certificates/authority/provisioner/sshpop.go

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2019-10-27 00:40:43 +00:00
package provisioner
import (
"context"
"crypto/ecdsa"
"crypto/rsa"
"crypto/x509"
"encoding/base64"
"encoding/pem"
"time"
"github.com/pkg/errors"
"github.com/smallstep/cli/crypto/pemutil"
"github.com/smallstep/cli/crypto/x509util"
"github.com/smallstep/cli/jose"
"golang.org/x/crypto/ed25519"
"golang.org/x/crypto/ssh"
)
// sshPOPPayload extends jwt.Claims with step attributes.
type sshPOPPayload struct {
jose.Claims
SANs []string `json:"sans,omitempty"`
Step *stepPayload `json:"step,omitempty"`
sshCert *ssh.Certificate
}
// SSHPOP is the default provisioner, an entity that can sign tokens necessary for
// signature requests.
type SSHPOP struct {
Type string `json:"type"`
Name string `json:"name"`
PubKeys []byte `json:"pubKeys"`
Claims *Claims `json:"claims,omitempty"`
claimer *Claimer
audiences Audiences
sshPubKeys []ssh.PublicKey
}
// GetID returns the provisioner unique identifier. The name and credential id
// should uniquely identify any SSH-POP provisioner.
func (p *SSHPOP) GetID() string {
return "sshpop/" + p.Name
}
// GetTokenID returns the identifier of the token.
func (p *SSHPOP) GetTokenID(ott string) (string, error) {
// Validate payload
token, err := jose.ParseSigned(ott)
if err != nil {
return "", errors.Wrap(err, "error parsing token")
}
// Get claims w/out verification. We need to look up the provisioner
// key in order to verify the claims and we need the issuer from the claims
// before we can look up the provisioner.
var claims jose.Claims
if err = token.UnsafeClaimsWithoutVerification(&claims); err != nil {
return "", errors.Wrap(err, "error verifying claims")
}
return claims.ID, nil
}
// GetName returns the name of the provisioner.
func (p *SSHPOP) GetName() string {
return p.Name
}
// GetType returns the type of provisioner.
func (p *SSHPOP) GetType() Type {
return TypeSSHPOP
}
// GetEncryptedKey returns the base provisioner encrypted key if it's defined.
func (p *SSHPOP) GetEncryptedKey() (string, string, bool) {
return "", "", false
}
// Init initializes and validates the fields of a SSHPOP type.
func (p *SSHPOP) Init(config Config) error {
switch {
case p.Type == "":
return errors.New("provisioner type cannot be empty")
case p.Name == "":
return errors.New("provisioner name cannot be empty")
case len(p.PubKeys) == 0:
return errors.New("provisioner root(s) cannot be empty")
}
var (
block *pem.Block
rest = p.PubKeys
)
for rest != nil {
block, rest = pem.Decode(rest)
if block == nil {
break
}
key, err := pemutil.ParseKey(pem.EncodeToMemory(block))
if err != nil {
return errors.Wrapf(err, "error parsing public key in provisioner %s", p.GetID())
}
switch q := key.(type) {
case *rsa.PublicKey, *ecdsa.PublicKey, ed25519.PublicKey:
sshKey, err := ssh.NewPublicKey(key)
if err != nil {
return errors.Wrap(err, "error converting pub key to SSH pub key")
}
p.sshPubKeys = append(p.sshPubKeys, sshKey)
default:
return errors.Errorf("Unexpected public key type %T in provisioner %s", q, p.GetID())
}
}
// Verify that at least one root was found.
if len(p.sshPubKeys) == 0 {
return errors.Errorf("no root public keys found in pub keys attribute for provisioner %s", p.GetName())
}
// Update claims with global ones
var err error
if p.claimer, err = NewClaimer(p.Claims, config.Claims); err != nil {
return err
}
p.audiences = config.Audiences.WithFragment(p.GetID())
return nil
}
// authorizeToken performs common jwt authorization actions and returns the
// claims for case specific downstream parsing.
// e.g. a Sign request will auth/validate different fields than a Revoke request.
func (p *SSHPOP) authorizeToken(token string, audiences []string) (*sshPOPPayload, error) {
jwt, err := jose.ParseSigned(token)
if err != nil {
return nil, errors.Wrapf(err, "error parsing token")
}
encodedSSHCert, ok := jwt.Headers[0].ExtraHeaders["sshpop"]
if !ok {
return nil, errors.New("token missing sshpop header")
}
encodedSSHCertStr, ok := encodedSSHCert.(string)
if !ok {
return nil, errors.New("error unexpected type for sshpop header")
}
sshCertBytes, err := base64.RawURLEncoding.DecodeString(encodedSSHCertStr)
if err != nil {
return nil, errors.Wrap(err, "error decoding sshpop header")
}
sshPub, err := ssh.ParsePublicKey(sshCertBytes)
if err != nil {
return nil, errors.Wrap(err, "error parsing ssh public key")
}
sshCert, ok := sshPub.(*ssh.Certificate)
if !ok {
return nil, errors.New("error converting ssh public key to ssh certificate")
}
data := bytesForSigning(sshCert)
var found bool
for _, k := range p.sshPubKeys {
if err = (&ssh.Certificate{Key: k}).Verify(data, sshCert.Signature); err == nil {
found = true
}
}
if !found {
return nil, errors.New("error: provisioner could could not verify the sshpop header certificate")
}
// Using the leaf certificates key to validate the claims accomplishes two
// things:
// 1. Asserts that the private key used to sign the token corresponds
// to the public certificate in the `sshpop` header of the token.
// 2. Asserts that the claims are valid - have not been tampered with.
var claims sshPOPPayload
if err = jwt.Claims(sshCert.Key, &claims); err != nil {
return nil, errors.Wrap(err, "error parsing claims")
}
// According to "rfc7519 JSON Web Token" acceptable skew should be no
// more than a few minutes.
if err = claims.ValidateWithLeeway(jose.Expected{
Issuer: p.Name,
Time: time.Now().UTC(),
}, time.Minute); err != nil {
return nil, errors.Wrapf(err, "invalid token")
}
// validate audiences with the defaults
if !matchesAudience(claims.Audience, audiences) {
return nil, errors.New("invalid token: invalid audience claim (aud)")
}
if claims.Subject == "" {
return nil, errors.New("token subject cannot be empty")
}
claims.sshCert = sshCert
return &claims, nil
}
// AuthorizeRevoke returns an error if the provisioner does not have rights to
// revoke the certificate with serial number in the `sub` property.
func (p *SSHPOP) AuthorizeRevoke(token string) error {
_, err := p.authorizeToken(token, p.audiences.Revoke)
return err
}
// AuthorizeSign validates the given token.
func (p *SSHPOP) AuthorizeSign(ctx context.Context, token string) ([]SignOption, error) {
claims, err := p.authorizeToken(token, p.audiences.Sign)
if err != nil {
return nil, err
}
// Check for SSH sign-ing request.
if MethodFromContext(ctx) == SignSSHMethod {
if !p.claimer.IsSSHCAEnabled() {
return nil, errors.Errorf("ssh ca is disabled for provisioner %s", p.GetID())
}
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.
if len(claims.SANs) == 0 {
claims.SANs = []string{claims.Subject}
}
dnsNames, ips, emails := x509util.SplitSANs(claims.SANs)
return []SignOption{
// modifiers / withOptions
newProvisionerExtensionOption(TypeSSHPOP, p.Name, ""),
profileLimitDuration{p.claimer.DefaultTLSCertDuration(), time.Unix(int64(claims.sshCert.ValidBefore), 0)},
// validators
commonNameValidator(claims.Subject),
defaultPublicKeyValidator{},
dnsNamesValidator(dnsNames),
emailAddressesValidator(emails),
ipAddressesValidator(ips),
newValidityValidator(p.claimer.MinTLSCertDuration(), p.claimer.MaxTLSCertDuration()),
}, nil
}
// AuthorizeRenewal returns an error if the renewal is disabled.
func (p *SSHPOP) AuthorizeRenewal(cert *x509.Certificate) error {
if p.claimer.IsDisableRenewal() {
return errors.Errorf("renew is disabled for provisioner %s", p.GetID())
}
return nil
}
// authorizeSSHSign returns the list of SignOption for a SignSSH request.
func (p *SSHPOP) authorizeSSHSign(claims *sshPOPPayload) ([]SignOption, error) {
if claims.Step == nil || claims.Step.SSH == nil {
return nil, errors.New("authorization token must be an SSH provisioning token")
}
opts := claims.Step.SSH
signOptions := []SignOption{
// validates user's SSHOptions with the ones in the token
sshCertificateOptionsValidator(*opts),
// 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))
}
t := now()
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()))
}
// Default to a user certificate with no principals if not set
signOptions = append(signOptions, sshCertificateDefaultsModifier{CertType: SSHUserCert})
return append(signOptions,
// Set the default extensions.
&sshDefaultExtensionModifier{},
// Checks the validity bounds, and set the validity if has not been set.
sshLimitValidityModifier(p.claimer, time.Unix(int64(claims.sshCert.ValidBefore), 0)),
// Validate public key.
&sshDefaultPublicKeyValidator{},
// Validate the validity period.
&sshCertificateValidityValidator{p.claimer},
// Require all the fields in the SSH certificate
&sshCertificateDefaultValidator{},
), nil
}
func bytesForSigning(cert *ssh.Certificate) []byte {
c2 := *cert
c2.Signature = nil
out := c2.Marshal()
// Drop trailing signature length.
return out[:len(out)-4]
}