Internet Engineering Task Force A. Nourse
Internet-Draft X. Liu
Intended status: Informational J. Vilhuber
Expires: July 25, 2009 C. Madson
Cisco Systems, Inc.
January 21, 2009
Cisco Systems' Simple Certificate Enrollment Protocol
draft-nourse-scep-18
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. The Goal of SCEP . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. SCEP Entity Types . . . . . . . . . . . . . . . . . . . . 6
2.1.1. Requesters . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1.1. Local Key/Certificate/CRL Storage and
Certificate-name uniqueness . . . . . . . . . . . 7
2.1.1.2. Requester authentication . . . . . . . . . . . . . 8
2.1.1.3. Requester Uses Existing CA-Issued or
Self-Signed Certificates . . . . . . . . . . . . . 9
2.1.1.4. Trusted CA Store . . . . . . . . . . . . . . . . . 10
2.1.2. Certificate Authority . . . . . . . . . . . . . . . . 10
2.1.3. Registration Authorities . . . . . . . . . . . . . . . 11
2.2. SCEP Operations Overview . . . . . . . . . . . . . . . . . 11
2.2.1. Requester Initialization . . . . . . . . . . . . . . . 11
2.2.1.1. RSA Key Pairs . . . . . . . . . . . . . . . . . . 11
2.2.1.2. non-RSA Keys . . . . . . . . . . . . . . . . . . . 11
2.2.1.3. Required Information . . . . . . . . . . . . . . . 11
2.2.2. CA/RA Certificate Distribution . . . . . . . . . . . . 12
2.2.3. Certificate Enrollment . . . . . . . . . . . . . . . . 12
2.2.4. Requester Certificate Revocation . . . . . . . . . . . 13
2.2.5. Certificate Access . . . . . . . . . . . . . . . . . 14
2.2.6. CRL Distribution . . . . . . . . . . . . . . . . . . . 14
2.3. PKI Operation Transactional Behavior . . . . . . . . . . . 15
2.3.1. Transaction Identifier . . . . . . . . . . . . . . . . 15
2.3.2. State Transitions in Certificate Enrollment . . . . . 15
2.3.3. Transaction Behavior of Certificate/CRL Access . . . . 16
2.4. Security . . . . . . . . . . . . . . . . . . . . . . . . . 16
3. Transport Protocol . . . . . . . . . . . . . . . . . . . . . . 17
3.1. HTTP "GET" and "POST" Message Format . . . . . . . . . . . 18
3.2. Response Message Format . . . . . . . . . . . . . . . . . 19
4. Secure Transportation: PKCS#7 . . . . . . . . . . . . . . . . 20
4.1. SCEP Message Format . . . . . . . . . . . . . . . . . . . 20
4.1.1. SCEP pkcsPKIEnvelope . . . . . . . . . . . . . . . . . 20
4.1.2. SCEP pkiMessage type . . . . . . . . . . . . . . . . . 21
4.2. Signed Transaction Attributes . . . . . . . . . . . . . . 21
4.2.1. transactionID . . . . . . . . . . . . . . . . . . . . 22
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4.2.2. messageType . . . . . . . . . . . . . . . . . . . . . 22
4.2.3. pkiStatus . . . . . . . . . . . . . . . . . . . . . . 22
4.2.4. failInfo . . . . . . . . . . . . . . . . . . . . . . . 23
4.2.5. senderNonce and responderNonce . . . . . . . . . . . . 23
5. SCEP Transaction Specification . . . . . . . . . . . . . . . . 23
5.1. Certificate Enrollment . . . . . . . . . . . . . . . . . . 23
5.1.1. PKCSReq Message Format . . . . . . . . . . . . . . . . 24
5.1.2. CertRep Message Format . . . . . . . . . . . . . . . . 24
5.1.2.1. PENDING Response . . . . . . . . . . . . . . . . . 25
5.1.2.2. FAILURE Response . . . . . . . . . . . . . . . . . 25
5.1.2.3. SUCCESS response . . . . . . . . . . . . . . . . . 25
5.2. Poll for Requester Initial Certificate . . . . . . . . . . 25
5.2.1. GetCertInitial Message Format . . . . . . . . . . . . 26
5.2.2. GetCertInitial Response Message Format . . . . . . . . 26
5.3. Certificate Access . . . . . . . . . . . . . . . . . . . . 26
5.3.1. GetCert Message Format . . . . . . . . . . . . . . . . 27
5.3.2. CertRep Message Format . . . . . . . . . . . . . . . . 27
5.4. CRL Access . . . . . . . . . . . . . . . . . . . . . . . . 28
5.4.1. GetCRL Message format . . . . . . . . . . . . . . . . 28
5.4.2. CertRep Message Format . . . . . . . . . . . . . . . . 28
5.5. Get Certificate Authority Certificate . . . . . . . . . . 29
5.5.1. GetCACert HTTP Message Format . . . . . . . . . . . . 29
5.5.2. Response . . . . . . . . . . . . . . . . . . . . . . . 29
5.5.2.1. CA Certificate Only Response . . . . . . . . . . . 29
5.5.2.2. CA and RA Certificates Response . . . . . . . . . 30
5.5.3. Get Next Certificate Authority Certificate . . . . . . 30
5.5.3.1. GetNextCACert HTTP Message Format . . . . . . . . 30
5.5.3.2. GetCACaps HTTP Message Format . . . . . . . . . . 30
5.6. Get Certificate Authority Certificate Chain . . . . . . . 30
5.6.1. GetCACertChain HTTP Message Format . . . . . . . . . . 30
5.6.2. Response . . . . . . . . . . . . . . . . . . . . . . . 30
5.6.3. Backwards Compatibility . . . . . . . . . . . . . . . 31
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
8. Security Considerations . . . . . . . . . . . . . . . . . . . 31
8.1. General Security . . . . . . . . . . . . . . . . . . . . . 31
8.2. Use of the CA keypair . . . . . . . . . . . . . . . . . . 31
8.3. ChallengePassword . . . . . . . . . . . . . . . . . . . . 32
8.4. transactionID . . . . . . . . . . . . . . . . . . . . . . 32
8.5. Unnecessary cryptography . . . . . . . . . . . . . . . . . 32
9. Intellectual Property . . . . . . . . . . . . . . . . . . . . 33
10. Normative References . . . . . . . . . . . . . . . . . . . . . 33
Appendix A. Cisco Requester Subject Name Definition . . . . . . . 34
Appendix B. IPSEC Client Enrollment Certificate Request . . . . . 34
Appendix C. Private OID Definitions . . . . . . . . . . . . . . . 36
Appendix D. CRL Query by means of LDAP . . . . . . . . . . . . . 36
Appendix E. SCEP State Transitions . . . . . . . . . . . . . . . 37
Appendix F. CA Capabilities . . . . . . . . . . . . . . . . . . . 40
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Appendix G. Certificate Renewal and CA Key Rollover . . . . . . . 41
Appendix H. PKIOperation via HTTP POST Message . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
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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 IKE [RFC2409]
and IKEv2 [RFC4306], and TLS [RFC4346]. 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, or certificate revocation from a
Certification Authority. Often there also is a need for protocols to
request a certificate or cert status info, although these functions
are often provided by distinct protocols, e.g., LDAP for X509
[RFC4523] or OCSP [RFC2560].
This specification defines a 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., in-band certificate revocation
transactions, that can significantly enhance the security achieved in
a PKI. The IETF protocol suite currently includes two certificate
management protocols with more comprehensive functionality: CMP
[RFC4210] and Certificate Management over CMS [RFC5272]. Where
interoperability with the installed base of SCEP implementations is
required, implementers are encouraged to support a comprehensive
standards track certificate management protocol in addition to the
protocol defined in this specification. This implementation strategy
balances near term requirements for interoperability with longer term
security goals.
1.1. Requirements Language
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 [RFC2119].
2. 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:
o CA and RA public key distribution
o Certificate enrollment
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o Certificate query
o 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 [RFC5280], is outside the scope of this
document. In Section Section 2.1, we first define PKI entity types
as well as the properties of each entity type. In Section
Section 2.2, the PKI operations are described at functional level.
Section Section 2.3 describes the transaction behavior of each PKI
operations. The complete PKI messages are covered in Section
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:
o Fully qualified domain name, for example, Router.cisco.com,
o IP Address,
o Serial number, and/or
o x.500 distinguished name.
The fully qualified domain name is required for a requester that
intends to use the certificate with, for example, IKE [RFC2409]. The
IP address, serial number, and x.500 distinguished name are optional
name attributes.
In the certificate enrollment request, the PKCS#10 [RFC2986] 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 subjectName field may be empty (if
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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+192.0.2.4" (read
Alice.cisco.com plus the IP address name attribute 192.0.2.4). From
a CA point of view, the Distinguished names assigned in these two
cases are distinct names.
Entity names which are specified as in the IPSEC profile (i.e., FQDN,
IP address and User FQDN) must be presented in the 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, and provide storage for,
asymmetric keypairs. 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.
A keypair 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 keypair for a given subjectName 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 keypairs with a given key usage. At any time, if the subject
name is changed, or if the keypair 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
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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 certificate 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 subjectName 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 Section 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
cryptographically secure manner. This requirement is needed to
prevent a "man in the middle" attack, in which an adversary can
manipulate the data as it travels between the protocol participants
and subvert the security of the protocol.
PKCS#10 [RFC2986] specifies the use of a ChallengePassword attribute
to be sent as part of the enrollment request. SCEP uses this
ChallengePassword to satisfy the above requirements for security.
The PKCS#7 [RFC2315] envelope protects the privacy of the challenge
password.
2.1.1.2.1. Manual enrollment authentication
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
[RFC2986] (before PKCS#7 [RFC2315] enveloping and signing) SHOULD be
compared out-of-band between the CA operator 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.
In this case the challenge password is only used to authenticate a
request for the certificate's revocation.
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2.1.1.2.2. Automated enrollment authentication
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 using the pre-shared secret scheme, the requester must enter the
pre-distributed secret as the ChallengePassword. It can then also be
used to authenticate a request for the certificate's revocation.
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 [RFC2315] as the
messaging protocol (see Section 4). PKCS#7 [RFC2315], however, is a
data format 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.
o If the requesting system already has a certificate issued by the
CA, that certificate SHOULD 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.
o If the requesting system has no certificate issued by the CA, but
has credentials from a different CA, that certificate MAY be
presented as credentials instead of a self-signed certificate.
Policy settings on the SCEP server will determine if the request
can be accepted or not.
o If the requester does not have an appropriate existing
certificate, then a self signed certificate must be used instead.
The self signed certificate MUST use the same subjectName as in
the pkcs10 request.
During the certificate enrollment, the requester will attach the
appropriate certificate to the signed certificate request.
When the Certificate Authority creates the PKCS#7 envelope on the
issued certificate, it should use the public key, issuer name, and
serial number conveyed in the above included certificate (from the
SignerInfo section of the inner pkcs#7 of the request). This will
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inform the end entity on which private key should be used 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 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 CAs, SCEP allows the users to configure
multiple trusted certificates. Trusted certificates are 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 purposes.
A Certificate Authority may enforce certain name policy. When using
a X.500 directory name as the subject name, all the name attributes
specified in the PKCS#10 [RFC2986] request should be included as
Relative DN. All the name attributes as defined in [RFC5280] should
be specified in the SubjectAltName. An example is provided in
Appendix A.
If there is no LDAP or HTTP query protocol support, the Certificate
Authority itself 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.
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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 [RFC2315], 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.
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. RSA Key Pairs
Before a requester can start a PKI transaction, it must have at least
one RSA key pair.
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 [RFC2986].
2.2.1.2. non-RSA Keys
SCEP does not support non-RSA keys. Though the protocol (being based
on PKCS#7) does not preclude them, RSA is the only algorithm
supported by current implementations.
2.2.1.3. 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,
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3. the HTTP proxy information if there is no direct Internet
connection to the server,
4. 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 [RFC5280].
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
exchanged between the requester and the CA/RA, the message to get the
CA/RA certificate cannot be secured using PKCS#7 [RFC2315]. 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 out-of-band. 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 Certificate: HTTP Get message
----------------------------->
CA/RA Certificate download: HTTP Response message
<---------------------------------------
Compute finger print and
call CA operator.
Receive call and check finger print
Get CA/RA Certificate
If an RA is in use, a degenerated PKCS#7 [RFC2315] 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 [RFC2986] and sends it to the CA/RA
enveloped using the PKCS#7 [RFC2315]. 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.
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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.
It is up to local CA policy (and CA implementation) as to whether a
certificate is granted automatically, or whether it is manually
granted by the administrator. The ChallengePassword MAY be used to
automatically authenticate the request, but does not have to.
Polling mode is entered whenever the server returns a PENDING
response.
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
SCEP currently only allows revocation as an out-of-band process. In
order to revoke a certificate, the requester must contact the CA
server operator, who MAY wish to verify the ChallengePassword (which
has been sent to the server as an attribute of the PKCS#10 [RFC2986]
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certificate request). If the ChallengePassword matches, the
certificate can be revoked.
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
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 certificate 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 two methods to query CRL:
1. If the CA supports CRL Distribution Points [RFC5280] (section
4.2.1.13), then the CRL MUST be retrieved via the mechanism
specified in the CDP. This is the preferred method. Please
refer to Appendix D for the examples of CRL Distribution Point.
2. If the CA does not support CDP's, a CRL query is composed by
creating a message consisting 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
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requests: The transaction to query CRL consists of one GetCRL PKI
message and one CertRep PKI message which contain only the CRL (no
certificates).
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 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:
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+-<------+
| |
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.
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:
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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
[RFC2315] and PKCS#10 [RFC2986] 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 [RFC2315] 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.
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.
3. Transport Protocol
In the SCEP protocol, HTTP is used as the transport protocol for the
PKI messages.
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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:
"GET " CGI-PATH CGI-PROG "?operation=" OPERATION "&message=" MESSAGE
where:
o CGI-PATH defines the actual CGI path to invoke the CGI program
which parses the request.
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.
o OPERATION is set to be the string
o
* "PKIOperation" when the GET message carries a PKI message to
request certificates or CRL
* "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 Certificate chain (respectively)
o MESSAGE is
o
* a base64-encoded PKI message , when OPERATION is "PKIOperation"
and method is GET.
* a CRL distribution point in URI format , when OPERATION is
GetCRL,
* a string which represents the certificate authority issuer
identifier otherwise.
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.
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Other types of requests are sent using the PKCS#7 [RFC2315] data
format. 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 [RFC2315] 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== HTTP/1.0
If supported by the CA, the message may also be sent via HTTP POST:
POST /cgi-bin/pkiclient.exe?operation=PKIOperation HTTP/1.0
Content-Length: 1234
<binary pkcs7 data>
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 [RFC2315] 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
[RFC2315] 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.
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If the CA supports it, PKIOperation may also be done via an HTTP
POST. This is described in Appendix H.
4. Secure Transportation: PKCS#7
PKCS#7 [RFC2315] 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 [RFC2315] enveloped PKI message format is specified.
All messages MUST be valid PKCS#7 [RFC2315] structures, unless
otherwise noted.
4.1. SCEP Message Format
An SCEP message consists of an information portion (which depends on
the type of SCEP message being sent) and a set of transaction-
specific Attributes (see section Section 4.2).
The message-specific information is used as EnvelopeContent in an
SCEP pkcsPKIEnvelope message (see section Section 4.1.1).
Next, the pkcsPKIEnvelope is used as Content for a pkiMessage SCEP
type (see section Section 4.1.2).
The transaction-specific attributes are encoded as a set of
authenticatedAttributes in the SignerInfo of the SignedData.
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.
4.1.1. SCEP pkcsPKIEnvelope
The SCEP messages are carried inside an Enveloped-data content type,
as defined in PKCS#7 Section 10, with the following restrictions:
o version shall be 0
o EncryptedContent shall be the SCEP message being transported (see
section Section 5)
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The message is encrypted using the public key of the recipient, i.e.
the RA or the CA the message is for.
NOTE: The PKCS#7 [RFC2315] EncryptedContent is specified as an octet
string, but SCEP entities must also accept a sequence of octet
strings as a valid alternate encoding.
4.1.2. SCEP pkiMessage type
A SCEP pkiMessage consists of an Signed-data content type, as defined
in PKCS#7 Section 9. The following restrictions apply:
o version shall be 1
o The signed content shall be a pkcsPKIEnvelope
o The SignerInfo MUST contain a set of authenticatedAttributes (see
PKCS#7 Section 9.2 as well as section Section 4.2 in this
document. All messages are required to contain a SCEP
transactionID attribute, an SCEP messageType, and, of course, any
attributes required by PKCS#7 section 9.2. It may have other
attributes as well, depending on the messageType.
The message is signed in one of two ways: The requester can generate
a self-signed certificate, or the requester can use a previously
issued certificate, if the RA/CA supports the RENEWAL option.
4.2. Signed Transaction Attributes
The following transaction attributes are encoded as authenticated
attributes, and are carried, as specified in PKCS#7 Section 9.2, in
the SignerInfo for this signedData.
Please refer to Appendix B for the OID definitions.
+----------------+-----------------+---------------------------+
| Attribute | Encoding | Comment |
+----------------+-----------------+---------------------------+
| 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 | OctetString | |
| recipientNonce | OctetString | |
+----------------+-----------------+---------------------------+
The attributes are detailed in the following sections.
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4.2.1. transactionID
The transactionID is an attribute which uniquely identifies a
transaction. This attribute is required in all PKI messages.
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.
For non-enrollment message (for example GetCert and GetCRL), the
transactionID should be a number unique to the client.
4.2.2. messageType
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:
o PKCSReq (19) -- PKCS#10 [RFC2986] certificate request
o CertRep (3) -- Response to certificate or CRL request
o GetCertInitial (20) -- Certificate polling in manual enrollment
o GetCert (21) -- Retrieve a certificate
o GetCRL (22) -- Retrieve a CRL
4.2.3. pkiStatus
All response message will include transaction status information
which is defined as pkiStatus attribute:
o SUCCESS (0) -- request granted
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o FAILURE (2) -- request rejected. This also requires a failInfo
attribute to be present, as defined in section 4.2.4.
o PENDING (3) -- request pending for manual approval
4.2.4. failInfo
The failInfo attribute will contain one of the following failure
reasons:
o badAlg (0) -- Unrecognized or unsupported algorithm ident
o badMessageCheck (1) -- integrity check failed
o badRequest (2) -- transaction not permitted or supported
o badTime (3) -- Message time field was not sufficiently close to
the system time
o badCertId (4) -- No certificate could be identified matching the
provided criteria
4.2.5. senderNonce and responderNonce
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.
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.
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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 a pkiMessage (see section Section 4.1.2), with
the messageType attribute set to PKCSReq.
The information portion of the pkiMessage is a PKCS#10 [RFC2986]
certificate request, which MUST contain at least the following items:
o the subject Distinguished Name
o the subject public key
o a ChallengePassword attribute. The Challenge Password may be used
to (out-of-band) authenticate the enrollment request itself, or in
an out-of-band revocation request for the issued certificate.
Of course the certificate request may also contain any additional
fields that make up a valid PKCS#10 request, including but not
limited to an ExtensionReq attribute which is a sequence of
extensions the requester expects to be included in its V3 certificate
extensions (for example key-usage and extended key-usages).
This pkiMessage of type PKCSReq MUST contain:
o A transactionID attribute, calculated as per section Section 4.2.1
o a messageType attribute set to PKCSReq
o a senderNonce, calculated as per section Section 4.2.5
The pkiMessage is then encrypted into a pkcsPKIEnvelope message (see
section Section 4.1.1).
5.1.2. CertRep Message Format
The response to an SCEP enrollment request (PKCSReq) is a CertRep
message.
If the request is granted, the pkiStatus is set to SUCCESS, and the
certificate is returned.
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If the request is rejected, the pkiStatus is set to FAILURE, and a
failInfo attribute is returned.
If the server requires manual approval of the request, the pkiStatus
is set to PENDING.
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. In addition to the attributes
required by PKCS#7, the following SCEP attributes are required:
o messageType set to CertReq
o transactionID copied from the PKCSReq message
o pkiStatus set to PENDING
o senderNonce as defined in section Section 4.2.5
o recipientNonce as defined in section Section 4.2.5
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.
5.1.2.3. SUCCESS response
In this case, the information portion of CertRep will be a
degenerated PKCS#7 [RFC2315] which contains the requester's newly
generated certificate. The CertRep is otherwise identical to the
PENDING reply, with the exception that pkiStatus is set to SUCCESS.
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.
Since GetCertInitial is part of the enrollment, the messages
exchanged during the polling period should carry the same
transactionID attribute as the previous PKCSReq.
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PreCondition: Either the requester has received a CertRep with
pkiStatus set to PENDING, or waiting for a previous PKCSReq has timed
out.
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.
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 (which was contained in the
original PKCS#10 sent via PKCSReq), combined with the transactionID
attribute, 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).
The content of the pkiMessage with messageType GetCertInitial is an
ASN.1 issuerAndSubject type (see figure Figure 1).
In addition to the authenticatedAttributes required for a valid
PKCS#7, the pkiMessage MUST include the following:
o messageType set to GetCertInitial
o transactionID
o senderNonce
issuerAndSubject
issuerAndSubject ::= SEQUENCE {
issuer Name,
subject Name
}
Figure 1
5.2.2. GetCertInitial Response Message Format
The response messages for GetCertInitial are the same as for PKCSReq.
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
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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.
The content of the pkiMessage with messageType GetCert is an ASN.1
IssuerAndSerial type, as specified in PKCS#7 Section 6.7.
In addition to the authenticatedAttributes required for a valid
PKCS#7, the pkiMessage MUST include the following:
o messageType set to GetCert
o transactionID
o senderNonce
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 (SUCCESS) or reject the request (FAILURE).
Also, the recipientInfo should use the CA issuer name and CA assigned
serial number to identify the requester's key pair since at this
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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
[RFC2315] which contains only the most recent CRL. The size of CRL
included in the CertRep should be determined by the implementation.
When a CRLDistributionPoint is used, the GetCRL exchange should not
be used. Instead, the CRLDistributionPoint, as set in the
certificate, should be queried (see [RFC5280] section 4.2.1.14).
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, combined into an ASN.1
issuerAndSerialNumber type, as defined in PKCS#7 Section 6.7. This
constitutes the content of the pkiMessage with messageType GetCRL.
In addition to the authenticatedAttributes required for a valid
PKCS#7, the pkiMessage MUST include the following:
o messageType set to GetCRL
o transactionID
o senderNonce
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 [RFC2315]
SignedData which contains only the raw DER encoded CRL.
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5.5. Get Certificate Authority Certificate
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:
o CGI-PATH defines the actual CGI path to invoke the CGI program
which parses the request.
o 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.
o 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.
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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 [RFC2315] SignedData is
used to carry the certificates to the requester, with a Content-Type
of application/x-x509-ca-ra-cert.
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 [RFC2315] 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 certificate (or RA certificate, if the CA is in RA mode.
Note that a PKCS#7 [RFC2315] 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
[RFC2315] SignedData to carry the certificates to the requester, with
a Content-Type of application/x-x509-ca-ra-cert-chain.
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5.6.3. Backwards Compatibility
Versions of SCEP prior to revision 3 do not support GetCACertChain or
GetNextCACert. 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.
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.
6. Acknowledgements
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.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
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 keypair is generally meant for (and is usually flagged as)
"certificate signing" (exclusively), rather than 'data signing' or
'data encryption'. The SCEP protocol, however, uses the CA keypair
indiscriminately to encrypt and sign PKCS#7 transport messages. This
is generally considered undesirable, as this weakens the CA keypair
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over time (it is generally accepted that using any key weakens it
over time, as it gives an attacker more data to work with).
While the CA keypair can be generated with the 'data encryption' and
'data signing' flags set, this is operationally not encouraged. It
would make using the key as a PKCS#7 transport key 'legal', but the
discussion from the previous paragraph still applies.
A solution 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
keypair. A physically separate RA allows the CA to be inside the
secure network, not accessible to hackers at all.
8.3. ChallengePassword
The ChallengePassword sent in the PKCS#10 enrollment request is
signed (via inclusion in a pkiMessage) and encrypted (via inclusion
in a pkcsPKIEnvelope). When saved by the CA, care should be taken to
protect this password.
If the ChallengePassword is used to automatically authenticate an
enrollment request, it is recommend that some form of one-time
password be used to minimize damage in the event the data is
compromised.
8.4. transactionID
A well-written CA/RA will 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
(or worse). A well-written CA/RA should never assume the data from a
requester is well-formed.
8.5. 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), where simply signing
the request would suffice (CRL's and Certificates, i.e. the
respective responses, are already signed by the CA and can be
verified by the recipient).
This may affect performance and scalability of the CA which could be
used as an attack vector on the CA (though not an anonymous one).
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The use of CDP's is recommended for CRL access, as well as other ways
of retrieving certificates (LDAP, direct HTTP access, etc).
9. Intellectual Property
This protocol 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.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
November 2000.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4523] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP) Schema Definitions for X.509 Certificates",
RFC 4523, June 2006.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
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(CMC)", RFC 5272, June 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
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 bound
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"}
Appendix B. IPSEC Client Enrollment Certificate Request
The following is the certificate enrollment request (PKCS#10
[RFC2986]) 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
: }
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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
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
: }
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Appendix C. Private OID Definitions
The OIDs used in SCEP are VeriSign self-maintained OIDs.
+-------------------+-----------------------------------------------+
| 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-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)} |
+-------------------+-----------------------------------------------+
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
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Appendix E. 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. 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 192.0.2.4.
Each enrollment transaction is uniquely associated with a transaction
identifier (carried in the transactionID signed attribute (see
section XXX). 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
identifier. 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 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
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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 certificate 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
Case when NVRAM is lost and client has to generate a new key pair,
there is no change of name information:
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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 certificate 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
GetCertInitial ----------> Crete has been signed
Client Installs Crete <---------- CertRep (13) SIGNED CERT
* Case 2
PKCSReq (13) ----------> Cert has been signed
Client Installs Cert <---------- CertRep (13) SIGNED CERT
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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.
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.
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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.
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 HTTP/1.0
Content-Length: <length of data>
<binary PKCS7 data>
The client can verify that the CA supports SCEP messages via POST by
looking for the "POSTPKIOperation" capability (See Appendix F).
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Authors' Addresses
Andrew Nourse
Cisco Systems, Inc.
510 McCarthy Drive
Milpitas, CA
USA
Email: nourse@cisco.com
Xiaoyi Liu
Cisco Systems, Inc.
510 McCarthy Drive
Milpitas, CA
USA
Email: xliu@cisco.com
Jan Vilhuber (editor)
Cisco Systems, Inc.
510 McCarthy Drive
Milpitas, CA
USA
Email: vilhuber@cisco.com
Cheryl Madson
Cisco Systems, Inc.
510 McCarthy Drive
Milpitas, CA
USA
Email: cmadson@cisco.com
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