PKIX Working Group Michael Myers(VeriSign)
draft-ietf-pkix-ocsp-03.txt Rich Ankney (CertCo)
Ambarish Malpani (Valicert)
Slava Galperin (Netscape)
Carlisle Adams (Entrust Technologies)
Expires in 6 months March 1998
X.509 Internet Public Key Infrastructure
Online Certificate Status Protocol - OCSP
<draft-ietf-pkix-ocsp-03.txt>
Status of this Memo
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1. Abstract
This document specifies a protocol useful in determining the
current status of a digital certificate without the use of CRLs.
Additional mechanisms addressing PKIX operational requirements are
specified in separate documents.
Section 2 provides an overview of the protocol. Section 3 goes
establishes functional requirements, while section 4 provides the
details of the protocol. In section 5 we cover security issues with the
protocol. Appendix A demonstrates OCSP over HTTP and appendix B
accumulates ASN.1 syntactic elements.
2. Protocol Overview
In lieu of or as a supplement to checking against a periodic CRL, it may
be necessary to obtain timely status regarding a certificates
revocation state (cf. PKIX Part 1, Section 3.3). Examples include high-
value funds transfer or the compromise of a highly sensitive key.
The Online Certificate Status Protocol (OCSP) enables applications
to determine the revocation state of an identified certificate. OCSP may
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be used to satisfy some of the operational requirements of providing
more timely revocation information than is possible with CRLs. An OCSP
client issues a status request to an OCSP responder and suspends
acceptance of the certificate in question until the responder
provides a response.
This protocol specifies the data that needs to be exchanged between an
application checking the revocation status of a certificate and the
server providing that status.
2.1 Request
An OCSP request contains the following data:
- protocol version
- service request
- target certificate identifier or a single end-entity certificate
- optional extensions which MAY be processed by the OCSP Responder
Upon receipt of a request, an OCSP Responder determines if: 1) the
message is well formed, 2) the responder is configured to provide the
requested service, and 3) the responder can perform the requested
service for the subject certificate. If any one of the prior conditions
are not met, the OCSP responder produces an error message; otherwise, it
returns a definitive response.
2.2 Response
All definitive response messages SHALL be digitally signed. The key
used to sign the response MUST belong to one of the following:
- the CA who issued the certificate in question
- a Trusted Responder whose public key is trusted by the requester
A definitive response message is composed of:
- response type identifier (to allow for different response types)
- version of the response
- name of the responder
- responses for each of the certificates in a request
- optional extensions
- signature algorithm OID
- signature computed across hash of the response
The response for each of the certificates in a request consists of
- target certificate identifier
- certificate status value
- response validity interval
- optional extensions
This specification defines the following definitive response indicators
for use in the certificate status value:
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- notRevoked
- revoked
- onHold
- expired
The notRevoked state indicates that the certificate is not revoked. It
does not necessarily mean that the certificate was ever issued. Nor
does it mean that the certificate is in its validity interval. A
notRevoked state by an OCSP responder DOES NOT absolve the application
of the responsibility of checking that the certificate is in its
validity period and has been correctly signed.
The revoked state indicates that the certificate has been revoked.
The onHold state corresponds to valid certificates that are
operationally suspended in accordance with PKIX Part 1.
A request that returns an expired state indicates that the validity of
the subject certificate has expired. Applications SHOULD check the
validity interval of a certificate and not perform an OCSP request if
the certificates validity has expired.
2.3 Exception Cases
In case of errors, the OCSP Responder may return an error message.
Errors can be of the following types:
- malformedRequest
- internalError
- tryLater
- notFound
- certRequired
- noCRL
A server produces the malformedRequest response if the request received
does not conform to the OCSP syntax.
The response internalError indicates that the OCSP responder reached
an inconsistent internal state. The query should be retried, potentially
with another responder.
In the event that the OCSP responder is operational, but unable to
return a status for the requested certificate, the tryLater response
can be used to indicate that the service exists, but is temporarily
unable to respond.
A recipient of a request may not be able to resolve a reference to the
subject certificate; a value of notFound is returned in such a case.
This value should not be taken as confirmation of the certificate's
existence.
The response certRequired is returned in cases where the server requires
the client to supply the certificate data itself in order to construct a
response.
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An extension is defined to enable delivery of CRLs with OCSP responses.
However, there is no requirement to list certificates on a CRL in order
to use OCSP to acquire revocation status on those certificates. The
error value noCRL is defined for this instance.
2.4 Response Pre-production
The response validity interval noted in the prior section is composed of
a {thisUpdate, nextUpdate} pair of elements in the response syntax.
Section 4.2 provides details of the response syntax.
OCSP responders MAY pre-produce signed responses specifying the current
status of certificates at the time the response was produced. The time
at which the response was produced SHALL be reflected in the thisUpdate
field of the response.
If responses are pre-produced, then for a given certificate, the
periodicity of this pre-production SHOULD match the response validity
interval of the most recently produced response.
[need to resolve the above statement with the following RCSP assertions,
esp. with respect to positive responses. Question put to the list.]
The time at which the response was known to be correct SHALL
be specified in the producedAt field of the response. This time is not
necessarily the same as the time at which the response was produced -
e.g. if the responder obtains a CRL from a CA and creates pre-produced
responses, the thisUpdate time should specify the thisUpdate time in
the CRL.
The producer of the response MAY include a value for nextUpdate. The
exact interval between thisUpdate and nextUpdate for given response is
a matter of local security and operational policy. If the nextUpdate
field is not present, the response is is known to be correct at the
thisUpdate time. Equivalently, the nextUpdate field is considered to be
the same as the thisUpdate field.
No assertions are being made about the current state of the certificate,
nor are any recommendations being made as to when the requestor should
check again with the responder. If the value of nextUpdate is set, it
is just a hint, not a guarantee, of when the responder expects to have
new information about that certificate's status.
3. Functional Requirements
3.1 Certificate Content
In order to convey to OCSP clients a well-known point of information
access, CAs SHALL provide the capability to include the
AuthorityInfoAccess extension (defined in PKIX Part 1, section 4.2.2.1)
in certificates that can be checked using OCSP. Alternatively, the
accessLocation for the OCSP provider may be configured locally at the
OCSP client.
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CAs that support an OCSP service, either hosted locally or provided by
an Authorized Responder, MAY provide a value for a
uniformResourceIndicator (URI) accessLocation and the OID value
id-ad-ocsp for the accessMethod in the AccessDescription SEQUENCE.
The value of the accessLocation field in the subject certificate
corresponds to the URL placed into an OCSP request.
3.3 Error Responses
Upon receipt of a request which fails to parse, the receiving OCSP
responder SHALL respond with an error message. Error responses MAY be
signed.
3.5 Signed Response Acceptance Requirements
Prior to accepting a signed response as valid, OCSP clients SHALL
confirm that:
1. The certificate identified in a received response corresponds to
that which was identified in the corresponding request;
2. The signature on the response is valid;
3. The identity of the signer matches the intended recipient of the
request.
4. Detailed Protocol
The ASN.1 syntax imports terms defined in the X.509 Certificate and CRL
Profile Internet Draft. For signature calculation, the data to be signed
is encoded using the ASN.1 distinguished encoding rules (DER) [X.690].
ASN.1 EXPLICIT tagging is used as a default unless specified otherwise.
The terms imported from elsewhere are: Version, Extensions,
CertificateSerialNumber, SubjectPublicKeyInfo, Name,
AlgorithmIdentifier, GeneralizedTime
4.1 Request Syntax
OCSPRequest ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
hashAlgorithm AlgorithmIdentifier,
requestList SEQUENCE OF Request,
requestExtensions [1] EXPLICIT Extensions OPTIONAL }
Version ::= INTEGER { v1(0) }
Request ::= CHOICE {
certID [0] EXPLICIT CertID,
cert [1] EXPLICIT Certificate }
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CertID ::= SEQUENCE {
issuerNameAndKeyHash Hash,
serialNumber CertificateSerialNumber }
IssuerNameAndKey ::= SEQUENCE {
issuer Name,
issuerPublicKey SubjectPublicKeyInfo }
Hash ::= OCTET STRING --hash of IssuerNameAndKey--
3.2 Response Syntax
This section specifies the ASN.1 specification for a confirmation
response. The actual formatting of the message could vary depending on
the transport mechanism used (http, smtp, ldap, etc.).
3.2.1 ASN.1 Specification of the OCSP Response
An OCSP response at a minimum consists of a responseStatus field
indicating the processing status of the prior request. If the value of
responseStatus is one of the error conditions, responseBytes are not
set.
OCSPResponse ::= SEQUENCE {
responseStatus OCSPResponseStatus,
responseBytes [0] EXPLICIT ResponseBytes OPTIONAL }
OCSPResponseStatus ::= ENUMERATED {
successful (0), --Response has valid confirmations
malformedRequest (1), --Illegal confirmation request
internalError (2), --Internal error in issuer
tryLater (3), --Try again later
notFound (4), --Certificate not on record
certRequired (5) --Must supply certificate }
3.2.1.1 BasicResponse
The value for responseBytes consists of an OBJECT IDENTIFIER and a
response syntax identified by that OID encoded as an OCTET STRING:
ResponseBytes ::= SEQUENCE {
responseType OBJECT IDENTIFIER,
response OCTET STRING }
For a basic OCSP responder, responseType will be id-pkix-ocsp-basic,
where:
id-pkix-ocsp OBJECT IDENTIFIER ::= { id-ad-ocsp }
id-pkix-ocsp-basic OBJECT IDENTIFIER ::= { id-pkix-ocsp 1 }
OCSP responders SHALL be capable of recognizing and responding to the
id-pkix-ocsp-basic response type. Correspondingly, OCSP clients SHALL be
capable of receiving and processing the id-pkix-ocsp-basic response
type.
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The value for response SHALL be the DER encoding of BasicOCSPResponse:
BasicOCSPResponse ::= SEQUENCE {
tbsResponseData ResponseData,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING,
certs [1] EXPLICIT SEQUENCE OF Certificate OPTIONAL }
The value for signature SHALL be computed on the hash of the DER
encoding ResponseData.
3.2.1.2 ResponseData
ResponseData ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
reponderID ResponderID,
responses SEQUENCE OF SingleResponse,
responseExtensions [1] EXPLICIT Extensions OPTIONAL }
ResponderID ::= CHOICE {
byName [0] Name,
byKey [1] KeyHash }
KeyHash ::= KeyIdentifier -SHA-1 hash as defined in PKIX Part.1
3.2.1.3 SingleResponse
[note: question put to the list regarding bandwidth issues associated
with sending certificates back; could just use certID directly since
requester already has certificates in question.]
SingleResponse ::= SEQUENCE {
request Request,
certStatus CertStatus,
producedAt GeneralizedTime,
nextUpdate [0] EXPLICIT GeneralizedTime OPTIONAL,
singleExtensions [2] EXPLICIT Extensions OPTIONAL }
CertStatus ::= CHOICE {
certStatusType [0] EXPLICIT CertStatusType (notRevoked | onHold),
statusWithTime [1] EXPLICIT StatusWithTime }
StatusWithTime ::= SEQUENCE {
certStatusType CertStatusType (revoked),
time GeneralizedTime }
CertStatusType ::= ENUMERATED {
notRevoked (0), --This serial number is not revoked
revoked (1), --Serial number was revoked
onHold (2), --Cert is on hold
expired (3) -- certificate is expired }
Applications SHOULD determine by observation of the certificates
validity interval that a certificate is expired. The expired value of
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CertStatusType defines a value to return when a request is received for
a subject certificate in this state.
3.2.2 Notes on OCSP Responses
If the certStatusType is revoked, onHold or expired, the time field of
StatusWithTime is the time of revocation, suspension or expiration
respectively. The date returned for expiration should match the
notAfter date of the certificates validity interval.
The thisUpdate and nextUpdate fields define a recommended validity
interval. This interval corresponds to the {thisUpdate, nextUpdate}
interval in CRLs. Responses whose nextUpdate value is earlier
than the local system time value SHOULD be considered unreliable.
Responses whose thusUpdate time is earlier than the local system time
SHOULD be considered unreliable. Responses where the nextUpdate value
is not set are equivalant to a CRL with no time for nextUpdate (see
section 2.3).
3.3 Mandatory and Optional Cryptographic Algorithms
Clients that request OCSP services SHALL be capable of processing
responses signed used DSA keys identified by the DSA sig-alg-oid
specified in section 7.2.2 of PKIX Part 1. Clients SHOULD also be
capable of processing RSA signatures as specified in section 7.2.1 of
PKIX Part 1. OCSP responders SHALL support the SHA1 hash algorithm.
3.4 Extensions
This section defines some standard extensions. Support for all
extensions is OPTIONAL. For each extension, the definition indicates
its syntax, processing performed by the OCSP Responder, and any
extensions which are included in the corresponding response.
3.4.1 Nonce
The nonce cryptographically binds a request and a response to prevent
replay attacks. The nonce is included as one of the requestExtensions
in requests, while in responses it would be included as one of the
responseExtensions. In both the request and the response, the nonce
will be identified by the object identifier id-pkix-ocsp-nonce, while
the extnValue is the value of the nonce.
id-pkix-ocsp-nonce OBJECT IDENTIFIER ::= { id-pkix-ocsp 2 }
3.4.2 Signed Requests
This extension allows the requester to sign a request. The requestor
includes an extension that has the signatureIdentifier, the actual bits
of the signature and a sequence of certificates to allow the OCSP
responder to verify the signature. The data to be signed is just the
basic request (none of the extensions). The OCSP Responder can verify
the signature, potentially using certificates that have been included
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with the extension. The signature on a request will be identified by id-
pkix-ocsp-signature, while the value will be SignatureData, where:
id-pkix-ocsp-signature OBJECT IDENTIFIER ::= { id-pkix-ocsp 5 }
SignatureData ::= SEQUENCE {
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING,
certs [0] EXPLICIT SEQUENCE OF Certificate OPTIONAL }
3.4.3 CRL References
It may be desirable for the OCSP responder to indicate the CRL on which
a revoked or onHold certificate is found. This can be useful where OCSP
is used between repositories, and also as an auditing mechanism. The
CRL may be specified by a URL (the URL at which the CRL is available),
a number (CRL number) or a time (the time at which the relevant CRL
was created). These extensions will be specified as singleExtensions.
The identifier for this extension will be id-pkix-ocsp-crl, while the
value will be CrlID.
id-pkix-ocsp-crl OBJECT IDENTIFIER ::= { id-pkix-ocsp 4 }
CrlID ::= SEQUENCE {
crlUrl [0] EXPLICIT IA5String OPTIONAL,
crlNum [1] EXPLICIT INTEGER OPTIONAL,
crlTime [2] EXPLICIT GeneralizedTime OPTIONAL }
For the choice crlUrl, the IA5String will specify the URL at which the
CRL is available. For crlNum, the INTEGER will specify the value of the
CRL number extension of the relevant CRL. For crlTime, the
GeneralizedTime will indicate the time at which the relevant CRL was
issued.
Note: There is no requirement to list certificates on a CRL in order to
use OCSP to acquire revocation status on those certificates. Therefore
inclusion of this extension in a request may yield no CRL information.
The error value noCRL is defined for this instance.
3.4.4 Acceptable Response Types
An OCSP client MAY wish to specify the kinds of response types it
understands. To do so, it SHOULD use an extension with the OID
id-pkix-ocsp-response, and the value AcceptableResponses. The OIDs
included in AcceptableResponses are the OIDs of the various response
types this client can accept (e.g., id-pkix-ocsp-basic).
id-pkix-ocsp-response OBJECT IDENTIFIER ::= { id-pkix-ocsp 3 }
AcceptableResponses ::= SEQUENCE OF { id OBJECT IDENTIFIER }
As noted in section 3.3, OCSP responders SHALL be capable of recognizing
and responding to the id-pkix-ocsp-basic response type. Correspondingly,
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OCSP clients SHALL be capable of receiving and processing the id-pkix-
ocsp-basic response type.
3.4.5 Other Extensions
CRL Entry Extensions - specified in Section 5.3 of PKIX part I - are
also supported as singleExtensions.
4. Security Considerations
For this service to be effective, certificate using systems must connect
to the certificate status service provider. In the event such a
connection cannot be obtained, certificate-using systems could implement
CRL processing logic as a fall-back position.
A denial of service vulnerability is evident with respect to a flood of
queries constructed to produce error responses. The production of a
cryptographic signature significantly affects response generation cycle
time, thereby exacerbating the situation.
Unsigned error responses can be produced more rapidly and thus reduce
the danger of this attack. However, unsigned error responses open up the
protocol to another denial of service attack, where the attacker sends
false error responses.
The use of precomputed responses allows replay attacks in which an
old (notRevoked) response is replayed prior to its expiration date but
after the certificate has been revoked. Deployments of OCSP should
carefully evaluate the benefit of precomputed responses against the
probability of a replay attack and the costs associated its successful
execution.
5. References
[HTTP] Hypertext Transfer Protocol -- HTTP/1.0. T. Berners-Lee,
R. Fielding & H. Frystyk, RFC 1945, May 1996.
[ABNF] Augmented BNF for Syntax Specifications: ABNF. D. Crocker,
P. Overell, RFC 2234, November 1997.
[MUSTSHOULD] Key words for use in RFCs to Indicate Requirement Levels,
S. Bradner, RFC 2119, March 1997.
[URL] Uniform Resource Locators (URL), T. Berners-Lee, L. Masinter,
M. McCahill, RFC 1738, December 1994.
7. Authors Address
Michael Myers
VeriSign, Inc.
1390 Shorebird Way
Mountain View, CA 94019
mmyers@verisign.com
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Rich Ankney
CertCo, LLC
13506 King Charles Dr.
Chantilly, VA 20151
rankney@erols.com
Ambarish Malpani
ValiCert, Inc.
3160 W. Bayshore Drive
Palo Alto, CA 94303
ambarish@valicert.com
Slava Galperin
Netscape Communications Corp.
MV-068
501 E. Middlefield Rd.
Mountain View, CA 94043
galperin@netscape.com
Carlisle Adams
Entrust Technologies
750 Heron Road, Suite E08
Ottawa, Ontario
K1V 1A7
Canada
cadams@entrust.com
Appendix A
A.1 OCSP over HTTP
This section describes the formatting that will be done to the request
and response to support HTTP.
A.1.1 Request
An OCSP request is an HTTP 1.0 POST method. The Content-Type header
has the value "application/ocsp-request" while the body of the message
is the DER encoding of the OCSPRequest.
A.1.2 Response
An HTTP-based OCSP response is composed of the appropriate HTTP headers,
followed by the DER encoding of the OCSPResponse. The Content-Type
header has the value "application/ocsp-response". The Content-Length
header SHOULD specify the length of the response. Other HTTP headers
MAY be present and MAY be ignored if not understood by the requestor.
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Appendix B: OCSP in ASN.1
OCSPRequest ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
hashAlgorithm AlgorithmIdentifier,
requestList SEQUENCE OF Request,
requestExtensions [1] EXPLICIT Extensions OPTIONAL }
Version ::= INTEGER { v1(0) }
Request ::= CHOICE {
certID [0] EXPLICIT CertID,
cert [1] EXPLICIT Certificate }
CertID ::= SEQUENCE {
issuerNameAndKeyHash Hash,
serialNumber CertificateSerialNumber }
IssuerNameAndKey ::= SEQUENCE {
issuer Name,
issuerPublicKey SubjectPublicKeyInfo }
Hash ::= OCTET STRING --hash of IssuerNameAndKey--
OCSPResponse ::= SEQUENCE {
responseStatus OCSPResponseStatus,
responseBytes [0] EXPLICIT ResponseBytes OPTIONAL }
OCSPResponseStatus ::= ENUMERATED {
successful (0), --Response has valid confirmations
malformedRequest (1), --Illegal confirmation request
internalError (2), --Internal error in issuer
tryLater (3), --Try again later
notFound (4), --Certificate not on record
certRequired (5) --Must supply certificate }
BasicOCSPResponse ::= SEQUENCE {
tbsResponseData ResponseData,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING,
certs [1] EXPLICIT SEQUENCE OF Certificate OPTIONAL }
ResponseData ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
reponderID ResponderID,
responses SEQUENCE OF SingleResponse,
responseExtensions [1] EXPLICIT Extensions OPTIONAL }
ResponderID ::= CHOICE {
byName [0] Name,
byKey [1] KeyHash }
KeyHash ::= KeyIdentifier -SHA-1 hash as defined in PKIX Part.1
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SingleResponse ::= SEQUENCE {
request Request,
certStatus CertStatus,
producedAt GeneralizedTime,
nextUpdate [0] EXPLICIT GeneralizedTime OPTIONAL,
singleExtensions [2] EXPLICIT Extensions OPTIONAL }
CertStatus ::= CHOICE {
certStatusType [0] EXPLICIT CertStatusType (notRevoked | onHold),
statusWithTime [1] EXPLICIT StatusWithTime }
StatusWithTime ::= SEQUENCE {
certStatusType CertStatusType (revoked),
time GeneralizedTime }
CertStatusType ::= ENUMERATED {
notRevoked (0), --This serial number is not revoked
revoked (1), --Serial number was revoked
onHold (2), --Cert is on hold
expired (3) -- certificate is expired }
--Extensions
SignatureData ::= SEQUENCE {
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING,
certs [0] EXPLICIT SEQUENCE OF Certificate OPTIONAL }
AcceptableResponses ::= SEQUENCE OF { id OBJECT IDENTIFIER }
CrlID ::= SEQUENCE {
crlUrl [0] EXPLICIT IA5String OPTIONAL,
crlNum [1] EXPLICIT INTEGER OPTIONAL,
crlTime [2] EXPLICIT GeneralizedTime OPTIONAL }
-- Object Identifiers
id-pkix-ocsp OBJECT IDENTIFIER ::= { id-ad-ocsp }
id-pkix-ocsp-basic OBJECT IDENTIFIER ::= { id-pkix-ocsp 1 }
id-pkix-ocsp-nonce OBJECT IDENTIFIER ::= { id-pkix-ocsp 2 }
id-pkix-ocsp-response OBJECT IDENTIFIER ::= { id-pkix-ocsp 3 }
id-pkix-ocsp-crl OBJECT IDENTIFIER ::= { id-pkix-ocsp 4 }
id-pkix-ocsp-signature OBJECT IDENTIFIER ::= { id-pkix-ocsp 5 }
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