Network Working Group P. Gutmann
Internet-Draft University of Auckland
Intended status: Standards Track M. Pritikin
Expires: March 23, 2016 Cisco
September 20, 2015
Simple Certificate Enrolment Protocol
draft-gutmann-scep-01.txt
Abstract
This document specifies the Simple Certificate Enrolment Protocol
(SCEP), a Public Key Infrastructure (PKI) communication protocol
which leverages existing technology by using CMS (formerly known as
PKCS #7) and PKCS #10 over HTTP. SCEP is the evolution of the
enrolment protocol sponsored by Cisco Systems, which now enjoys wide
support in both client and server implementations, as well as being
relied upon by numerous other industry standards that work with
certificates.
Status of This Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on March 23, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions Used in This Document . . . . . . . . . . . . 5
2. SCEP Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. SCEP Entities . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Requester . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2. Certification Authority . . . . . . . . . . . . . . . 6
2.1.3. Registration Authority . . . . . . . . . . . . . . . 6
2.1.4. CA/RA Certificate Distribution . . . . . . . . . . . 7
2.2. Requester authentication . . . . . . . . . . . . . . . . 8
2.3. Enrolment authorization . . . . . . . . . . . . . . . . . 9
2.4. Certificate Enrolment/Renewal/Update . . . . . . . . . . 10
2.4.1. Client State Transitions . . . . . . . . . . . . . . 10
2.5. Certificate Access . . . . . . . . . . . . . . . . . . . 12
2.6. CRL Access . . . . . . . . . . . . . . . . . . . . . . . 13
2.7. Certificate Revocation . . . . . . . . . . . . . . . . . 14
2.8. Mandatory-to-Implement Functionality . . . . . . . . . . 14
3. SCEP Secure Message Objects . . . . . . . . . . . . . . . . . 14
3.1. SCEP pkiMessage . . . . . . . . . . . . . . . . . . . . . 15
3.1.1. Signed Transaction Attributes . . . . . . . . . . . . 16
3.1.1.1. transactionID . . . . . . . . . . . . . . . . . . 18
3.1.1.2. messageType . . . . . . . . . . . . . . . . . . . 19
3.1.1.3. pkiStatus . . . . . . . . . . . . . . . . . . . . 19
3.1.1.4. failInfo . . . . . . . . . . . . . . . . . . . . 20
3.1.1.5. senderNonce and recipientNonce . . . . . . . . . 20
3.1.2. SCEP pkcsPKIEnvelope . . . . . . . . . . . . . . . . 20
3.2. SCEP pkiMessage types . . . . . . . . . . . . . . . . . . 21
3.2.1. PKCSReq/RenewalReq/UpdateReq . . . . . . . . . . . . 21
3.2.2. CertRep . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.2.1. CertRep SUCCESS . . . . . . . . . . . . . . . . . 22
3.2.2.2. CertRep FAILURE . . . . . . . . . . . . . . . . . 23
3.2.2.3. CertRep PENDING . . . . . . . . . . . . . . . . . 23
3.2.3. CertPoll (GetCertInitial) . . . . . . . . . . . . . . 23
3.2.4. GetCert . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.5. GetCRL . . . . . . . . . . . . . . . . . . . . . . . 24
3.3. Degenerate certificates-only CMS Signed-Data . . . . . . 25
3.4. CA Capabilities . . . . . . . . . . . . . . . . . . . . . 25
3.4.1. GetCACaps HTTP Message Format . . . . . . . . . . . . 25
3.4.2. CA Capabilities Response Format . . . . . . . . . . . 25
4. SCEP Transactions . . . . . . . . . . . . . . . . . . . . . . 27
4.1. Get CA Certificate . . . . . . . . . . . . . . . . . . . 27
4.1.1. Get CA Certificate Response Message Format . . . . . 28
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4.1.1.1. CA Certificate Response Message Format . . . . . 28
4.1.1.2. CA/RA Certificate Response Message Format . . . . 28
4.2. Certificate Enrolment/Renewal/Update . . . . . . . . . . 28
4.2.1. Certificate Enrolment/Renewal/Update Response Message 28
4.3. Poll for Requester Initial Certificate . . . . . . . . . 29
4.3.1. Polling Response Message Format . . . . . . . . . . . 29
4.4. Certificate Access . . . . . . . . . . . . . . . . . . . 30
4.4.1. Certificate Access Response Message Format . . . . . 30
4.5. CRL Access . . . . . . . . . . . . . . . . . . . . . . . 30
4.5.1. CRL Access Response Message Format . . . . . . . . . 30
4.6. Get Next Certification Authority Certificate . . . . . . 30
4.6.1. Get Next CA Response Message Format . . . . . . . . . 31
5. SCEP Transport . . . . . . . . . . . . . . . . . . . . . . . 31
5.1. HTTP GET and POST Message Formats . . . . . . . . . . . . 31
5.1.1. Response Message Format . . . . . . . . . . . . . . . 32
5.2. SCEP HTTP Messages . . . . . . . . . . . . . . . . . . . 33
5.2.1. GetCACert . . . . . . . . . . . . . . . . . . . . . . 33
5.2.1.1. GetCACert Response . . . . . . . . . . . . . . . 33
5.2.1.1.1. CA Certificate Only Response . . . . . . . . 33
5.2.1.1.2. CA and RA Certificates Response . . . . . . . 33
5.2.2. PKCSReq/RenewalReq/UpdateReq . . . . . . . . . . . . 34
5.2.2.1. PKCSReq/RenewalReq/UpdateReq Response . . . . . . 34
5.2.3. CertPoll . . . . . . . . . . . . . . . . . . . . . . 34
5.2.3.1. CertPoll Response . . . . . . . . . . . . . . . . 35
5.2.4. GetCert . . . . . . . . . . . . . . . . . . . . . . . 35
5.2.4.1. GetCert Response . . . . . . . . . . . . . . . . 35
5.2.5. GetCRL . . . . . . . . . . . . . . . . . . . . . . . 35
5.2.5.1. GetCRL Response . . . . . . . . . . . . . . . . . 35
5.2.6. GetNextCACert . . . . . . . . . . . . . . . . . . . . 35
5.2.6.1. GetNextCACert Response . . . . . . . . . . . . . 35
6. Contributors/Acknowledgements . . . . . . . . . . . . . . . . 36
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
8. Security Considerations . . . . . . . . . . . . . . . . . . . 36
8.1. General Security . . . . . . . . . . . . . . . . . . . . 37
8.2. Use of the CA keypair . . . . . . . . . . . . . . . . . . 37
8.3. Challenge Password . . . . . . . . . . . . . . . . . . . 38
8.4. Transaction ID . . . . . . . . . . . . . . . . . . . . . 38
8.5. Nonces and Replay . . . . . . . . . . . . . . . . . . . . 38
8.6. GetCACaps Issues . . . . . . . . . . . . . . . . . . . . 38
8.7. Unnecessary cryptography . . . . . . . . . . . . . . . . 38
8.8. GetNextCACert . . . . . . . . . . . . . . . . . . . . . . 39
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
9.1. Normative References . . . . . . . . . . . . . . . . . . 39
9.2. Informative References . . . . . . . . . . . . . . . . . 40
Appendix A. SCEP State Transitions . . . . . . . . . . . . . . . 40
Appendix B. Background Notes . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
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1. Introduction
Public key technology is widely available and increasingly widely
deployed. X.509 certificates serve as the basis for several
standards-based security protocols in the IETF, such as TLS [14], S/
MIME [13], and and IKE/IPsec [12]. When an X.509 certificate is
issued by other than the certificate subject (a self-issued
certificate), there typically is a need for a certificate management
protocol. Such a protocol enables a PKI client to request a
certificate, certificate renewal, certificate update, or certificate
revocation from a Certification Authority (CA).
This specification defines a protocol, Simple Certificate Enrolment
Protocol (SCEP), for certificate management and certificate and CRL
queries in a closed environment. While widely deployed, this
protocol omits some certificate management features, e.g. certificate
revocation transactions, which can significantly enhance the security
achieved in a PKI. The IETF protocol suite currently includes two
further certificate management protocols with more comprehensive
functionality: Certificate Management Protocol (CMP) [10] and
Certificate Management over CMS (CMC) [9]. Environments that do not
require interoperability with SCEP implementations MAY consider using
the above-mentioned certificate management protocols, however anyone
considering this step should be aware that the high level of
complexity of these two protocols has resulted in serious
interoperability problems and corresponding lack of industry support.
SCEP's simplicity, while being a drawback in terms of its limited
functionality, also makes deployment relatively straightforward, so
that it enjoys widespread industry support and ready interoperability
across a wide range of platforms. While implementers are encouraged
to investigate one of the more comprehensive alternative certificate
management protocols in addition to the protocol defined in this
specification, anyone wishing to deploy them should proceed with
caution, and consider support and interoperability issues before
committing to their use.
The protocol supports the following general operations:
o CA and Registration Authority (RA) public key distribution.
o Certificate enrolment.
o Certificate renewal/update.
o Certificate query.
o CRL query.
SCEP makes extensive use of CMS [3] and PKCS #10 [6].
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1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
2. SCEP Overview
This section provides a high level overview of the functionality of
SCEP.
2.1. SCEP Entities
The entity types defined in SCEP are
o The Requester, or client (Section 2.1.1).
o The Server, which may be either a Certification Authority (CA)
(Section 2.1.2) or a Registration Authority (RA) (Section 2.1.3).
2.1.1. Requester
The requester is sometimes called a "client" in this document. It is
the client of the SCEP exchange.
The requester MAY submit SCEP messages for itself or it MAY submit
SCEP messages on behalf of peers as described in Registration
Authority (Section 2.1.3). This section focuses on the requester
that is obtaining certificates for its own use.
Before a requester can start a PKI transaction, it MUST have at least
one appropriate key pair for use when signing the SCEP pkiMessage
(Section 3.1).
The message types, being based on CMS [3] and PKCS #10 [6], fully
support algorithm agility but the requester has to use a key type
that is supported by the server. Specifically, they must employ a
PKC algorithm capable of both encryption and signing. RSA is the
only widely-used algorithm that has these properties.
A requester MUST have the following information locally configured:
1. The Certification Authority IP address or fully qualified domain
name.
2. The Certification Authority HTTP CGI script path (this usually
has a default value, see Section 5.1).
3. The identifying information that is used for authentication of
the Certification Authority in Section 4.1.1, typically a
certificate fingerprint. This information MAY be obtained from
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the user, or presented to the end user for manual authorization
during the protocol exchange (e.g. the user indicates acceptance
of a fingerprint via a user-interface element).
The requester MAY maintain multiple independent configurations
appropriate for multiple Certification Authorities. Doing so does
not effect the protocol operation and is not in scope of this
document.
2.1.2. Certification Authority
A SCEP Certification Authority (CA) is the entity that signs client
certificates. A certification authority MAY enforce any arbitrary
policies and apply them to certification requests. The certification
authority MAY reject any request. If the client has already been
issued a certificate for this keypair the server MAY return the
previously created certificate. The requester MUST NOT assume any of
the fields in the certification request, except for the public key,
will be the same in the certificate issued.
The certification authority MAY include a cRLDistributionPoint
extension in every certificate it issues, make CRLs available via
HTTP [11] or LDAP, or answer CRL queries itself. In the latter case
it SHOULD be online at all times.
Since the client is expected to perform encryption and signature
verification using the CA certificate, the keyUsage extension in the
CA certificate MUST indicate that it is valid for digitalSignature
and keyEncipherment use alongside the usual CA usages of keyCertSign
and/or cRLSign.
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 transactionID. The CA SHOULD return the
status of the original transaction, including the certificate if it
was granted.
2.1.3. Registration Authority
A SCEP Registration Authority (RA) is a SCEP server that performs
validation and authorization checks of the SCEP requester but
forwards the certification requests to the CA. The RA's name does
not appear in the issuer field of resulting certificates.
Distribution of RA certificates is covered in Section 2.1.4. In
order to securely communicate with an RA using SCEP Secure Message
Objects (Section 3) the client specifies the RA as the recipient of
subsequent SCEP pkiMessages (see Section 3.1.2).
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In order to service certification requests the RA must pass the
requests to the CA server for signing. The RA MAY use SCEP to
communicate with the CA, in which case the RA acts as both a SCEP
server (between the client and the RA) and a SCEP requester (between
the RA and the CA). The RA MAY respond to client certificate
requests with a PENDING response while communicating with the CA; for
example if the CA must manually authorize a certification request and
thus returns PENDING to the RA the RA may respond with PENDING to the
client while polling the CA.
[Question: How does the client know whether an RA is in use? The
spec talks about the use of an RA as if both sides somehow know
that an RA rather than a CA is being used, but there's no obvious
way for the client to know this. The presence of multiple certs
in the cert chain can't be used as an indicator because some CAs
use distinct encryption and signing certs].
2.1.4. CA/RA Certificate Distribution
If the CA and/or RA certificates have not previously been acquired by
the requester in some other means, the requester MUST retrieve the
CA/RA certificates before any PKI operation (Section 3) can be
started.
Since no public key has yet been exchanged between the requester and
the CA/RA, the messages cannot be secured using CMS [3], and the data
is instead transferred in the clear.
If an RA is in use, a certificates-only CMS [3] Signed-Data message
with a certificate chain consisting of both RA and CA certificates is
returned. Otherwise the CA certificate itself is returned. The
transport protocol (Section 5) MUST indicate which one is returned.
The SCEP server CA certificate MAY be provided out-of-band to the
SCEP requester. Alternatively, the CA certificate fingerprint MAY be
used to authenticate a CA Certificate distributed by the GetCACert
response (Section 4.1) or via HTTP [11]. The fingerprint is created
by calculating a SHA-1, SHA-256, or SHA-512 hash over the whole CA
certificate.
After the requester gets the CA certificate, it SHOULD authenticate
the CA certificate by comparing the CA certificate fingerprint with
the locally configured, out-of-band distributed, identifying
information. RA certificates, if any, are signed by the CA so there
is no need to authenticate them against the out-of-band data.
Clients SHOULD verify the RA certificate signatures before use during
protocol exchanges.
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Because a long time can pass between queries from a requester to a
CA/RA and because RA certificates can change at any time, it is
recommended that a requester not store RA certificates. Instead, the
requester SHOULD retrieve the CA/RA certificates before each
operation.
2.2. 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
cryptographically secure manner. This requirement is needed to
prevent a man-in-the-middle (MITM) attack, in which an adversary can
manipulate the data as it travels between the protocol participants
and subvert the security of the protocol.
The communication between the requester and the certification
authority are secured using SCEP Secure Message Objects (Section 3)
which specifies how CMS [3] is used to encrypt and sign the data. In
order to perform the signing operation the client uses an appropriate
local certificate:
1. If the requester does not have an appropriate existing
certificate then a locally generated self-signed certificate MUST
be used. The self-signed certificate SHOULD use the same subject
name as in the PKCS #10 request. In this case the messageType is
PKCS10Req (see Section 3.1.1.2).
2. If the requesting system already has a certificate issued by the
SCEP server, and the server supports renewal (see Section 2.4),
that certificate SHOULD be used. In this case the messageType is
RenewalReq (see Section 3.1.1.2).
3. If the requesting system has no certificate issued by the new CA,
but has credentials from an alternate CA the certificate issued
by the alternate CA MAY be used. Policy settings on the new CA
will determine if the request can be accepted or not. This is
useful when enrolling with a new administrative domain using a
certificate from the old domain as credentials. In this case the
messageType is UpdateReq (see Section 3.1.1.2).
Note that although the above text describes three different types of
operations, in practice most implementations always apply the first
one even if an existing certificate already exists. For this reason
support for the first case is mandatory while support for the latter
two are optional (see Section 2.8).
During the certificate enrolment process, the requester MUST use the
selected certificate's key when signing the CMS [3] envelope (see
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Section 3). The server's CertResp then uses the same certificate's
public key when encrypting the response (see Section 3.2.2).
[Question: This is another area where the semantics were never
defined, what happens during a renewal or update? For an
enrolment the signing cert contains the key that's also in the
request, but what about for a renewal or update where they're
quite probably different keys? Should the envelope be
encrypted to the key in the request or the signing key?].
When the certification authority creates the CMS [3] envelope
containing the issued certificate, it SHOULD use the public key and
identifying information conveyed in the above included certificate.
This will inform the end entity of which private key is needed to
open the envelope. 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 encryption
certificate to be used to encrypt both responses. In any case, the
RecipientInfo on the envelope MUST reflect the key used to encrypt
the request.
[Question: Another undefined area, how is this dual-cert
operation supposed to work? Does the CA look up a previously-
issued encryption cert? Does anyone even care about this?].
2.3. Enrolment authorization
PKCS #10 [6] specifies a PKCS #9 [5] challengePassword attribute to
be sent as part of the enrolment request. When utilizing the
challengePassword, the server distributes a shared secret to the
requester which will uniquely associate the enrolment request with
the requester.
Inclusion of the challengePassword by the SCEP client is OPTIONAL and
allows for unauthenticated authorization of enrolment requests
(which, however, requires manual approval of each certificate issue,
see below), or for renewal or update requests which are authenticated
by being signed with an existing certificate. The CMS [3] envelope
protects the privacy of the challengePassword.
A client that is performing certificate renewal or update as per
Section 2.4 SHOULD omit the challengePassword but MAY send the
originally distributed password in the challengePassword attribute.
In the former case the SCEP CA MUST authenticate the request based on
the certificate used to sign the renewal or update request. In the
latter case the SCEP CA MAY use either the challengePassword or the
previously issued certificate (or both), depending on CA policy, to
authenticate the request. The SCEP server MUST NOT attempt to
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authenticate a client based on a self-signed certificate unless it
has been verified through out-of-band means such as a certificate
fingerprint.
To perform the authorization in manual mode the requester's messages
are placed in the PENDING state until the CA operator authorizes or
rejects them. Manual authorization is used when the client has only
a self-signed certificate that hasn't been previously authenticated
by the CA and/or a challengePassword is not available. The SCEP
server MAY either reject unauthorized certification requests or mark
them for manual authorization according to CA policy.
2.4. Certificate Enrolment/Renewal/Update
A requester starts an enrolment (Section 3.2.1) transaction by
creating a certificate request using PKCS #10 [6] and sends it to the
CA/RA enveloped using CMS [3] (Section 3).
If the CA supports certificate renewal or update then a new
certificate with new validity dates can be issued, even though the
old one is still valid, if the CA policy permits. The server MAY
automatically revoke the old client certificate. To renew or update
an existing certificate, the client uses the RenewalReq or UpdateReq
message (see Section 3.2) and signs it with the existing client
certificate. The client SHOULD use a new keypair when requesting a
new certificate, but MAY request a new certicate using the old
keypair.
If the CA/RA returns a CertRep (Section 3.2.2) message with status
set to PENDING, the requester enters into polling mode by
periodically sending a CertPoll (Section 3.2.3) PKI message to the
CA/RA, until the CA/RA operator completes the manual authentication
(approving or denying the request).
In general, the requester will send a single PKCSReq/RenewalReq/
UpdateReq (Section 3.2.1) message, followed by 0 or more CertPoll
(Section 3.2.3) messages, if polling mode is entered.
In general, the CA/RA will send 0 or more CertRep (Section 3.2.2)
messages with status set to PENDING, followed by a single CertRep
(Section 3.2.2) with status set to either SUCCESS or FAILURE.
2.4.1. Client State Transitions
The requester state transitions during enrolment operation are
indicated in Figure 1.
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CertPoll
+----<---+
| | CertRep(PENDING),
| | CertPoll send-timeout,
| | new-poll timer
| |
[CERT-NONEXISTENT] -----+---> [CERT-REQ-PENDING] [CERT-ISSUED]
^ PKCSReq | | ^
| RenewalReq | | |
| UpdateReq | +---------------+
| | CertRep(SUCCESS)
+--------------------------+
CertRep(FAILURE),
PKCS/Update/RenewalReq send-timeout,
max-time/max-polls exceeded
Figure 1: State Transition Diagram
The certificate issue process starts at the state CERT-NONEXISTENT.
Sending a PKCSReq/RenewalReq/UpdateReq message changes the state to
CERT-REQ-PENDING. If there is no response, or sending is not
possible, the state reverts back to CERT-NONEXISTENT.
Receiving a CertRep message with pkiStatus set to SUCCESS changes the
state to CERT-ISSUED.
Receiving a CertRep message with pkiStatus set to FAILURE changes the
state to CERT-NONEXISTENT.
If the server sends back a CertRep message with pkiStatus set to
PENDING, the requester will keep polling by sending a CertPoll
message to the server, until either a CertRep message with status set
to SUCCESS or FAILURE is received, or the maximum number of polls has
been exceeded.
If the maximum number of polls has been exceeded or a CertRep message
with pkiStatus set to FAILURE is received while in the CERT-REQ-
PENDING state, the end entity will transition to the CERT-NONEXISTENT
state, and the SCEP client can eventually initiate another enrolment
request. It is important to note that, as long as the requester does
not change its subject name or keys, the same transactionID may be
used in the "new" transaction. This is important because based on
this transactionID, the certification authority can recognize this as
an existing transaction instead of a new one.
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A successful transaction in automatic mode:
REQUESTER CA SERVER
PKCSReq: PKI cert. enrolment msg
--------------------------------> CertRep: pkiStatus = SUCCESS
certificate attached
<------------------------------
Receive issued certificate.
A successful transaction in manual mode:
REQUESTER CA SERVER
PKCSReq: PKI cert. enrolment msg
--------------------------------> CertRep: pkiStatus = PENDING
<------------------------------
CertPoll: polling msg
--------------------------------> CertRep: pkiStatus = PENDING
<------------------------------
................ <manual identity authentication> ...............
CertPoll: polling msg
--------------------------------> CertRep: pkiStatus = SUCCESS
certificate attached
<------------------------------
Receive issued certificate.
2.5. Certificate Access
A certificate query message is defined for clients to retrieve a copy
of their own certificate from the CA. It allows clients that do not
store their certificates locally to obtain a copy when needed. This
functionality is not intended to provide a general purpose
certificate store access service, which may be achieved via HTTP [11]
or LDAP.
To query a certificate from the certification authority, a requester
sends a request consisting of the certificate's issuer name and
serial number. This assumes that the requester has saved the issuer
name and the serial number of the issued certificate from the
previous enrolment transaction. The transaction to query a
certificate consists of one GetCert (Section 3.2.4) message and one
CertRep (Section 3.2.2) message, as shown below.
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REQUESTER CA SERVER
GetCert: PKI certificate query msg
-------------------------------> CertRep: pkiStatus = SUCCESS
certificate attached
<-----------------------------
Receive the certificate.
2.6. CRL Access
SCEP clients MAY request a CRL via one of three methods:
1. If the CA supports CRL Distribution Points (CRLDPs) [7], then the
CRL MAY be retrieved via the mechanism specified in the CRDLP.
2. If the CA supports HTTP [11], then the CRL MAY be retrieved via
the AuthorityInfoAcces [7] location specified in the certificate.
3. Only if the CA does not support CRDLPs or HTTP access should a
CRL query be composed by creating a GetCRL message consisting of
the issuer name and serial number from the certificate whose
revocation status is being queried.
The server SHOULD NOT support the GetCRL method because:
o It does not scale well due to the unnecessary cryptography (see
Section 8).
o It requires the CA to be a high-availability service.
o Only limited information to determine the CRL scope is provided
(see [7]).
The message is sent to the SCEP server in the same way as the other
SCEP requests. The transaction to retrieve a CRL consists of one
GetCRL PKI message and one CertRep PKI message, which contains only
the CRL (no certificates) in a degenerate certificates-only CMS [3]
Signed-Data message (Section 3.3), as shown below.
REQUESTER CA SERVER
GetCRL: PKI CRL query msg
---------------------------------->
CertRep: CRL attached
<-----------------------------
Receive the CRL
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2.7. Certificate Revocation
SCEP does not specify a method to request certificate revocation. In
order to revoke a certificate, the requester must contact the CA
using a non-SCEP defined mechanism.
2.8. Mandatory-to-Implement Functionality
At a minimum, all SCEP implementations compliant with this
specification MUST support GetCACert (Section 4.1), PKCSReq
(Section 3.2.1) (and its associated response messages), communication
of binary data via HTTP POST (Section 5.1), and the AES and SHA-256
algorithms to secure pkiMessages (Section 3.1).
For historical reasons implementations MAY support communications of
binary data via HTTP GET (Section 5.1), and the triple DES and SHA-1
algorithms to secure pkiMessages (Section 3.1).
3. SCEP Secure Message Objects
CMS [3] is a general enveloping mechanism that enables both signed
and encrypted transmission of arbitrary data. SCEP messages that
require confidentiality use two layers of CMS [3], as shown in
Figure 2. By applying both enveloping and signing transformations,
the SCEP message is protected both for the integrity of its end-to-
end transaction 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 invoking security handling procedures specific to
the information portion being processed.
Some messages do not require enveloping, in which case the Enveloped-
Data in Figure 2 is omitted.
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pkiMessage {
contentType = signedData
content {
pkcsPKIEnvelope { -- Optional
contentType = envelopedData
content {
recipientInfo
contentType = data
content {
messageData -- Typically PKCS #10 request
}
}
}
signerInfo {
signedAttrs {
transactionID
messageType
pkiStatus
failInfo
senderNonce
recipientNonce
}
signature
}
}
}
Figure 2: CMS Layering
When a particular SCEP message carries data, this data is carried in
the messageData. CertRep messages will lack any signed content and
consist only of a pkcsPKIEnvelope (Section 3.1.2).
Note: The remainder of this document will refer only to
'messageData', but it is understood to always be encapsulated in the
pkcsPKIEnvelope (Section 3.1.2). The format of the data in the
messageData is defined by the messageType attribute (see Section 3.1)
of the Signed-Data. If there is no messageData to be transmitted,
the entire pkcsPKIEnvelope MUST be omitted.
3.1. SCEP pkiMessage
The basic building block of all secured SCEP messages is the SCEP
pkiMessage. It consists of a CMS [3] Signed-Data content type. The
following restrictions apply:
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o The contentType in contentInfo MUST be data ({pkcs-7 1}) as
defined in CMS [3].
o The signed content, if present (e.g. FAILURE and PENDING CertRep
messages will lack any signed content), MUST be a pkcsPKIEnvelope
(Section 3.1.2), and MUST match the messageType attribute.
o The SignerInfo MUST contain a set of authenticatedAttributes (see
CMS [3] as well as Section 3.1.1 in this document).
At a minimum, all messages MUST contain the following
authenticatedAttributes:
o A transactionID attribute (see Section 3.1.1.1).
o A messageType attribute (see Section 3.1.1.2).
o A senderNonce attribute (see Section 3.1.1.5).
o Any attributes required by CMS [3].
If the message is a response, it MUST also include the following
authenticatedAttributes:
o A pkiStatus attribute (see Section 3.1.1.3).
o A recipientNonce attribute (see Section 3.1.1.5).
3.1.1. Signed Transaction Attributes
The following transaction attributes are encoded as authenticated
attributes, and are carried, as specified in CMS [3], in the
SignerInfo for this Signed-Data.
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+----------------+-----------------+--------------------------------+
| Attribute | Encoding | Comment |
+----------------+-----------------+--------------------------------+
| transactionID | PrintableString | Unique ID for this transaction |
| | | as a text string |
| | | |
| messageType | PrintableString | Decimal value as a numeric |
| | | text string |
| | | |
| pkiStatus | PrintableString | Decimal value as a numeric |
| | | text string |
| | | |
| failInfo | PrintableString | Decimal value as a numeric |
| | | text string |
| | | |
| senderNonce | OCTET STRING | Random nonce as a 16-byte |
| | | binary data string |
| | | |
| recipientNonce | OCTET STRING | Random nonce as 16-byte binary |
| | | data string |
+----------------+-----------------+--------------------------------+
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The OIDs used for these attributes are as follows:
+-------------------+-----------------------------------------------+
| Name | ASN.1 Definition |
+-------------------+-----------------------------------------------+
| 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-transactionID | OBJECT_IDENTIFIER ::= {id-attributes |
| | transactionID(7)} |
| | |
| 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)} |
+-------------------+-----------------------------------------------+
The attributes are detailed in the following sections.
3.1.1.1. transactionID
A PKI operation is a transaction consisting of the messages exchanged
between a requester and the server. The transactionID is a text
string generated by the client when starting a transaction. The
client MUST generate a unique string as the transaction identifier,
which MUST be used for all PKI messages exchanged for a given
enrolment, encoded as a PrintableString.
One means of generating the transactionID is as a SHA-1, SHA-256, or
SHA-512 hash of the public key value in the enrolment request when
encoded as an X.509 SubjectPublicKeyInfo [7] (in other words the
exact binary form in which it appears in both the request and the
resulting certificate) and then coverting it into a text string using
base64 encoding or ASCII hex digits. This allows the SCEP client to
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automatically generate the same transactionID for any given public
key. The SCEP protocol requires that transactionIDs be unique, so
that subsequent polling queries can be matched with previous
transactions. When separate signing and encryption certificates are
requested by the client, using distinct keypairs ensures that
distinct transactionIDs are also used when the transactionID is
created by hashing the X.509 SubjectPublicKeyInfo.
[Question: Again with the separate-certificate stuff...].
When using the certificate query and CRL query messages defined in
this protocol, the transactionID is required so that the requester
can match the response message with the outstanding request message.
For a non-enrolment message (for example GetCert and GetCRL), the
transactionID SHOULD be some value unique to the client.
3.1.1.2. messageType
The messageType attribute specifies the type of operation performed
by the transaction. This attribute MUST be included in all PKI
messages. The following message types are defined:
o CertRep ("3") -- Response to certificate or CRL request.
o RenewalReq ("17") -- PKCS #10 [6] certificate request for renewal
of an existing certificate.
o UpdateReq ("18") -- PKCS #10 [6] certificate request for update of
a certificate issued by a different CA.
o PKCSReq ("19") -- PKCS #10 [6] certificate request.
o CertPoll ("20") -- Certificate polling in manual enrolment.
o GetCert ("21") -- Retrieve a certificate.
o GetCRL ("22") -- Retrieve a CRL.
Undefined message types are treated as an error.
3.1.1.3. pkiStatus
All response messages MUST include transaction status information,
which is defined as pkiStatus attribute:
o SUCCESS ("0") -- request granted.
o FAILURE ("2") -- request rejected. When pkiStatus is FAILURE, the
failInfo attribute, as defined in Section 3.1.1.4, MUST also be
present.
o PENDING ("3") -- request pending for manual approval.
Undefined pkiStatus attributes are treated as an error.
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3.1.1.4. failInfo
The failInfo attribute MUST contain one of the following failure
reasons:
o badAlg ("0") -- Unrecognized or unsupported algorithm identifier.
o badMessageCheck ("1") -- integrity check failed.
o badRequest ("2") -- transaction not permitted or supported.
o badTime ("3") -- The signingTime attribute from the CMS [3]
authenticatedAttributes was not sufficiently close to the system
time (see Section 3.1.1.6).
o badCertId ("4") -- No certificate could be identified matching the
provided criteria.
[Question: Is there any demand for a free-form UTF8String
attribute to explain what really went wrong? Trying to sort
out an error when all you ever get back is the near-universal
badRequest is almost impossible, adding a failInfoText
attribute to address this could be quite useful since it
would allow expressing information such as a failure to meet
CA policy, or indeed anything more complex than "no go away"].
Undefined failInfo attributes are treated as an error.
3.1.1.5. senderNonce and recipientNonce
The attributes of senderNonce and recipientNonce are a 16 byte random
number generated for each transaction. These are intended to prevent
replay attacks.
When a sender sends a PKI message to a recipient, a senderNonce MUST
be included in the message. The recipient MUST copy the senderNonce
into the recipientNonce of the reply as a proof of liveliness. The
original sender MUST verify that the recipientNonce of the reply
matches the senderNonce it sent in the request. If the nonce does
not match, the message MUST be rejected.
[Question: What does this do for polling? Polling messages can
get lost so nonces will go out of sync, is there a need to
chain XXXReqs to polls via nonces? If not, why do we have two
nonces?].
3.1.2. SCEP pkcsPKIEnvelope
The information portion of a SCEP message is carried inside an
Enveloped-Data content type, as defined in CMS [3], with the
following restrictions:
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o contentType in encryptedContentInfo MUST be data ({pkcs-7 1}) as
defined in CMS [3].
o encryptedContent MUST be the SCEP message being transported (see
Section 4), and must match the messageType authenticated Attribute
in the pkiMessage.
The CMS [3] content-encryption key is encrypted using the public key
of the recipient of the message, i.e. the RA or the CA public key (if
sent from the requester), or the requester public key (if sent as a
reply to the requester).
3.2. SCEP pkiMessage types
All of the messages in this section are pkiMessages (Section 3.1),
where the type of the message MUST be specified in the 'messageType'
authenticated Attribute. Each section defines a valid message type,
the corresponding messageData formats, and mandatory authenticated
attributes for that type.
3.2.1. PKCSReq/RenewalReq/UpdateReq
The messageData for this type consists of a PKCS #10 [6]
Certification Request. The certification request MUST contain at
least the following items:
o The subject Distinguished Name.
o The subject public key.
o For a PKCSReq and if authorisation based on a password is being
used, a challengePassword attribute.
In addition to the authenticatedAttributes required for a valid CMS
[3] message, the pkiMessage MUST include the following attributes:
o A transactionID (Section 3.1.1.1) attribute.
o A messageType (Section 3.1.1.2) attribute set to PKCSReq,
RenewalReq, or UpdateReq as appropriate.
o A senderNonce (Section 3.1.1.5) attribute.
The pkcsPKIEnvelope for this message type is protected using the
public key of the recipient as detailed in Section 3.1.2, e.g. either
the CA or RA public key.
3.2.2. CertRep
The messageData for this type consists of a degenerate certificates-
only CMS [3] Signed-Data message (Section 3.3). The exact content
required for the reply depends on the type of request this message is
a reply to. They are detailed in Section 3.2.2.1 and in Section 4.
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In addition to the authenticatedAttributes required for a valid CMS
[3], this pkiMessage MUST include the following attributes:
o The transactionID (Section 3.1.1.1) attribute copied from the
request we are responding to.
o A messageType (Section 3.1.1.2) attribute set to CertRep.
o A senderNonce (Section 3.1.1.5) attribute.
o A recipientNonce attribute (Section 3.1.1.5) copied from the
senderNonce from the request that this is a response to.
o A pkiStatus (Section 3.1.1.3) set to the status of the reply.
The pkcsPKIEnvelope for this message type is protected using the
public key of the recipient as detailed in Section 3.1.2. For
example if a self-signed certificate was used to send the original
request then this self-signed certificate's public key is used to
encrypt the content-encryption key of the SUCCESS response's
pkcsPKIEnvelope.
Note that although it may appear that the senderNonce serves no
purpose in this message, it is required if the CertRep contains a
PENDING status since the nonce will be used in subsequent polling
operations.
3.2.2.1. CertRep SUCCESS
When the pkiStatus attribute is set to SUCCESS, the messageData for
this message consists of a degenerate certificates-only CMS [3]
Signed-Data message (Section 3.3). The content of this degenerate
certificates-only Signed-Data depends on what the original request
was, as outlined below.
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+--------------+----------------------------------------------------+
| Request-type | Reply-contents |
+--------------+----------------------------------------------------+
| PKCSReq | The reply MUST contain at least the issued |
| | certificate in the certificates field of the |
| | Signed-Data. The reply MAY contain additional |
| | certificates, but the issued certificate MUST be |
| | the leaf certificate. The reply MUST NOT contain |
| | a CRL. |
| | |
| RenewalReq | Same as PKCSReq |
| | |
| UpdateReq | Same as PKCSReq |
| | |
| CertPoll | Same as PKCSReq |
| | |
| GetCert | The reply MUST contain at least the requested |
| | certificate in the certificates field of the |
| | Signed-Data. The reply MAY contain additional |
| | certificates, but the requested certificate MUST |
| | be the leaf certificate. The reply MUST NOT |
| | contain a CRL. |
| | |
| GetCRL | The reply MUST contain the CRL in the crls field |
| | of the Signed-Data. The reply MUST NOT contain a |
| | certificate. |
+--------------+----------------------------------------------------+
3.2.2.2. CertRep FAILURE
When the pkiStatus attribute is set to FAILURE, the reply MUST also
contain a failInfo (Section 3.1.1.4) attribute set to the appropriate
error condition describing the failure. The pkcsPKIEnvelope
(Section 3.1.2) MUST be omitted.
3.2.2.3. CertRep PENDING
When the pkiStatus attribute is set to PENDING, the pkcsPKIEnvelope
(Section 3.1.2) MUST be omitted.
3.2.3. CertPoll (GetCertInitial)
This message is used for certificate polling. For unknown reasons it
was referred to as "GetCertInitial" in earlier drafts. The
messageData for this type consists of an IssuerAndSubject:
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issuerAndSubject ::= SEQUENCE {
issuer Name,
subject Name
}
The issuer is set to the subjectName of the CA (in other words the
intended issuerName of the certificate that's being requested). The
Subject is set to the subjectName used when requesting the
certificate.
In addition to the authenticatedAttributes required for a valid CMS
[3], this pkiMessage MUST include the following attributes:
o The same transactionID (Section 3.1.1.1) attribute from the
original PKCSReq/RenewalReq/UpdateReq message.
o A messageType (Section 3.1.1.2) attribute set to CertPoll.
o A senderNonce (Section 3.1.1.5) attribute.
o A recipientNonce attribute (Section 3.1.1.5) copied from the
senderNonce from the request that this is a response to.
3.2.4. GetCert
The messageData for this type consists of an IssuerAndSerialNumber as
defined in CMS [3] which uniquely identifies the certificate being
requested.
In addition to the authenticatedAttributes required for a valid CMS
[3], this pkiMessage MUST include the following attributes:
o A transactionID (Section 3.1.1.1) attribute.
o A messageType (Section 3.1.1.2) attribute set to GetCert.
o A senderNonce (Section 3.1.1.5) attribute.
A self-signed certificate MAY be used in the signed envelope. This
enables the requester to request their own certificate if they were
unable to store it previously.
3.2.5. GetCRL
The messageData for this type consists of a IssuerAndSerialNumber as
defined in CMS [3] containing the issuer name and serial number of
the certificate whose revocation status is being checked.
In addition to the authenticatedAttributes required for a valid CMS
[3], this pkiMessage MUST include the following attributes:
o A transactionID (Section 3.1.1.1) attribute.
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o A messageType (Section 3.1.1.2) attribute set to GetCRL.
o A senderNonce (Section 3.1.1.5) attribute.
3.3. Degenerate certificates-only CMS Signed-Data
CMS [3] includes a degenerate case of the CMS [3] Signed-Data content
type, in which there are no signers. The use of such a degenerate
case is to disseminate certificates and CRLs. For SCEP the content
field of the ContentInfo value of a degenerate certificates-only
Signed-Data MUST be omitted.
When carrying certificates, the certificates are included in the
'certificates' field of the Signed-Data. When carrying a CRL, the
CRL will be included in the 'crls' field of the Signed-Data.
3.4. CA Capabilities
In order to provide support for future enhancements to the protocol,
CAs SHOULD implement the GetCACaps message to allow clients to query
which functionality is available from the CA.
3.4.1. GetCACaps HTTP Message Format
This message requests capabilities from a CA, with the format:
"GET" CGI-PATH CGI-PROG "?operation=GetCACaps"
with the message components as described in Section 5. The response
is a list of text capabilities, as defined in Section 3.4.2. CA
servers SHOULD support the GetCACaps message and MUST support it when
they implement any extended functonality beyond the mandatory-to-
implement basics Section 2.8.
3.4.2. CA Capabilities Response Format
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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 |
+--------------------+----------------------------------------------+
| "AES" | CA Supports the AES encryption algorithm. |
| | |
| "DES3" | CA Supports the triple DES encryption |
| | algorithm. |
| | |
| "GetNextCACert" | CA Supports the GetNextCACert message. |
| | |
| "POSTPKIOperation" | PKIOPeration messages may be sent via HTTP |
| | POST. |
| | |
| "Renewal" | CA Supports the Renewal CA operation. |
| | |
| "SHA-1" | CA Supports the SHA-1 hashing algorithm. |
| | |
| "SHA-256" | CA Supports the SHA-256 hashing algorithm. |
| | |
| "SHA-512" | CA Supports the SHA-512 hashing algorithm. |
| | |
| "Update" | CA Supports the Update CA operation. |
+--------------------+----------------------------------------------+
The client SHOULD use SHA-256 or SHA-512 in preference to SHA-1
hashing, and AES in preference to triple DES if they are supported by
the CA.
Announcing some of these capabilities is redundant since they're
required as mandatory-to-implement functionality (see Section 2.8),
but it may be useful to announce them in order to deal with old
implementations that would otherwise default to obsolete, insecure
algorithms and mechanisms.
The server MUST use the texual case specified here, but clients
SHOULD ignore the textual case when processing this message. A
client MUST be able to accept and ignore any unknown keywords that
might be sent back by a CA.
If the CA supports none of the above capabilities the SCEP server
SHOULD return an empty message. A server MAY simply return an HTTP
error. A client that receives an empty message or an HTTP error
SHOULD interpret the response as if none of the requested
capabilities are supported by the CA.
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(Note that at least one widely-deployed server implementation
supports several of the above operations but doesn't support the
GetCACaps message to indicate that it supports them. This means that
the equivalent of GetCACaps must be performed through server
fingerprinting, which can be done using the ID string "Microsoft-
IIS").
The Content-type of the reply SHOULD be "text/plain". Clients SHOULD
ignore the Content-type, as older server implementations of SCEP may
send various Content-types.
Example:
GET /cgi-bin/pkiclient.exe?operation=GetCACaps
might return:
AES
SHA-256
GetNextCACert
POSTPKIOperation
This means that the CA supports modern crypto algorithms, the
GetNextCACert message, and allows PKIOperation messages
(PKCSReq/RenewalReq/UpdateReq, GetCert, CertPoll, ...) to be sent
using HTTP POST.
4. SCEP Transactions
This section describes the SCEP Transactions, without explaining the
transport. The transport of each message is discussed in Section 5.
Some of the transaction-requests have no data to send, i.e. the only
data is the message-type itself (e.g. a GetCACert message has no
additional data).
In this section, each SCEP transaction is specified in terms of the
complete messages exchanged during the transaction.
4.1. Get CA Certificate
To get the CA certificate(s), the requester sends a GetCACert message
to the server. There is no request data associated with this message
(see Section 5.2.1).
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4.1.1. Get CA Certificate Response Message Format
The response depends on whether the responding server has RA
certificates or only a single CA certificate. The server MUST
indicate which response it is sending via the transport protocol used
(see Section 5.2.1).
All returned certificates MUST conform to PKIX [7].
If the requester does not have a certificate path to a trust anchor
certificate, the SHA-1, SHA-256, or SHA-512 fingerprint of the
returned CA certificate (communicated via out-of-band means) may be
used to verify it.
4.1.1.1. CA Certificate Response Message Format
If the server does not have any RA Certificates, the response
consists of a single X.509 CA certificate.
4.1.1.2. CA/RA Certificate Response Message Format
If the server has RA Certificates, the response consists of a
degenerate certificates-only CMS [3] Signed-Data (Section 3.3)
containing the CA and RA certificates, with the RA certificate(s) as
the leaf certificate(s).
4.2. Certificate Enrolment/Renewal/Update
A PKCSReq/RenewalReq/UpdateReq (Section 3.2.1) message is used to
perform a certificate enrolment, renewal, or update transaction.
The reply MUST be a CertRep (Section 3.2.2) message sent back from
the server, indicating SUCCESS, FAILURE, or PENDING.
Precondition: Both the requester and the certification authority have
completed their initialization process. The requester has already
been configured with the CA/RA certificate.
Postcondition: The requester receives the certificate, the request is
rejected, or the request is pending. A pending response might
indicate that manual authentication is necessary.
4.2.1. Certificate Enrolment/Renewal/Update Response Message
If the request is granted, a CertRep (Section 3.2.2) message with
pkiStatus set to SUCCESS is returned. The reply MUST also contain
the certificate (and MAY contain any other certificates needed by the
requester). The issued certificate MUST be the first in the list.
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If the request is rejected, a CertRep (Section 3.2.2) message with
pkiStatus set to FAILURE is returned. The reply MUST also contain a
failInfo attribute.
If the the CA is configured to manually authenticate the requester, a
CertRep (Section 3.2.2) message with pkiStatus set to PENDING MAY be
returned. The CA MAY return a PENDING for other reasons.
4.3. Poll for Requester Initial Certificate
Triggered by a CertRep (Section 3.2.2) with pkiStatus set to PENDING,
a requester will enter the polling state by periodically sending
CertPoll messages (Section 3.2.3) to the server, until either the
request is granted and the certificate is sent back, or the request
is rejected, or some preconfigured time limit for polling or maximum
number of polls is exceeded.
CertPoll messages exchanged during the polling period MUST carry the
same transactionID attribute as the previous PKCSReq/RenewalReq/
UpdateReq. A server receiving a CertPoll for which it does not have
a matching PKCSReq/RenewalReq/UpdateReq MUST ignore this request.
Since at this time the certificate has not been issued, the requester
can only use its own subject name (which was contained in the
original PKCS# 10 sent via PKCSReq/RenewalReq/UpdateReq) to identify
the polled certificate request. In theory there can be multiple
outstanding requests from one requester (for example, if different
keys and different key-usages were used to request multiple
certificates), so the transactionID must also be included to
disambiguate between multiple requests. In practice however it's
safer for the requester to not have multiple requests outstanding at
any one time, since this tends to confuse some servers.
PreCondition: The requester has received a CertRep with pkiStatus set
to PENDING.
PostCondition: The requester has either received a valid response,
which could be either a valid certificate (pkiStatus = SUCCESS), or a
FAILURE message, or the polling period times out.
4.3.1. Polling Response Message Format
The response messages for CertPoll are the same as in Section 4.2.1.
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4.4. Certificate Access
A requester can query an issued certificate from the SCEP server, as
long as the requester knows the issuer name and the issuer assigned
certificate serial number.
This transaction consists of one GetCert (Section 3.2.4) message sent
to the server by a requester, and one CertRep (Section 3.2.2) message
sent back from the server.
PreCondition: The certification authority has issued the queried
certificate and the issuer assigned serial number is known.
PostCondition: Either the certificate is sent back or the request is
rejected.
4.4.1. Certificate Access Response Message Format
In this case, the CertRep from the server is same as in
Section Section 4.2.1, except that the server will only either grant
the request (SUCCESS) or reject the request (FAILURE).
4.5. CRL Access
Clients can request a CRL from the SCEP server as described in
Section 2.6.
PreCondition: The certification authority certificate has been
downloaded to the end entity.
PostCondition: CRL sent back to the requester.
4.5.1. CRL Access Response Message Format
The CRL is sent back to the requester in a CertRep (Section 3.2.2)
message. The information portion of this message is a degenerate
certificates-only Signed-Data (Section 3.3) that contains only the
most recent CRL in the crls field of the Signed-Data.
4.6. Get Next Certification Authority Certificate
When the CA certificate expires all certificates that have been
signed by it are no longer valid. CA key rollover provides a
mechanism by which the server MAY distribute a new CA certificate
which is valid in the future; when the current certificate has
expired. When a CA certificate is about to expire, clients need to
retrieve the CA's next CA certificate (i.e. the rollover
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certificate). This is done via the GetNextCACert message. There is
no request data associated with this message (see Section 5.2.6).
Clients MUST store the not-yet-valid CA certificate, and any not-yet-
valid client certificates obtained, until such time that they are
valid, at which point clients switch over to using the newly valid
certificates.
4.6.1. Get Next CA Response Message Format
The response consists of a Signed-Data CMS [3], signed by the current
CA (or RA) signing key. Clients MUST validate the signature on the
the Signed-Data CMS [3] before accepting any of its contents.
The content of the Signed-Data CMS [3] message is a degenerate
certificates-only Signed-Data (Section 3.3) message containing the
new CA certificate and any new RA certificates, as defined in
Section 5.2.1.1.2, to be used when the current CA certificate
expires.
If the CA (or RA) does not have the rollover certificate(s) it MUST
reject the request. It SHOULD also remove the GetNextCACert setting
from the capabilities until it does have rollover certificates.
If there are any RA certificates in this response, clients MUST check
that these RA certificates are signed by the CA, and MUST check
authorization of these RA certificates (see Section 2.1.3).
5. SCEP Transport
HTTP [4] is used as the transport protocol for SCEP Message Objects.
5.1. HTTP GET and POST Message Formats
SCEP uses the HTTP "GET" and "POST" messages to exchange information
with the CA. The following defines the syntax of a HTTP GET and POST
messages sent from a requester to a certification authority server:
"GET" CGI-PATH CGI-PROG "?operation=" OPERATION "&message=" MESSAGE
"POST" CGI-PATH CGI-PROG "?operation=" OPERATION
where:
o CGI-PATH defines the actual CGI path to invoke the CGI program
that parses the request.
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o 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, or ignore both if it's not issuing certificates via a
web server. Typically, setting CGI-PATH/CGI-PROG to "/cgi-bin/
pkiclient.exe" will satisfy most servers.
o OPERATION depends on the SCEP transaction and is defined in the
following sections.
o MESSAGE depends on the SCEP transaction and is defined in the
following sections.
Early SCEP drafts performed all communications via "GET" messages,
including non-idempotent ones that should have been sent via "POST"
messages. This has caused problems because of the way that the
(supposedly) idempotent GET interacts with caches and proxies, and
because the extremely large GET requests created by encoding CMS
messages may be truncated in transit. These issues are typically not
visible when testing on a LAN, but crop up during deployment over
WANs. If the remote CA supports it, any of the CMS [3]-encoded SCEP
messages SHOULD be sent via HTTP POST instead of HTTP GET. This is
allowed for any SCEP message except GetCACert, GetNextCACert, or
GetCACaps, and avoids the need for base64- and URL-encoding that's
required for GET messaging. The client can verify that the CA
supports SCEP messages via POST by looking for the "POSTPKIOperation"
capability (See Section 3.4.2).
If your client or server uses HTTP GET and encounters HTTP-related
problems such as messages being truncated, seeing errors such as HTTP
414 ("Request URI too long"), or simply having the message not sent/
received at all, when standard requests to the server (for example
via a web browser) work, then this is a symptom of the problematic
use of HTTP GET. The solution to this problem is typically to move
to HTTP POST instead. In addition when using GET it's recommended to
test your implementation over the public internet from as many
locations as possible to determine whether the use of GET will cause
problems with communications.
When using GET messages to communicate binary data, base64 encoding
as specified in [2] MUST be used. The base64 encoded data is
distinct from "base64url" and may contain URI reserved characters,
thus it MUST be escaped as specified in [8] in addition to being
bas64 encoded.
5.1.1. Response Message Format
For each GET or POST operation, the CA/RA server MUST return a
Content-Type and appropriate response data, if any.
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5.2. SCEP HTTP Messages
This section describes the OPERATION and MESSAGE values for SCEP
exchanges.
5.2.1. GetCACert
The OPERATION MUST be set to "GetCACert".
5.2.1.1. GetCACert Response
The response for GetCACert is different between the case where the CA
directly communicates with the requester during the enrolment, and
the case where a RA exists and the requester communicates with the RA
during the enrolment.
5.2.1.1.1. CA Certificate Only Response
The response will have a Content-Type of "application/x-x509-ca-
cert".
The body of this response consists of an X.509 CA certificate, as
defined in Section 4.1.1.1:
"Content-Type:application/x-x509-ca-cert"
<binary X.509>
5.2.1.1.2. CA and RA Certificates Response
The response will have a Content-Type of "application/x-x509-ca-ra-
cert".
The body of this response consists of a degenerate certificates-only
CMS [3] Signed-Data (Section 3.3) message containing both CA and RA
certificates, as defined in Section 4.1.1.2:
"Content-Type:application/x-x509-ca-ra-cert"
<binary CMS>
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5.2.2. PKCSReq/RenewalReq/UpdateReq
The OPERATION MUST be set to "PKIOperation". Note that when used
with HTTP POST, the only OPERATION possible is "PKIOperation", so
many servers don't check these values, or even notice their absence.
[Question: Should this be made optional? "POSTPKIOperation"
already hard-codes POST == PKIOperation, and since many servers
don't check it, it seems like more of a MAY than a MUST].
The MESSAGE consists of a PKCSReq, RenewalReq, or UpdateReq SCEP
message. When implemented using HTTP POST this might look as
follows:
POST /cgi-bin/pkiclient.exe?operation=PKIOperation HTTP/1.1
Content-Length: <length of data>
<binary CMS data>
When implemented using HTTP GET this might look as follows:
GET /cgi-bin/pkiclient.exe?operation=PKIOperation& \
message=MIAGCSqGSIb3DQEHA6CAMIACAQAxgDCBzAIBADB2MG \
IxETAPBgNVBAcTCE......AAAAAA== HTTP/1.1
5.2.2.1. PKCSReq/RenewalReq/UpdateReq Response
The response will have a Content-Type of "application/x-pki-message".
The body of this response consists of a CertRep SCEP message defined
in Section 4.2.1. The following is an example of the response:
"Content-Type:application/x-pki-message"
<binary CertRep msg>
5.2.3. CertPoll
The OPERATION MUST be set to "PKIOperation". The MESSAGE consists of
a CertPoll SCEP message.
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5.2.3.1. CertPoll Response
The body of this response consists of a CertRep SCEP message defined
in Section 4.3.1.
5.2.4. GetCert
The OPERATION MUST be set to "PKIOperation". The MESSAGE consists of
a GetCert SCEP message.
5.2.4.1. GetCert Response
The body of this response consists of a CertRep SCEP message defined
in Section 4.4.1.
5.2.5. GetCRL
The OPERATION MUST be set to "PKIOperation". The MESSAGE consists of
a GetCRL SCEP message.
5.2.5.1. GetCRL Response
The body of this response consists of a CertRep SCEP message defined
in Section 4.5.1.
5.2.6. GetNextCACert
The OPERATION MUST be set to "GetNextCACert".
5.2.6.1. GetNextCACert Response
The response will have a Content-Type of "application/x-x509-next-ca-
cert".
The body of this response consists of a Signed-Data CMS [3], as
defined in Section 4.6.1. (This is similar to the GetCert response
but does not include any of the attributes defined in Section 3.1.1).
"Content-Type:application/x-x509-next-ca-cert"
<binary CMS>
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6. Contributors/Acknowledgements
The editor would like to thank all the previous editors, authors and
contributors: Cheryl Madson, Xiaoyi Liu, David McGrew, David Cooper,
Andy Nourse, Max Pritikin, Jan Vilhuber, etc for their work
maintaining the draft over the years. Numerous other people have
contributed during the long life cycle of the draft and all deserve
thanks.
The earlier 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 early
versions of this protocol and this document.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
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 enrolment 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. The adversary
is computationally limited, but that can manipulate data during
transmission (that is, can act as a MITM). The precise meaning of
'computationally limited' depends on the implementer's choice of one-
way hash functions and cryptographic algorithms.
The first and second goals are met through the use of CMS [3] and
PKCS #10 [6] encryption and digital signatures using authenticated
public keys. The CA's public key is authenticated via out-of-band
means such as the checking of the CA fingerprint, as specified in
Section 2.1.2, and the SCEP client's public key is authenticated
through manual or pre-shared secret authentication, as specified in
Section 2.2. The third goal is met through the use of a
challengePassword for revocation, which is chosen by the SCEP client
and communicated to the CA protected by the CMS [3] Enveloped-Data,
as specified in Section 2.7.
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[Question: Uhh, the protocol doesn't support revocation
requests, should this be removed to match what it actually
does or should be spec be updated to match the description
here?].
The motivation of the first security goal is straightforward. The
motivation for the second security goal is to protect the identity
information in the enrolment requests and issued certificates.
Subsequent protocols can use the certificate in ways that either
expose the identity information, or protect it, depending on the
security requirements of those protocols. The motivation for the
third security goal is to protect the SCEP clients from denial of
service attacks.
8.1. General Security
Common key-management 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. This is especially true for CA keys, which, when
compromised, compromise the security of all relying parties.
8.2. Use of the CA keypair
A CA key pair is generally meant for (and is usually flagged as)
certificate (and CRL) signing exclusively, rather than data signing
or encryption. The SCEP protocol, however, uses the CA private key
to both encrypt and sign CMS [3] transport messages. This is
generally considered undesirable, as it widens the possibility of an
implementation weakness, and provides:
o Another place that the private key must be used (and hence is
slightly more vulnerable to exposure).
o Another place where a side channel attack (say, timing or power
analysis) might be used.
o Another place that the attacker might somehow insert their own
data and get it signed by the CA's private key (note that this
issue is purely theoretical, since the CMS data signed by the CA
is nothing remotely like a certificate and couldn't be passed off
as such).
One solution to this problem is to use RA keys to secure the SCEP
transport (i.e. message signing and encrypting), which allows the CA
keys to be used only for their intended purpose of certificate
signing. An RA can be implemented in two ways, physically separate
or implicit. In the implicit case, the CA simply creates an extra
key pair. A physically separate RA allows the CA to be inside the
secure network, not accessible to hackers at all.
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The corresponding downside of using an RA is that it makes the client
side considerably more complex, as the key used by the CA to issue
certificates is no longer the same one used by the client to
communicate with the CA. This requires that the client keep track of
multiple keys rather than a single CA key.
8.3. Challenge Password
The challengePassword sent in the PKCS #10 enrolment request is
signed and encrypted by way of being encapsulated in a pkiMessage.
When saved by the CA, care should be taken to protect this password.
If the challengePassword is used to automatically authenticate an
enrolment request, it is recommended that some form of one-time
password be used to minimize damage in the event the data is
compromised.
8.4. Transaction ID
CAs/RAs SHOULD NOT rely on the transactionID to be correct or as
specified in this document. Requesters with buggy software might add
additional undetected duplicate requests to the CA's queue. A well-
written CA/RA should never assume the data from a requester is well-
formed.
8.5. Nonces and Replay
In order to detect replay attacks, both sides need to maintain state
information sufficient to detect an unexpected nonce value.
8.6. GetCACaps Issues
The GetCACaps response is not signed. This allows an attacker to
perform downgrade attacks on the cryptographic capabilities of the
client/CA exchange.
8.7. Unnecessary cryptography
Some of the SCEP exchanges use signing and encryption operations that
are not necessary. In particular the GetCert and GetCRL exchanges
are encrypted and signed in both directions. The information
requested is public and thus signing the requests is of questionable
value but also CRLs and Certificates, i.e. the respective responses,
are already signed by the CA and can be verified by the recipient
without requiring additional signing and encryption.
This may affect performance and scalability of the CA and could be
used as an attack vector on the CA (though not an anonymous one).
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The use of CRLDPs as well as other ways of retrieving certificates
such as HTTP access and LDAP are recommended for CRL access.
8.8. GetNextCACert
GetNextCACert depends on a 'flag moment' at which every client in the
PKI infrastructure switches from the current CA certificate (and
client certificate) to the new CA certificate and client
certificates. Proper monitoring of the network infrastructure can
ensure that this will proceed as expected but any errors in
processing or implementation can result in a failure of the PKI
infrastructure.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[3] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 5652, September 2009.
[4] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[5] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
November 2000.
[6] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
November 2000.
[7] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "PKCS #10: Certification Request
Syntax Specification Version 1.7", RFC 5280, May 2008.
[8] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
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9.2. Informative References
[9] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.
[10] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
[11] Gutmann, P., "Internet X.509 Public Key Infrastructure
Operational Protocols: Certificate Store Access via HTTP",
RFC 4387, February 2006.
[12] Alighieri, D., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, March 1300.
[13] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[14] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Appendix A. SCEP State Transitions
SCEP state transitions are indexed by the transactionID attribute.
The design goal is to ensure the synchronization between the CA and
the requester under various error situations.
Each enrolment transaction is uniquely associated with a
transactionID (carried in the transactionID signed attribute (see
Section 3.1.1.1). Because the enrolment transaction could be
interrupted by various errors, including network connection errors or
client reboot, the SCEP client generates a fixed transaction
identifier as specified in Section 3.1.1.1 which is included in the
PKCSReq/RenewalReq/UpdateReq. If the CA returns a response of
PENDING, the requester will poll by periodically sending a CertPoll
with the same transaction identifier until either a response other
than PENDING is obtained or the configured maximum time has elapsed.
This mechanism retains the same transaction identifier throughout the
enrolment transaction.
If the client times out or reboots, the client administrator will
start another transaction with the same key pair. The second
enrolment will have the same transactionID. At the server side,
instead of accepting the PKCSReq/RenewalReq/UpdateReq as a new
request, it can respond as if another CertPoll message had been sent
with that transaction ID. The second PKCSReq/RenewalReq/UpdateReq
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should be taken as a resynchronization message to allow the process
to resume as the same transaction.
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.
The first transaction, for example, would read like this:
"Client Sends PKCSReq message with transactionID 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 certificate locally."
Successful Enrolment Case: no manual authentication
PKCSReq (1) ----------> CA Signs Cert
Client Installs Cert <---------- CertRep (1) SIGNED CERT
Successful Enrolment Case: manual authentication required
PKCSReq (10) ----------> Cert Request goes into Queue
Client Polls <---------- CertRep (10) PENDING
CertPoll (10) ----------> Still pending
Client Polls <---------- CertRep (10) PENDING
CertPoll (10) ----------> Still pending
Client Polls <---------- CertRep (10) PENDING
CertPoll (10) ----------> Still pending
Client Polls <---------- CertRep (10) PENDING
CertPoll (10) ----------> Cert has been signed
<---------- CertRep (10) SIGNED CERT
Client Installs Cert
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Resync Case 1 - CA Receives PKCSReq, sends PENDING, eventually grants
the certificate and returns SUCCESS, with the certificate. The
SUCCESS gets lost:
PKCSReq (3) ----------> Cert Request goes into queue
<---------- CertRep (3) PENDING
CertPoll (3) ----------> Still pending
<---------- CertRep (3) PENDING
CertPoll (3) ----------> Cert has been signed
X-------- CertRep(3) SIGNED CERT
(Time Out)
PKCSReq (3) ----------> Cert already granted
<---------- CertRep (3) SIGNED CERT
Client Installs Cert
Resync Case 2 - CA Receives PKCSReq, sends PENDING, PENDING reply
gets lost:
PKCSReq (3) ----------> Cert Request goes into queue
X-------- CertRep (3) PENDING
(Time Out)
PKCSReq (3) ---------->
<---------- CertRep (3) PENDING
etc...
Case when the Certificate is lost, the CA arbitrarily refuses to sign
a replacement (enforcing name-uniqueness) until the original
certificate has been revoked (there is no change of name
information):
PKCSReq (4) ----------> CA Signs Cert
<---------- CertRep (4) SIGNED CERT
Client Installs Cert
(Client looses Cert)
PKCSReq (5) ----------> There is already a valid cert with
this DN.
<---------- CertRep (5) BAD REQUEST
Admin Revokes
PKCSReq (5) ----------> CA Signs Cert
<---------- CertRep (5) SIGNED CERT
Client Installs Cert
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CA certificate rollover case:
GetNextCACert ---------->
<---------- New CA certificate
PKCSReq* ----------> CA Signs certificate with NEW
key
Client Stores Cert <---------- CertRep - Certificate issued
for installation when from NEW CA certificate and key
existing cert expires. pair
*enveloped for new CA or RA cert and key pair. The CA will use the
envelope to determine which key and certificate to use to issue the
client certificate.
Appendix B. Background Notes
This specification has spent more than fifteen years in the draft
stage. Its original goal, provisioning IPsec routers with RSA
certificates, has long since changed to general device/embedded
system/IoT use. To fit this role, extra features were bolted on in a
haphazard manner through the addition of a growing list of appendices
and by inserting additional, often conflicting, paragraphs in various
locations in the body text. Since existing features were never
updated as newer ones were added, the specification accumulated large
amounts of historical baggage over time. If OpenPGP was described as
"a museum of 1990s crypto" then the SCEP draft was its graveyard.
About five years ago the specification, which even at that point had
seen only sporadic re-posts of the existing document, was more or
less abandoned by its original sponsors. Due to its widespread use
in large segments of the industry, the specification was rebooted in
2015, cleaning up fifteen years of accumulated cruft, fixing errors,
clarifying ambiguities, and bringing the algorithms and standards
used into the current century (prior to the update, the de-facto
lowest-common denominator algorithms used for interoperability were
the forty-year-old single DES and broken MD5 hash algorithms).
Other changes include:
o Resolved contradictions in the text, for example a requirement
given as a MUST in one paragraph and a SHOULD in the next, a MUST
NOT in one paragraph and a MAY a few paragraphs later, a SHOULD
NOT contradicted later by a MAY, and so on.
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o Merged several later fragmentary addenda placed in appendices (for
example the handling of certificate renewal and update) with the
body of the text.
o Updated the algorithms to ones dating from at least this century.
o Did the same for normative references to other standards.
o Corrected incorrect references to other standards, e.g.
IssuerAndSerial -> IssuerAndSerialNumber.
o Corrected errors such as a statement that when both signature and
encryption certificates existed, the signature certificate was
used for encryption.
o Condensed redundant discussions of the same topic spread across
multiple sections into a single location. For example the
description of RA certificate handling previously existed in three
different locations, with slightly different reqirements in each
one.
o Relaxed some requirements that didn't serve any obvious purpose
and that major implementations didn't seem to be enforcing. For
example the requirement that the self-signed certificate used with
a request MUST contain a subject name that matched the one in the
PKCS #10 request was relaxed to a SHOULD because a number of
implementations either ignored the issue entirely or at worst
performed some minor action like creating a log entry after which
they continued anyway.
o Clarified sections that were unclear or even made no sense, for
example the requirement for a "hash on the public key [sic]"
encoded as a PrintableString.
o Clarified certificate renewal and update. These represent a
capability that was bolted onto the original protocol with (at
best) vaguely-defined semantics, including a requirement by the
server to guess whether a particular request was a renewal or not
(updates were even more vaguely defined). In response to
developer feedback that they either avoided renewal/update
entirely because of this uncertainty or hardcoded in particular
behaviour on a per-server basis, this specification explicitly
identifies renewal and update requests as such, and provides
proper semantics for both. Note that this is still a work in
progress due to the lack of clarity of the original spec in this
area, see some of the questions inline with the text.
Authors' Addresses
Peter Gutmann
University of Auckland
Department of Computer Science
Auckland
New Zealand
Email: pgut001@cs.auckland.ac.nz
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Max Pritikin
Cisco Systems, Inc
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