors.Wrap(err, "error decoding certificate")
+	}
+	certData, err := base64.StdEncoding.DecodeString(s)
+	if err != nil {
+		return errors.Wrap(err, "error decoding certificate")
+	}
+	pub, err := ssh.ParsePublicKey(certData)
+	if err != nil {
+		return errors.Wrap(err, "error decoding certificate")
+	}
+	cert, ok := pub.(*ssh.Certificate)
+	if !ok {
+		return errors.Errorf("error decoding certificate: %T is not an *ssh.Certificate", pub)
+	}
+	c.Certificate = cert
+	return nil
+}
+
+// Validate validates the SignSSHRequest.
+func (s *SignSSHRequest) Validate() error {
+	switch {
+	case s.CertType != "" && s.CertType != provisioner.SSHUserCert && s.CertType != provisioner.SSHHostCert:
+		return errors.Errorf("unknown certType %s", s.CertType)
+	case len(s.PublicKey) == 0:
+		return errors.New("missing or empty publicKey")
+	case len(s.OTT) == 0:
+		return errors.New("missing or empty ott")
+	default:
+		return nil
+	}
+}
+
+// ParsePublicKey returns the ssh.PublicKey from the request.
+func (s *SignSSHRequest) ParsePublicKey() (ssh.PublicKey, error) {
+	// Validate pub key.
+	data := make([]byte, base64.StdEncoding.DecodedLen(len(s.PublicKey)))
+	if _, err := base64.StdEncoding.Decode(data, s.PublicKey); err != nil {
+		return nil, errors.Wrap(err, "error decoding publicKey")
+	}
+
+	// Trim padding from end of key.
+	data = bytes.TrimRight(data, "\x00")
+	publicKey, err := ssh.ParsePublicKey(data)
+	if err != nil {
+		return nil, errors.Wrap(err, "error parsing publicKey")
+	}
+
+	return publicKey, nil
+}
+
+// SignSSH is an HTTP handler that reads an SignSSHRequest with a one-time-token
+// (ott) from the body and creates a new SSH certificate with the information in
+// the request.
+func (h *caHandler) SignSSH(w http.ResponseWriter, r *http.Request) {
+	var body SignSSHRequest
+	if err := ReadJSON(r.Body, &body); err != nil {
+		WriteError(w, BadRequest(errors.Wrap(err, "error reading request body")))
+		return
+	}
+
+	logOtt(w, body.OTT)
+	if err := body.Validate(); err != nil {
+		WriteError(w, err)
+		return
+	}
+
+	publicKey, err := body.ParsePublicKey()
+	if err != nil {
+		WriteError(w, BadRequest(err))
+		return
+	}
+
+	opts := provisioner.SSHOptions{
+		CertType:    body.CertType,
+		Principals:  body.Principals,
+		ValidBefore: body.ValidBefore,
+		ValidAfter:  body.ValidAfter,
+	}
+
+	signOpts, err := h.Authority.AuthorizeSign(body.OTT)
+	if err != nil {
+		WriteError(w, Unauthorized(err))
+		return
+	}
+
+	cert, err := h.Authority.SignSSH(publicKey, opts, signOpts...)
+	if err != nil {
+		WriteError(w, Forbidden(err))
+		return
+	}
+
+	w.WriteHeader(http.StatusCreated)
+	// logCertificate(w, cert)
+	JSON(w, &SignSSHResponse{
+		Certificate: SSHCertificate{cert},
+	})
+}

authority/authority.go

@@ -1,6 +1,7 @@
 package authority
 
 import (
+	"crypto"
 	"crypto/sha256"
 	"crypto/x509"
 	"encoding/hex"
@@ -20,6 +21,8 @@ type Authority struct {
 	config               *Config
 	rootX509Certs        []*x509.Certificate
 	intermediateIdentity *x509util.Identity
+	sshCAUserCertSignKey crypto.Signer
+	sshCAHostCertSignKey crypto.Signer
 	validateOnce         bool
 	certificates         *sync.Map
 	startTime            time.Time
@@ -117,6 +120,9 @@ func (a *Authority) init() error {
 		}
 	}
 
+	a.sshCAHostCertSignKey = a.intermediateIdentity.Key.(crypto.Signer)
+	a.sshCAUserCertSignKey = a.intermediateIdentity.Key.(crypto.Signer)
+
 	// Store all the provisioners
 	for _, p := range a.config.AuthorityConfig.Provisioners {
 		if err := a.provisioners.Store(p); err != nil {

authority/config.go

@@ -29,10 +29,16 @@ var (
 	}
 	defaultDisableRenewal   = false
 	globalProvisionerClaims = provisioner.Claims{
-		MinTLSDur:      &provisioner.Duration{Duration: 5 * time.Minute},
-		MaxTLSDur:      &provisioner.Duration{Duration: 24 * time.Hour},
-		DefaultTLSDur:  &provisioner.Duration{Duration: 24 * time.Hour},
-		DisableRenewal: &defaultDisableRenewal,
+		MinTLSDur:         &provisioner.Duration{Duration: 5 * time.Minute}, // TLS certs
+		MaxTLSDur:         &provisioner.Duration{Duration: 24 * time.Hour},
+		DefaultTLSDur:     &provisioner.Duration{Duration: 24 * time.Hour},
+		DisableRenewal:    &defaultDisableRenewal,
+		MinUserSSHDur:     &provisioner.Duration{Duration: 5 * time.Minute}, // User SSH certs
+		MaxUserSSHDur:     &provisioner.Duration{Duration: 24 * time.Hour},
+		DefaultUserSSHDur: &provisioner.Duration{Duration: 4 * time.Hour},
+		MinHostSSHDur:     &provisioner.Duration{Duration: 5 * time.Minute}, // Host SSH certs
+		MaxHostSSHDur:     &provisioner.Duration{Duration: 30 * 24 * time.Hour},
+		DefaultHostSSHDur: &provisioner.Duration{Duration: 30 * 24 * time.Hour},
 	}
 )
 

authority/provisioner/claims.go

@@ -8,10 +8,18 @@ import (
 
 // Claims so that individual provisioners can override global claims.
 type Claims struct {
+	// TLS CA properties
 	MinTLSDur      *Duration `json:"minTLSCertDuration,omitempty"`
 	MaxTLSDur      *Duration `json:"maxTLSCertDuration,omitempty"`
 	DefaultTLSDur  *Duration `json:"defaultTLSCertDuration,omitempty"`
 	DisableRenewal *bool     `json:"disableRenewal,omitempty"`
+	// SSH CA properties
+	MinUserSSHDur     *Duration `json:"minUserSSHCertDuration,omitempty"`
+	MaxUserSSHDur     *Duration `json:"maxUserSSHCertDuration,omitempty"`
+	DefaultUserSSHDur *Duration `json:"defaultUserSSHCertDuration,omitempty"`
+	MinHostSSHDur     *Duration `json:"minHostSSHCertDuration,omitempty"`
+	MaxHostSSHDur     *Duration `json:"maxHostSSHCertDuration,omitempty"`
+	DefaultHostSSHDur *Duration `json:"defaultHostSSHCertDuration,omitempty"`
 }
 
 // Claimer is the type that controls claims. It provides an interface around the
@@ -31,10 +39,16 @@ func NewClaimer(claims *Claims, global Claims) (*Claimer, error) {
 func (c *Claimer) Claims() Claims {
 	disableRenewal := c.IsDisableRenewal()
 	return Claims{
-		MinTLSDur:      &Duration{c.MinTLSCertDuration()},
-		MaxTLSDur:      &Duration{c.MaxTLSCertDuration()},
-		DefaultTLSDur:  &Duration{c.DefaultTLSCertDuration()},
-		DisableRenewal: &disableRenewal,
+		MinTLSDur:         &Duration{c.MinTLSCertDuration()},
+		MaxTLSDur:         &Duration{c.MaxTLSCertDuration()},
+		DefaultTLSDur:     &Duration{c.DefaultTLSCertDuration()},
+		DisableRenewal:    &disableRenewal,
+		MinUserSSHDur:     &Duration{c.MinUserSSHCertDuration()},
+		MaxUserSSHDur:     &Duration{c.MaxUserSSHCertDuration()},
+		DefaultUserSSHDur: &Duration{c.DefaultUserSSHCertDuration()},
+		MinHostSSHDur:     &Duration{c.MinHostSSHCertDuration()},
+		MaxHostSSHDur:     &Duration{c.MaxHostSSHCertDuration()},
+		DefaultHostSSHDur: &Duration{c.DefaultHostSSHCertDuration()},
 	}
 }
 
@@ -78,6 +92,66 @@ func (c *Claimer) IsDisableRenewal() bool {
 	return *c.claims.DisableRenewal
 }
 
+// DefaultUserSSHCertDuration returns the default SSH user cert duration for the
+// provisioner. If the default is not set within the provisioner, then the
+// global default from the authority configuration will be used.
+func (c *Claimer) DefaultUserSSHCertDuration() time.Duration {
+	if c.claims == nil || c.claims.DefaultUserSSHDur == nil {
+		return c.global.DefaultUserSSHDur.Duration
+	}
+	return c.claims.DefaultUserSSHDur.Duration
+}
+
+// MinUserSSHCertDuration returns the minimum SSH user cert duration for the
+// provisioner. If the minimum is not set within the provisioner, then the
+// global minimum from the authority configuration will be used.
+func (c *Claimer) MinUserSSHCertDuration() time.Duration {
+	if c.claims == nil || c.claims.MinUserSSHDur == nil {
+		return c.global.MinUserSSHDur.Duration
+	}
+	return c.claims.MinUserSSHDur.Duration
+}
+
+// MaxUserSSHCertDuration returns the maximum SSH user cert duration for the
+// provisioner. If the maximum is not set within the provisioner, then the
+// global maximum from the authority configuration will be used.
+func (c *Claimer) MaxUserSSHCertDuration() time.Duration {
+	if c.claims == nil || c.claims.MaxUserSSHDur == nil {
+		return c.global.MaxUserSSHDur.Duration
+	}
+	return c.claims.MaxUserSSHDur.Duration
+}
+
+// DefaultHostSSHCertDuration returns the default SSH host cert duration for the
+// provisioner. If the default is not set within the provisioner, then the
+// global default from the authority configuration will be used.
+func (c *Claimer) DefaultHostSSHCertDuration() time.Duration {
+	if c.claims == nil || c.claims.DefaultHostSSHDur == nil {
+		return c.global.DefaultHostSSHDur.Duration
+	}
+	return c.claims.DefaultHostSSHDur.Duration
+}
+
+// MinHostSSHCertDuration returns the minimum SSH host cert duration for the
+// provisioner. If the minimum is not set within the provisioner, then the
+// global minimum from the authority configuration will be used.
+func (c *Claimer) MinHostSSHCertDuration() time.Duration {
+	if c.claims == nil || c.claims.MinHostSSHDur == nil {
+		return c.global.MinHostSSHDur.Duration
+	}
+	return c.claims.MinHostSSHDur.Duration
+}
+
+// MaxHostSSHCertDuration returns the maximum SSH Host cert duration for the
+// provisioner. If the maximum is not set within the provisioner, then the
+// global maximum from the authority configuration will be used.
+func (c *Claimer) MaxHostSSHCertDuration() time.Duration {
+	if c.claims == nil || c.claims.MaxHostSSHDur == nil {
+		return c.global.MaxHostSSHDur.Duration
+	}
+	return c.claims.MaxHostSSHDur.Duration
+}
+
 // Validate validates and modifies the Claims with default values.
 func (c *Claimer) Validate() error {
 	var (

authority/provisioner/jwk.go

@@ -12,7 +12,12 @@ import (
 // jwtPayload extends jwt.Claims with step attributes.
 type jwtPayload struct {
 	jose.Claims
-	SANs []string `json:"sans,omitempty"`
+	SANs []string     `json:"sans,omitempty"`
+	Step *stepPayload `json:"step,omitempty"`
+}
+
+type stepPayload struct {
+	SSH *SSHOptions `json:"ssh,omitempty"`
 }
 
 // JWK is the default provisioner, an entity that can sign tokens necessary for
@@ -134,6 +139,12 @@ func (p *JWK) AuthorizeSign(token string) ([]SignOption, error) {
 	if err != nil {
 		return nil, err
 	}
+
+	// Check for SSH token
+	if claims.Step != nil && claims.Step.SSH != nil {
+		return p.authorizeSSHSign(claims)
+	}
+
 	// NOTE: This is for backwards compatibility with older versions of cli
 	// and certificates. Older versions added the token subject as the only SAN
 	// in a CSR by default.
@@ -159,3 +170,37 @@ func (p *JWK) AuthorizeRenewal(cert *x509.Certificate) error {
 	}
 	return nil
 }
+
+func (p *JWK) authorizeSSHSign(claims *jwtPayload) ([]SignOption, error) {
+	t := now()
+	opts := claims.Step.SSH
+	signOptions := []SignOption{
+		// validates user's SSHOptions with the ones in the token
+		&sshCertificateOptionsValidator{opts},
+		// set the default extensions
+		&sshDefaultExtensionModifier{},
+		// set the key id to the token subject
+		sshCertificateKeyIDModifier(claims.Subject),
+	}
+
+	// Add modifiers from custom claims
+	if opts.CertType != "" {
+		signOptions = append(signOptions, sshCertificateCertTypeModifier(opts.CertType))
+	}
+	if len(opts.Principals) > 0 {
+		signOptions = append(signOptions, sshCertificatePrincipalsModifier(opts.Principals))
+	}
+	if !opts.ValidAfter.IsZero() {
+		signOptions = append(signOptions, sshCertificateValidAfterModifier(opts.ValidAfter.RelativeTime(t).Unix()))
+	}
+	if !opts.ValidBefore.IsZero() {
+		signOptions = append(signOptions, sshCertificateValidBeforeModifier(opts.ValidBefore.RelativeTime(t).Unix()))
+	}
+
+	return append(signOptions,
+		// checks the validity bounds, and set the validity if has not been set
+		&sshCertificateValidityModifier{p.claimer},
+		// require all the fields in the SSH certificate
+		&sshCertificateDefaultValidator{},
+	), nil
+}

authority/provisioner/sign_ssh_options.go

@@ -0,0 +1,290 @@
+package provisioner
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/pkg/errors"
+	"golang.org/x/crypto/ssh"
+)
+
+const (
+	// SSHUserCert is the string used to represent ssh.UserCert.
+	SSHUserCert = "user"
+
+	// SSHHostCert is the string used to represent ssh.HostCert.
+	SSHHostCert = "host"
+)
+
+// SSHCertificateModifier is the interface used to change properties in an SSH
+// certificate.
+type SSHCertificateModifier interface {
+	SignOption
+	Modify(cert *ssh.Certificate) error
+}
+
+// SSHCertificateOptionModifier is the interface used to add custom options used
+// to modify the SSH certificate.
+type SSHCertificateOptionModifier interface {
+	SignOption
+	Option(o SSHOptions) SSHCertificateModifier
+}
+
+// SSHCertificateValidator is the interface used to validate an SSH certificate.
+type SSHCertificateValidator interface {
+	SignOption
+	Valid(crt *ssh.Certificate) error
+}
+
+// SSHCertificateOptionsValidator is the interface used to validate the custom
+// options used to modify the SSH certificate.
+type SSHCertificateOptionsValidator interface {
+	SignOption
+	Valid(got SSHOptions) error
+}
+
+// SSHOptions contains the options that can be passed to the SignSSH method.
+type SSHOptions struct {
+	CertType    string       `json:"certType"`
+	Principals  []string     `json:"principals"`
+	ValidAfter  TimeDuration `json:"validAfter,omitempty"`
+	ValidBefore TimeDuration `json:"validBefore,omitempty"`
+}
+
+// Type returns the uint32 representation of the CertType.
+func (o SSHOptions) Type() uint32 {
+	return sshCertTypeUInt32(o.CertType)
+}
+
+// Modify implements SSHCertificateModifier and sets the SSHOption in the ssh.Certificate.
+func (o SSHOptions) Modify(cert *ssh.Certificate) error {
+	switch o.CertType {
+	case "": // ignore
+	case SSHUserCert:
+		cert.CertType = ssh.UserCert
+	case SSHHostCert:
+		cert.CertType = ssh.HostCert
+	default:
+		return errors.Errorf("ssh certificate has an unknown type: %s", o.CertType)
+	}
+	cert.ValidPrincipals = o.Principals
+	if !o.ValidAfter.IsZero() {
+		cert.ValidAfter = uint64(o.ValidAfter.Time().Unix())
+	}
+	if !o.ValidBefore.IsZero() {
+		cert.ValidBefore = uint64(o.ValidBefore.Time().Unix())
+	}
+	if cert.ValidAfter > 0 && cert.ValidBefore > 0 && cert.ValidAfter > cert.ValidBefore {
+		return errors.New("ssh certificate valid after cannot be greater than valid before")
+	}
+	return nil
+}
+
+// match compares two SSHOptions and return an error if they don't match. It
+// ignores zero values.
+func (o SSHOptions) match(got SSHOptions) error {
+	if o.CertType != "" && got.CertType != "" && o.CertType != got.CertType {
+		return errors.Errorf("ssh certificate type does not match - got %v, want %v", got.CertType, o.CertType)
+	}
+	if len(o.Principals) > 0 && len(got.Principals) > 0 && !equalStringSlice(o.Principals, got.Principals) {
+		return errors.Errorf("ssh certificate principals does not match - got %v, want %v", got.Principals, o.Principals)
+	}
+	if !o.ValidAfter.IsZero() && !got.ValidAfter.IsZero() && !o.ValidAfter.Equal(&got.ValidAfter) {
+		return errors.Errorf("ssh certificate valid after does not match - got %v, want %v", got.ValidAfter, o.ValidAfter)
+	}
+	if !o.ValidBefore.IsZero() && !got.ValidBefore.IsZero() && !o.ValidBefore.Equal(&got.ValidBefore) {
+		return errors.Errorf("ssh certificate valid before does not match - got %v, want %v", got.ValidBefore, o.ValidBefore)
+	}
+	return nil
+}
+
+// sshCertificateKeyIDModifier is an SSHCertificateModifier that sets the given
+// Key ID in the SSH certificate.
+type sshCertificateKeyIDModifier string
+
+func (m sshCertificateKeyIDModifier) Modify(cert *ssh.Certificate) error {
+	cert.KeyId = string(m)
+	return nil
+}
+
+// sshCertificateCertTypeModifier is an SSHCertificateModifier that sets the
+// certificate type to the SSH certificate.
+type sshCertificateCertTypeModifier string
+
+func (m sshCertificateCertTypeModifier) Modify(cert *ssh.Certificate) error {
+	cert.CertType = sshCertTypeUInt32(string(m))
+	return nil
+}
+
+// sshCertificatePrincipalsModifier is an SSHCertificateModifier that sets the
+// principals to the SSH certificate.
+type sshCertificatePrincipalsModifier []string
+
+func (m sshCertificatePrincipalsModifier) Modify(cert *ssh.Certificate) error {
+	cert.ValidPrincipals = []string(m)
+	return nil
+}
+
+// sshCertificateValidAfterModifier is an SSHCertificateModifier that sets the
+// ValidAfter in the SSH certificate.
+type sshCertificateValidAfterModifier uint64
+
+func (m sshCertificateValidAfterModifier) Modify(cert *ssh.Certificate) error {
+	cert.ValidAfter = uint64(m)
+	return nil
+}
+
+// sshCertificateValidBeforeModifier is an SSHCertificateModifier that sets the
+// ValidBefore in the SSH certificate.
+type sshCertificateValidBeforeModifier uint64
+
+func (m sshCertificateValidBeforeModifier) Modify(cert *ssh.Certificate) error {
+	cert.ValidBefore = uint64(m)
+	return nil
+}
+
+// sshDefaultExtensionModifier implements an SSHCertificateModifier that sets
+// the default extensions in an SSH certificate.
+type sshDefaultExtensionModifier struct{}
+
+func (m *sshDefaultExtensionModifier) Modify(cert *ssh.Certificate) error {
+	if cert.Extensions == nil {
+		cert.Extensions = make(map[string]string)
+	}
+	cert.Extensions["permit-X11-forwarding"] = ""
+	cert.Extensions["permit-agent-forwarding"] = ""
+	cert.Extensions["permit-port-forwarding"] = ""
+	cert.Extensions["permit-pty"] = ""
+	cert.Extensions["permit-user-rc"] = ""
+	return nil
+}
+
+// sshCertificateValidityModifier is a SSHCertificateModifier checks the
+// validity bounds, setting them if they are not provided. It will fail if a
+// CertType has not been set or is not valid.
+type sshCertificateValidityModifier struct {
+	*Claimer
+}
+
+func (m *sshCertificateValidityModifier) Modify(cert *ssh.Certificate) error {
+	var d, min, max time.Duration
+	switch cert.CertType {
+	case ssh.UserCert:
+		d = m.DefaultUserSSHCertDuration()
+		min = m.MinUserSSHCertDuration()
+		max = m.MaxUserSSHCertDuration()
+	case ssh.HostCert:
+		d = m.DefaultHostSSHCertDuration()
+		min = m.MinHostSSHCertDuration()
+		max = m.MaxHostSSHCertDuration()
+	case 0:
+		return errors.New("ssh certificate type has not been set")
+	default:
+		return errors.Errorf("unknown ssh certificate type %d", cert.CertType)
+	}
+
+	if cert.ValidAfter == 0 {
+		cert.ValidAfter = uint64(now().Unix())
+	}
+	if cert.ValidBefore == 0 {
+		t := time.Unix(int64(cert.ValidAfter), 0)
+		cert.ValidBefore = uint64(t.Add(d).Unix())
+	}
+
+	diff := time.Duration(cert.ValidBefore-cert.ValidAfter) * time.Second
+	switch {
+	case diff < max:
+		return errors.Errorf("ssh certificate duration cannot be lower than %s", min)
+	case diff > max:
+		return errors.Errorf("ssh certificate duration cannot be greater than %s", max)
+	default:
+		return nil
+	}
+}
+
+// sshCertificateOptionsValidator validates the user SSHOptions with the ones
+// usually present in the token.
+type sshCertificateOptionsValidator struct {
+	*SSHOptions
+}
+
+func (want *sshCertificateOptionsValidator) Valid(got SSHOptions) error {
+	return want.match(got)
+}
+
+// sshCertificateDefaultValidator implements a simple validator for all the
+// fields in the SSH certificate.
+type sshCertificateDefaultValidator struct{}
+
+// Valid returns an error if the given certificate does not contain the necessary fields.
+func (v *sshCertificateDefaultValidator) Valid(crt *ssh.Certificate) error {
+	switch {
+	case len(crt.Nonce) == 0:
+		return errors.New("ssh certificate nonce cannot be empty")
+	case crt.Key == nil:
+		return errors.New("ssh certificate key cannot be nil")
+	case crt.Serial == 0:
+		return errors.New("ssh certificate serial cannot be 0")
+	case crt.CertType != ssh.UserCert && crt.CertType != ssh.HostCert:
+		return errors.Errorf("ssh certificate has an unknown type: %d", crt.CertType)
+	case crt.KeyId == "":
+		return errors.New("ssh certificate key id cannot be empty")
+	case len(crt.ValidPrincipals) == 0:
+		return errors.New("ssh certificate valid principals cannot be empty")
+	case crt.ValidAfter == 0:
+		return errors.New("ssh certificate valid after cannot be 0")
+	case crt.ValidBefore == 0:
+		return errors.New("ssh certificate valid before cannot be 0")
+	case len(crt.Extensions) == 0:
+		return errors.New("ssh certificate extensions cannot be empty")
+	case crt.SignatureKey == nil:
+		return errors.New("ssh certificate signature key cannot be nil")
+	case crt.Signature == nil:
+		return errors.New("ssh certificate signature cannot be nil")
+	default:
+		return nil
+	}
+}
+
+// sshCertTypeName returns the string representation of the given ssh.CertType.
+func sshCertTypeString(ct uint32) string {
+	switch ct {
+	case 0:
+		return ""
+	case ssh.UserCert:
+		return SSHUserCert
+	case ssh.HostCert:
+		return SSHHostCert
+	default:
+		return fmt.Sprintf("unknown (%d)", ct)
+	}
+}
+
+// sshCertTypeUInt32
+func sshCertTypeUInt32(ct string) uint32 {
+	switch ct {
+	case SSHUserCert:
+		return ssh.UserCert
+	case SSHHostCert:
+		return ssh.HostCert
+	default:
+		return 0
+	}
+}
+
+func equalStringSlice(a, b []string) bool {
+	var l int
+	if l = len(a); l != len(b) {
+		return false
+	}
+	visit := make(map[string]struct{}, l)
+	for i := 0; i < l; i++ {
+		visit[a[i]] = struct{}{}
+	}
+	for i := 0; i < l; i++ {
+		if _, ok := visit[b[i]]; !ok {
+			return false
+		}
+	}
+	return true
+}

authority/provisioner/timeduration.go

@@ -57,6 +57,17 @@ func (t *TimeDuration) SetTime(tt time.Time) {
 	t.t, t.d = tt, 0
 }
 
+// IsZero returns true the TimeDuration represents the zero value, false
+// otherwise.
+func (t *TimeDuration) IsZero() bool {
+	return t.t.IsZero() && t.d == 0
+}
+
+// Equal returns if t and other are equal.
+func (t *TimeDuration) Equal(other *TimeDuration) bool {
+	return t.t.Equal(other.t) && t.d == other.d
+}
+
 // MarshalJSON implements the json.Marshaler interface. If the time is set it
 // will return the time in RFC 3339 format if not it will return the duration
 // string.

authority/ssh.go

@@ -0,0 +1,114 @@
+package authority
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/binary"
+	"encoding/hex"
+	"net/http"
+	"strings"
+
+	"github.com/pkg/errors"
+	"github.com/smallstep/certificates/authority/provisioner"
+	"github.com/smallstep/cli/crypto/randutil"
+	"golang.org/x/crypto/ssh"
+)
+
+func generateSSHPublicKeyID(key ssh.PublicKey) string {
+	sum := sha256.Sum256(key.Marshal())
+	return strings.ToLower(hex.EncodeToString(sum[:]))
+}
+
+// SignSSH creates a signed SSH certificate with the given public key and options.
+func (a *Authority) SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, signOpts ...provisioner.SignOption) (*ssh.Certificate, error) {
+	var mods []provisioner.SSHCertificateModifier
+	var validators []provisioner.SSHCertificateValidator
+
+	for _, op := range signOpts {
+		switch o := op.(type) {
+		// modify the ssh.Certificate
+		case provisioner.SSHCertificateModifier:
+			mods = append(mods, o)
+		// modify the ssh.Certificate given the SSHOptions
+		case provisioner.SSHCertificateOptionModifier:
+			mods = append(mods, o.Option(opts))
+		// validate the ssh.Certificate
+		case provisioner.SSHCertificateValidator:
+			validators = append(validators, o)
+		// validate the given SSHOptions
+		case provisioner.SSHCertificateOptionsValidator:
+			if err := o.Valid(opts); err != nil {
+				return nil, &apiError{err: err, code: http.StatusUnauthorized}
+			}
+		default:
+			return nil, &apiError{
+				err:  errors.Errorf("signSSH: invalid extra option type %T", o),
+				code: http.StatusInternalServerError,
+			}
+		}
+	}
+
+	nonce, err := randutil.ASCII(32)
+	if err != nil {
+		return nil, err
+	}
+
+	var serial uint64
+	if err := binary.Read(rand.Reader, binary.BigEndian, &serial); err != nil {
+		return nil, errors.Wrap(err, "error reading random number")
+	}
+
+	// Build base certificate with the key and some random values
+	cert := &ssh.Certificate{
+		Nonce:  []byte(nonce),
+		Key:    key,
+		Serial: serial,
+	}
+
+	// Use opts to modify the certificate
+	if err := opts.Modify(cert); err != nil {
+		return nil, err
+	}
+
+	// Use provisioner modifiers
+	for _, m := range mods {
+		if err := m.Modify(cert); err != nil {
+			return nil, &apiError{err: err, code: http.StatusInternalServerError}
+		}
+	}
+
+	// Get signer from authority keys
+	var signer ssh.Signer
+	switch cert.CertType {
+	case ssh.UserCert:
+		signer, err = ssh.NewSignerFromSigner(a.sshCAUserCertSignKey)
+	case ssh.HostCert:
+		signer, err = ssh.NewSignerFromSigner(a.sshCAHostCertSignKey)
+	default:
+		return nil, &apiError{
+			err:  errors.Errorf("unexpected ssh certificate type: %d", cert.CertType),
+			code: http.StatusInternalServerError,
+		}
+	}
+	cert.SignatureKey = signer.PublicKey()
+
+	// Get bytes for signing trailing the signature length.
+	data := cert.Marshal()
+	data = data[:len(data)-4]
+
+	// Sign the certificate
+	sig, err := signer.Sign(rand.Reader, data)
+	if err != nil {
+		return nil, err
+	}
+	cert.Signature = sig
+
+	// User provisioners validators
+	for _, v := range validators {
+		if err := v.Valid(cert); err != nil {
+			return nil, &apiError{err: err, code: http.StatusUnauthorized}
+		}
+	}
+
+	return cert, nil
+}

ca/client.go

@@ -373,6 +373,28 @@ func (c *Client) Sign(req *api.SignRequest) (*api.SignResponse, error) {
 	return &sign, nil
 }
 
+// SignSSH performs the SSH certificate sign request to the CA and returns the
+// api.SignSSHResponse struct.
+func (c *Client) SignSSH(req *api.SignSSHRequest) (*api.SignSSHResponse, error) {
+	body, err := json.Marshal(req)
+	if err != nil {
+		return nil, errors.Wrap(err, "error marshaling request")
+	}
+	u := c.endpoint.ResolveReference(&url.URL{Path: "/sign-ssh"})
+	resp, err := c.client.Post(u.String(), "application/json", bytes.NewReader(body))
+	if err != nil {
+		return nil, errors.Wrapf(err, "client POST %s failed", u)
+	}
+	if resp.StatusCode >= 400 {
+		return nil, readError(resp.Body)
+	}
+	var sign api.SignSSHResponse
+	if err := readJSON(resp.Body, &sign); err != nil {
+		return nil, errors.Wrapf(err, "error reading %s", u)
+	}
+	return &sign, nil
+}
+
 // Renew performs the renew request to the CA and returns the api.SignResponse
 // struct.
 func (c *Client) Renew(tr http.RoundTripper) (*api.SignResponse, error) {