certificates/scep/authority.go

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package scep
import (
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"context"
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"crypto/subtle"
"crypto/x509"
"errors"
"fmt"
"net/url"
microx509util "github.com/micromdm/scep/v2/cryptoutil/x509util"
microscep "github.com/micromdm/scep/v2/scep"
"go.mozilla.org/pkcs7"
"go.step.sm/crypto/x509util"
"github.com/smallstep/certificates/authority/provisioner"
)
// Authority is the layer that handles all SCEP interactions.
type Authority struct {
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prefix string
dns string
intermediateCertificate *x509.Certificate
caCerts []*x509.Certificate // TODO(hs): change to use these instead of root and intermediate
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service *Service
signAuth SignAuthority
}
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type authorityKey struct{}
// NewContext adds the given authority to the context.
func NewContext(ctx context.Context, a *Authority) context.Context {
return context.WithValue(ctx, authorityKey{}, a)
}
// FromContext returns the current authority from the given context.
func FromContext(ctx context.Context) (a *Authority, ok bool) {
a, ok = ctx.Value(authorityKey{}).(*Authority)
return
}
// MustFromContext returns the current authority from the given context. It will
// panic if the authority is not in the context.
func MustFromContext(ctx context.Context) *Authority {
if a, ok := FromContext(ctx); !ok {
panic("scep authority is not in the context")
} else {
return a
}
}
// AuthorityOptions required to create a new SCEP Authority.
type AuthorityOptions struct {
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// Service provides the certificate chain, the signer and the decrypter to the Authority
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Service *Service
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// DNS is the host used to generate accurate SCEP links. By default the authority
// will use the Host from the request, so this value will only be used if
// request.Host is empty.
DNS string
// Prefix is a URL path prefix under which the SCEP api is served. This
// prefix is required to generate accurate SCEP links.
Prefix string
}
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type optionsKey struct{}
func newOptionsContext(ctx context.Context, o *AuthorityOptions) context.Context {
return context.WithValue(ctx, optionsKey{}, o)
}
func optionsFromContext(ctx context.Context) *AuthorityOptions {
o, ok := ctx.Value(optionsKey{}).(*AuthorityOptions)
if !ok {
panic("scep options are not in the context")
}
return o
}
// SignAuthority is the interface for a signing authority
type SignAuthority interface {
Sign(cr *x509.CertificateRequest, opts provisioner.SignOptions, signOpts ...provisioner.SignOption) ([]*x509.Certificate, error)
LoadProvisionerByName(string) (provisioner.Interface, error)
}
// New returns a new Authority that implements the SCEP interface.
func New(signAuth SignAuthority, ops AuthorityOptions) (*Authority, error) {
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authority := &Authority{
prefix: ops.Prefix,
dns: ops.DNS,
signAuth: signAuth,
}
// TODO: this is not really nice to do; the Service should be removed
// in its entirety to make this more interoperable with the rest of
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// step-ca, I think.
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if ops.Service != nil {
authority.caCerts = ops.Service.certificateChain
// TODO(hs): look into refactoring SCEP into using just caCerts everywhere, if it makes sense for more elaborate SCEP configuration. Keeping it like this for clarity (for now).
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authority.intermediateCertificate = ops.Service.certificateChain[0]
authority.service = ops.Service
}
return authority, nil
}
var (
// TODO: check the default capabilities; https://tools.ietf.org/html/rfc8894#section-3.5.2
defaultCapabilities = []string{
"Renewal", // NOTE: removing this will result in macOS SCEP client stating the server doesn't support renewal, but it uses PKCSreq to do so.
"SHA-1",
"SHA-256",
"AES",
"DES3",
"SCEPStandard",
"POSTPKIOperation",
}
)
// LoadProvisionerByName calls out to the SignAuthority interface to load a
// provisioner by name.
func (a *Authority) LoadProvisionerByName(name string) (provisioner.Interface, error) {
return a.signAuth.LoadProvisionerByName(name)
}
// GetLinkExplicit returns the requested link from the directory.
func (a *Authority) GetLinkExplicit(provName string, abs bool, baseURL *url.URL, inputs ...string) string {
return a.getLinkExplicit(provName, abs, baseURL, inputs...)
}
// getLinkExplicit returns an absolute or partial path to the given resource and a base
// URL dynamically obtained from the request for which the link is being calculated.
func (a *Authority) getLinkExplicit(provisionerName string, abs bool, baseURL *url.URL, inputs ...string) string {
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link := "/" + provisionerName
if abs {
// Copy the baseURL value from the pointer. https://github.com/golang/go/issues/38351
u := url.URL{}
if baseURL != nil {
u = *baseURL
}
// If no Scheme is set, then default to http (in case of SCEP)
if u.Scheme == "" {
u.Scheme = "http"
}
// If no Host is set, then use the default (first DNS attr in the ca.json).
if u.Host == "" {
u.Host = a.dns
}
u.Path = a.prefix + link
return u.String()
}
return link
}
// GetCACertificates returns the certificate (chain) for the CA
func (a *Authority) GetCACertificates(ctx context.Context) ([]*x509.Certificate, error) {
// TODO: this should return: the "SCEP Server (RA)" certificate, the issuing CA up to and excl. the root
// Some clients do need the root certificate however; also see: https://github.com/openxpki/openxpki/issues/73
//
// This means we might need to think about if we should use the current intermediate CA
// certificate as the "SCEP Server (RA)" certificate. It might be better to have a distinct
// RA certificate, with a corresponding rsa.PrivateKey, just for SCEP usage, which is signed by
// the intermediate CA. Will need to look how we can provide this nicely within step-ca.
//
// This might also mean that we might want to use a distinct instance of KMS for doing the key operations,
// so that we can use RSA just for SCEP.
//
// Using an RA does not seem to exist in https://tools.ietf.org/html/rfc8894, but is mentioned in
// https://tools.ietf.org/id/draft-nourse-scep-21.html. Will continue using the CA directly for now.
//
// The certificate to use should probably depend on the (configured) provisioner and may
// use a distinct certificate, apart from the intermediate.
p, err := provisionerFromContext(ctx)
if err != nil {
return nil, err
}
if len(a.caCerts) == 0 {
return nil, errors.New("no intermediate certificate available in SCEP authority")
}
certs := []*x509.Certificate{}
certs = append(certs, a.caCerts[0])
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// NOTE: we're adding the CA roots here, but they are (highly likely) different than what the RFC means.
// Clients are responsible to select the right cert(s) to use, though.
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if p.ShouldIncludeRootInChain() && len(a.caCerts) > 1 {
certs = append(certs, a.caCerts[1])
}
return certs, nil
}
// DecryptPKIEnvelope decrypts an enveloped message
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func (a *Authority) DecryptPKIEnvelope(ctx context.Context, msg *PKIMessage) error {
p7c, err := pkcs7.Parse(msg.P7.Content)
if err != nil {
return fmt.Errorf("error parsing pkcs7 content: %w", err)
}
envelope, err := p7c.Decrypt(a.intermediateCertificate, a.service.decrypter)
if err != nil {
return fmt.Errorf("error decrypting encrypted pkcs7 content: %w", err)
}
msg.pkiEnvelope = envelope
switch msg.MessageType {
case microscep.CertRep:
certs, err := microscep.CACerts(msg.pkiEnvelope)
if err != nil {
return fmt.Errorf("error extracting CA certs from pkcs7 degenerate data: %w", err)
}
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msg.CertRepMessage.Certificate = certs[0]
return nil
case microscep.PKCSReq, microscep.UpdateReq, microscep.RenewalReq:
csr, err := x509.ParseCertificateRequest(msg.pkiEnvelope)
if err != nil {
return fmt.Errorf("parse CSR from pkiEnvelope: %w", err)
}
// check for challengePassword
cp, err := microx509util.ParseChallengePassword(msg.pkiEnvelope)
if err != nil {
return fmt.Errorf("parse challenge password in pkiEnvelope: %w", err)
}
msg.CSRReqMessage = &microscep.CSRReqMessage{
RawDecrypted: msg.pkiEnvelope,
CSR: csr,
ChallengePassword: cp,
}
return nil
case microscep.GetCRL, microscep.GetCert, microscep.CertPoll:
return errors.New("not implemented")
}
return nil
}
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// SignCSR creates an x509.Certificate based on a CSR template and Cert Authority credentials
// returns a new PKIMessage with CertRep data
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func (a *Authority) SignCSR(ctx context.Context, csr *x509.CertificateRequest, msg *PKIMessage) (*PKIMessage, error) {
// TODO: intermediate storage of the request? In SCEP it's possible to request a csr/certificate
// to be signed, which can be performed asynchronously / out-of-band. In that case a client can
// poll for the status. It seems to be similar as what can happen in ACME, so might want to model
// the implementation after the one in the ACME authority. Requires storage, etc.
p, err := provisionerFromContext(ctx)
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if err != nil {
return nil, err
}
// check if CSRReqMessage has already been decrypted
if msg.CSRReqMessage.CSR == nil {
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if err := a.DecryptPKIEnvelope(ctx, msg); err != nil {
return nil, err
}
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csr = msg.CSRReqMessage.CSR
}
// Template data
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sans := []string{}
sans = append(sans, csr.DNSNames...)
sans = append(sans, csr.EmailAddresses...)
for _, v := range csr.IPAddresses {
sans = append(sans, v.String())
}
for _, v := range csr.URIs {
sans = append(sans, v.String())
}
if len(sans) == 0 {
sans = append(sans, csr.Subject.CommonName)
}
data := x509util.CreateTemplateData(csr.Subject.CommonName, sans)
data.SetCertificateRequest(csr)
data.SetSubject(x509util.Subject{
Country: csr.Subject.Country,
Organization: csr.Subject.Organization,
OrganizationalUnit: csr.Subject.OrganizationalUnit,
Locality: csr.Subject.Locality,
Province: csr.Subject.Province,
StreetAddress: csr.Subject.StreetAddress,
PostalCode: csr.Subject.PostalCode,
SerialNumber: csr.Subject.SerialNumber,
CommonName: csr.Subject.CommonName,
})
// Get authorizations from the SCEP provisioner.
ctx = provisioner.NewContextWithMethod(ctx, provisioner.SignMethod)
signOps, err := p.AuthorizeSign(ctx, "")
if err != nil {
return nil, fmt.Errorf("error retrieving authorization options from SCEP provisioner: %w", err)
}
opts := provisioner.SignOptions{}
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templateOptions, err := provisioner.TemplateOptions(p.GetOptions(), data)
if err != nil {
return nil, fmt.Errorf("error creating template options from SCEP provisioner: %w", err)
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}
signOps = append(signOps, templateOptions)
certChain, err := a.signAuth.Sign(csr, opts, signOps...)
if err != nil {
return nil, fmt.Errorf("error generating certificate for order: %w", err)
}
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// take the issued certificate (only); https://tools.ietf.org/html/rfc8894#section-3.3.2
cert := certChain[0]
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// and create a degenerate cert structure
deg, err := microscep.DegenerateCertificates([]*x509.Certificate{cert})
if err != nil {
return nil, err
}
// apparently the pkcs7 library uses a global default setting for the content encryption
// algorithm to use when en- or decrypting data. We need to restore the current setting after
// the cryptographic operation, so that other usages of the library are not influenced by
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// this call to Encrypt(). We are not required to use the same algorithm the SCEP client uses.
encryptionAlgorithmToRestore := pkcs7.ContentEncryptionAlgorithm
pkcs7.ContentEncryptionAlgorithm = p.GetContentEncryptionAlgorithm()
e7, err := pkcs7.Encrypt(deg, msg.P7.Certificates)
if err != nil {
return nil, err
}
pkcs7.ContentEncryptionAlgorithm = encryptionAlgorithmToRestore
// PKIMessageAttributes to be signed
config := pkcs7.SignerInfoConfig{
ExtraSignedAttributes: []pkcs7.Attribute{
{
Type: oidSCEPtransactionID,
Value: msg.TransactionID,
},
{
Type: oidSCEPpkiStatus,
Value: microscep.SUCCESS,
},
{
Type: oidSCEPmessageType,
Value: microscep.CertRep,
},
{
Type: oidSCEPrecipientNonce,
Value: msg.SenderNonce,
},
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{
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Type: oidSCEPsenderNonce,
Value: msg.SenderNonce,
},
},
}
signedData, err := pkcs7.NewSignedData(e7)
if err != nil {
return nil, err
}
// add the certificate into the signed data type
// this cert must be added before the signedData because the recipient will expect it
// as the first certificate in the array
signedData.AddCertificate(cert)
authCert := a.intermediateCertificate
// sign the attributes
if err := signedData.AddSigner(authCert, a.service.signer, config); err != nil {
return nil, err
}
certRepBytes, err := signedData.Finish()
if err != nil {
return nil, err
}
cr := &CertRepMessage{
PKIStatus: microscep.SUCCESS,
RecipientNonce: microscep.RecipientNonce(msg.SenderNonce),
Certificate: cert,
degenerate: deg,
}
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// create a CertRep message from the original
crepMsg := &PKIMessage{
Raw: certRepBytes,
TransactionID: msg.TransactionID,
MessageType: microscep.CertRep,
CertRepMessage: cr,
}
return crepMsg, nil
}
// CreateFailureResponse creates an appropriately signed reply for PKI operations
func (a *Authority) CreateFailureResponse(ctx context.Context, csr *x509.CertificateRequest, msg *PKIMessage, info FailInfoName, infoText string) (*PKIMessage, error) {
config := pkcs7.SignerInfoConfig{
ExtraSignedAttributes: []pkcs7.Attribute{
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{
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Type: oidSCEPtransactionID,
Value: msg.TransactionID,
},
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{
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Type: oidSCEPpkiStatus,
Value: microscep.FAILURE,
},
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{
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Type: oidSCEPfailInfo,
Value: info,
},
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{
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Type: oidSCEPfailInfoText,
Value: infoText,
},
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{
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Type: oidSCEPmessageType,
Value: microscep.CertRep,
},
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{
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Type: oidSCEPsenderNonce,
Value: msg.SenderNonce,
},
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{
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Type: oidSCEPrecipientNonce,
Value: msg.SenderNonce,
},
},
}
signedData, err := pkcs7.NewSignedData(nil)
if err != nil {
return nil, err
}
// sign the attributes
if err := signedData.AddSigner(a.intermediateCertificate, a.service.signer, config); err != nil {
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return nil, err
}
certRepBytes, err := signedData.Finish()
if err != nil {
return nil, err
}
cr := &CertRepMessage{
PKIStatus: microscep.FAILURE,
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FailInfo: microscep.FailInfo(info),
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RecipientNonce: microscep.RecipientNonce(msg.SenderNonce),
}
// create a CertRep message from the original
crepMsg := &PKIMessage{
Raw: certRepBytes,
TransactionID: msg.TransactionID,
MessageType: microscep.CertRep,
CertRepMessage: cr,
}
return crepMsg, nil
}
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// MatchChallengePassword verifies a SCEP challenge password
func (a *Authority) MatchChallengePassword(ctx context.Context, password string) (bool, error) {
p, err := provisionerFromContext(ctx)
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if err != nil {
return false, err
}
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if subtle.ConstantTimeCompare([]byte(p.GetChallengePassword()), []byte(password)) == 1 {
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return true, nil
}
// TODO: support dynamic challenges, i.e. a list of challenges instead of one?
// That's probably a bit harder to configure, though; likely requires some data store
// that can be interacted with more easily, via some internal API, for example.
return false, nil
}
// GetCACaps returns the CA capabilities
func (a *Authority) GetCACaps(ctx context.Context) []string {
p, err := provisionerFromContext(ctx)
if err != nil {
return defaultCapabilities
}
caps := p.GetCapabilities()
if len(caps) == 0 {
return defaultCapabilities
}
// TODO: validate the caps? Ensure they are the right format according to RFC?
// TODO: ensure that the capabilities are actually "enforced"/"verified" in code too:
// check that only parts of the spec are used in the implementation belonging to the capabilities.
// For example for renewals, which we could disable in the provisioner, should then also
// not be reported in cacaps operation.
return caps
}