Internet Engineering Task Force M. Pritikin, Ed.
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track July 11, 2011
Expires: January 12, 2012
Enrollment over Secure Transport
draft-pritikin-est-02
Abstract
This document specifies a protocol for certificate Enrollment over
Secure Transport (EST). EST is a certificate enrollment protocol
that operates over HTTPS, and thus should be trivially accessible by
modern clients. The Certificate Management over CMS (CMC) "Simple
PKI Request" and "Simple PKI Response" messages are leveraged. EST
is designed to be easily implemented by clients and servers running
other common enrollment mechanisms such as Simple Certificate
Enrollment Protocol (SCEP). Renewal and rekey mechanisms are
described consistent with Certificate Management Protocol (CMP).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 12, 2012.
Copyright Notice
Copyright (c) 2011 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
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 7
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Secure Transport . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. HTTPS-Based Server Authentication . . . . . . . . . . . . 9
3.2. Server Authentication and Authorization . . . . . . . . . 10
3.3. HTTPS-Based Client Authentication . . . . . . . . . . . . 11
3.4. HTTP-Based Client Authentication . . . . . . . . . . . . . 11
3.5. Client Authorization . . . . . . . . . . . . . . . . . . . 12
3.6. Proof-of-Possession . . . . . . . . . . . . . . . . . . . 12
3.7. Peer Authentication . . . . . . . . . . . . . . . . . . . 13
4. HTTP URLs . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Distribution of CA certificates . . . . . . . . . . . . . 15
5.1.1. Distribution of CA certificates response . . . . . . . 15
5.2. Simple Enrollment of Clients . . . . . . . . . . . . . . . 16
5.2.1. Simple Re-Enrollment of Clients . . . . . . . . . . . 16
5.2.2. Simple Enroll and Re-Enroll Response . . . . . . . . . 17
5.3. Full CMC . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.3.1. Full CMC Request . . . . . . . . . . . . . . . . . . . 18
5.3.2. Full CMC Response . . . . . . . . . . . . . . . . . . 18
6. Cryptographic Algorithms . . . . . . . . . . . . . . . . . . . 18
7. Contributors/Acknowledgements . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Server Discovery . . . . . . . . . . . . . . . . . . 22
Appendix B. External TLS concentrator . . . . . . . . . . . . . . 22
Appendix C. CGI Server implementation . . . . . . . . . . . . . . 23
Appendix D. Updating SCEP implementations . . . . . . . . . . . . 23
Appendix E. Key Update mechanisms . . . . . . . . . . . . . . . . 25
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This document profiles certificate enrollment for clients using
Certificate Management over CMS [RFC5272] messages over a secure
transport. This profile describes a simple yet functional
certificate management protocol targeting simple PKI clients that
need to acquire client certificate(s) and associated infrastructure
certificate(s).
"CMC: Transport Protocols" [RFC5273] provides some guidance for
running CMC over HTTP [RFC2617] but notes only that "clients MAY
attempt to send HTTP requests using TLS 1.0 [TLS] or later, although
servers are not required to support TLS". No attempt is made in that
document to specify how the client and server might take advantage of
a secured transport to better leverage the Simple PKI messages. This
profile specifies secure transport mechanisms and how values from the
TLS exchange, the HTTP exchange, and the CMC Simple PKI messages
layers are used for authentication (and authorization) purposes by
the server.
The aspects profiled from TLS/HTTPS, CMS and CMP are summarized in
Figure 1:
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Profiled Layers:
Protocol:
+---------------------------------------------------+
| |
| 3) Message types |
| CMC "Simple PKI" messages. |
| PEM encoded certifiate chains. |
| Optionally "Full" CMC messages. |
| |
+---------------------------------------------------+
| |
| 2) HTTP headers and URLs for control |
| URLs used to specify the PKI operation |
| (including renew/rekey). |
| Content-Type headers specify the message type. |
| Headers profiled for control/error messages. |
| Username/password methods supported for |
| client proof-of-identity. |
| |
| |
+- ----(combination is known as HTTPS)--+
| |
| |
| 1) TLS for transport security |
| Provides proof-of-identity for |
| EST Server authentication and |
| EST Client authentication. |
| "Channel binding" type techniques used to |
| during Proof-of-Possesion. |
| |
+---------------------------------------------------+
| |
| TCP/IP layer etc included in diagram for context |
| |
+---------------------------------------------------+
Application Logic:
+------------------------------------+
| |
| 4) Certificate Chain Validation |
| Certificate chains that include |
| rekey/renewed certificates as |
| specified in CMP. |
| |
+------------------------------------+
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Figure 1
The following provides a high level overview describing how these
layers are used. Each aspect is profiled in detail in the sections
below.
1) TLS for transport security:
CMC section 3.1 notes that "the Simple PKI Request MUST NOT be
used if a proof-of-identity needs to be included". This precludes
use of these messages if inline authentication and/or
authorization is required, unless a secured transport that
provides proof-of-identity is also specified. Many simple clients
engaged in certificate enrollment operations will have a TLS
client implementation available for secure transport, so use of
TLS is specified herein. This document specifies a method for
authorizing client enrollment requests using existing
certificates. Such existing certificates may have been issued by
the CA (from which the client is requesting a certificate) or they
may have been issued under a distinct PKI (e.g. an IEEE 802.1AR
IDevID [IDevID] credential). Additionally this document specifies
username/password authentication methods beyond those included in
CMC. Authentication and authorization mechanisms required for
certificate issuance (and renew/rekey) are provided by HTTP and
TLS (HTTPS) mechanisms as described in this document.
Proof-of-possession is a distinct issue from proof-of-identity and
is addressed in Section 3.6.
This document also defines an appropriate transport for the full
CMC specification compliant with CMC Transport Protocols.
2) HTTP Headers and URLs for control:
This profile supports two operations indicated by specific URLs:
* Distribution of CA certificates
* Authorized enrollment and re-enrollment of clients
This document profiles HTTP headers to indicate the message type
and to provide the protocol control messages. Support for the
HTTP username/password methods is profiled.
CMC also states that: "No special services are provided for doing
either renewal (new certificates with the same key) or rekeying
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(new certificates on new keys) of clients. Instead a renewal/
rekey message looks the same as any enrollment message, with the
identity proof being supplied by existing certificates from the
CA." This profile clarifies the renewal and rekey behavior of
both the client and server. It does so by specifying the HTTP
control mechanisms required of the client and server without
require a distinct message type.
3) Message Types:
Some messages types used here are defined in CMC and include
subsets of the PKCS#10 Certification Request [RFC2986] and the
PKCS#7 [RFC2315] message specifications.
This document profiles the use of two Certificate Management over
CMS messages: "Simple PKI Request" and "Simple PKI Response" and
does not require full implementation of all CMC features. This is
consistent with the CMC protocol specification of "simple"
messages for clients to use "in the event no other services are
needed". Additional simple message formats are defined in this
document. To support distribution of the CA certificate chain a
simple PEM format is specified. Full CMC messages MAY be used as
specified below.
HTTP Content-Type headers are as specified in CMC: Transport
Protocols, Table 1. This document introduces new content types
for the simple format messages not specified by CMC.
4) Certificate Chain Validation:
A small clarification of the application layer certificate chain
validation logic is provided by a normative reference to CMP. The
certificate renewal and rekey certificate chaining mechanisms
documented in CMP [RFC4210] are referenced.
An EST server providing certificate management functions is operated
by (or on behalf of) a CA or RA.
An EST server MAY provide additional, non-EST services on other URLs.
The server also MAY support full CMC messages over HTTP.
[[EDNOTE: Comments such as this one, included within double brackets
and initiated with an 'EDNOTE', are for editorial use and shall be
removed as the document is polished.]]
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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. Requirements
[[EDNOTE: The following section is still included here for
succinctness. It will eventually be published independently as
draft-pritikin-estr-00.]]
The following describes goals and technical requirements for initial
PKI certificate enrollment along with a rationale for each
requirement.
G1 "Completeness". Server and client implementations compliant with
this document MUST be able to interoperate without reference to
subsequent profiles or additional future specifications.
The goal of this enrollment protocol is to provide a simple and easy-
to-implement method for end-entities to request, obtain and update a
certificate issued from a specified Certification Authority. The
following certificate management operations are required. Additional
operations NEED NOT be supported (via this protocol) although the
protocol design SHOULD be extensible:
M1 "Distribution of current CA certificates". Clients MUST be able
to obtain the current certificate for the CA under which the
client's certificate was issued. Certificates have a finite
lifetime and will need to be updated on a periodic basis. It must
be possible for the client to obtain the updated CA certificates.
M2 "Enrollment". A client MUST be able to use the protocol to submit
a certificate request and obtain a certificate.
M3 "Renew/Rekey". Certificates have a finite lifetime and will need
to be updated on a periodic basis. A client MUST be able to use
the protocol for certificate renewal or rekey operations.
End-Entity Proof of Identity authentication mechanisms:
A1 "Username/Password". It MUST be possible to identify a username
specified client as being in possession of an associated symmetric
key. This allows users currently in possession of a username and
password to obtain a certificate.
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A2 "Password". It MUST be possible to identity a client wihtout
reference to a "username". A common operational model is to
distribute a "one time password" that is presented to a CA or RA
to authorize enrollment.
A3 "Existing Certificate". It MUST be possible to authenticate a
client by making use of an existing certificate associated with
the client. A certificate used for client identification need not
be issued under the same PKI as the certificate that is being
requested. This allows clients that are already in a PKI to use a
certificate from that PKI to obtain additional certificates.
Additionally this capability allows a client who has a certificate
issued by a 3rd party, such as a certificate issued by a device
manufacturer, to leverage that credential during initial
enrollment.
A4 "Username/password and Certificate". It MUST be possible to
authenticate a client by using a combination of a username and
password and an existing certificate. This is a combination of A1
and A3. This supports "two factor authentication" where the
client proves possession of the private keys for an existing
certificate stored within a hardware device and knowledge of a
username/password.
A5 "Password and certificate". It MUST be possible to authenticate a
client by using a combination of a secrete value that is not
associated with a "username" and an existing certificate. This is
a combination of A2 and A3. This requirement is similar to A4
except that the client is in possession of a "one time password".
End-Entity Proof of Possession:
P1 Proof-of-Possession of subject keys MUST be supported. As
discussed in Appendix C of [RFC4211] Proof-of-Possession "means
that the CA is adequately convinced that the entity requesting a
certificate for the public key Y, has access to the corresponding
private key X".
Key algorithms:
K1 "Algorithm agility". The protocol MUST support algorithm agility.
It must be possible to employ different cryptographic algorithms
for securing the transport or for signing the certificates. The
protocol SHOULD demonstrate this agility by being shown to work
with existing RSA based solutions as well as providing for other
algorithms such as Elliptic Curve cryptography.
Server Identity mechanism:
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I1 "RA certificate". It MUST be possible for a client to verify
authorization of the EST server as a representative of the CA.
The protocol operations allow the client to send a username/
password or (one time) password to the EST server. These values
cannot be safely transmitted to an unauthorized server.
3. Secure Transport
HTTPS MUST be used. TLS 'session resumption' SHOULD be supported.
HTTPS is defined in HTTP Over TLS [RFC2818] and is a definition of
how HTTP messages are carried over TLS. HTTPS (HTTP over TLS) is a
commonly used transport and can be easily layered on top of extremely
simple client or server code. In some environments HTTPS can be
utilized through an external process. Specifying HTTPS as the
secured transport for PKI enrollment messages introduces two
potential 'layers' for communication of authorization data or for
status/informative responses during the protocol exchange: TLS and
HTTPS. This profile specifies when information is used from each
layer.
3.1. HTTPS-Based Server Authentication
The client MUST validate the HTTPS server certificate presented
during the TLS [RFC5246] defined Server Certificate message or the
client MUST independently validate the response contents. The cipher
suites are detailed in Section 6.
There are multiple methods of validation depending on the current
state of the client:
1. If the client has a store of trust anchors, which may be in the
form of certificates, for validating HTTPS connections the client
MAY validate the HTTPS server certificate using the standard HTTP
logic of checking the server's identity as presented in the
server's Certificate message against the URL provisioned for the
EST server (see HTTPS Over TLS, Section 3.1 Server Identity.
This method makes it possible for clients with a large store of
HTTPS certificates to securely obtain the CA server certificate
by leveraging the HTTPS security model. Note that the EST server
URL MUST be made available to the client in a secure fashion and
many systems are configured with many trust anchors from a wide
range of CAs and this would make such systems vulnerable to
spoofing of the ETS server certificate by an attacker that is
ablet to obtain an erroneous certificate from a lax CA. As
detailed in Section 9 clients are RECOMMENDED to ship with a
carefully chosen list of initial trust anchors. Proper selection
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of initial trust anchors is out of scope of this document.
[[EDNOTE: is there an RFC discussing this problem in the HTTPS
space that we can reference?]]
2. If the client already has one or more trust anchors associated
with this EST server, the client MAY validate the EST server
certificate using these trust anchors. The EST server URL MAY be
made available to the client in an insecure fashion.
3. If the client does not yet have a trust anchor associated with
this EST server then the client MAY provisionally accept the TLS
connection, but the HTTP content data must be accepted manually
as described in Section 5.1. HTTP authentication requests MUST
NOT be responded to.
If one of these validation methods succeeds the CA certificate are
stored and made available for future use. If none of these
validation methods succeeds the client MUST reject the EST server
response and SHOULD log or inform the end user.
The EST server MUST present an end-entity certificate such as the CMC
Local Registration Authority (LRA) certificate. The client MUST
support validating the EST server certificate using the "Verifying
Certificates" logic specified in CMP section 4.4. Appendix E
provides an informative summary of key renewal and the associated
validation logic.
3.2. Server Authentication and Authorization
The client MUST check the EST server authorization.
If the client has a securely configured and authorized URI for the
EST server it SHOULD check the URI "against the server's identity as
presented in the server's Certificate message" (Section 3.1 Server
Identity [RFC2818]). The securely configured URI provides the
authorization statement and the server's authenticated identity
confirms it is the authorized server.
If this check fails, or if the URI was configured using an insecure
method, then the client MUST verify the server's authorization by
checking that the [RFC5280] defined certificate policy extension
sequence contains the 'LRA Authorization' policy OID.
The LRA Authorization policy OID is defined as: id-cmc [[EDNOTE: TBD,
perhaps 35]]. The LRA Authorization policy information MUST NOT
contain any optional qualifiers.
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3.3. HTTPS-Based Client Authentication
The server MUST send a TLS section 7.4.4 "Certificate Request" and
the client MUST respond. The client MUST respond with a certificate
that allows it to subsequently send the a TLS Section 7.4.8
"Certificate Verify" (i.e. the client MUST use an end entity "client
certificate that has signing capability"). The server MUST verify
the Certificate Verify message.
The certificate presented by the client MAY be from the same PKI as
the Server Certificate, from a completely different PKI, or as a last
resort the client MAY respond with a self-signed certificate. The
certificate supplied during authentication is used during client
authorization (Section 3.5).
The server MUST support the validation of the EST client certificate
using normal certificate validation logic including rekey/renew
support as specified in CMP section 4.4. Appendix E provides an
informative summary of key renewal and the associated validation
logic.
3.4. HTTP-Based Client Authentication
As specified in CMC: Transport Protocols the server "MUST NOT assume
client support for any type of HTTP authentication such as cookies,
Basic authentication, or Digest authentication". Clients intended
for deployments where password authentication is advantageous SHOULD
support the Basic and Digest authentication mechanism. Servers MAY
provide configuration mechanisms for administrators to enable Basic
and Digest authentication methods.
Servers that support Basic and Digest authentication methods reject
requests using the HTTP defined WWW-Authenticate response-header
(Section 14.47). At which point the client SHOULD repeat the
request, including the appropriate HTTP [RFC2617] Authorization
Request Header (Section 3.2.2).
Support for Basic authentication as specified in HTTP allows the
server access to the client's cleartext password. This provides
integration with legacy username password databases but requires
exposing the plaintext password to the EST server. The client MUST
NOT respond to this request unless the EST server has been
authenticated (as per Section 3.2).
Clients MAY set the username to the empty string ("") if they wish to
present a "one time password" or "PIN" that is not associated with a
username.
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3.5. Client Authorization
When the CA server receives a CMC Simple PKI Enrollment or re-
enrollment message, the decision to issue a certificates is always a
matter of local policy. Thus the CA can use any data it wishes in
making that determination. The EST protocol exchange provides the
EST server access to the TLS client Certificate in addition to the
HTTP user authentication credentials to help in that determination.
The communication channel between the TLS implementation and the EST
software implementation is out-of-scope of this document.
3.6. Proof-of-Possession
As discussed in Appendix C of CRMF [RFC4211] Proof-of-Possession
"means that the CA is adequately convinced that the entity requesting
a certificate for the public key Y, has access to the corresponding
private key X".
This specification provides proof-of-possession by including
information specific to the current TLS session within the signed
certification request. This proves to the server that the TLS client
has possession of the private key associated with the certification
request and that the client was able to sign the certification
request after the TLS session was established. The value included
within the certification request is very similar to "tls-unique" as
defined in Channel Bindings for TLS [RFC5929]. The value is defined
as:
tls-unique-securerenegotiation: The first TLS Finished message
sent in the _first_ TLS handshake of the TLS connection that is
being bound to is the TLS "channel binding" value. Any TLS
renegotiation MUST use "secure_renegotiation" [RFC5746] (thus
maintaining the binding). Mandating secure renegotiation allows
implementations to avoid the synchronization issues encountered
with tls-unique.
The client generating the request SHOULD obtain the tls-unique-
securerenegotation value, encode it using base64 encoding, and place
the resulting string in the certification request challenge password
field.
The server SHOULD verify the tls-unique-securerenegotation
information. This ensures that the authenticated TLS client is in
possession of the private key used to sign the certification request.
The tls-unique-securerenegotiation value is encoded into the
certification request by the client but back-end infrastructure
elements that process the request might not have access to the
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initial TLS session. Such infrastructure elements validate the
source of the certification request to determine if proof-of-
possession checks have already been performed. For example if the
client authentication results in an authenticated client identity of
an RA that is known to independently verify the proof-of-possession,
then the back-end infrastructure does not need to perform proof-of-
possession checks a second time.
Implementation Note: The tls-unique value is consistent with tls-
unique-securerenegotiation until after a renegotiation (at which
point the tls-unique value is the TLS Finished message of the "most
recent TLS handshake" instead of the "first handshake"). A valid
tls-unique-securerenegotiation value can be obtained by careful use
of the implementation's tls-unique channel binding TLS APIs so long
as renegotiation has not yet taken place.
The use of tls-unique-securerenegotiation makes it possible for
servers to wait to request TLS client authentication until after the
URI has been parsed, as is commonly implemented.
3.7. Peer Authentication
The EST server can itself be an EST client when an RA uses EST to
communicate with back-end infrastructure elements. Authentication of
credentials identifying an EST peer is in scope in that appropriate
generic credential authentication in an environment supporting Root
CA Key Update is mandated. EST clients validating peer (other EST
client) certificates MUST support the Root CA Key Update verification
mechanisms specified in CMP section 4.4 when validating the peer
certificates. Appendix E provides an informative summary on key
renewal.
4. HTTP URLs
EST uses the HTTP "GET" and "POST" messages to communicate with the
EST server. The following describes the syntax of these messages:
"GET" BASEPATH OPERATIONPATH
"POST" BASEPATH OPERATIONPATH
where:
o BASEPATH is a common path for all EST operations
o OPERATIONPATH specifies the specific operation.
When an URL is formed the BASEPATH and OPERATIONPATH are combined to
form the abs_path [RFC2616]. The server and port and MUST be pre-
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configured or otherwise discovered by the client as described in
Appendix A. The means by which clients acquire the base URL are
outside the scope of this document. The following are two example
base URLs:
o https://example.org/BASEPATH
o https://example.org:8080/arbitrary/base/path
These can be conveniently distributed as they are in a form with
which many end users are already familiar. The following operation
URLs for client to access are defined relative to the EST base URL:
o /CACerts - The server responds to an HTTPS GET with the CA
certificates as defined in Distribution of CA certificates
(Section 5.1).
o /simpleEnroll - The client sends a CMC Simple PKI Enrollment
message as specified in Enrollment of Clients (Section 5.2), the
response is a CMC Simple PKI Response. message as specified in
Enroll Response (Section 5.2.2).
o /simpleReEnroll - Exactly the same as 'simpleEnroll' except that
the request is for re-enrollment or re-issuance purposes.
o /fullCMC - Provides for a CMC transport (optional).
The following examples are valid complete URLs under this
specification:
o https://example.org/BASEPATH/CACerts
o https://example2.org/arbitrary/base/path/simpleEnroll
o https://example2.org/arbitrary/base/path/simpleReEnroll
o https://example3.org/example/ca/fullCMC
The mechanisms by which the EST server interacts with an HTTPS server
to handle GET and POST operations at these URLs is outside the scope
of this document. The use of distinct URLs simplifies implementation
for servers that do not perform client authentication when
distributing "CACerts" responses.
Implementation note: A simple Common Gateway Interface (CGI)
application at each fully specified path, with the HTTPS server
configured to provide Section 3.3, is sufficient for a working
example (the web service can forward the Section 3.6 proof-of-
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possession information to the application using the CGI interface).
[[EDNOTE: This does not use the mechanism specified in "Defining
Well-Known Uniform Resource Identifiers (URIs)" [RFC5785]. That
would be a possibility here for a base URL of
"https://example.org/.well-known/EST" but such would preclude the
flexibility associated with multiple base urls being handled by the
same server unless some form of "?designator=value" is included.]]
5. Messages
5.1. Distribution of CA certificates
Before engaging in enrollment operations, clients MUST request trust
anchor information by sending an HTTPS GET message to the EST base
URL with the relative path extension 'CACerts'. Clients SHOULD
request an up to date response before stored information has expired.
The EST server SHOULD NOT require client authentication or
authorization to reply to this request.
The client MUST authenticate the EST server as specified in
Authentication and Authorization (Section 3). If the authentication
and authorization is successful, the client accepts the response and
stores it. If the authentication and authorization is not
successful, then when the response is received the client MUST
extract the CA certificate and engage the end-user or otherwise
authorize the credential using out-of-band pre-configuration data
such as a CA certificate "fingerprint" (e.g., a SHA-1, SHA-256, SHA-
512, or MD5 hash on the whole CA certificate).
The client MUST NOT accept the CA certificate without validating it
via one of the mechanisms described above.
Subsequent connections to the EST server validate the TLS server
certificate using the stored CA certificates as described in
Authentication and Authorization (Section 3).
5.1.1. Distribution of CA certificates response
The EST server MUST respond to the client HTTPS GET message with
trust anchor information in the form of a certificate. Additionally
the server MUST include any "Root CA Key Update" CMP certificates
(Appendix E provides an informative summary of "Root CA Key Update").
The response format is a text file containing a list of certificates
each formatted as specified in Section 6.1 of [RFC4945]. Each
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certificate is delimited by a newline. The content-type of
"application/x-est-cacerts" MUST be specified.
5.2. Simple Enrollment of Clients
At any time the client MAY request a certificate from the EST base
URL with the relative path extension "simpleEnroll'.
When HTTPS POSTing to the 'Enroll' location the client MUST include a
CMC Simple PKI Enrollment request as specified in CMC Section 3.1 (a
PKCS#10 Certification Request).
The content-type of "application/x-est-pkcs10" MUST be specified.
The format of the request is as specified in Section 6.4 of
[RFC4945].
The server MUST authenticate the client as specified in
Authentication and Authorization (Section 3). The server applies
whatever authorization or policy logic it chooses determining if the
certificate should be issued. The server MAY choose to issue a
certificate different from the certificate request as specified in
CMC Section 3.1. The client MAY request an additional certificate
even when using an existing certificate in the TLS client
authentication.
The client MUST authenticate the EST server as specified in
Section 3.1.
If the EST server forwards the request to a back-end process it
SHOULD communicate the authentication results. For example using the
CMC "RA POP Witness Control" in a CMC Full PKI Request message.
5.2.1. Simple Re-Enrollment of Clients
At any time a client MAY request renew/rekey of its certificate from
the EST base URL with the relative path extension "simpleReEnroll'.
The certificate request is the same format as for the "simpleEnroll"
path extension with the same content-type.
The EST server MUST handle enrollment requests submitted to the
"simpleReEnroll" URL as renewal or rekey requests rather than
depending only on the method of identifying a renewal or rekey
request specified in Section 2 of RFC5272 [RFC5272], that "renewal
and rekey requests look the same as any certification request, except
that the identity proof is supplied by existing certificates from a
trusted CA". The proof of client identity is supplied by client
authentication during the HTTPS handshake. When attempting to renew
or rekey the client MUST use its existing certificate for TLS client
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authentication.
[[EDNOTE: draft-turner-suiteb-cmc defines a method of recognizing an
re-enroll based on PKCS10 contents, see section 4.1. The method
described herein is explicit.]]
If the server forwards the request to a back-end process it SHOULD
communicate that this is a renew/rekey attempt. Implementation note:
if using this protocol to communicate with a CA the /simpleReEnroll
URL is used.
5.2.2. Simple Enroll and Re-Enroll Response
The server responds to a 'simpleEnroll' or 'simpleReEnroll' request
with the client's newly issued certificate or it provides an error
response.
If the enrollment is successful the server response MUST have a
response code of 200 with a content-type of "application/x-est-x509".
The response data is the certificate formatted as specified in
Section 6.1 of [RFC4945]. The issued certificate MAY be signed by a
new CA key established as described in CMP.
When rejecting a request the server MUST specify either an HTTP 4xx/
401 error, or an HTTP 5xx error. A simple CMC response with content-
type of "application/pkcs7-mime" MAY be included in the response data
for any error response. If the content-type is not set the response
data MUST be a plain text human readable error message. A client MAY
elect to not parse a CMC error response in favor of a generic error
message.
If the server responds with an HTTP 202 this indicates that the
request has been accepted for processing but that a response is not
yet available. The server MUST include a Retry-After header as
defined for 503 responses and MAY include informative human readable
content. The client MUST wait at least the specified 'retry-after'
time before repeating the same request. The client repeats the
initial enrollment request after the appropriate 'retry-after'
interval as expired. The client SHOULD log or inform the end user of
this event. The server is responsible for maintaining all state
necessary to recognize and handle retry operations as the client is
stateless in this regard (it simply sends the same request repeatedly
until it receives a different response code).
All other return codes are handled as specified in HTTP.
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5.3. Full CMC
At any time the client MAY request a certificate from the EST base
URL with the relative path extension "fullCMC".
The client MUST authenticate the server as specified in Server
Authentication (Section 3.1) if an HTTPS url is used.
The server SHOULD authenticate the client as specified in
Authentication and Authorization (Section 3). The server MAY depend
on CMC client authentication methods instead.
In addition to the normal CMC proof-of-identity mechanisms the client
SHOULD include the Section 3.6 value.
5.3.1. Full CMC Request
When HTTP(S) POSTing to the 'fullCMC' location the client MUST
include a valid CMC message. The content-type MUST be set to
"application/pkcs7-mime" as specified in CMC: Transport Protocols.
5.3.2. Full CMC Response
The server responds with the client's newly issued certificate or
provides an error response.
If the enrollment is successful the server response MUST have a
response code of 200 with a content-type of "application/pkcs7-mime"
as specified in CMC: Transport Protocols. The response data includes
either the CMC Simple PKI Response or the CMC Full PKI Response.
When rejecting a request the server MAY specify either an HTTP 4xx/
401 error, an HTTP 5xx error or a response code 200. A CMC response
with content-type of "application/pkcs7-mime" MUST be included in the
response data for any error response. The client MUST parse the CMC
response to determine the current status.
All other return codes are handled as specified in Section 5.2.2 or
HTTP [RFC2616].
6. Cryptographic Algorithms
This section details the specific cryptographic algorithms and cipher
suite requirements.
When the TLS connection is established the supported cipher suite
codes are exchanged in the ClientHello and ServerHello messages. The
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negotiated cipher suite denotes the algorithms used during client and
server authentication and these are negotiated to match the
credentials available to the peers.
The client SHOULD offer the Suite B compliant cipher suites as
indicated in [RFC5430], Section 4 "Suite B Compliance and
Interoperability Requirements". For greatest interoperability the
client SHOULD also offer TLS_RSA_WITH_AES_128_CBC_SHA.
When the client accesses the "simpleReEnroll" method the cipher suite
MUST be appropriate for the existing certificate. The certificate
type used determines the appropriate signatureAlgorithm for the
PKCS#10 Certification Request. For example if a [RFC5430] cipher
suite is used the signatureAlgorithm MAY be ecdsa-with-sha256 for
P-256 certification requests, or MAY be ecdsa-with-sha384 for P-384
certification requests.
[[EDNOTE: This is in alignment with draft-turner-suitb-cmc-03 section
4.1. To encourage algorithm agility, discussions of the MUST/SHOULD
algorithms should be in a distinct document.]]
7. Contributors/Acknowledgements
The editor would like to thank Stephen Kent, Vinod Arjun, Jan
Vilhuber and others for their feedback and prototypes of early
drafts.
8. IANA Considerations
(This section is incomplete)
The following aspects should be registered with IANA Considerations:
The LRA Authorization certificate policy extension OID as discussed
in Section 3.2 requires registration with IANA.
[[EDNOTE: The URLs specified in Section 1 probably do not need to be
registered with IANA.]]
9. Security Considerations
(This section is incomplete)
"Badges? We ain't got no badges. We don't need no badges! I don't
have to show you any stinkin' badges!" -- The Treasure of the Sierra
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Madre.
As described in CMC Section 6.7, "For keys that can be used as
signature keys, signing the certification request with the private
key serves as a POP on that key pair". The inclusion of tls-unique-
securerenegotiation within the certification request provides
timeliness to the proof-of-possession. For support of keys that can
not be used for signing the certification request the full CMC
specification MUST be used.
As described in Section 3.3 clients use an existing certificate for
TLS client authentication. If a certificate with appropriate key
usage is not available the client MAY generate one. If a self-signed
certificate with appropriate key usage is used the server SHOULD
require HTTP based client authentication according to server policy
as described in Section 3.3 and Section 3.5. The server MAY fall
back on manual authorization by the server administrator.
As described in Section 3.1 servers use an existing certificate for
TLS server authentication. When the server certificate is issued by
a mutually trusted PKI hierarchy validation proceeds as specified in
Section 3.2. In this situation the client has validated the server
as being a valid responder for the URI configured but can not
directly verify that the responder is authorized as an RA within the
to-be-enrolled PKI hierarchy. A client may thus be enticed to expose
username/password or certificate enrollment requests to an
unauthorized server (if the server presents a valid HTTPS certificate
for an erroneous URL that the client has been tricked into using).
Proof-of-Identity and Proof-of-Possession checks by the CA prevent an
illegitimate RA from leveraging such misconfigured clients to act as
a man-in-the-middle during client authenticated operations but it is
possible for such illegitimate RAs to send the client doctored
messages or erroneous CA certificate lists. If the illegitimate RA
has successfully phished a username/password or PIN from the client
it might try to use these values to enroll its own keypair with the
real PKI hierarchy. EST servers identified with an externally issued
server certificate SHOULD require HTTPS based client authentication
(Section 3.3). Similarly EST clients SHOULD use an existing client
certificate to identify themselves and otherwise prevent "private
data" (obviously including passwords but also including private
identity information) from being exposed during the enrollment
exchange a weak server authorization method is used.
10. References
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10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
November 2000.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
[RFC4945] Korver, B., "The Internet IP Security PKI Profile of
IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.
[RFC5273] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC): Transport Protocols", RFC 5273, June 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5430] Salter, M., Rescorla, E., and R. Housley, "Suite B Profile
for Transport Layer Security (TLS)", RFC 5430, March 2009.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
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"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
10.2. Informative References
[IDevID] IEEE Std, "IEEE 802.1AR Secure Device Identifier",
December 2009, <http://standards.ieee.org/findstds/
standard/802.1AR-2009.html>.
[RFC4211] Schaad, J., "Internet X.509 Public Key Infrastructure
Certificate Request Message Format (CRMF)", RFC 4211,
September 2005.
Appendix A. Server Discovery
(informative)
(This section is incomplete)
Clients MAY use DNS-SD or similar discovery algorithms to determine
the EST base URL. In such cases it is expected that method 2
(Section 3.1) be used during server authentication.
Appendix B. External TLS concentrator
(informative)
In some deployments it may be beneficial to use a TLS concentrator to
offload the TLS processing from the server. In such a deployment the
TLS client authentication result must, in some way, be forwarded to
the server.
The TLS server SHOULD NOT reject the connection based on PKIX
validation of the client certificate. The client certificate SHOULD
be passed to the EST layer for verification and authorization. This
allows support of external TLS concentrators, or an external web
server, that might provide an independent TLS implementation.
The TLS concentrator MUST validate the TLS Section 7.4.8 'Certificate
Verify'.
A TLS concentrator MUST insert the client certificate into the HTTP
header. The TLS concentrator MUST first remove any existing client
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certificates, possibly inserted by a nefarious client, from the HTTP
headers before forwarding the HTTP connection to the server.
[TBD - need to better understand what would happen in the case of
proxy's or multiple concentrators. Or specifically state that as out
of scope.]
[TBD - the HTTP header field names etc shall be specified here]
The EST server MUST be specifically configured by the administrator
to accept this mechanism.
Appendix C. CGI Server implementation
(informative)
In some deployments it may be beneficial to use a HTTPS server that
runs the EST server as a CGI application. In such a deployment the
HTTPS server client authentication result must, in some way, be
forwarded to the server.
An HTTPS server MUST insert the client certificate into environment
variables before calling a server CGI application.
[TBD - describe the CGI environment variables here. Can likely
follow the apache example].
An HTTP server MUST insert the client certificate into environment
variables before calling a server CGI application.
[TBD - describe the CGI environment variables here. Can likely
follow the apache example].
Appendix D. Updating SCEP implementations
(informative)
SCEP has been used instead of a full implementation of CMC for the
same simplicity reasons discussed in Section 1. Such implementations
would benefit from being updated to this specification in the
following ways:
o Implementing a subset of CMC provides an enhancement path if the
full CMC functionality is required.
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o The use of HTTPS as a transport is often perceived as more secure.
Although the SCEP protocol specification includes mechanisms (and
complexity) to address security issues avoiding a vendor
requirement to educate systems administrators is beneficial.
Implementors can benefit from the wide availability of existing
HTTPS/TLS libraries.
o SCEP servers can use their CA certificate to protect SCEP traffic
in ways that are not appropriate. (See SCEP draft Section 8.2).
This specification precludes those misuses.
o The SCEP draft Appendix D renew and rekey functionalities imply a
'flag moment' where the PKI infrastructure transitions from an
(expired) CA certificate to a new CA certificate. This
specification specifies the better mechanism defined in CMP.
Updating an SCEP client implementation to support this protocol
involves the following changes to the SCEP implementation. There is
no server side indication that SCEP clients should be so modified so
this depends on a client side configuration:
o The SCEP client supports HTTPS server authentication and
authorization as detailed Section 3.1.
o The SCEP client supports HTTPS client authentication as detailed
in Section 3.3.
o When performing the "Get CA Cert" SCEP transaction the client
supports the Section 5.1 described CMC Simple PKI Response (ref
CMC 4.1, which is extremely similar to the SCEP "CA/RA Certificate
Response Message Format" if not exactly the same).
o When performing the certificate enrollment via SCEP PKCSReq the
outgoing message is simplified to be only the inner PKCS10 (ref
CMC section 3.2.1.2.1).
o When handling the certificate enrollment response the response
format is simplified to be only the SCEP inner 'messageData'
containing the actual certificates in the degenerate PKCS7 form.
(ref CMC 4.1) The only 'authenticatedAttributes' value of
remaining importance is the 'pkiStatus' and this value is now
found in the HTTP header as defined in Section 5.2.2.
o Polling is simplified with clients repeatedly establishing the
full HTTPS connection; no polling specific state information is
encoded into the EST messages.
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o GetCert is deprecated.
o GetCRL is deprecated.
These simplifications to an existing SCEP implementation result in an
SCEP client that is compliant with CMC when using the EST transport.
Implementation note: The use of tls-unique-securerenegotiation
precludes the use of SCEP 'challenge-password' within the pkcs10 for
password/PIN assertion. Instead these values must be asserted with
the Section 3.4 described mechanism. A side effect of this is that a
client communicating with an EST server can not embed an SCEP
'challenge-password' within the PKCS#10. An EST service running as
an RA thus can not forward the PKCS#10 using SCEP to an SCEP server
that expects the 'challenge-password' to be populated.
Appendix E. Key Update mechanisms
(informative)
(This section is incomplete)
The CMP section 4.4 defined Root CA Key Update mechanisms are
repeated here for easier reference.
Author's Address
Max Pritikin (editor)
Cisco Systems, Inc.
510 McCarthy Drive
Milpitas, CA
USA
Email: pritikin@cisco.com
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