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INTERNET DRAFT                                           Xiaoyi Liu
draft-nourse-scep-12.txt                                 Cheryl Madson
expires 22 Feb 2006                                      David  McGrew
(revised 22 Aug 2005)                                    Andrew Nourse
                                                         Cisco Systems

Category: Informational                                  22-August-2005


Cisco Systems' Simple Certificate Enrollment Protocol(SCEP):

Status of this Memo

This document is an Internet-Draft and is NOT offered in accordance
with Section 10 of RFC2026, and the author does not provide the IETF
with any rights other than to publish as an Internet-Draft

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This memo provides information for the Internet community.  This memo
does not specify an Internet standard of any kind.  Distribution of
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By submitting this Internet-Draft, each author represents
that any applicable patent or other IPR claims of which he
or she is aware have been or will be disclosed, and any of
which he or she becomes aware will be disclosed, in
accordance with Section 6 of BCP 79.

Abstract

This document specifies the Simple Certificate Enrollment Protocol,
a PKI communication protocol which leverages existing technology by
using PKCS#7 and PKCS#10.  SCEP is the evolution of the enrollment
protocol developed by Verisign, Inc. for Cisco Systems, Inc.
It now enjoys wide support in both client and CA implementations.


Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . .   2
   2.   The Goal of SCEP . . . . . . . . . . . . . . . . . . . . .   3
   2.1  SCEP Entity types  . . . . . . . . . . . . . . . . . . . .   3
   2.2  SCEP Operations Overview . . . . . . . . . . . . . . . . .   7
   2.3  PKI Operation Transactional Behavior . . . . . . . . . . .  10
   2.4  Security . . . . . . . . . . . . . . . . . . . . . . . . .  12
   3.   Transport Protocol . . . . . . . . . . . . . . . . . . . .  13
   4.   Secure Transportation: PKCS #7 . . . . . . . . . . . . . .  14
   4.1  SCEP Message Format  . . . . . . . . . . . . . . . . . . .  14

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   4.2  Signed Transaction Attributes  . . . . . . . . . . . . . .  15
   5.   SCEP Transaction Specification . . . . . . . . . . . . . .  16
   5.1  Certificate Enrollment . . . . . . . . . . . . . . . . . .  16
   5.2  Poll for Requester Initial Certificate . . . . . . . . . .  22
   5.3  Certificate Access . . . . . . . . . . . . . . . . . . . .  26
   5.4  CRL Access   . . . . . . . . . . . . . . . . . . . . . . .  27
   5.5  Get Certificate Authority Certificate  . . . . . . . . . .  31
   5.6  Get Certificate Authority Certificate Chain  . . . . . . .  33
   6.   Security Considerations  . . . . . . . . . . . . . . . . .  33
   7.   Intellectual Propoerty . . . . . . . . . . . . . . . . . .  33
   8.   References . . . . . . . . . . . . . . . . . . . . . . . .  33
   Appendix A. Cisco Requester Subject Name Definition . . . . . .  34
   Appendix B. IPSEC Client Enrollment Certificate Request . . . .  35
   Appendix C. Private OID Definitions   . . . . . . . . . . . . .  36
   Appendix D. Obtaining CRL by LDAP Query . . . . . . . . . . . .  36
   Appendix E. SCEP State Transitions  . . . . . . . . . . . . . .  37
   Appendix F. CA Capabilities . . . . . . . . . . . . . . . . . .  40
   Appendix G. Certificate Renewal and CA Key Rollover . . . . . .  41
   Appendix H. PKIOperation via HTTP POST Message. . . . . . . . .  42
   Appendix Y. Author Contact Information. . . . . . . . . . . . .  43
   Appendix Z. Copyright Section . . . . . . . . . . . . . . . . .  43

Section 1. Introduction

Public key technology is becoming more widely deployed and is becoming
the basis for standards based security, such as the Internet Engineering
Task Force's IPSEC and IKE protocols.  With the use of public key
certificates in network security protocols comes the need for a
certificate management protocol that Public Key Infrastructure (PKI)
clients and Certificate Authority servers can use to support certificate
life cycle operations such as certificate enrollment and revocation, and
certificate and CRL access.

In the following, Section 2 gives an overview of the PKI operations,
and Section 2.4 describes the security goals of the protocol and the
mechanisms used to achieve them.  The transport protocol and the
security protocol PKCS#7 are described at Section 3 and Section 4,
respectively.  The last section, Section 5, specifies each PKI
operation in terms of the message formats and the data structures of
each operation.

The appendices provide detailed specifications and examples. Requester
subject names are specified in Appendix A, attribute OIDs are
specified in Appendix C , and the SCEP state transitions are described
in Appendix E.  An example of a certificate enrollment request is
provided in Appendix B, and an example LDAP query URL encoding is
provided in Appendix D.

The authors would like to thank Peter William of ValiCert, Inc.
(formerly of Verisign, Inc) and Alex Deacon of Verisign, Inc. and
Christopher Welles of IRE, Inc. for their contributions to this protocol
and to this document.

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2.0 The Goal of SCEP
The goal of SCEP is to support the secure issuance of certificates to
network devices in a scalable manner, using existing technology whenever
possible.  The protocol supports the following operations:

  CA and RA public key distribution
  Certificate enrollment
  Certificate revocation
  Certificate query
  CRL query

Certificate and CRL access can be achieved by using the LDAP protocol
(as specified in Appendix D), or by using the query messages defined in
SCEP.  The use of HTTP certificate and CRL access, and the support of
CDP as specified in RFC2459, will be specified in a future version of
this document.  In Section 2.1, we first define PKI entity types as well
as the properties of each entity type. In Section 2.2, the PKI
operations are described at functional level. Section 2.3 describes the
transaction behavior of each PKI operations.  The complete PKI messages
are covered in Section 5.

2.1 SCEP Entity types

The entity types defined in SCEP are the "requester" type (i.e., IPSEC
clients), the Certificate Authority (CA) entity type, and the
Registration Authority entity type (RA).  A requester is sometimes
called a "SCEP client" in the following.

2.1.1 Requesters

A requester is an entity whose name is defined in a certificate
subject name field and optionally, in SubjectAltName, a X.509
certificate V3 extension.  As a requester, a SCEP client is identified
by a subject name consisting of the following naming attributes:

  Fully qualified domain name, for example, router.cisco.com
 IP address,  Serial number, and/or x.500 distinguished name

The fully qualified domain name is required for a requester that intends
to use the certificate for ISAKMP.  The IP address, serial number, and
x.500 distinguished name are optional name attributes.  In the
certificate enrollment request, the PKCS#10 subject field contains the
required and optional name attributes.  The distinguished name, if any,
should be the subject name field, while any domain name, serial number,
or IP address supplied should be in the subjectAltName field. The
subject name field may be empty (if there is no distinguished name)
or the subjectAltName may be omitted, but not both.

It is important to note that a client named as Alice.cisco.com is
different than a client named as Alice.cisco.com plus the IP address
name attribute 117.96.1.219. From CA point of view, the Distinguished
names assigned in these two cases are distinct names.


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Entity names which are specified as in the IPSEC profile (i.e., FQDN, IP
address and User FQDN) must be presented in certificate's SubjectAltName
extension. Multiple IPSEC entity names, (if any) are encoded as multiple
values of a single SubjectAltName extension.  The CA has the authority
to assign a distinguished name to a requester, whether or not one was
included in the request. The assigned DN should contain the SCEP client
names as the relative DN.

The attribute identifiers and an example of SCEP client subject name are
specified in Appendix A. Appendix B has an example from Cisco VPN Client
enrollment request.

2.1.1.1  Local Key/Certificate/CRL Storage and Certificate-name uniqueness

A requester is required to generate asymmetric key pairs and to provide
storage to store its private keys.  If the requester does not have enough
permanent memory to save its certificate, then it should be able to query
its own certificate from the CA or an LDAP server, once the certificate
has been issued.  The public key pairs can be generated with a specific
key usage. The key usage is conveyed to the CA through the certificate
enrollment request.  All current SCEP client implementations expect that
there will be only one pair of keys for a given subject name
and key usage combination and CA, at any time.  This property is called
the certificate-name uniqueness property, and it implies that a CA that
implements SCEP will enforce the unique mapping between a SCEP client
subject name and its key pairs with a given key usage. At any time, if
the subject name is changed, or if the key is updated, the existing
certificate would have to be revoked before a new one could be issued.

It is desirable that the CA enforce certificate-name uniqueness, but
it is not mandatory.  However a CA that does not enforce uniqueness
must provide some other mechanism to prevent the re-transmission of an
enrollment request by a SCEP client from creating a second certificate
or certificate request, nor can the second request merely be rejected.
If a client times out from polling for a pending request it can
resynchronize by reissuing the original request with the original
subject name, key, and transaction ID.  This should return the status of
the original transaction, including the certificate if it was granted.
It should not create a new transaction unless the original cert has been
revoked, or the transaction arrives more than halfway through the
validity time of the original certificate.

An enrollment request that occurs more than halfway through the validity
time of an existing certificate for the same subject name and key usage
MAY be interpreted as a re-enrollment or renewal request and accepted.
A new certificate with new validity dates may be issued, even though
the old one is still valid, if the CA policy permits, as described in
2.1.1.3.  See also appendix G.

2.1.1.2 Requester authentication

As with every protocol that uses public-key cryptography, the
association between the public keys used in the protocol and the
identities with which they are associated must be authenticated in a
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cryptographically secure manner.  This requirement is needed to
prevent a "man in the middle" attack, in which an adversary that can
manipulate the data as it travels between the protocol participants
can subvert the security of the protocol.  To satisfy this
requirement, SCEP provides two authentication methods: manual
authentication, and authentication based on pre-shared secret.  In the
manual mode, the requester is required to wait until its identity can
be verified by the CA operator using any reliable out-of-band
method. To prevent a "man-in-the-middle" attack, a SHA-1, SHA-256,
SHA-512, or MD5 `fingerprint' generated on the PKCS#10 (before PKCS #7
enveloping and signing) must be compared out-of-band between the
server and the requester.  SCEP clients and CAs (or RAs, if
appropriate) must display this fingerprint to the operator to enable
this verification if manual mode is used.  Failing to provide this
information leaves the protocol vulnerable to attack by sophisticated
adversaries.  When utilizing a pre-shared secret scheme, the server
should distribute a shared secret to the requester which can uniquely
associate the enrollment request with the given end entity. The
distribution of the secret must be private: only the end entity should
know this secret. The actual binding mechanism between the requester
and the secret is subject to the server policy and implementation.
When creating the enrollment request, the requester is asked to
provide a challenge password. When using the pre-shared secret scheme,
the requester must enter the re-distributed secret as the password. In
the manual authentication case, the challenge password only used to
authenticate a request for the certificate's revokation.  This
challenge password is included as a PKCS#10 attribute, and is sent to
the server as encrypted data.  The PKCS#7 envelope protects the
privacy of the challenge password with DES encryption.

2.1.1.3  Requester Uses Existing CA-Issued or Self-Signed Certificates

In this protocol, the communication between the requester and the
certificate authority is secured by using PKCS#7 as the messaging
protocol. PKCS#7, however, is a protocol which assumes the
communicating entities already possess the peer's certificates and
requires both parties use the issuer names and issuer assigned
certificate serial numbers to identify the certificate in order to
verify the signature and decrypt the message.  If the requesting
system already has a certificate issued by the CA, that certificate
may be presented as credentials for the renewal of that certificate if
the CA supports the "Renewal" capability and the CA policy permits the
certificate to be renewed. If the requester has no certificate issued
by the CA, or if the CA does not support and permit renewal, the
requestor must generate a self-signed certificate with the requester
subject name (the same name later used in the PKCS#10) as both issuer
and subject name.  During the certificate enrollment, the requester
will first post itself as the signing authority by attaching the
self-signed certificate to the signed certificate request.  When the
Certificate Authority makes the envelope on the issued certificate
using the public key included in the self-signed certificate, it
should use the same issuer name and serial number as conveyed in the
self-signed certificate to inform the end entity on which private key
should be used to open the envelope.
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Note that when a client enrolls for separate encryption and signature
certificates, it may use the signature certificate to sign both
requests, and then expect its signature key to be used to encrypt
both responses.  In any case, the recipientinfo on the envelope should
reflect the key used to encrypt the request.

2.1.1.4 Trusted CA Store

To support interoperability between IPSEC peers whose certificates are
issued by different CA, SCEP allows the users to configure multiple
trusted certificates.  Trusted certificates are have been configured as
such in the client, based on some out-of-band means such as a "fingerprint".
These trusted certificates are used to verify certificate chains that end
in those certificates.

2.1.2 Certificate Authority

A Certificate Authority(CA) is an entity whose name is defined in the
certificate issuer name field. Before any PKI operations can begin,
the CA generates its own public key pair and creates a self-signed CA
certificate, or causes another CA to issue a certificate to it.
Associated with the CA certificate is a fingerprint which will be used
by the requester to authenticate the received CA certificate if it is
self-signed.  The fingerprint is created by calculating a SHA-1,
SHA-256, SHA-512, or MD5 hash on the whole CA certificate.  Before any
requester can start its enrollment, this CA certificate has to be
configured at the entity side securely.  For IPSEC clients, the client
certificates must have SubjectAltName extension.  To utilize LDAP as a
CRL query protocol, the certificates must have a CRL Distribution
Point. Key usage is optional. Without key usage, the public key is
assumed as a general purpose public key and it can be used for all the
purposes.

A Certificate Authority may enforce certain name policy. When using
X.500 directory name as the subject name, all the name attributes
specified in the PKCS#10 request should be included as Relative DN. All
the name attributes as defined in RFC2459 should be specified in the
SubjectAltName.  An example is provided in Appendix A.

 If there is no LDAP query protocol support, the Certificate Authority
should answer certificate and CRL queries, and to this end it should be
online all the time.

The updating of the CA's public key is addressed in Appendix G.

2.1.3 Registration Authorities

In an environment where an RA is present, a requester performs
enrollment through the RA. In order to setup a secure channel with an RA
using PKCS#7, the RA certificate(s) have to be obtained by the client
in addition to the CA certificate(s).

In the following, the CA and RA are specified as one entity in the
context of PKI operation definitions.
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2.2 SCEP Operations Overview

In this section, we give a high level overview of the PKI operations as
defined in SCEP.

2.2.1 Requester Initialization

The requester initialization includes the key pair generation and the
configuring of the required information to communicate with the
certificate authority.

2.2.1.1  Key Pairs

Before a requester can start PKI transaction, it must have at least one
asymmetric key pair, using the selected algorithm (the RSA algorithm is
required in SCEP, and is the only algorithm in current implementations).

Key pairs may be intended for particular purposes, such as encryption only,
or signing only.  The usage of any associated certificate can be restricted
by adding key usage and extended key usage attributes to the PKCS#10.

2.2.1.2 Required Information

A requester is required to have the following information configured
before starting any PKI operations:

1. the certificate authority IP address or fully-qualified domain name,
2. the certificate authority HTTP CGI script path, and
   the HTTP proxy information in case there is no direct Internet
   connection to the server,
3. If CRLs are being published by the CA to an LDAP directory server,
   and there is a CRL Distribution Point containing only an X.500 directory
   name, then the client will need to know the LDAP server fully-qualified
   domain name or IP address.  CRL Distribution Points are discussed in
   more detail in RFC 2459.


2.2.2  CA/RA Certificate Distribution

Before any PKI operation can be started, the requester needs to get
the CA/RA certificates. At this time, since no public key has been

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exchanged between the requester and the CA/RA, the message to get the
CA/RA certificate can not be secured using PKCS#7 protocol. Instead, the
CA/RA certificate distribution is implemented as a clear HTTP Get
operation. After the requester gets the CA certificate, it has to
authenticate the CA certificate by comparing the finger print with the
CA/RA operator. Since the RA certificates are signed by the CA, there is
no need to authenticate the RA certificates.

This operation is defined as a transaction consisting of one HTTP Get
message and one HTTP Response message:

         REQUESTER                           CA SERVER
  Get CA/RA Cert: HTTP Get message
     ----------------------------->
                              CA/RA Cert download: HTTP Response message
                                <---------------------------------------
     Compute finger print and
     call CA operator.
                                    Receive call and check finger print

If an RA is in use, a degenerated PKCS#7 with a certificate chain
consisting of both RA and CA certificates is sent back to the end
entity. Otherwise the CA certificate is directly sent back as the
HTTP response payload.


2.2.3 Certificate Enrollment

A requester starts an enrollment transaction by creating a certificate
request using PKCS#10 and sends it to the CA/RA enveloped using the
PKCS#7. After the CA/RA receives the request, it will either
automatically approve the request and send the certificate back, or it
will require the requester to wait until the operator can manually
authenticate the identity of the requester.  Two attributes are
included in the PKCS#10 certificate request - a Challenge Password
attribute and an optional ExtensionReq attribute which will be a
sequence of extensions the requester would like to be included in its
V3 certificate extensions.  The Challenge Password may be used to
authenticate either the enrollment request itself, or a verbal
revocation request for the issued certificate in the event of key
compromise or other reason.

In the automatic mode, the transaction consists of one PKCSReq PKI
Message, and one CertRep PKI message. In the manual mode, the requester
enters into polling mode by periodically sending a GetCertInitial PKI
message to the server, until the server operator completes the manual
authentication, after which the CA will respond to GetCertInitial by
returning the issued certificate.  A  CA MAY run in automatic mode for
preapproved requests, and manual mode for the rest.  A request with a
non-null password is not necessarily a pre-approved request. It is up
to the CA server to decide.   Polling mode is entered whenever the
server returns a PENDING response.

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The transaction in automatic mode:

         REQUESTER                           CA SERVER

PKCSReq: PKI cert. enrollment msg
     --------------------------------> CertRep: pkiStatus = SUCCESS
                                       certificate attached
                                       <------------------------------
     Receive issued certificate.

The transaction in manual mode:

         REQUESTER                           CA SERVER
     PKCSReq: PKI cert. enrollment msg
     --------------------------------> CertRep: pkiStatus = PENDING
                                       <------------------------------
     GetCertInitial: polling msg
     --------------------------------> CertRep: pkiStatus = PENDING
                                       <------------------------------
     ................. <manual identity authentication................

     GetCertInitial: polling msg
     --------------------------------> CertRep: pkiStatus = SUCCESS
                                       certificate attached
                                       <------------------------------
     Receive issued certificate.

2.2.4 Requester Certificate Revocation

A requester should be able to revoke its own certificate. Currently
the revocation is implemented as a manual process. In order to revoke a
certificate, the requester makes a phone call to the CA server
operator. The operator will come back asking the ChallengePassword
(which has been sent to the server as an attribute of the PKCS#10
certificate request). If the ChallengePassword matches, the certificate
is revoked. The reason of the revocation is documented by CA/RA.

2.2.5 Certificate  Access

There are two methods to query certificates. The first method is to use
LDAP as a query protocol. Using LDAP to query assumes the client
understand the LDAP scheme supported by the CA. The SCEP client assumes
that the subject DN name in the certificate is used as the URL to query the
certificate. The standard attributes (userCertificate and caCertificate)
are used as filter.

For the environment where LDAP is not available, a certificate query
message is defined to retrieve the certificates from the CA.

To query a certificate from the certificate authority, a requester
sends a request consisting of the certificate's issuer name and the
serial number. This assumes that the requester has saved the issuer

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name and the serial number of the issued certificate from the previous
enrollment transaction.  The transaction to query a certificate consists
of one GetCert PKI message and one CertRep PKI message:

         REQUESTER                           CA SERVER
     GetCert: PKI cert query msg
     -------------------------------> CertRep:  pkiStatus = SUCCESS
                                      certificate
attached
                                      <-----------------------------
     Receive the certificate.

2.2.6 CRL Distribution

The CA/RA will not "push" the CRL to the end entities. The query of the
CRL can only be initialized by the requester.

There are three methods to query CRL.

The CRL may be retrieved by a simple HTTP GET.  If the CA supports this
method, it should encode the URL into a CRL Distribution Point extension
in the certificates it issues.  Support for this method should be
incorporated in new and updated clients, but may not be in older
versions.

The second method is to query CRL using LDAP. This assumes the CA server
supports CRL LDAP publishing and issues the CRL Distribution Point in
the certificate.  The CRL Distribution Point is encoded as a DN. Please
refer to Appendix D for the examples of CRL Distribution Point.

The third method is implemented for the CA which does not support LDAP
CRL publishing or does not implement the CRL Distribution Point. In this
case, a CRL query is composed by creating a message consists of the CA
issuer name and the CA's certificate serial number.  This method is
deprecated because it does not scale well and requires the CA to be a
high-availability service.

The message is sent to the CA in the same way as the other SCEP
requests: The transaction to query CRL consists of one GetCRL PKI
message and one CertRep PKI message which have no certificates but CRL.

         REQUESTER                           CA SERVER
     GetCRL: PKI CRL query msg
     ----------------------------------> CertRep:  CRL attached
                                       <--------------------------------

2.3  PKI Operation Transactional Behavior

As described before, a PKI operation is a transaction consisting of the
messages exchanged between a requester and the CA/RA. This section
will specify the transaction behavior on both the requester and the


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certificate authority server.  Because the protocol is basically a two
way communication protocol without a confirmation message from the
initiating side, state and state resynchronization rules have to be
defined, in case any error happens at either side.  Before the state
transition can be defined, the notion of transaction identifier has to
be defined first.

2.3.1 Transaction Identifier

A transaction identifier is a string generated by the entity when
starting a transaction. Since all the PKI operations defined in this
protocol are initiated by the requester, it is the responsibility of
the requester to generate a unique string as the transaction
identifier. All the PKI messages exchanged for a given PKI transaction
must carry the same transaction identifier.  The transaction
identifier is generated as a SHA-1, SHA-256, SHA-512 or MD5 hash on
the public key value for which the enrollment request is made. This
allows the SCEP client to reuse the same transaction identifier if it
is reissuing a request for the same certificate (i.e. a certificate
with the same subject, issuer, and key).  The SCEP protocol requires
that transaction identifiers be unique, so that queries can be matched
up with transactions.  For this reason, in those cases in which
separate signing and encryption certificates are issued to the same
requester, the keys must be different.

2.3.2 State Transitions in Certificate Enrollment

The requester state transitions during enrollment operation are
indicated in the diagram below:
                                    +-<------+
                                    |        |
                                   GetCertInitial triggered by timeout or
                                    |        |    manual authentication
                                    |        |
     [CERT-NONEXISTANT] ------> [CERT-REQ-PENDING] ---> [CERT-ISSUED]
           |           PKCSReq          |        CertRep with SUCCESS
           |                            |
           |                            |
           +--------<-------------------+
           request rejected, timeout, or error

As described in the section 2.2.3, certificate enrollment starts at the
state CERT-NONEXISTANT. Sending PKCSReq changes the state to
CERT-REQ-PENDING. Receiving CertRep with SUCCESS status changes the
state to CERT-ISSUED. In the case the server sending back the response
with pending status, the requester will keep polling certificate
response by sending GetCertInitial to the server, until either a CertRep
with SUCCESS status is received, or the maximum polling number has been
exceeded.

If an error or timeout occurs in the CERT-REQ-PENDING state, the end
entity will transition to the CERT-NONEXISTANT state.


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The client administrator will, eventually, start up another enrollment
request.  It is important to note that, as long as the requester does
not change its subject name or keys, the same transaction id will be
used in the "new" transaction. This is important because based on this
transaction id, the certificate authority server can recognize this as
an existing transaction instead of a new one.


2.3.3 Transaction Behavior of Certificate/CRL Access

There is no state maintained during certificate access and CRL access
transaction. When using the certificate query and CRL query messages
defined in this protocol, the transaction identifier is still required
so that the requester can match the response message with the
upstanding request message. When using LDAP to query the certificate and
the CRL, the behavior is specified by the LDAP protocol.

2.4 Security

The security goals of SCEP are that no adversary can:

o subvert the public key/identity binding from that intended,
o discover the identity information in the enrollment requests and
  issued certificates,
o cause the revocation of certificates with any non-negligible
  probability.

Here an adversary is any entity other than the requester and the CA
(and optionally the RA) participating in the protocol that is
computationally limited, but that can manipulate data during
transmission (that is, a man-in-the-middle).  The precise meaning of
'computationally limited' depends on the implementer's choice of
cryptographic hash functions and ciphers.  The required algorithms are
RSA, DES and MD5.   Depending on the CA Capabilities, Triple-DES may
be used instead of DES, and SHA-1, SHA-256, or SHA-512 may be used
instead of MD5.  [See Appendix F].

The first and second goals are met through the use of PKCS#7 and PKCS#10
encryption and digital signatures using authenticated public keys.  The
CA's public key is authenticated via the checking of the CA fingerprint,
as specified in Section 2.1.2, and the SCEP client's public key is
authenticated through the manual authentication or pre-shared secret
authentication, as specified in Section 2.1.1.2.  The third goal is met
through the use of a Challenge Password for revocation, that is chosen
by the SCEP client and communicated to the CA protected by the PKCS#7
encryption, as specified in Section 2.2.4.

The motivation of the first security goal is straightforward.  The
motivation for the second security goal is to protect the identity
information in the enrollment requests and certificates.  For example,
two IPSEC hosts behind a firewall may need to exchange certificates, and
may need to enroll certificates with a CA that is outside of a firewall.

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Most networks with firewalls seek to prevent IP addresses and DNS
information from the trusted network leaving that network.  The second
goal enables the hosts in this example to enroll with a CA outside the
firewall without revealing this information.  The motivation for the
third security goal is to protect the SCEP clients from denial of
service attacks.

Section 3 Transport Protocol

In the SCEP protocol, HTTP is used as the transport protocol for the PKI
messages.

3.1 HTTP "GET" and "POST" Message Format

The following is the syntax definition of a HTTP GET message sent from
a requester to a certificate authority server:

Request = "GET " CGI-PATH CGI-PROG "?operation=" OPERATION "&message=" MESSAGE
where:
  CGI-PATH defines the actual CGI path to invoke the CGI program which
  parses the request.
  CGI-PROG is set to be the string "pkiclient.exe".  This is intended
  to be the program that the CA will use to handle the SCEP transactions,
  though the CA may ignore CGI-PROG and use only the CGI-PATH.
  OPERATION is set to be the string "PKIOperation" when the GET message
  carries a PKI message to request certificates or CRL; OPERATION is set
  to be the string "GetCACaps", "GetCACert", "GetNextCACert" or
  "GetCACertChain" when the GET operation is used to get CA capabilities,
  CA/RA certificate, the replacement CA/RA certificates for when the
  current ones expire, or the CA Cert chain  (respectively).

  When OPERATION is "PKIOperation", MESSAGE is a base64-encoded PKI message,
  When OPERATION is GetCACert, MESSAGE is a CRL distribution
  point in URI format,  otherwise, MESSAGE is a string which represents
  the certificate authority issuer identifier.

SCEP uses the HTTP "GET" and "POST" messages to request information from the CA.
Requests for CA certificates or capabilities are sent in the clear, using "GET",
with the OPERATION and MESSAGE fields identifying the requested data.
CRLs may also be requested in the clear if the CA supports it.

Other types of requests are sent using the PKCS#7 secure protocol.
These may be issued by means of a GET operation with
OPERATION and MESSAGE parameters in the Request-URL. OPERATION
identifies the type of GET operation, and MESSAGE is actually the PKCS#7
message Base64-Encoded.

For example. a requester may submit a message via HTTP to the server
as follows:

GET /cgi-bin/pkiclient.exe?operation=PKIOperation&message=MIAGCSqGSIb3D
QEHA6CAMIACAQAxgDCBzAIBADB2MGIxETAPBgNVBAcTCE ......AAAAAA==
Liu/Madson/McGrew/Nourse                                        [Page 13a]

If supported by the CA, the message may also be sent via HTTP POST:

POST /cgi-bin/pkiclient.exe?operation=PKIOperation

This is further described in Appendix H.
To determine if the CA supports POST, use the GetCACaps message described
in Appendix F.


3.2 Response Message Format

For each GET operation, the CA/RA server will return a MIME object via
HTTP. For a GET operation with PKIOperation as its type, the response is
tagged as having a Content Type of application/x-pki-message.  The body
of this message is a BER encoded binary PKI message. The following is an
example of the response:

"Content-Type:application/x-pki-message\n\n"<BER-encoded PKI msg>

In the case of GET operation with a type of GetCACert the MIME content
type returned will depend on whether or not an RA is in use.  If there
is no RA, only the CA certificate is sent back in the response, and
the response has the content type tagged as
application/x-x509-ca-cert. the body of the response is a DER encoded
binary X.509 certificate. For example:

"Content-Type:application/x-x509-ca-cert\n\n"<BER-encoded X509>

If there is an RA, the RA certificates are sent back together with the
CA certificates, a certificate-only PKCS#7 SignedData is sent back in
the response where the SignerInfo is empty. Section 5 has the detailed
definition of the message format in this case.  The content type is
application/x-x509-ca-ra-cert.

The response to GetNextCACert is always a certificates-only PKCS#7
SignedData with a content type of application/x-x509-ca-ra-cert.
If there is an RA, The signer is the current RA certificate.  Otherwise,
the signer is the current CA certificate.

If the CA supports it, PKIOperation may also be done via an HTTP POST.
This is described in Appendix H.

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Section 4  Secure Transportation: PKCS#7

PKCS#7 is a general enveloping mechanism that enables both signed and
encrypted transmission of arbitrary data. It is widely implemented and
included in the RSA tool kit.  In this section, the general PKCS#7
enveloped PKI message format is specified.  The complete PKCS#7 message
format for each PKI transaction will be covered in Section 5.

4.1 SCEP Message Format

As a transaction message, a SCEP message has a set of transaction
specific attributes and an information portion. Employing PKCS#7
protocol, the transaction specific attributes are encoded as a set of
authenticated attributes of the SignedData. The information portion will
first be encrypted to become Enveloped Data, and then the digest of the
enveloped information portion is included as one of the message digest
attributes and being signed together with the other transaction specific
attributes.

By applying both enveloping and signing transformations, a SCEP message
is protected both for the integrity of its end-end-transition
information and the confidentiality of its information portion. The
advantage of this technique over the conventional transaction message
format is that, the signed transaction type information and the status
of the transaction can be determined prior to invoke security handling
procedures specific to the information portion being processed.

The following is an example of a SCEP message with its enveloped and
signed data portion represented by pkcsPKISigned and
pkcsPKIEnveloped. The out-most of any PKI message is a blob of
ContentInfo, with its content type set to SignedData and the actual
signed data as the content.

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  pkiMessage ContentInfo ::= {
      contentType {pkcs-7 signedData(2)}
      content pkcsPKISigned
  }
  pkcsPKISigned SignedData ::= {
      version 1
      digestAlgorithm { iso(1) member-body(2) US(840) rsadsi(113549)
                        digestAlgorithm(2) 5}
      contentInfo {
          contentType {pkcs-7 1} --  data content identifier
          content pkcsPKIEnvelope -- enveloped information portion
      }
      certificates   -- signer certificate chain
      signerInfo     -- including signed transaction info and the digest
                     -- of the enveloped information portion as the
                     -- authenticated attributes
  }
  pkcsPKIEnveloped EnvelopedData ::= {
      version 0
      recipientInfos -- information required to open the envelop
      encryptedContentInfo {
          contentType {pkcs-7 1}  -- data content identifier
          contentEncryptionAlgorithm
          encryptedContent     -- encrypted information portion
      }
  }

4.2 Signed Transaction Attributes

The following transaction attributes are encoded as authenticated
attributes.  Please refer to Appendix B for the OID definitions.

transactionID      PrintableString  -- Decimal value as a string
  messageType        PrintableString  -- Decimal value as a string
  pkiStatus          PrintableString  -- Decimal value as a string
  failinfo           PrintableString  -- Decimal value as a string
  senderNonce        Octet String
  recipientNonce     Octet String

where:

  The transactionID is an attribute which uniquely identify a
  transaction.  This attribute is required in all PKI messages.

  The messageType attribute specify the type of operation performed by the
  transaction. This attribute is required in all PKI
  messages. Currently, the following message types are defined:

    PKCSReq (19)  -- Permits use of PKCS#10 certificate request
    CertRep (3)   -- Response to certificate or CRL request
    GetCertInitial (20)  -- Certificate polling in manual enrollment
    GetCert (21)  -- Retrieve a certificate
    GetCRL  (22)  -- Retrieve a CRL

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  All response message will include transaction status information which
  is defined as pkiStatus attribute:

          SUCCESS (0)   -- request granted
          FAILURE (2)   -- request rejected
          PENDING (3)   -- request pending for manual approval.

  If the status in the response is FAILURE, the failinfo attribute will
  contain one of the following failure reasons:

        badAlg (0)  -- Unrecognized or unsupported algorithm ident
        badMessageCheck (1)  -- integrity check failed
        badRequest (2)  -- transaction not permitted or supported
        badTime (3)  -- Message time field was not sufficiently close
                        to the system time
        badCertId (4)  -- No certificate could be identified matching
                          the provided criteria

  The attributes of senderNonce and recipientNonce are the 16 byte
  random numbers generated for each transaction to prevent the replay
  attack.

When a requester sends a PKI message to the server, a senderNonce is
included in the message. After the server processes the request, it will
send back the requester senderNonce as the recipientNonce and generates
another nonce as the senderNonce in the response message.  Because the
proposed pki protocol is a two-way communication protocol, it is clear
that the nonce can only be used by the requester to prevent the
replay. The server has to employ extra state related information to
prevent a replay attack.

Section 5. SCEP Transaction Specification

In this section each SCEP transaction is specified in terms of the
complete messages exchanged during the transaction.

5.1 Certificate Enrollment

The certificate enrollment transaction consists of one PKCSReq message
sent to the certificate authority from a requester, and one CertRep
message sent back from the server. The pkiStatus returned in the
response message is either SUCCESS, or FAILURE, or PENDING.  The
information portion of a PKCSReq message is a PKCS#10 certificate
request, which contains the subject Distinguished Name, the subject
public key, and two attributes, a ChallengePassword attribute to be used
for revocation, and an optional ExtensionReq attribute which will be a
sequence of extensions the requester expects to be included in its V3
certificate extensions. One of the extension attribute specifies the key
usage.  If the request is granted, the pkiStatus is set to SUCCESS, and
the certificate is returned in CertRep; if the request is rejected, the


Liu/Madson/McGrew/Nourse                                        [Page 17]


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pkiStatus is set to FAILURE; if the server requires manual approval of
the request, the pkiStatus is set to PENDING. The messages exchanged
in the manual authentication mode is further specified in Section 5.2.

Precondition:
  Both the requester and the certificate authority have completed their
  initialization process. The requester has already been configured
  with the CA/RA certificate.

Postcondition:
  Either the certificate is received by the requester, or the end
  entity is notified to do the manual authentication, or the request
  is rejected.

5.1.1 PKCSReq Message Format

A PKCSReq message is created by following the steps defined below:

1.  Create a PKCS#10 certificate request which is signed by the end
    entity's private key, corresponding to the public key included in
    the PKCS#10 certificate request. This constitutes the information
    portion of PKCSReq.

2.  Encrypt the PKCS#10 certificate request using a randomly generated
    content-encryption key. This content-encryption key is then
    encrypted by the CA's* public key and included in the recipientInfo.
    This step completes the "envelope" for the PKCS#10 certificate
    request.

3.  Generate a unique string as the transaction id.

4.  Generate a 16 byte random number as senderNonce.

5.  Generate message digest on the enveloped PKCS#10 certificate request
    using the selected digest algorithm.

6.  Create SignedData by adding the requester's self- or CA-certificate
    as the  signer's public key certificate. Include the message type,
    transaction id, the senderNonce and the message digest as the
    authenticated attributes and sign the attributes using the end
    entity's private key. This completes the SignedData.

7.  The SignedData is prepended with the ContenInfo blob which indicates
    a SignedData object. This final step completes the create of a
    complete PKCSReq PKI message.

In the following, the PKCSReq message is defined following the ASN.1
notation.

For readability, the values of a field is either represented by a quoted
string which specifies the intended value, or a constant when the value
is known.


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  -- PKCSReq information portion
  pkcsCertReq CertificationRequest ::= {   -- PKCS#10
      version 0
      subject  "the requester's subject name"
      subjectPublicKeyInfo {
          algorithm {pkcs-1 1}  -- rsa encryption
          subjectPublicKey "DER encoding of the requester's public key"
      }
      attributes {
          challengePassword {{pkcs-9 7} "password string" }
          extensions
      }
      signatureAlgorithm {pkcs-1 4} -- MD5WithRSAEncryption
      signature "bit string which is created by signing inner content
                of the defined pkcsCertReq using requester's private
                key, corresponding to the public key included in
                subjectPublicKeyInfo."
  }
  -- Enveloped information portion
  pkcsCertReqEnvelope EnvelopeData ::= {   -- PKCS#7
      version 0
      recipientInfo {
          version 0
          issuerAndSerialNumber {
              issuer  "the CA issuer name"
              serialNumber  "the CA certificate serial number"
          }
          keyEncryptionAlgorithm  {pkcs-1 1}  -- rsa encryption
          encryptedKey "content-encryption key
                        encrypted by CA public key"
      }
      encryptedContentInfo {
          contentType {pkcs-7 1}  -- data content
          contentEncryptionAlgorithm  "object identifier
                                       for DES encryption"
          encryptedContent  "encrypted pkcsCertReq using the content-
                             encryption key"
      }
  }
  -- Signed PKCSReq
  pkcsCertReqSigned SignedData ::= { -- PKCS#7
      version 1
      digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                       digestAlgorithm(2) 5}
      contentInfo {
          contentType {pkcs-7 1} -- data content identifier
          content  pkcsCertReqEnvelope
      }
     certificate {   -- requester self-signed or CA-issued certificate
          version 3
          serialNumber  "the transaction id associated with enrollment"
          signature {pkcs-1 4}  -- md5WithRSAEncryption


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     issuer " the requester's subject name"
          validity {
              notBefore "a UTC time"
              notAfter  "a UTC time"
          }
          subject  "the requester's subject name"
          subjectPublicKeyInfo {
              algorithm {pkcs-1 1}
              subjectPublicKey "DER encoding of requester's public key"
          }
          signatureAlgorithm {pkcs-1 4}
          signature "the signature generated by using the requester's
                     private key corresponding to the public key in
                     this certificate."
      }
      signerInfo  {
          version 1
          issuerAndSerialNumber {
              issuer "the requester's subject name"
              serialNumber "the transaction id associated
                            with the enrollment"
          }
          digestAlgorithm {iso(0) member-body(2) US(840) rsadsi(113549)
                           digestAlgorithm(2) 5}
          authenticateAttributes {
              contentType  {{pkcs-9 3} {pkcs-7 1}}
              messageDigest {{pkcs-9 4} "an octet string"}
              transaction-id {{id-attributes transId(7)} "printable
                                                          string"}
                             -- this transaction id will be used
                             -- together with the subject name as
                             -- the identifier of the requester's key
                             -- pair during enrollment
              messageType {{id-attributes messageType(2)} "PKCSReq"}
              senderNonce {{id-attributes senderNonce(5)}
                           "a random number encoded as a string"}
          }
          digestEncryptionAlgorithm {pkcs-1 1} -- rsa encryption
          encryptedDigest "encrypted digest of the authenticated
                           attributes using requester's private key"
      }
  }
  pkcsReq PKIMessage ::= {
      contentType {pkcs-7 2}
      content pkcsCertReqSigned
  }








Liu/Madson/McGrew/Nourse                                        [Page 20]


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5.1.2 CertRep Message Format

The response to an SCEP enrollment request is a CertRep message.

5.1.2.1 PENDING Response

When the CA is configured to manually authenticate the requester,
the CertRep is returned with the attribute pkiStatus set to PENDING.
The data portion for this message is null. Only the transaction
required attributes are sent back.

CertRepSigned  SignedData ::= { -- PKCS#7
    version 1
    digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                     digestAlgorithm(2) 5}
    contentInfo {contentType {pkcs-7 1}  -- empty content
    }
    signerInfo {
        version 1
        issuerAndSerialNumber {
           issuer "name of CA that issued the CA [RA] cert"
           serialNumber "the serial number of the CA [RA] cert"
        }
        digestAlgorithm (iso(1) member-body(2) US(840) rsadsi(113549)
                         digestAlgorithm(2) 5}
        authenticateAttributes {
            contentType {{pkcs-9 3} {pkcs-7 1}}
            messageDigest {{pkcs-9 4} NULL}
            messageType {{id-attribute messageType(0)} "CertRep"}
            transaction-id {{id-attributes transid(7)} "printablestring"}
                           --- same transaction id used in PKCSReq
            pkiStatus  {{id-attributes pkiStatus(3)} "PENDING"}
            recipientNonce {{id-attributes recipientNonce(6)}<16 bytes>}
            senderNonce {{id-attributes senderNonce(5)} <16 bytes>}
        }
        digestEncrytionAlgorithm {pkcs-1 1}
        encryptedDigest  "encrypted message digest of the authenticated
                         attributes using the CA's [RA's] private key"
     }
}
CertRep PKIMessage ::= {
    contentType {pkcs-7 2}
    content CertRepSigned
}

5.1.2.2 Failure Response

In this case, the CertRep sent back to the requester is same as in
the PENDING case, except that the pkiStatus attribute is set to FAILURE,
and the failInfo attribute should be included:

    pkistatus {{id-attributes pkiStatus(3)} "FAILURE"}
    failInfo {{id-attributes failInfo(4)} "the reason to reject"}

Liu/Madson/McGrew/Nourse                                        [Page 21]


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5.1.2.3  SUCCESS response

In this case, the information portion of CertRep will be a degenerated
PKCS#7 which contains the requester's certificate. It is then enveloped
and signed as below:

pkcsCertRep SignedData ::= { -- PKCS#7
    version 1
    digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                     digestAlgorithm(2) 5}
    contentInfo { -- empty content since this is degenerated PKCS#7
        contentType {pkcs-7 1}
    }
    certificates {
        certificate { -- issued requester's certificate  // must be first
            version 3
            serialNumber "issued requester's certificate serial number"
            signature {pkcs-1 4}  -- md5WithRSAEncryption
            issuer "the certificate authority issuer name"
            validity {
                notBefore "UTC time"
                notAfter "UTC time"
            }
            subject  "the requester subject name as given in PKCS#10"
            subjectPublicKeyInfo {
                algorithm {pkcs-1 1}
                subjectPublicKey "a DER encoding of requester public
                                  key as given in PKCS#10"
            }
            extensions  " the extensions as given in PKCS#10"
            signatureAlgorithm {pkcs-1 4}
            signature " the certificate authority signature"
        }
        certificate "the certificate authority certificate" (optional)
        certificate "the registration authority certificate(s)" (optional)
    }
}
pkcsCertRepEnvelope EnvelopedData ::= {  -- PKCS#7
    version 0
    recipientInfo {
        version 0
        issuerAndSerialNumber { -- use issuer name and serial number as
                                -- conveyed in requester's self-signed
                                -- certificate, included in the PKCSReq
            issuer  "the requester's subject name"
            serialNumber "the serial number defined by the requester in
                          its self-signed certificate"
        }
        keyEncryptionAlgorithm {pkcs-1 1}
        encryptedKey "content-encrypt key encrypted by the requester's
                      public key which is same key as authenticated in
                      the requester's certificate"
    }


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    encryptedContentInfo {
        contentType {pkcs-7 1}  -- data content identifier
        contentEncryptionAlgorithm "OID for DES encryption"
        encryptedContent "encrypted pkcsCertRep using content encryption
                          key"
    }
}
pkcsCertRepSigned SignedData ::= { -- PKCS#7
    version 1
    digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                     digestAlgorithm(2) 5}
    contentInfo {
        contentType {pkcs-7 1}
        content pkcsCertRepEnvelope
    }
    signerInfo {
        version 1
        issuerAndSerialNumber {
            issuer "the certificate authority issuer name"
            serialNumber "the CA certificate's serial number"
        }
        digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549)
                         digestAlgorithm(2) 5}
        authenticateAttributes {
            contentType {{pkcs-9 3} {pkcs-7 1}}
            messageDigest {{pkcs-9 4} "a octet string"}
            messageType {{id-attribute messageType(2)} "CertRep"}
            transaction-id {{id-attributes transId(7)} "printable
                                                        string"}
                          -- same transaction id as given in PKCSReq
            pkiStatus {{id-attributes pkiStatus(3) "SUCCESS"}
            recipientNonce {{id-attribute recipientNonce(6)}<16 bytes>}
            senderNonce {{ id-attributes senderNonce(5) <16 bytes>}
        }
        digestEncryptionAlgorithm {pkcs-1 1}
        encryptedDigest "encrypted digest of authenticate attributes
                         using CA's private key "
    }
}
CertRep PKIMessage ::= {
    contentType {pkcs-7 2}
    content pkcsCertRepSigned
}

5.2 Poll for Requester Initial Certificate

Either triggered by the PENDING status received from the CertRep, or by
the non-response timeout for the previous PKCSReq, a requester will
enter the polling state by periodically sending GetCertInitial to the
server, until either the request is granted and the certificate is sent
back, or the request is rejected, or the configured time limit for
polling is exceeded.


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Since GetCertInitial is part of the enrollment, the messages exchanged
during the polling period should carry the same transaction identifier
as the previous PKCSReq.

PreCondition
    Either the requester has received a CertRep with pkiStatus set to be
    PENDING, or the previous PKCSReq has timed out.

PostContition
    The requester has either received the certificate, or be rejected of
    its request, or the polling period ended as a failure.

5.2.1 GetCertInitial Message Format

Since at this time the certificate has not been issued, the requester
can only use the requester's subject name, combined with the
transaction identifier, to identify the polled certificate request.

The certificate authority server must be able to uniquely identify the
polled certificate request.  A subject name can have more than one
outstanding certificate request (with different key usage attributes).

-- Information portion

pkcsGetCertInitial issuerAndSubject ::= {
    issuer "the certificate authority issuer name"
    subject "the requester subject name as given in PKCS#10"
}
pkcsGetCertInitialEnvelope EnvelopedData ::= {
    version 0
    recipientInfo {
        version 0
        issuerAndSerialNumber {
            issuer  "the CA issuer name"
            serialNumber "the CA certificate serial number"
        }
        keyEncryptionAlgorithm {pkcs-1 1}
        encryptedKey "content-encrypt key encrypted by CA's public key"
    }
    encryptedContentInfo {
        contentType {pkcs-7 1}  -- data content
        contentEncryptionAlgorithm "OID for DES encryption"
        encryptedContent "encrypted getCertInital"
    }
}
pkcsGetCertInitialSigned SignedData ::= { -- PKCS#7
    version 1
    digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                     digestAlgorithm(2) 5}
    contentInfo {
        contentType {pkcs-7 1}


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        content pkcsGetCertIntialEnvelope
    }
     certificate {   -- the requester's self-signed certificate
         version 3
         serialNumber  "the transaction id associated with enrollment"
         signature {pkcs-1 4}  -- md5WithRSAEncryption
         issuer " the requester's subject name"
         validity {
             notBefore "a UTC time"
             notAfter  "a UTC time"
         }
         subject  "the requester's subject name"
         subjectPublicKeyInfo {
             algorithm {pkcs-1 1}
             subjectPublicKey "DER encoding of requester's public key"
         }
         signatureAlgorithm {pkcs-1 4}
         signature "the signature generated by using the requester's
                    private key corresponding to the public key in
                    this certificate."
     }
    signerInfo {
        version 1
        issuerAndSerialNumber {
            issuer "requester's subject name"
            serialNumber "the transaction id used in previous PKCSReq"
        }
        digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549)
                         digestAlgorithm(2) 5}
        authenticateAttributes {
            contentType {{pkcs-9 3} {pkcs-7 1}}
            messageDigest {{pkcs-9 4} "an octet string"}
                            -- digest of getCertInitial
            messageType {{id-attribute messageType(2)} "GetCertInitial"}
            transaction-id {{id-attributes transId(7)} "printable
                                                        string"}
                          -- same transaction idused in previous PKCSReq
            senderNonce {{id-attribute senderNonce(3)} 0x<16 bytes>}
        }
        digestEncryptionAlgorithm {pkcs-1 1}
        encryptedDigest "encrypted digest of authenticateAttributes"
    }
}
GetCertInitial PKIMessage ::= {
    contentType {pkcs-7 2}
    content pkcsGetCertInitialSigned
}



5.2.2 GetCertInitial Response Message Format

The response messages for GetCertInitial are the same as for PKCSReq.

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5.3  Certificate Access

The certificate query message defined in this section is an option when
the LDAP server is not available to provide the certificate query.  A
requester should be able to query an issued certificate from the
certificate authority, as long as the issuer name and the issuer
assigned certificate serial number is known to the requesting end
entity. This transaction is not intended to provide the service as a
certificate directory service. A more complicated query mechanism would
have to be defined in order to allow a requester to query a certificate
using various different fields.

This transaction consists of one GetCert message sent to the server by
a requester, and one CertRep message sent back from the server.

PreCondition
   The queried certificate have been issued by the certificate authority
   and the issuer assigned serial number is known.

PostCondition
    Either the certificate is sent back or the request is rejected.


5.3.1 GetCert Message Format

The queried certificate is identified by its issuer name and the issuer
assigned serial number. If this is a query for an arbitrary requester's
certificate, the requesting requester should includes its own CA issued
certificate in the signed envelope. If this is a query for its own
certificate (assume the requester lost the issued certificate, or does
not have enough non-volatile memory to save the certificate), then the
self-signed certificate has to be included in the signed envelope.

  pkcsGetCert issuerAndSerialNumber ::= {
      issuer "the certificate issuer name"
      serialNumber "the certificate serial number"
  }
  pkcsGetCertEnvelope EnvelopedData ::= {
      version 0
      recipientInfo {
          version 0
          issuerAndSerialNumber {
              issuer  "the CA [RA] issuer name"
              serialNumber "the CA [RA] certificate serial number"
          }
          keyEncryptionAlgorithm {pkcs-1 1}
          encryptedKey "content-encrypt key encrypted
                        by CA [RA] public key"
      }





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      encryptedContentInfo {
          contentType {pkcs-7 1}  -- data content
          contentEncryptionAlgorithm "OID for DES encryption"
          encryptedContent "encrypted pkcsGetCert using the content
                            encryption key"
      }
  }
  pkcsGetCertSigned SignedData ::= {
      version 1
      digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                       digestAlgorithm(2) 5}
      contentInfo {
          contentType {pkcs-7 1}
          content pkcsGetCertEnvelope
      }
      certificates {
          certificate "CA issued certificate"
                      or "self-signed certificate"
      }
      signerInfo {
          version 1
          issuerAndSerialNumber {
              issuer "the requester's subject name"
              serialNumber "requester's certificate serial number"
          }
          digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549)
                           digestAlgorithm(2) 5}
          authenticateAttributes {
              contentType {{pkcs-9 3} {pkcs-7 1}}
              messageDigest {{pkcs-9 4} "an octet string"}
                              -- digest of pkcsGetCertEnvelope
              messageType {{id-attribute messageType(2)} "GetCert"}
              transaction-id {{id-attributes transId(7)} "printable
                                                          string"}
              senderNonce {{id-attribute senderNonce(3)} <16 bytes>}
          }
          digestEncryptionAlgorithm {pkcs-1 1}
          encryptedDigest "encrypted digest of authenticateAttributes"
      }
  }
  GetCert PKIMessage ::= {
      contentType {pkcs-7 2}
      content pkcsGetCertSigned
  }









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5.3.2 CertRep Message Format

In this case, the CertRep from the server is same as the CertRep for the
PKCSReq, except that the server will only either grant the request or
reject the request. Also, the recipientInfo should use the CA issuer
name and CA assigned serial number to identify the requester's key pair
since at this time, the requester has received its own certificate.

5.4  CRL Access

The CRL query message defined in this section is an option when the LDAP
server is not available to provide the CRL query.  In the PKI protocol
proposed here, only the requester can initiate the transaction to
download CRL. A requester sends GetCRL request to the server and the
server sends back CertRep whose information portion is a degenerated
PKCS#7 which contains only the most recent CRL. The size of CRL included
in the CertRep should be determined by the implementation.

PreCondition
    The certificate authority certificate has been downloaded to the end
    entity.

PostCondition
    CRL sent back to the requester.

5.4.1 GetCRL Message format

The CRL is identified by using both CA's issuer name and the CA
certificate's serial number:

  pkcsGetCRL issuerAndSerialNumber {
      issuer "the certificate authority issuer name"
      serialNumber "certificate authority certificate's serial number"
  }

When the CRLDistributionPoint is supported, the pkcsGetCRL is defined as
the following:

  pkcsGetCRL SEQUENCE {
      crlIssuer  issuerAndSerialNumber
      distributionPoint CE-CRLDistPoints
  }

where CE-CRLDisPoints is defined in X.509, but must contain only one
CRL distribution point.










Liu/Madson/McGrew/Nourse                                        [Page 28]


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  pkcsGetCRLEnvelope EnvelopedData ::= {
      version 0
      recipientInfo {
          version 0
          issuerAndSerialNumber {
              issuer  "the certificate authority (or RA) issuer name"
              serialNumber "the CA (RA) certificate's serial number"
          }
          keyEncryptionAlgorithm {pkcs-1 1}
          encryptedKey "content-encrypt key encrypted by CA (RA) public key"
      }
      encryptedContentInfo {
          contentType {pkcs-7 1}  -- data content
          contentEncryptionAlgorithm "OID for DES encryption"
          encryptedContent "encrypted pkcsGetCRL"
      }
  }
  pkcsGetCRLSigned SignedData ::= {
      version 1
      digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                       digestAlgorithm(2) 5}
      contentInfo {
          contentType {pkcs-7 1}
          content pkcsGetCRLEnvelope
      }
      certificates {
         certificate "CA-issued or self-signed requester's certificate"
      }
      signerInfo {
          version 1
          issuerAndSerialNumber {
              issuer "the requester's issuer name"
              serialNumber "the requester's certificate serial number"
          }
          digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549)
                           digestAlgorithm(2) 5}
          authenticateAttributes {
              contentType {{pkcs-9 3} {pkcs-7 1}}
              messageDigest {{pkcs-9 4} 0x<16/20 bytes>}
                              -- digest of pkcsGetCRLEnvelope
              messageType {{id-attribute messageType(2)} "GetCRL"}
              transaction-id {{id-attributes transId(7)} "printable
                                                          string"}
              senderNonce {{id-attribute senderNonce(3)} <16 bytes>}
          }
          digestEncryptionAlgorithm {pkcs-1 1}
          encryptedDigest "encrypted digest of authenticateAttributes"
      }
  }
  GetCRL PKIMessage ::= {
      contentType {pkcs-7 2}
      content pkcsGetCRLSigned
  }

Liu/Madson/McGrew/Nourse                                        [Page 29]


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5.4.2  CertRep Message Format

The CRL is sent back to the requester through CertRep message. The
information portion of this message is a degenerated PKCS#7 SignedData
which contains only a CRL.

  pkcsCertRep SignedData ::= {
      version 1
      digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                       digestAlgorithm(2) 5}
      contentInfo {
          contentType {pkcs-7 1}
      }
      crl {
          signature {pkcs-1 4}
          issuer "the certificate authority issuer name"
          lastUpdate "UTC time"
          nextUpdate "UTC time"
          revokedCertificate {
              -- the first entry
              userCertificate "certificate serial number"
              revocationData "UTC time"
              ....
              -- last entry
              userCertificate "certificate serial number"
              revocationData "UTC time"
      }
  }
  pkcsCertRepEnvelope EnvelopedData ::= {
      version 0
      recipientInfo {
          version 0
          issuerAndSerialNumber {
              issuer  "the requester's issuer name"
              serialNumber "the requester certificate serial number"
          }
          keyEncryptionAlgorithm {pkcs-1 1}
          encryptedKey "content-encrypt key encrypted by requester's
                        public key "
      }
      encryptedContentInfo {
          contentType {pkcs-7 1}  -- data content
          contentEncryptionAlgorithm "OID for DES encryption"
          encryptedContent "encrypted pkcsCertRep using requester's
                            public key"
      }
  }






Liu/Madson/McGrew/Nourse                                        [Page 30]


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  pkcsCertRepSigned SignedData ::= { -- PKCS#7
      version 1
      digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                       digestAlgorithm(2) 5}
      contentInfo {
          contentType {pkcs-7 1}
          content pkcsCertRepEnvelope
      }
      signerInfo {
          version 1
          issuerAndSerialNumber {
              issuer "the certificate authority issuer name"
              serialNumber "the CA certificate's serial number"
          }
          digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549)
                           digestAlgorithm(2) 5}
          authenticateAttributes {
              contentType {{pkcs-9 3} {pkcs-7 1}}
              messageDigest {{pkcs-9 4} "an octet string"}
                            -- digest of pkcsCertRepEnvelope
              messageType {{id-attribute messageType(2)} "CertRep"}
              transaction-id {{id-attributes transId(7)} "printable
                                                          string"}
                            -- same transaction id as given in PKCSReq
              pkiStatus {{id-attributes pkiStatus(3) "SUCCESS"}
              recipientNonce{{id-attribute recipientNonce(6)}<16 bytes>}
              senderNonce {{id-attribute senderNonce (5) 0x<16 bytes>}
          }
          digestEncryptionAlgorithm {pkcs-1 1}
          encryptedDigest "encrypted digest of authenticatedAttributes
                           using CA private key"
      }
  }


NOTE:The PKCS#7 EncryptedContent is specified as an octet string, but
SCEP entities must also accept a sequence of octet strings as a valid
alternate encoding.

This alternate encoding must be accepted wherever PKCS #7 Enveloped
Data is specified in this document.











Liu/Madson/McGrew/Nourse                                        [Page 31]


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5.5 Get Certificate Authority Certificate

Before any transaction begins, end entities have to get the CA (and
possibly RA) certificate(s) first.  Since the requester may have no CA
certificates or CA public keys at all, this message can not be
encrypted and the response must be authenticated by out-of-band means.
These certs are obtained by means of an HTTP GET message.  To get the
CA certificate, the requester does a "HTTP GET" with a URL that
identifies a CGI script on the server and an optional CA issuer
identifier as the parameter to the CGI script.  The response is either
a single X.509 CA certificate ("CA mode"), or a PKCS7 message
containing the CA certificate and RA certificates ("RA mode").  The
client can determine which mode the CA operates in by which response
it gets.  Once the CA certificate is received by the requester, a
fingerprint is generated using the SHA1, SHA256, SHA512 or  MD5 hash
algorithm on the whole CA certificate.  If the requester does not have
a certificate path to a trusted CA certificate, this fingerprint may
be used to verify the certificate, by some positive out-of-band means,
such as a phone call.

5.5.1 GetCACert HTTP Message Format
   "GET" CGI-PATH CGI-PROG "?operation=GetCACert" "&message=" CA-IDENT
    where:
        CGI-PATH defines the actual CGI path to invoke the CGI program
        which parses the request.
        CGI-PROG is set to be the string "pkiclient.exe" and this is
        expected to be the program that the CA will use to handle the
        SCEP transactions.
        CA-IDENT is any string which is understood by the CA.
        For example, it could be a domain name like ietf.org.
        If a certificate authority has multiple CA certificates
        this field can be used to distinguish which is required.
        Otherwise it may be ignored.

5.5.2 Response

The response for GetCACert is different between the case where the CA
directly communicated with the requester during the enrollment, and the
case where a RA exists and the requester communicates with the RA
during the enrollment.

5.5.2.1 CA Certificate Only Response

A binary X.509 CA certificate is sent back as a MIME object with a
Content-Type of application/x-x509-ca-cert.

5.5.2.2 CA and RA Certificates Response

When an RA exists,  both CA and RA certificates must be sent back in
the response to the GetCACert request.  The RA certificate(s) must be
signed by the CA.   A certificates-only PKCS#7 SignedData is used to
carry the certificates to the requester, with a Content-Type of
application/x-x509-ca-ra-cert.

Liu/Madson/McGrew/Nourse                                        [Page 32]


5.5.3 Get Next Certificate Authority Certificate

5.5.3.1 GetNextCACert HTTP Message Format
   "GET" CGI-PATH CGI-PROG "?operation=GetNextCACert" "&message=" CA-IDENT

The response to this message is a PKCS#7 certificates-only message containing
a CA certificate (and possibly RA certificates) to be used when the current CA
certificate expires, signed with the current CA cert (or RA certificate, if
the CA is in RA mode.  Note that a PKCS#7 is returned even in CA mode.

5.5.3.2 GetCACaps HTTP Message Format
   "GET" CGI-PATH CGI-PROG "?operation=GetCACaps" "&message=" CA-IDENT

This message requests capabilities from CA.  The response is a list of
text capabilities, as defined in Appendix F.  Support for this message
is optional, but if it is not supported, the client should assume that
none of the capabilities in Appendix F are supported.

5.6 Get Certificate Authority Certificate Chain

GetCACertChain provides  a way to get the entire certificate chain.

5.6.1 GetCACertChain HTTP Message Format

   "GET" CGI-SCRIPT "?" "operation=GetCACertChain" "&" "message" CA-IDENT
    where CGI-SCRIPT and CA-IDENT are as described for GetCACert.

5.6.2 Response

The response for GetCACertChain is a certificates-only PKCS#7 SignedData
to carry the certificates to the requester, with a Content-Type of
application/x-x509-ca-ra-cert-chain.

5.6.3 Backwards Compatability

Versions of SCEP prior to revision 3 do not support GetCACertChain.
Certificate Authorities written to these prior versions will not be
able to process the message and may return an HTML error.

To avoid this, clients should send the GetCACert message first.  If the
returned certificate is self-signed or is signed by a Certificate
Authority that is trusted by the client, then it is not necessary to
send the GetCACertChain message and it should not be sent.

If a Certificate Authority is configured with a certificate that is
not either self-signed or has a self-signed issuer, then it should
support this message.  In other words, it should be supported if the
CA hierarchy is more than two-deep.

An old CA in a two-deep hierarchy might still get this message from
a client if the client did not trust either that CA or its issuer.
In that event, the certificate cannot be trusted anyway. In any case
the CA must not crash or hang upon the receipt of the message and the
client must be able to handle whatever error is returned by the CA,
including an HTML error or an ungraceful disconnect.

Liu/Madson/McGrew/Nourse                                        [Page 33]


Cisco Systems' Simple Certificate Enrollment Protocol           Aug 2005

The following is the ASN.1 definition of Cert-Only PKCS#7:

  certOnly SignedData ::= {
      version 1
      digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549)
                       digestAlgorithm(2) 5}

contentInfo {
          contentType {pkcs-7 1} -- data content identifier
          content  -- NULL
      }
      certificates   -- the RA and CA certificates.
  }

  CARACerts PKIMessage ::= {  -- special pki message sent in the clear
      contentType {pkcs-7 2}
      content certOnly
  }


6.0 Security Considerations

This entire document is about security. Common security considerations
such as keeping private keys truly private and using adequate lengths
for symmetric and asymmetric keys must be followed in order to maintain
the security of this protocol.


7.0 Intellectual Property

This protcol includes the optional use of Certificate Revocation List
Distribution Point (CRLDP) technology, which is a patented technology
of Entrust Technologies, Inc. (Method for Efficient Management of
Certificate Revocation Lists and Update Information (U.S. Patent
5,699,431)). Please contact Entrust Technologies, Inc.
(www.entrust.com) for more information on licensing CRLDP technology.


8.0 References

[PKCS7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
1.5", RFC 2315, March 1998.

[PKCS10] Kaliski, B., "PKCS #10: Certification Request Syntax Version
1.5", RFC 2314, March 1998.

[RFC2459] Housley, R., ec. al., "Internet X.509 Public Key
Infrastructure Certificate and CRL Profile", RFC 2459, January 1999.






Liu/Madson/McGrew/Nourse                                        [Page 34]


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Appendix A: Cisco Requester Subject Name Definition

The ip address and the FQDN of a SCEP client should be included in the
V3 extension subjectAltName. When the subjectAltName extension attribute
is present, both the subjectAltName fields and the subjectName field could
have the IP address and the FQDN information.

When the X.500 directory is used by the CA to define the name space, the
subject name defined above become a RDN which is part of DN binded to
the requester's public key in the certificate.


A sample of DN assigned by Entrust CA is given below (assume the same
ciscoRouterAlice is used as the requester defined subject name):

    OU = InteropTesting, O = Entrust Technologies, C = CA
    RDN = {"alice.cisco.com", "172.21.114.67", "22334455"}


Liu/Madson/McGrew/Nourse                                        [Page 35]


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Appendix B:  IPSEC Client Enrollment Certificate Request

The following is the certificate enrollment request (PKCS#10) as created
by Cisco VPN Client:

-----END NEW CERTIFICATE REQUEST-----
   0 30  439: SEQUENCE {
   4 30  288:   SEQUENCE {
   8 02    1:     INTEGER 0
  11 30   57:     SEQUENCE {
  13 31   55:       SET {
  15 30   53:         SEQUENCE {
  17 06    3:           OBJECT IDENTIFIER commonName (2 5 4 3)
  22 13   46:           PrintableString
            :             'For Xiaoyi, IPSEC attrs in alternate name
                           extn'
            :           }
            :         }
            :       }
  70 30  158:     SEQUENCE {
  73 30   13:       SEQUENCE {
  75 06    9:         OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1
                                                       1 1)
  86 05    0:         NULL
            :         }
  88 03  140:       BIT STRING 0 unused bits
            :         30 81 88 02 81 80 73 DB 1D D5 65 AA EF C7 D4 8E
            :         AA 6E EB 46 AC 91 2A 0F 50 51 17 AD 50 A2 2A F2
            :         CE BE F1 E4 22 8C D7 61 A1 6C 87 61 62 92 CB A6
            :         80 EA B4 0F 09 9D 18 5F 39 A3 02 0E DB 38 4C E4
            :         8A 63 2E 72 8B DC BE 9E ED 6C 1A 47 DE 13 1B 0F
            :         83 29 4D 3E 08 86 FF 08 2B 43 09 EF 67 A7 6B EA
            :         77 62 30 35 4D A9 0F 0F DF CC 44 F5 4D 2C 2E 19
            :         E8 63 94 AC 84 A4 D0 01 E1 E3 97 16 CD 86 64 18
            :                 [ Another 11 bytes skipped ]
            :       }
 231 A0   63:     [0] {
 233 30   61:       SEQUENCE {
 235 06    9:         OBJECT IDENTIFIER extensionReq (1 2 840 113549 1 9
                                                      14)
 246 31   48:         SET {
 248 30   46:           SEQUENCE {
 250 30   44:             SEQUENCE {
 252 06    3:               OBJECT IDENTIFIER subjectAltName (2 5 29 17)
 257 04   37:               OCTET STRING
                              30 23 87 04 01 02 03 04 81 0D 65 6D 61 69


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                              6C 40 69 72 65 2E 63 6F 6D 82 0C 66 71 64
                              6E 2E 69 72 65 2E 63 6F 6D
            :               }
            :             }
            :           }
            :         }
            :       }
            :     }

 296 30   13:   SEQUENCE {
 298 06    9:     OBJECT IDENTIFIER md5withRSAEncryption (1 2 840 113549
                                                          1 1 4)
 309 05    0:     NULL
            :     }
 311 03  129:   BIT STRING 0 unused bits
            :     19 60 55 45 7F 72 FD 4E E5 3F D2 66 B0 77 13 9A
            :     87 86 75 6A E1 36 C6 B6 21 71 68 BD 96 F0 B4 60
            :     95 8F 12 F1 65 33 16 FD 46 8A 63 19 90 40 B4 B7
            :     2C B5 AC 63 17 50 28 F0 CD A4 F0 00 4E D2 DE 6D
            :     C3 4F F5 CB 03 4D C8 D8 31 5A 7C 01 47 D2 2B 91
            :     B5 48 55 C8 A7 0B DD 45 D3 4A 8D 94 04 3A 6C B0
            :     A7 1D 64 74 AB 8A F7 FF 82 C7 22 0A 2A 95 FB 24
            :     88 AA B6 27 83 C1 EC 5E A0 BA 0C BA 2E 6D 50 C7
            :   }


Appendix C:   Private OID Definitions

The OIDs used in defining pkiStatus are VeriSign self-maintained
OIDs. Please note, work is in progress to replace the VeriSign owned
object identifiers with the standard object identifiers. Once the
standarlization is completed, this documentation will be updated.

id-VeriSign   OBJECT_IDENTIFIER ::= {2 16 US(840) 1 VeriSign(113733)}
id-pki        OBJECT_IDENTIFIER ::= {id-VeriSign pki(1)}
id-attributes OBJECT_IDENTIFIER ::= {id-pki attributes(9)}
id-messageType  OBJECT_IDENTIFIER ::= {id-attributes messageType(2)}
id-pkiStatus    OBJECT_IDENTIFIER ::= {id-attributes pkiStatus(3)}
id-failInfo     OBJECT_IDENTIFIER ::= {id-attributes failInfo(4)}
id-senderNonce  OBJECT_IDENTIFIER ::= {id-attributes senderNonce(5)}
id-recipientNonce OBJECT_IDENTIFIER ::= {id-attributes recipientNonce(6)}
id-transId        OBJECT_IDENTIFIER ::= {id-attributes transId(7)}
id-extensionReq   OBJECT_IDENTIFIER ::= {id-attributes extensionReq(8)}


Liu/Madson/McGrew/Nourse                                        [Page 37]


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 Appendix D:   CRL Query by means of LDAP

 In order to retrieve the CRL by means of LDAP, the client needs to know
 where in the directory it is stored.  The certificate must contain a
 CRL Distribution Point extension encoded as a DN or as an LDAP URI.

For example, the certificate issued by Entrust VPN contains
the following DN as the CRL distribution point:



CN = CRL1, O = cisco, C = US.

 The asn.1 encoding of this distribution point is:

     30 2C 31 0B 30 09 06 03 55 04 06 13 02 55 53 31 0E 30 0C 06
     03 55 04 0A 13 05 63 69 73 63 6F 31 0D 30 0B 06 03 55 04 03
     13 04 43 52 4C 31


The ldap form would be:

ldap://servername/CN=CRL1,O=cisco,C=US



Appendix E:  SCEP State Transitions

SCEP state transitions are based on transaction identifier. The design
goal is to ensure the synchronization between the CA and the requester
under various error situations.


An identity is defined by the combination of FQDN, the IP address and
the client serial number.  FQDN is the required name attribute. It is
important to notice that, a client named as Alice.cisco.com is different
from the client named as Alice.cisco.com plus IPAddress 117.96.1.219.

Each enrollment transaction is uniquely associated with a transaction
identifier. Because the enrollment transaction could be interrupted by
various errors, including network connection errors or client reboot,
the SCEP client generates a transaction identifier by calculating a
hash on the public key value for which the enrollment is requested. This
retains the same transaction identifier throughout the enrollment
transaction, even if the client has rebooted or timed out, and issues a
new enrollment request for the same key pair.  It also provides the way
for the CA to uniquely identify a transaction in its database.  At the
requester side, it generates a transaction identifier which is included
in PKCSReq.  If the CA returns a response of PENDING, the requester
will poll by periodically sending out GetCertInitial with the same
transaction identifier until either a response other than PENDING is
obtained, or the configured maximum time has elapsed.

If the client times out or the client reboots, the client administrator
will start another enrollment transaction with the same key pair.  The
second enrollment will have the transaction idenifier.  At the server
side, instead of accepting the PKCSReq as a new enrollment request, it
should respond as if another GetCertInitial message had been sent with
that transaction ID.  In another word, the second PKCSReq should be
taken as a resynchronization message to allow the enrollment resume as
the same transaction.

It is important to keep the transaction id unique since SCEP requires the
same policy and same identity be applied to the same subject name and


Liu/Madson/McGrew/Nourse                                        [Page 38]


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key pair binding.  In the current implementation, an SCEP client can
only assume one identity. At any time, only one key pair, with a given
key usage, can be associated with the same identity.

The following gives several examples of client to CA transactions.

Client actions are indicated in the left column, CA actions are
indicated in the right column.  A blank action signifies that no message
was received.  Note that these examples assume that the CA enforces the
certificate-name uniqueness property defined in Section 2.1.1.1.

The first transaction, for example, would read like this:
 "Client Sends PKCSReq message with transaction ID 1 to the
  CA.  The CA signs the certificate and constructs a CertRep Message
  containing the signed certificate with a transaction ID 1.  The client
  receives the message and installs the cert locally."

Successful Enrollment Case: no manual authentication
PKCSReq (1)             ----------> CA Signs Cert
Client Installs Cert    <---------- CertRep (1) SIGNED CERT



Successful Enrollment Case:  manual authentication required
PKCSReq (10)            ----------> Cert Request goes into Queue
Client Polls            <---------- CertRep (10) PENDING
GetCertInitial (10)     ----------> Still pending
Client Polls            <---------- CertRep (10) PENDING
GetCertInitial (10)     ----------> Still pending
Client Polls            <---------- CertRep (10) PENDING
GetCertInitial (10)     ----------> Still pending
Client Polls            <---------- CertRep (10) PENDING
GetCertInitial (10)     ----------> Cert has been signed
Client Installs Cert    <---------- CertRep (10) SIGNED CERT



Resync Case - CA Receive and Signs PKCSReq, Client Did not receive
CertRep:

PKCSReq (3)           ----------> Cert Request goes into queue
                      <---------- CertRep (3) PENDING
GetCertInitial (3)    ---------->
                      <---------- CertRep (3) PENDING
GetCertInitial (3)    ----------->
                      <----------- CA signed Cert and sent back
                                   CertRep(3)
(Time Out)
PKCSReq (3)           ----------> Cert already signed, sent back to
                                  client
Client Installs Cert  <---------- CertRep (3) SIGNED CERT



Liu/Madson/McGrew/Nourse                                        [Page 39]


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Case when NVRAM is lost and client has to generate a new key pair, there
is no change of name information:

PKCSReq (4)           ----------> CA Signs Cert
Client Installs Cert  <---------- CertRep (4) SIGNED CERT
(Client looses Cert)
PKCSReq (5)           ----------> There is already a valid cert with
                                  this DN.
Client Admin Revokes  <---------- CertRep (5) OVERLAPPING CERT ERROR
PKCSReq (5)           ----------> CA Signs Cert
Client Installs Cert  <---------- CertRep (5) SIGNED CERT


Case when client admin resync the enrollment using a different PKCS#10:
PKCSReq (6)             ----------> CA Signs Cert
                        <---------- CertRep (6) SIGNED CERT
(Client timeout and admin starts another enrollment with a different
 PKCS#10, but the same transaction id)
PKCSReq (6)  with different PKCS#10
                        ----------> There is already a valid cert with
                                    this entity (by checking FQDN).
                        <---------- CertRep (6) INVALID PKCS#10 CERT
                                     ERROR
Client admin either revokes the existing cert
or corrects the error by enrolling with
the same PKCS#10 as the first PKCSReq(6)
PKCSReq (6)             ----------> CA find the existing Cert
Client Installs Cert    <---------- CertRep (6) SIGNED CERT


Resync case when server is slow in response:
PKCSReq (13)    ---------->  Cert Request goes into Queue
                <----------  CertRep (13) PENDING
GetCertInitial  ----------> Still pending
                <----------  CertRep (13) PENDING
GetCertInitial  ---------->  Still pending
                <----------  CertRep (13) PENDING
GetCertInitial  ---------->  Still pending
                <----------  CertRep (13) PENDING
GetCertInitial  ---------->  Still pending
(TimeOut)       <----------  CertRep (13) PENDING
* Case 1
PKCSReq (13)          ---------->  Still pending
Client polls          <----------  CertRep (13) PENDING
CertCertInitial       ---------->  Cert has been signed
Client Installs Cert  <----------  CertRep (13) SIGNED CERT
* Case 2
PKCSReq (13)          ----------> Cert has been signed
Client Installs Cert  <---------- CertRep (13) SIGNED CERT




Liu/Madson/McGrew/Nourse                                        [Page 40]


Cisco Systems' Simple Certificate Enrollment Protocol           Aug 2005

Appendix F. CA Capabilities

The response for a GetCACaps message is a list of CA capabilities, in
plain text, separated by <LF> characters, as follows (quotation marks
are NOT sent):

Keyword               Description

"GetNextCACert"       CA Supports the GetNextCACert message.
"POSTPKIOperation"    PKIOPeration messages may be sent via HTTP POST.
"Renewal"             Clients may use current certificate and key to
                      authenticate an enrollment request for a new
                      certificate.
"SHA-512"             CA Supports the SHA-512 hashing algorithm in
                      signatures and fingerprints.
"SHA-256"             CA Supports the SHA-256 hashing algorithm in
                      signatures and fingerprints.
"SHA-1"               CA Supports the SHA-1 hashing algorithm in
                      signatures and fingerprints.
"DES3"                CA Supports triple-DES for encryption.

The client should use SHA-1, SHA-256, or SHA-512 in preference to MD5
hashing if it is supported by the CA.

A client must be able to accept and ignore any unknown keywords that
might be sent back by a CA that implements a future version of SCEP.

If none of the above capabilities are supported by the CA, no data
is returned.

The appropriate HTML headers are returned in any case.

Example:

GET /cgi-bin/pkiclient.exe?operation=GetCACaps&message=myca

might return:

GetNextCACert
POSTPKIOperation

This means that the CA supports the GetNextCACert message and allows
PKIOperation messages (PKCSreq, GetCert, GetCertInitial...) to be sent
using HTTP POST.


Liu/Madson/McGrew/Nourse                                        [Page 41]


Cisco Systems' Simple Certificate Enrollment Protocol           Aug 2005

Appendix G. Certificate Renewal and CA Key Rollover

To renew a client certificate, use the PKCSreq message and sign it with
the existing client certificate instead of a self-signed certificate.

To obtain the new CA certificate prior to the expiration of the current
one, use the GetNextCACert message if the CA supports it.

To obtain a new client certificate signed by the new CA certificate,
use the new CA or RA certificate in the message envelope.


Example:

GetNextCACert           ---------->
                        <---------- CertRep (3) New CA certificate

PKCSReq* (1)            ----------> CA Signs certificate with NEW key
Client Stores Cert      <---------- CertRep (3) Certificate issued
for installation when               from NEW CA certificate and keypair.
existing cert expires.


*enveloped for new CA or RA cert and keypair.  The CA will use the
envelope to determine which key and certificate to use to issue the
client certificate.


Liu/Madson/McGrew/Nourse                                        [Page 42]


Cisco Systems' Simple Certificate Enrollment Protocol           Aug 2005

Appendix H. PKIOperation via HTTP POST Message

If the remote CA supports it, any of the PKCS#7-encoded SCEP messages
may be sent via HTTP POST instead of HTTP GET.   This is allowed for
any SCEP message except GetCACert, GetCACertChain, GetNextCACert,
or GetCACaps.  In this form of the message, Base 64 encoding is not
used.

POST /cgi-bin/pkiclient.exe?operation=PKIOperation
<binary PKCS7 data>

The client can verify that the CA supports SCEP messages via POST by
looking for the "POSTPKIOperation" capability (See Appendix F).






Liu/Madson/McGrew/Nourse                                        [Page 43]

Cisco Systems' Simple Certificate Enrollment Protocol           Aug 2005

Appendix Y. Author Contact Information

Xiaoyi Liu                      Cheryl Madson
Cisco                           Cisco
510 McCarthy Drive              510 McCarthy Drive
Milpitas, CA                    Milpitas, CA.
xliu@cisco.com                  cmadson@cisco.com


David McGrew                    Andrew Nourse
Cisco                           Cisco
170 West Tasman Drive           510 McCarthy Drive
San Jose, CA 94134              Milpitas, CA.
mcgrew@cisco.com                nourse@cisco.com




Appendix Z.  Copyright Section

Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.




This draft expires 22 Feb 2006

[End of draft-nourse-scep-12.txt]


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