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Versions: (draft-pritikin-est) 00 01 02 03 06 07 08 09 RFC 7030

PKIX                                                    M. Pritikin, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                             P. Yee, Ed.
Expires: July 7, 2012                                       AKAYLA, Inc.
                                                         January 4, 2012


                    Enrollment over Secure Transport
                         draft-ietf-pkix-est-00

Abstract

   This document profiles certificate enrollment for clients using
   Certificate Management over CMS (CMC) messages over a secure
   transport.  This profile, called Enrollment over Secure Transport
   (EST), describes a simple yet functional certificate management
   protocol targeting simple Public Key Infrastructure clients that need
   to acquire client certificate(s) and associated Certification
   Authority (CA) certificate(s).

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 7, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  6
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Secure Transport . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  TLS-Based Server Authentication  . . . . . . . . . . . . .  9
     3.2.  Server Authentication and Authorization  . . . . . . . . . 10
     3.3.  TLS-Based Client Authentication  . . . . . . . . . . . . . 11
     3.4.  HTTP-Based Client Authentication . . . . . . . . . . . . . 11
     3.5.  Client Authorization . . . . . . . . . . . . . . . . . . . 12
     3.6.  Proof-of-Possession  . . . . . . . . . . . . . . . . . . . 12
     3.7.  Linking Identity and POP information . . . . . . . . . . . 12
   4.  HTTP URIs  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Distribution of CA certificates  . . . . . . . . . . . . . 15
       5.1.1.  Distribution of CA certificates response . . . . . . . 15
     5.2.  Simple Enrollment of Clients . . . . . . . . . . . . . . . 16
       5.2.1.  Simple Re-Enrollment of Clients  . . . . . . . . . . . 16
       5.2.2.  Simple Enroll and Re-Enroll Response . . . . . . . . . 17
     5.3.  Full CMC . . . . . . . . . . . . . . . . . . . . . . . . . 18
       5.3.1.  Full CMC Request . . . . . . . . . . . . . . . . . . . 18
       5.3.2.  Full CMC Response  . . . . . . . . . . . . . . . . . . 18
   6.  Cryptographic Algorithms . . . . . . . . . . . . . . . . . . . 18
   7.  Contributors/Acknowledgements  . . . . . . . . . . . . . . . . 19
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     10.2. Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Server Discovery  . . . . . . . . . . . . . . . . . . 22
   Appendix B.  External TLS concentrator . . . . . . . . . . . . . . 22
   Appendix C.  CGI Server implementation . . . . . . . . . . . . . . 23
   Appendix D.  Updating SCEP implementations . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25











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1.  Introduction

   This document specifies a protocol for certificate Enrollment over
   Secure Transport (EST).  EST is a certificate enrollment protocol
   that operates over HTTPS, and thus should be trivially accessible by
   most clients.  Certificate Management over CMS (CMC) [RFC5272]
   "Simple PKI Request" and "Simple PKI Response" messages are
   leveraged.  EST is designed to be easily implemented by clients and
   servers running other common enrollment mechanisms such as the non-
   standard Simple Certificate Enrollment Protocol (SCEP).

   "CMC: Transport Protocols" [RFC5273] provides some guidance for
   running CMC over HTTP [RFC2616] but notes only that "clients MAY
   attempt to send HTTP requests using TLS 1.0 [TLS] or later, although
   servers are not required to support TLS".  No attempt is made in that
   document to specify how the client and server might take advantage of
   a secured transport to better leverage the Simple PKI messages.  This
   profile specifies secure transport mechanisms and how values from the
   TLS exchange, the HTTP exchange, and the Simple PKI messages layers
   are used for authentication (and authorization) purposes by the
   server.  For some simple operations, TLS client authentication is
   required.  When TLS client authentication cannot be leveraged as
   required by a particular operation, EST also provides a conduit for
   full CMC operations.  It is assumed that reader is familiar with the
   terms and concepts found in Certificate Management over CMS (CMC)
   [RFC5272], Certificate Request Message Format (CRMF) [RFC4211], etc.
   Unlike [RFC5273], EST uses both HTTPS GET and POST to support its
   functions.

   The aspects profiled from HTTPS (HTTP over TLS) and CMC are
   summarized in Figure 1:




















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   Profiled Layers:

   Protocol:
   +---------------------------------------------------+
   |                                                   |
   | 3) Message types                                  |
   |   CMC "Simple PKI" messages                       |
   |   Base64-encoded certificate chain               |
   |   Optionally "Full" CMC messages                  |
   |                                                   |
   +---------------------------------------------------+
   |                                                   |
   | 2) HTTP headers and URIs for control              |
   |   URIs used to specify the PKI operation          |
   |     (including renew/rekey).                      |
   |   Content-Type headers specify the message type.  |
   |   Headers profiled for control/error messages.    |
   |   Username/password methods supported for         |
   |     client proof-of-identity.                     |
   |                                                   |
   |                                                   |
   +-             ----(combination is known as HTTPS)--+
   |                                                   |
   |                                                   |
   | 1) TLS for transport security                     |
   |   Provides proof-of-identity for                  |
   |     EST Server authentication and                 |
   |     EST Client authentication.                    |
   |   "Channel binding" type techniques used to       |
   |     during Proof-of-Possession.                   |
   |                                                   |
   +---------------------------------------------------+
   |                                                   |
   | TCP/IP layer etc included in diagram for context  |
   |                                                   |
   +---------------------------------------------------+


                                 Figure 1

   Base64 [RFC4648] is used as specified in Section 4 of that RFC.

   The following provides a high level overview describing how these
   layers are used.  Each aspect is profiled in detail in the sections
   below.






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   1) TLS for transport security:

      CMC section 3.1 notes that "the Simple PKI Request MUST NOT be
      used if a proof-of-identity needs to be included".  This precludes
      use of these messages if inline authentication and/or
      authorization is required, unless a secured transport that
      provides proof-of-identity is also specified.  Many simple clients
      engaged in certificate enrollment operations will have a TLS
      client implementation available for secure transport, so use of
      TLS is specified herein.  This document specifies a method for
      authorizing client enrollment requests using existing
      certificates.  Such existing certificates may have been issued by
      the Certification Authority (CA) (from which the client is
      requesting a certificate) or they may have been issued under a
      distinct PKI (e.g., an IEEE 802.1AR IDevID [IDevID] credential).
      Additionally this document specifies username/password
      authentication methods beyond those included in CMC.
      Authentication and authorization mechanisms required for
      certificate issuance (and renew/rekey) are provided by HTTP and
      HTTPS mechanisms as described in this document.

      Proof-of-possession is a distinct issue from proof-of-identity and
      is addressed in Section 3.6.

      This document also defines transport for the full CMC
      specification compliant with CMC Transport Protocols.


   2) HTTP Headers and URIs for control:

      This profile supports two operations indicated by specific URIs:


      *  Distribution of CA certificates

      *  Authorized enrollment and re-enrollment of clients

      This document profiles the HTTP content-type header to indicate
      the message type and to provide the protocol control messages.
      Support for the HTTP username/password methods is profiled.

      CMC does not provide explicit messages for renewal and rekey.
      This profile clarifies the renewal and rekey behavior of both the
      client and server.  It does so by specifying the HTTP control
      mechanisms required of the client and server without requiring a
      distinct message type.

      Various media types as indicated in the HTTP content-type header



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      are used to transport EST messages.  For simple certificate
      enrollment and re-enrollment requests, application/pkcs10 (defined
      in [RFC5967]) is used as specified in Section 5.2.  Certificate
      responses to enrollment and re-enrollment requests are carried as
      application/pkix-cert (defined in [RFC2585]) as specified in
      Section 5.2.2.  FullCMC requests and responses are both
      transported as application/pkcs7-mime (as given in [RFC5273].
      Requests for CA certificates generate a response with the media
      type multipart/parallel.  Within each parallel part is an entity
      of media type application/pkix-cert.  See Section 5.1.


   3) Message Types:

      Some message types used here are defined in CMC and include
      subsets of the PKCS#10 Certification Request [RFC2986] and the
      PKCS#7 [RFC2315] message specifications.

      This document profiles the use of two Certificate Management over
      CMS messages: "Simple PKI Request" and "Simple PKI Response" and
      does not require full implementation of all CMC features.  This is
      consistent with the CMC protocol specification of "simple"
      messages for clients to use "in the event no other services are
      needed".  To support distribution of the CA certificate chain a
      simple Base 64 format is specified.  Full CMC messages MAY also be
      used.

      HTTP Content-Type headers are as specified in [RFC5273], Table 1.
      This document reuses media types for the simple format messages as
      specified by Internet X.509 Public Key Infrastructure Operational
      Protocols: FTP and HTTP [RFC2585] and The application/pkcs10 Media
      Type [RFC5967].  See the next section for details.


   An EST server providing certificate management functions is operated
   by (or on behalf of) a CA or RA.

   An EST server MAY provide additional, non-EST services on other URIs.
   The server also MAY support full CMC messages over HTTPS.

   [[EDNOTE: Comments such as this one, included within double brackets
   and initiated with an 'EDNOTE', are for editorial use and shall be
   removed as the document is polished.]]

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in [RFC2119].


2.  Requirements

   [[EDNOTE: The following section is still included here for
   succinctness.  It will eventually be published independently as
   draft-ietf-pkix-estr-00.]]

   The following describes goals and technical requirements for initial
   PKI certificate enrollment along with a rationale for each
   requirement.

   G1 "Completeness".  Server and client implementations compliant with
      this document MUST be able to interoperate without reference to
      subsequent profiles or additional future specifications.

   The goal of this enrollment protocol is to provide a simple and easy-
   to-implement method for end-entities to request, obtain, and update a
   certificate issued from a specified Certification Authority.  The
   following certificate management operations are required.  Additional
   operations NEED NOT be supported (via this protocol) although the
   protocol design SHOULD be extensible:

   M1 "Distribution of current CA certificates".  Clients MUST be able
      to obtain the current certificate for the CA under which the
      client's certificate was issued.  Certificates have a finite
      lifetime and will need to be updated on a periodic basis.  It must
      be possible for the client to obtain the updated CA certificates.

   M2 "Enrollment".  A client MUST be able to use the protocol to submit
      a certificate request and obtain a certificate.

   M3 "Renew/Rekey".  Certificates have a finite lifetime and will need
      to be updated on a periodic basis.  A client MUST be able to use
      the protocol for certificate renewal or rekey operations.

   End-Entity Proof of Identity authentication mechanisms:

   A1 "Username/Password".  It MUST be possible to identify a username
      specified client as being in possession of an associated symmetric
      key.  This allows users currently in possession of a username and
      password to obtain a certificate.

   A2 "Password".  It MUST be possible to identify a client without
      reference to a "username".  A common operational model is to
      distribute a "one-time password" that is presented to a CA or RA
      to authorize enrollment.



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   A3 "Existing Certificate".  It MUST be possible to authenticate a
      client by making use of an existing certificate associated with
      the client.  A certificate used for client identification need not
      be issued under the same PKI as the certificate that is being
      requested.  This allows clients that are already in a PKI to use a
      certificate from that PKI to obtain additional certificates.
      Additionally this capability allows a client who has a certificate
      issued by a 3rd party, such as a certificate issued by a device
      manufacturer, to leverage that credential during initial
      enrollment.

   A4 "Username/password and Certificate".  It MUST be possible to
      authenticate a client by using a combination of a username and
      password and an existing certificate.  This is a combination of A1
      and A3.  This supports "two-factor authentication" where the
      client proves possession of the private keys for an existing
      certificate stored within a hardware device and knowledge of a
      username/password.

   A5 "Password and certificate".  It MUST be possible to authenticate a
      client by using a combination of a secret value that is not
      associated with a "username" and an existing certificate.  This is
      a combination of A2 and A3.  This requirement is similar to A4
      except that the client is in possession of a "one-time password".

   End-Entity Proof of Possession:

   P1 Proof-of-Possession of subject keys MUST be supported.  As
      discussed in Appendix C of [RFC4211], Proof-of-Possession "means
      that the CA is adequately convinced that the entity requesting a
      certificate for the public key Y, has access to the corresponding
      private key X".

   Key algorithms:

   K1 "Algorithm agility".  The protocol MUST support algorithm agility.
      It must be possible to employ different cryptographic algorithms
      for securing the transport or for signing the certificates.  The
      protocol SHOULD demonstrate this agility by being shown to work
      with existing RSA-based solutions as well as providing for other
      algorithms such as Elliptic Curve cryptography.

   Server Identity mechanism:








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   I1 "RA certificate".  It MUST be possible for a client to verify
      authorization of the EST server as a representative of the CA.
      The protocol operations allow the client to send a username/
      password or (one-time) password to the EST server.  These values
      cannot be safely transmitted to an unauthorized server.


3.  Secure Transport

   HTTPS MUST be used.  TLS 'session resumption' SHOULD be supported.

   HTTPS is defined in HTTP Over TLS [RFC2818] and is a definition of
   how HTTP messages are carried over TLS.  HTTPS is a commonly used
   secure transport and can be easily layered on top of extremely simple
   client or server code.  In some environments HTTPS can be utilized
   through an external process.  Specifying HTTPS as the secured
   transport for PKI enrollment messages introduces two potential
   'layers' for communication of authorization data or for status/
   informative responses during the protocol exchange: TLS and HTTPS.
   This profile specifies when information is used from each layer.

3.1.  TLS-Based Server Authentication

   The client MUST validate the TLS server certificate presented during
   the TLS [RFC5246] exchange-defined Server Certificate message or the
   client MUST independently validate the response contents.  Validation
   is performed as given in [RFC5280] and [RFC6125].  The cipher suites
   are detailed in Section 6.

   There are multiple methods of validation depending on the current
   state of the client:

   1.  If the client has a store of trust anchors, which may be in the
       form of certificates, for validating TLS connections the client
       MAY validate the TLS server certificate using the standard HTTPS
       logic of checking the server's identity as presented in the
       server's Certificate message against the URI provisioned for the
       EST server (see HTTP Over TLS, Section 3.1 Server Identity and
       [RFC6125]).  This method makes it possible for clients with a
       store of trust anchors, possibly in the the form of certificates,
       to securely obtain the CA certificate by leveraging the HTTPS
       security model.  The EST server URI MUST be made available to the
       client in a secure fashion so that the client only obtains EST
       functions from a desired server.

   2.  If the client already has one or more trust anchors associated
       with this EST server, the client MUST validate the EST server
       certificate using these trust anchors.  The EST server URI MAY be



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       made available to the client in an insecure fashion.  The EST
       server certificate MUST contain the id-kp-cmcRA [CMC RFC5272bis]
       extended key usage extension.

   3.  If the client does not yet have a trust anchor associated with
       this EST server then the client MAY provisionally accept the TLS
       connection, but the HTTP content data MUST be accepted manually
       as described in Section 5.1.  HTTP authentication requests MUST
       NOT be responded to.

   Methods 1 and 2 are essentially validation as given in [RFC5280] with
   the addition of authorization.  Method 1 is as described in [RFC6125]
   Section 6.6.1 "Match Found".  Method 2 is described in [RFC6125] as
   "No Match Found, Pinned Certificate".  Method 3 is described in
   [RFC6125] as "Fallback" and describes the process of "pinning" the
   recieved certificate.

   If one of these validation methods succeeds the CA certificates are
   stored and made available for future use.  If none of these
   validation methods succeeds the client MUST reject the EST server
   response and SHOULD log and/or inform the end user.

   The EST server MUST present an end-entity certificate such as a CMC
   Registration Authority (RA) certificate.

3.2.  Server Authentication and Authorization

   The client MUST check the EST server authorization before accepting
   the server's response.

   If the client has a securely configured and authorized URI for the
   EST server it MUST check the URI "against the server's identity as
   presented in the server's Certificate message" (Section 3.1 Server
   Identity [RFC2818] and [RFC6125]).  The securely configured URI
   provides the authorization statement and the server's authenticated
   identity confirms it is the authorized server.

   If this check fails, or if the URI was configured using an insecure
   method, then the client MUST verify the server's authorization by
   checking that the [RFC5280] defined certificate policy extension
   sequence contains the 'RA Authorization' policy OID.

   The RA Authorization policy OID is defined as: id-cmc [[EDNOTE: TBD,
   perhaps 35]].  The RA Authorization policy information MUST NOT
   contain any optional qualifiers.






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3.3.  TLS-Based Client Authentication

   Clients MUST support client-side certificate authentication
   [RFC5246].  To authenticate the client, the server sends the
   certificate request message to the client, the client returns a
   client certificate and a certificate verify message to the server,
   and the server verifies the certificate verify message with the
   certificate in the client certificate message.  As required by
   [RFC5246], the client certificate needs to indicate support for
   digital signatures.

   The certificate presented by the client MAY be from the same PKI as
   the Server Certificate, from a completely different PKI, or as a last
   resort the client MAY respond with a self-signed certificate.  The
   certificate supplied during authentication is used during client
   authorization (Section 3.5).

3.4.  HTTP-Based Client Authentication

   As specified in CMC: Transport Protocols [RFC5273] the server "MUST
   NOT assume client support for any type of HTTP authentication such as
   cookies, Basic authentication, or Digest authentication".  Clients
   intended for deployments where password authentication is
   advantageous SHOULD support the Basic and Digest authentication
   mechanism.  Servers MAY provide configuration mechanisms for
   administrators to enable Basic [RFC2616] and Digest [RFC2617]
   authentication methods.  Basic and Digest authentication MUST be
   performed over TLS [RFC5246].

   Servers that support Basic and Digest authentication methods MAY
   reject requests using the HTTP defined WWW-Authenticate response-
   header (Section 14.47).  At that point the client SHOULD repeat the
   request, including the appropriate HTTP [RFC2617] Authorization
   Request Header (Section 3.2.2).

   Support for Basic authentication as specified in HTTP allows the
   server access to the client's cleartext password.  This provides
   integration with legacy username/password databases but requires
   exposing the plaintext password to the EST server.  The client MUST
   NOT respond to this request unless the client has authenticated the
   EST server (as per Section 3.2).

   Clients MAY set the username to the empty string ("") if they wish to
   present a "one-time password" or "PIN" that is not associated with a
   username.






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3.5.  Client Authorization

   When the EST server receives a CMC Simple PKI Request or rekey/renew
   message, the decision to issue a certificates is always a matter of
   local policy.  Thus the CA can use any data it wishes in making that
   determination.  The EST protocol exchange provides the EST server
   access to the TLS client certificate in addition to any HTTP user
   authentication credentials to help in that determination.  The
   communication channel between the TLS server implementation and the
   EST software implementation is out-of-scope of this document.

3.6.  Proof-of-Possession

   As discussed in Appendix C of CRMF [RFC4211], Proof-of-Possession
   "means that the CA is adequately convinced that the entity requesting
   a certificate for the public key Y, has access to the corresponding
   private key X".

   The signed enrollment request provides a "Signature"-based proof-of-
   posession.  The mechanism described in Section 3.7 strengthens this
   by optionally including identity linking information within the data
   covered by the enrollment request signature (thus ensuring that the
   enrollment request was signed after the TLS tunnel was established).

3.7.  Linking Identity and POP information

   This specification provides a method of linking identity and proof-
   of-possession by including information specific to the current
   authenticated TLS session within the signed certification request.
   This proves to the server that the authenticated TLS client has
   possession of the private key associated with the certification
   request and that the client was able to sign the certification
   request after the TLS session was established.  This is an
   alternative to the [RFC5272] Section 6.3-defined "Linking Identity
   and POP information" method available if fullCMC messages are used.

   The client generating the request SHOULD obtain the tls-unique value
   as defined in Channel Bindings for TLS [RFC5929] from the TLS
   subsystem, encode it using base64 encoding, and place the resulting
   string in the certification request challenge password field.

   The tls-unique specification includes a synchronization issue as
   described in Channel Bindings for TLS [RFC5929] section 3.1.  This
   problem is avoided for EST implementations.  The tls-unique value
   used MUST be from the first TLS handshake.  EST client and servers
   must use their tls-unique implementation specific synchronization
   methods to obtain this first tls-unique value.




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   Any TLS renegotiation MUST use "secure_renegotiation" [RFC5746] (thus
   maintaining the binding).  Mandating secure renegotiation secures
   this method of avoiding the synchronization issues encountered when
   using the most recent tls-unique value (which is defined as the the
   value from the most recent TLS handshake).

   The server SHOULD verify the tls-unique information.  This ensures
   that the authenticated TLS client is in possession of the private key
   used to sign the certification request.

   The tls-unique value is encoded into the certification request by the
   client but back-end infrastructure elements that process the request
   after the EST server might not have access to the initial TLS
   session.  Such infrastructure elements validate the source of the
   certification request to determine if POP checks have already been
   performed.  For example if the EST client authentication results in
   an authenticated client identity of an RA that is known to
   independently verify the proof-of-possession then the back-end
   infrastructure does not need to perform proof-of-possession checks a
   second time.  If the EST server forwards a request to a back-end
   process it SHOULD communicate the authentication results.  This
   communication might use the CMC "RA POP Witness Control" in a CMC
   Full PKI Request message or other mechanisms which are out-of-scope
   of this document.


4.  HTTP URIs

   EST uses the HTTPS "GET" and "POST" messages to communicate with the
   EST server.  The following describes the syntax of these messages:
   "GET" BASEPATH OPERATIONPATH
   "POST" BASEPATH OPERATIONPATH

   where:

   o  BASEPATH is a common path for all EST operations

   o  OPERATIONPATH specifies the specific operation.

   When a URI is formed, the BASEPATH and OPERATIONPATH are combined to
   form the abs_path [RFC2616].  The means by which clients acquire the
   BASEPATH URI are outside the scope of this document.  The following
   are two example base URIs:

   o  With BASEPATH having the value of /arbitrary/base/path

   o  https://example.org/arbitrary/base/path




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   o  https://example2.org:8080/arbitrary/base/path

   These can be conveniently distributed as they are in a form with
   which many end users are already familiar.  The following operation
   paths for clients to access are defined relative to the EST base URL:

   o  /CACerts - The server responds to an HTTPS GET with the CA
      certificates as defined in Distribution of CA certificates
      (Section 5.1).

   o  /simpleEnroll - The client sends a CMC Simple PKI Request message
      as specified in Enrollment of Clients (Section 5.2), the response
      is a CMC Simple PKI Response message as specified in Enroll
      Response (Section 5.2.2).

   o  /simpleReEnroll - Exactly the same as 'simpleEnroll' except that
      the request is for re-enrollment or re-issuance purposes.  Only
      certificates that are suitable for TLS client authentication can
      be re-enrolled using this operation because of the reliance on the
      TLS authentication.  For other types of certificates, use of the
      fullCMC operation is required.

   o  /fullCMC - Provides for Full CMC messages (OPTIONAL).

   The following examples are valid complete URIs under this
   specification:

   o  With BASEPATH having the value /base/path

   o  https://example.org/base/path/CACerts

   o  https://example2.org:8080/base/path/simpleEnroll

   o  https://example3.org/base/path/fullCMC

   The mechanisms by which the EST server interacts with an HTTPS server
   to handle GET and POST operations at these URIs is outside the scope
   of this document.  The use of distinct URIs simplifies implementation
   for servers that do not perform client authentication when
   distributing "CACerts" responses.

   Implementation note: A simple Common Gateway Interface (CGI)
   application at each fully specified path, with the HTTPS server
   configured to provide Section 3.3, is sufficient for a working
   example (the web service can forward the Section 3.6 proof-of-
   possession information to the application using the CGI interface).
   Additional dicussion regarding the use of CGI can be found in
   Appendix C.



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   [[EDNOTE: This does not use the mechanism specified in "Defining
   Well-Known Uniform Resource Identifiers (URIs)" [RFC5785].  That
   would be a possibility here for a base URL of
   "https://example.org/.well-known/EST" but such would preclude the
   flexibility associated with multiple base URLs being handled by the
   same server unless some form of "?designator=value" is included.]]


5.  Messages

5.1.  Distribution of CA certificates

   Before engaging in enrollment operations, clients MUST request trust
   anchor information (in the form of certificates) by sending an HTTPS
   GET message to the EST base URI with the relative path extension
   '/CACerts'.  Clients SHOULD request an up-to-date response before
   stored information has expired.

   The EST server SHOULD NOT require client authentication or
   authorization to reply to this request.

   The client MUST authenticate the EST server as specified in
   Authentication and Authorization (Section 3).  If the authentication
   and authorization is successful, the client accepts the response and
   stores it.  If the authentication and authorization is not
   successful, then when the response is received the client MUST
   extract the CA certificate and engage the end-user or otherwise
   authorize the credential using out-of-band pre-configuration data
   such as a CA certificate "fingerprint" (e.g., a SHA-1, SHA-256, SHA-
   512, or MD5 hash on the whole CA certificate).

   The client MUST NOT accept the CA certificate without validating it
   via one of the mechanisms described in Section 3.1.

   Subsequent connections to the EST server validate the TLS server
   certificate using the stored CA certificates as described in
   Authentication and Authorization (Section 3).

5.1.1.  Distribution of CA certificates response

   The EST server MUST respond to the client HTTPS GET message with CA
   trust anchor information in the form of a certificate.  Additionally
   the server MUST include any "Root CA Key Update" CMP certificates.

   The response format is the media type multipart/parallel.  Within
   each parallel part is an entity of media type application/pkix-cert.
   One part MUST be the the current self-signed CA certificate.
   Additional parts MAY be included.  If additional parts are included



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   they MUST be the three additional CMP-defined Root CA Key Update
   certificates: OldWithOld, OldWithNew, and NewWithOld.

   The client can always find the current self-signed CA certificate by
   examining the certificates received.  The NewWithNew certificate is
   self-signed and has the latest NotAfter date.

   The NewWithNew certificate is the certificate that is extracted and
   authorized using out-of-band information as described in Section 5.1.
   When out-of-band validation occurs each of the other three
   certificates MUST be validated using normal [RFC5280] certificate
   path validation (using the NewWithNew certificate as the trust
   anchor) before they can be used to build certificate paths during
   peer certificate validation.

5.2.  Simple Enrollment of Clients

   At any time the client MAY request a certificate from the EST base
   URI with the OPERATIONPATH "/simpleEnroll'.

   When HTTPS POSTing to the 'SimpleEnroll' location the client MUST
   include a CMC Simple PKI Request as specified in CMC Section 3.1
   (i.e., a PKCS#10 Certification Request).

   The content-type of "application/pkcs10" MUST be specified.  The
   format of the request is as specified in Section 6.4 of [RFC4945].

   The server MUST authenticate the client as specified in
   Authentication and Authorization (Section 3).  The server applies
   whatever authorization or policy logic it chooses determining if the
   certificate should be issued.  The client MAY request an additional
   certificate even when using an existing certificate in the TLS client
   authentication.

   The client MUST authenticate the EST server as specified in
   Section 3.1.

5.2.1.  Simple Re-Enrollment of Clients

   At any time a client MAY request renew/rekey of its certificate from
   the EST base URI with the OPERATIONPATH "/simpleReEnroll'.  The
   certificate request is the same format as for the "simpleEnroll" path
   extension with the same content-type.

   The EST server MUST handle enrollment requests submitted to the
   "simpleReEnroll" URI as renewal or rekey requests rather than
   depending only on the method of identifying a renewal or rekey
   request specified in Section 2 of [RFC5272], that "renewal and rekey



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   requests look the same as any certification request, except that the
   identity proof is supplied by existing certificates from a trusted
   CA".  The proof of client identity is supplied by client
   authentication during the HTTPS handshake.  When attempting to renew
   or rekey the client MUST use its existing certificate for TLS client
   authentication.

   [[EDNOTE: draft-turner-suiteb-cmc defines a method of recognizing a
   re-enroll based on PKCS10 contents, see section 4.1.  The method
   described herein is explicit.]]

5.2.2.  Simple Enroll and Re-Enroll Response

   The server responds to a 'simpleEnroll' or 'simpleReEnroll' request
   with the client's newly issued certificate or it provides an error
   response.

   If the enrollment is successful the server response MUST have a
   response code of 200 with a content-type of "application/pkix-cert".
   The response data is the certificate formatted as specified in
   Section 6.1 of [RFC4945].

   When rejecting a request the server MUST specify either an HTTP 4xx/
   401 error, or an HTTP 5xx error.  A CMC Simple PKI Response with an
   HTTP content-type of "application/pkcs7-mime" MAY be included in the
   response data for any error response.  If the content-type is not set
   the response data MUST be a plain text human-readable error message.
   A client MAY elect not to parse a CMC error response in favor of a
   generic error message.

   If the server responds with an HTTP 202 this indicates that the
   request has been accepted for processing but that a response is not
   yet available.  The server MUST include a Retry-After header as
   defined for 503 responses and MAY include informative human-readable
   content.  The client MUST wait at least the specified 'retry-after'
   time before repeating the same request.  The client repeats the
   initial enrollment request after the appropriate 'retry-after'
   interval has expired.  The client SHOULD log or inform the end user
   of this event.  The server is responsible for maintaining all state
   necessary to recognize and handle retry operations as the client is
   stateless in this regard (it simply sends the same request repeatedly
   until it receives a different response code).

   All other return codes are handled as specified in HTTP.







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5.3.  Full CMC

   At any time the client MAY request a certificate from the EST base
   URI with the OPERATIONPATH "/fullCMC".

   The client MUST authenticate the server as specified in Server
   Authentication (Section 3.1).

   The server SHOULD authenticate the client as specified in
   Authentication and Authorization (Section 3).  The server MAY depend
   on CMC client authentication methods instead.

5.3.1.  Full CMC Request

   When HTTPS POSTing to the "fullCMC" location the client MUST include
   a valid CMC message.  The content-type MUST be set to "application/
   pkcs7-mime" as specified in [RFC5273].

5.3.2.  Full CMC Response

   The server responds with the client's newly issued certificate or
   provides an error response.

   If the enrollment is successful the server response MUST have a
   response code of 200 with a content-type of "application/pkcs7-mime"
   as specified in [RFC5273].  The response data includes either the CMC
   Simple PKI Response or the CMC Full PKI Response.

   When rejecting a request the server MAY specify either an HTTP 4xx/
   401 error, an HTTP 5xx error, or a response code 200.  A CMC response
   with content-type of "application/pkcs7-mime" MUST be included in the
   response data for any error response.  The client MUST parse the CMC
   response to determine the current status.

   All other return codes are handled as specified in Section 5.2.2 or
   HTTP [RFC2616].


6.  Cryptographic Algorithms

   This section details the specific cryptographic algorithms and cipher
   suite requirements.

   The client SHOULD offer the Suite B compliant cipher suites as
   indicated in [RFC5430], Section 4 "Suite B Compliance and
   Interoperability Requirements".  For greatest interoperability the
   client SHOULD also offer TLS_RSA_WITH_AES_128_CBC_SHA.




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   When the client accesses the "simpleReEnroll" method the TLS cipher
   suite in use MUST be appropriate for the existing certificate.  The
   certificate type used determines the appropriate signatureAlgorithm
   for the PKCS#10 Certification Request.  For example if a [RFC5430]
   cipher suite is used the signatureAlgorithm MAY be ecdsa-with-sha256
   for P-256 certification requests, or MAY be ecdsa-with-sha384 for
   P-384 certification requests.

   [[EDNOTE: This is in alignment with draft-turner-suitb-cmc-03 section
   4.1.  To encourage algorithm agility, discussions of the MUST/SHOULD
   algorithms should be in a distinct document.]]


7.  Contributors/Acknowledgements

   The editors would like to thank Stephen Kent, Vinod Arjun, Jan
   Vilhuber, Sean Turner, and others for their feedback and prototypes
   of early drafts.


8.  IANA Considerations

   (This section is incomplete)

   The following aspects should be registered with IANA Considerations:

   The RA Authorization certificate policy extension OID as discussed in
   Section 3.2 requires registration with IANA.

   [[EDNOTE: The URLs specified in Section 1 probably do not need to be
   registered with IANA.]]


9.  Security Considerations

   (This section is incomplete)

   "Badges?  We ain't got no badges.  We don't need no badges!  I don't
   have to show you any stinkin' badges!" -- The Treasure of the Sierra
   Madre.

   As described in CMC Section 6.7, "For keys that can be used as
   signature keys, signing the certification request with the private
   key serves as a POP on that key pair".  The inclusion of tls-unique
   within the certification request provides timeliness to the proof-of-
   possession.  For support of keys that can not be used for signing the
   certification request the full CMC specification MUST be used.




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   As described in Section 3.3 clients use an existing certificate for
   TLS client authentication.  If a certificate with appropriate key
   usage is not available the client MAY generate one.  If a self-signed
   certificate with appropriate key usage is used the server SHOULD
   require HTTP-based client authentication according to server policy
   as described in Section 3.3 and Section 3.5.  The server MAY fall
   back on manual authorization by the server administrator.

   As described in Section 3.1 servers use an existing certificate for
   TLS server authentication.  When the server certificate is issued by
   a mutually trusted PKI hierarchy validation proceeds as specified in
   Section 3.2.  In this situation the client has validated the server
   as being a valid responder for the URI configured but can not
   directly verify that the responder is authorized as an RA within the
   to-be-enrolled PKI hierarchy.  A client may thus be enticed to expose
   username/password or certificate enrollment requests to an
   unauthorized server (if the server presents a valid HTTPS certificate
   for an erroneous URL that the client has been tricked into using).
   Proof-of-Identity and Proof-of-Possession checks by the CA prevent an
   illegitimate RA from leveraging such misconfigured clients to act as
   a man-in-the-middle during client authenticated operations but it is
   possible for such illegitimate RAs to send the client doctored
   messages or erroneous CA certificate lists.  If the illegitimate RA
   has successfully phished a username/password or PIN from the client
   it might try to use these values to enroll its own keypair with the
   real PKI hierarchy.  EST servers identified with an externally issued
   server certificate SHOULD require HTTPS-based client authentication
   (Section 3.3).  Similarly EST clients SHOULD use an existing client
   certificate to identify themselves and otherwise prevent "private
   data" (obviously including passwords but also including private
   identity information) from being exposed during the enrollment
   exchange a weak server authorization method is used.


10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, March 1998.

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, May 1999.




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   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              November 2000.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210, September 2005.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC4945]  Korver, B., "The Internet IP Security PKI Profile of
              IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, June 2008.

   [RFC5273]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC): Transport Protocols", RFC 5273, June 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5430]  Salter, M., Rescorla, E., and R. Housley, "Suite B Profile
              for Transport Layer Security (TLS)", RFC 5430, March 2009.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, February 2010.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.



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   [RFC5967]  Turner, S., "The application/pkcs10 Media Type", RFC 5967,
              August 2010.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

10.2.  Informative References

   [IDevID]   IEEE Std, "IEEE 802.1AR Secure Device Identifier",
              December 2009, <http://standards.ieee.org/findstds/
              standard/802.1AR-2009.html>.

   [RFC4211]  Schaad, J., "Internet X.509 Public Key Infrastructure
              Certificate Request Message Format (CRMF)", RFC 4211,
              September 2005.


Appendix A.  Server Discovery

   (informative)

   (This section is incomplete)

   Clients MAY use DNS-SD or similar discovery algorithms to determine
   the EST base URL.  In such cases it is expected that method 2
   (Section 3.1) be used during server authentication.


Appendix B.  External TLS concentrator

   (informative)

   In some deployments it may be beneficial to use a TLS concentrator to
   offload the TLS processing from the server.  In such a deployment the
   TLS client authentication result must, in some way, be forwarded to
   the server.

   The TLS server SHOULD NOT reject the connection based on PKIX
   validation of the client certificate.  The client certificate SHOULD
   be passed to the EST layer for verification and authorization.  This
   allows support of external TLS concentrators, or an external web
   server, that might provide an independent TLS implementation.

   The TLS concentrator MUST validate the TLS Section 7.4.8 'Certificate
   Verify'.



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   A TLS concentrator MUST insert the client certificate into the HTTP
   header.  The TLS concentrator MUST first remove any existing client
   certificates, possibly inserted by a nefarious client, from the HTTP
   headers before forwarding the HTTP connection to the server.

   [TBD - need to better understand what would happen in the case of
   proxy's or multiple concentrators.  Or specifically state that as out
   of scope.]

   [TBD - the HTTP header field names etc shall be specified here]

   The EST server MUST be specifically configured by the administrator
   to accept this mechanism.


Appendix C.  CGI Server implementation

   (informative)

   In some deployments it may be beneficial to use a HTTPS server that
   runs the EST server as a CGI application.  In such a deployment the
   HTTPS server client authentication result must, in some way, be
   forwarded to the server.

   An HTTPS server MUST insert the client certificate into environment
   variables before calling a server CGI application.

   [TBD - describe the CGI environment variables here.  Can likely
   follow the apache example].

   An HTTP server MUST insert the client certificate into environment
   variables before calling a server CGI application.

   [TBD - describe the CGI environment variables here.  Can likely
   follow the apache example].


Appendix D.  Updating SCEP implementations

   (informative)

   SCEP has been used instead of a full implementation of CMC for the
   same simplicity reasons discussed in Section 1.  Such implementations
   would benefit from being updated to this specification in the
   following ways:

   o  Implementing a subset of CMC provides an enhancement path if the
      full CMC functionality is required.



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   o  The use of HTTPS as a transport is often perceived as more secure.
      Although the SCEP protocol specification includes mechanisms (and
      complexity) to address security issues avoiding a vendor
      requirement to educate systems administrators is beneficial.
      Implementors can benefit from the wide availability of existing
      HTTPS/TLS libraries.

   o  SCEP servers can use their CA certificate to protect SCEP traffic
      in ways that are not appropriate.  (See SCEP draft Section 8.2).
      This specification precludes those misuses.

   o  The SCEP draft Appendix D renew and rekey functionalities imply a
      'flag moment' where the PKI infrastructure transitions from an
      (expired) CA certificate to a new CA certificate.  This
      specification specifies the better mechanism defined in CMP.

   Updating an SCEP client implementation to support this protocol
   involves the following changes to the SCEP implementation.  There is
   no server-side indication that SCEP clients should be so modified so
   this depends on a client-side configuration:

   o  The SCEP client supports HTTPS server authentication and
      authorization as detailed Section 3.1.

   o  The SCEP client supports HTTPS client authentication as detailed
      in Section 3.3.

   o  When performing the "Get CA Cert" SCEP transaction the client
      supports the Section 5.1 described CMC Simple PKI Response (ref
      CMC 4.1, which is extremely similar to the SCEP "CA/RA Certificate
      Response Message Format" if not exactly the same).

   o  When performing the certificate enrollment via SCEP PKCSReq the
      outgoing message is simplified to be only the inner PKCS10 (ref
      CMC section 3.2.1.2.1).

   o  When handling the certificate enrollment response the response
      format is simplified to be only the SCEP inner 'messageData'
      containing the actual certificates in the degenerate PKCS7 form.
      (ref CMC 4.1) The only 'authenticatedAttributes' value of
      remaining importance is the 'pkiStatus' and this value is now
      found in the HTTP header as defined in Section 5.2.2.

   o  Polling is simplified with clients repeatedly establishing the
      full HTTPS connection; no polling specific state information is
      encoded into the EST messages.





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   o  GetCert is deprecated.

   o  GetCRL is deprecated.

   These simplifications to an existing SCEP implementation result in an
   SCEP client that is compliant with CMC when using the EST transport.

   Implementation note: The use of tls-unique-securerenegotiation
   precludes the use of SCEP 'challenge-password' within the pkcs10 for
   password/PIN assertion.  Instead these values must be asserted with
   the Section 3.4 described mechanism.  A side effect of this is that a
   client communicating with an EST server can not embed an SCEP
   'challenge-password' within the PKCS#10.  An EST service running as
   an RA thus can not forward the PKCS#10 using SCEP to an SCEP server
   that expects the 'challenge-password' to be populated.


Authors' Addresses

   Max Pritikin (editor)
   Cisco Systems, Inc.
   510 McCarthy Drive
   Milpitas, CA  95035
   USA

   Email: pritikin@cisco.com


   Peter E. Yee (editor)
   AKAYLA, Inc.
   7150 Moorland Drive
   Clarksville, MD  21029
   USA

   Email: peter@akayla.com
















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