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Internet Draft                                       Ambarish Malpani
draft-ietf-pkix-scvp-01.txt                                  ValiCert
August 9, 1999                                           Paul Hoffman
Expires in six months                                  VPN Consortium

          Simple Certificate Validation Protocol (SCVP)

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.


Abstract

The SCVP protocol allows a client to offload certificate handling to a
server. The server can give a variety of valuable information about the
certificate, such as whether or not the certificate is valid, a chain
to a trusted root, and so on.


1. Introduction

Certificate validation is a difficult problem. If certificate handling
is to be widely deployed in a variety of applications and environments,
the amount of processing an application needs to perform before it can
accept a certificate must be reduced. There are a variety of
applications that can use public key certificates but are burdened by
the overhead of validating the certificates when all the application
really wants is the public key and name from the certificate, and a
determination of whether or not the certificate may be used for a
particular purpose. There are other applications that can perform
certificate path validation but have no reliable method of obtaining a
current chain to a trusted root.

1.1 SCVP overview and requirements

The primary goals of SCVP are to make it easier for  applications to
deploy systems using a PKI and to allow centralization of administering
PKI policies. Parts of SCVP can be used by clients that do much of the
PKI processing themselves and simply want a useful but untrusted server
that will collect information for them. Other parts can be used by
clients that have complete trust in the server to both offload the work
of certificate validation and to ensure that policies are enforced in a
consistent fashion across an enterprise.

Untrusted SCVP servers can give client the certificate chains needed
for path validation. They can also give clients revocation information
such as CRLs and OCSP responses that the client can use in the client's
path validation. These services can be valuable to client systems that
do not include the protocols needed to find and download all of the
intermediate certificates, CRLs, and OCSP responses needed for the
client to perform complete path validation.

Trusted SCVP servers can perform full certificate validation for the
client. If a client uses these services, it inherently trusts the SCVP
server as much as it would its own path validation software (if it
contained such software). There are two main reasons that a client may
want to trust such an SCVP server:

- The client does not want to incur the overhead of including path
validation software and running it for each certificate it receives.

- The client is in an enterprise that wants to centralize its PKI
validation policies, such as which root certificates are trusted and
which types of policy checking are performed during path validation.

1.2 Terminology

Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD NOT
are used in capital letters. This  conforms to the definitions in
[MUSTSHOULD]. [MUSTSHOULD] defines the use of these key words to help
make the intent of standards track documents as clear as possible. The
same key words are used in this document to help implementors achieve
interoperability.

1.3 Open Issues

The following is a list of issues that were raised on earlier versions
of this document that have not been fully dealt with here. Comments on
these issues are particularly welcome.

- CertBundle items can have certificates represented either in their
entirety or as a hash of the cert. We included cert hashes as a way of
making the request and responses smaller. However, each side of the
request and response can only use a cert hash if it is sure that the
other side already has the cert. This preexisting knowledge can reduce
the size of the messages but can also cause problems if the two sides
are not completely correct about what the other side knows. The
CertBundle item is now only used in TrustedRoots, RequestSignature,
ResponseSignature, and ReplyWantBack items. Is it OK to use hashes in
all these items? Can we use hashes safely in other places which now
require full certificates?

- Extensions can be marked as critical. The usefulness and problems of
criticality have been long debated and there has not been a great deal
of consensus. In SCVP, marking a request extension as critical says to
the server "don't give me an answer unless you understand this
extension", and marking a response as critical says "don't use this
response unless you understand this extension". Without the critical
bit in the extensions, either the semantics of extensions would have to
be changed to essentially say "all extensions are critical" (which is
overkill for some extensions that might really be optional), or the
semantics would have to be changed to say "you can never rely on the
other party understanding an extension", which would limit the
usefulness of some extensions.

- In this version of the spec, all responses must be signed. In the
previous version, signing was optional if the protocol was being used
in an otherwise secure environment; this was criticized as being too
hard to enforce. However, there are many responses on which a signature
might not be needed and a server could be more efficient if it did not
need to sign those responses. One way around this is to go back to an
optional response signature and carefully list the places that the
signature is and is not needed. Another is to create a NULL
signature algorithm so that "signing" takes almost no effort.


2. Protocol

The SCVP protocol uses a simple request-response model. That is, a SCVP
client creates a single request and sends it to the server; the server
creates a single response and sends it to the client. Typical use of
SCVP is expected to be over HTTP, and possibly email. This document
registers MIME types for SCVP requests and responses.


3. Requests

A SCVP client's request to the server MUST be a single FullRequest
item. The FullRequest item contains the entire request. A FullRequest
item is carried in an application/scvp-request MIME body part.

3.1 FullRequest

The FullRequest item encapsulates the client's request. The FullRequest
item contains a TBSRequest item, an RequestHash item, and an optional
RequestSignature item. The RequestHash and the RequestSignature items
are performed over the TBSRequest item.

3.2 TBSRequest

The TBSRequest item contains the part of the client's request that is
hashed by the RequestHash item and optionally signed by the
RequestSignature item. The TBSRequest item contains a Version item, a
Query item, a TypesOfCheck item, and a WantBack item. It can also
contain an optional RequestNonce item and an optional Extensions item.

In this specification, the item(s) in the Query item are certificates.
The TypesOfCheck item tells the server what types of checking it should
do on the item(s) in the Query item. The WantBack item tells the server
what the client wants to know about the item(s). Extensions in the
TBSRequest item are used to extend the request, such as to request a
different type of item.

3.3 Version

The Version item tells the version of SCVP used in a request or a
response. The value of the Version item for this specification is 1.

3.4 Query

The Query item specifies the object of the request. One type of object
is defined in this specification: CertsQuery. The CertsQuery is a
request for information on one or more certificate. A CertsQuery
contains a list of certificates, and can also contain zero or one of
each of the following items: ValidityTime, IntermediateCerts,
TrustedRoots, RevocationInfo, UsageID, ConfigurationIdentifier, and
Extensions.

The list of certificates in the Query item tells the server the
certificate(s) the client wants a reply for. The optional ValidityTime
item tells the time at which the client wants to know about. The
optional IntermediateCerts, TrustedRoots, RevocationInfo, UsageID, and
ConfigurationIdentifier items tell the server how to process the
request. The Extension item in the Query item applies only to the
certificates in the list of certificates. If there is more than one
certificate in the list, the other items all apply to every certificate
in the list.

3.5 CertBundle

The CertBundle item contains one or more certificates, represented as
FullCert items and/or CertHash items. The order of the certificate(s)
in the bundle is not important. The items that use CertBundle items
specify the types of certificates that can be in the CertBundle items.

3.6 FullCert

The FullCert item contains a complete certificate. The FullCert item
contains an identifier for the type of certificate and the octets of
the certificate itself. One type of certificate, for PKIX [PKIX], is
defined, but other types of certificates (such as for OpenPGP
[OpenPGP]) may be defined in the future.

3.7 CertHash

The CertHash item contains the hash of a certificate. The CertHash item
has an identifier for the type of certificate and the SHA-1 [SHA-1]
hash of the octets of the certificate itself. One type of certificate,
for PKIX, is defined, but other types of certificates (such as for
OpenPGP) may be defined in the future.

The client SHOULD only include CertHash items for certificates that the
client knows the server is aware of. It makes no sense to send the hash
of a certificate unless the server already has a complete copy of the
certificate.

3.8 Extensions

The Extensions item specifies a list of extensions to the SCVP
protocol, for example to request additional information about the
certificate(s) in the CertsQuery. The Extensions item contains a
sequence of Extension items, each of which contain an ExtensionID item,
an ExtensionCritical item, and an ExtensionValue item.

3.9 ExtensionID

The ExtensionID item is an identifier for the extension. It contains
the OID of the extension.

3.10 ExtensionCritical

The ExtensionCritical item tells whether the extension is critical. The
values for the item are:

False  Not critical
True   Critical

In a request, if the ExtensionCritical item is true, the server MUST
NOT process the request unless it understands the extension. In a
reply, if the ExtensionCritical item is true, the client MUST NOT
process the reply if unless it understands the extension.

3.11 ExtensionValue

The ExtensionParameters item gives parameters for an extension. It
contains a sequence of octets.

3.12 IntermediateCerts

The IntermediateCerts item specifies to the server the intermediate
certificates that the server MAY use when forming a certificate chain.
The certificates in the IntermediateCerts item can be used by the
server in addition to any other certificates that the server knows of
when building chains. The IntermediateCerts item contains a list of
certificates. The certificates in the IntermediateCerts MUST NOT be
self-signed certificates.

The purpose of the IntermediateCerts item is to help the server create
validation chains. For example, the server may not have access to all
intermediate certificates for an end-entity certificate, but the client
has what it thinks are candidates for intermediate certificates because
the end entity gave them to the client. Another example is that the
client may have more up-to-date information than the server about
certificates that have been rolled-over.

3.13 TrustedRoots

The TrustedRoots item specifies to the server the root certificates
that the server MUST use. If a TrustedRoots item is included in a
CertsQuery item, the server MUST NOT use any roots other than the
certificates in the TrustedRoots item when forming a certificate chain
for validation. The TrustedRoots item contains a CertBundle item.

The purpose of the TrustedRoots item is to tell the server which roots
the client trusts. Many end-entity certificates have one or more
intermediate certificates in the chain to a the root. If the client is
not aware of the intermediate certificates, the TrustedRoots item tells
the server only validate the chain that the server builds if the chain
ends at one of the root certificates given. The TrustedRoots item can
also tell a server which roots a client trusts in the case where the
certificate in question might make different chains to different roots
due to cross-certification.

3.14 RevocationInfo

The RevocationInfo item specifies to the server revocation information
such as CRLs and OCSP [OCSP] responses that the server MAY use when
validating certificate chains. The purpose of the RevocationInfo item
is to provide revocation information to the server that the server may
not have access to, such as an OCSP response that the client received
on its own OCSP request. Note that the information in the
RevocationInfo item might not be used by the server, such as if the
information is for certificates that the server does not use in chain
building. Each type of revocation proof that can be requested is
indicated by an OID from a standard arc.

{revinfo-arc 1}  CRL
{revinfo-arc 2}  OCSP response

[[[Need to specify the format of the extensions for both CRLs and
for OCSP responses.]]]

3.15 UsageID

The UsageID item specifies to the server the policy ID that the server
MUST use when forming a certificate chain. The UsageID item contains
the OID of the policy.

3.16 ConfigurationIdentifier

The ConfigurationIdentifier item tells the server the SCVP options that
the client wants the server to use. The client can use this option
instead of specifying other SCVP configuration such as UsageID,
TrustedRoots, RevocationInfo, and so on. The value of this item is
determined by private agreement between the client and the server and
is not specified in this document. For example, the value might be the
hash of some set of options, or it might be a short identifier for a
common set of options. Further, the server might want to have
identifiers that indicate that some settings are used in addition to
others given in the request; in this way, the configuration identifier
might be a shorthand for some SCVP options.

3.17 TypesOfCheck

The TypesOfCheck item describes the kind of checking that the client
wants the server to do on the certificate(s) in the Query item. If the
TypesOfCheck item is given in a request, it can contain one or more
types of checks. For each type of check specified in the request, the
server MUST return information on what it found during the check. Each
type of checks is indicated by an OID from a standard arc.

{type-arc 0}  Path validation to a trusted root
{type-arc 1}  Revocation status

3.18 WantBack

The WantBack item describes the kind of information the client wants
from the server for the certificate(s) in the Query item. If the
WantBack item is given in a request, it can contain one or more types
of information. For each type of information specified in the request,
the server MUST return information on what it found during the check.
Each type of information that can be requested is indicated by an OID
from a standard arc.

{want-arc 1}  Certificate chain used to validate the certificate
{want-arc 2}  Proof of revocation status

For example, a request might include a TypesOfCheck item that does not
specify path validation, and include a WantBack item that specifies the
certificate chain used to validate. The response would not include a
status for the validation, but would include a certificate chain that
the server thinks might validate. This set of options might be used by
a client that wants to do its own path validation.

3.19 ValidityTime

The ValidityTime indicates the time for which the client wants the
information to be relevant. For example, when asking for validation of
a certificate, the client might ask "was this certificate valid at this
time". The information in the CertReply item in the response MUST be
formatted as if the server created the response at the time indicated
in the ValidityTime, except that if the server doesn't have historical
information about that time, the information will be for a time later
than the ValidityTime. A client MUST be able to handle responses that
have ThisUpdate and NextUpdate items that are later than the requested
ValidityTime.

3.20 RequestNonce

The RequestNonce item is an identifier generated by the client for the
request; the server MUST return the same RequestNonce in the signed
part of the server's response. The RequestNonce item is simply a
sequence of octets. The client SHOULD include a RequestNonce item in
every request to prevent an attacker acting as a man-in-the-middle from
replaying old responses from the server. The value of the nonce SHOULD
change with every request sent from the client.

3.21 RequestHash

The RequestHash item is the SHA-1 hash of the TBSRequest item. The
RequestHash item serves the following purposes:

- It helps a client know that the request was  not maliciously modified
  when the client gets the response back

- It allows the client to associate a response with a request when
  using connectionless protocols

The purpose of the RequestHash is not for authentication of the
client.

The server MUST check the hash to verify that the request was not
modified in transit, and MUST return the RequestHash item in the
response.

3.22 RequestSignature

The RequestSignature item is the signature of the TBSRequest item. The
RequestSignature item contains a SignatureName item that is the name of
the client, a SignatureAlgorithm item that is the algorithm used to
sign the request, and a SignatureBits item that is the signature
itself. The RequestSignature item may contain an optional KeyID item
that identifies the signing key. The RequestSignature item may also
contain an optional CertBundle that represents a chain of certs for the
key used to sign the request.

A RequestSignature item can be used to authenticate the client to the
server and for non-repudiation of the client's request, such as for
accounting purposes. A server might require all requests to be signed
if the server did not want to respond to request unless they were from
authenticated clients. A client does not need to sign a request in
order to be sure that the response from a server is to the request that
the client issued; the RequestHash is sufficient as long as the server
checks the value of the RequestHash and signs its response to the
client.

3.23 SignatureName

The SignatureName item holds the name of a signer. It is used to
associate a key with the request.

3.24 SignatureAlgorithm

The SignatureAlgorithm identifies the algorithm used to sign a request
or response. The SigningAlgorithm item contains the OID of the
algorithm and any necessary parameters for the algorithm.

3.25 SignatureBits

The SignatureBits item holds the octets of a signature. The structure
of the SignatureBits item is determined by the value of the
SignatureAlgorithm item.

3.26 KeyID

The KeyID item contains the hash of a public key. The purpose of the
KeyID is to identify the specific public key used by the named signer in
the case that the signer has many public keys. The KeyID item has the
SHA-1 hash of the octets of the key itself. [[[What bits are to be
hashed?]]]


4. Responses

A SCVP server's response to the client MUST be a single FullResponse
item. The FullResponse item contains the entire response. A
FullResponse item is carried in an application/scvp-response MIME body
part.

4.1 FullResponse

The FullResponse item encapsulates the server's response. The
FullResponse item contains a TBSResponse item and a ResponseSignature
item.

4.2 TBSResponse

The TBSResponse item contains the part of the server's response that is
signed by the ResponseSignature item. The item contains a Version item,
a ProducedAt item, a ResponseStatus item, and a RequestHash item. The
item can also contain an optional ReplyObjects item, an optional
RequestNonce item, and an optional Extensions item. The TBSResponse item
MUST contain exactly one CertReply item for each certificate requested
in the request. The RequestNonce item MUST be included if the request
had a RequestNonce item.

4.3 ProducedAt

The ProducedAt item tells the time at which the whole response was
produced. The ProducedAt item represents the date at Greenwich Mean
Time.

4.4 ResponseStatus

The ResponseStatus item gives status information to the client about
its request. The ResponseStatus item has a numeric status code and an
optional string that is a sequence of characters from the ISO/IEC
10646-1 character set encoded with the UTF-8 transformation format
defined in [UTF8].

The optional string may be used to transmit status information, but it
is optional. The client MAY choose to display the string to the client.
However, because there is no way to know the languages understood by
the user, the string may be of little or no use to them.

The complete list of status codes for the ResponseStatus item is:

0  The request was fully processable
1  The request included unrecognized items; continuing

10  Too busy; try again whenever you feel like it
11  Too busy; try again later than 10 seconds from now
12  Too busy; try again later than 60 seconds from now

20  The structure of the request was wrong
21  The version of request is not supported by this server
22  The request included unrecognized items; aborting
23  The key given in the RequestSignature is not recognized
24  The signature did not match the body of the request
25  The hash in the RequestHash did not compute
26  The request was not authorized

4.5 ReplyObjects

The ReplyObjects item returns objects to the client. In this
specification, the ReplyObjects item is always a CertReply, which tells
the client about a single certificate from the request. The CertReply
item contains a CertHash item identifying the certificate, a
ReplyStatus item, a ThisUpdate item, and a NextUpdate item. There may
also be the following optional items: ValidationStatus,
RevocationStatus, PublicKey, CertSubject, ValidationChain,
RevocationProof, and Extensions.

The presence or absence of the ValidationStatus, RevocationStatus,
PublicKey, CertSubject, ValidationChain, and RevocationProof items in
the CertReply item is controlled by the TypesOfCheck, and WantBack
items in the request. A server MUST include one of the above items for
each related item requested in the TypesOfCheck, and WantBack items. An
Extensions item in the response MAY be in response to an Extensions
item in the request, but the server MAY include an Extensions item in a
response even if the request did not have an Extensions item.

4.6 ReplyStatus

The ReplyStatus item gives status information to the client about the
request for the specific certificate. Note that the ResponseStatus item
is different than the ReplyStatus item. The ResponseStatus item is the
status of the whole request, while the ReplyStatus item is the status
for the individual certificate.

The complete list of status codes for the ReplyStatus item is:

0  Success: a definitive answer follows
1  Failure: the certificate type is not recognized
2  Failure: an item wanted in TypesOfCheck is not recognized
3  Failure: an item wanted in WantBack is not recognized
4  Failure: the certificate was malformed
5  Failure: the mandatory UsageID is not recognized
6  Failure: the ConfigurationIdentifier is not recognized
7  Failure: unauthorized request

Status code 4 is used to tell the client that the request was properly
formed but the certificate in question was not. This is useful to
clients that cannot parse a certificate.

4.7 ThisUpdate

The ThisUpdate item tells the time at which the information in the
CertReply was correct. The ThisUpdate item represents the date at
Greenwich Mean Time.

4.8 NextUpdate

The NextUpdate item tells the time until which the server expects the
information in the CertReply to be valid. The NextUpdate item
represents the date at Greenwich Mean Time. [[[Is there a desire
for another item that says "the server takes liability for this
response up to this particular time?]]]

4.9 ReplyTypesOfCheck

The ReplyTypesOfCheck contains the responses to the client's
TypesOfCheck item in the request. It has the same form as the
Extensions item, and the OIDs in the ReplyTypesOfCheck item MUST match
the OIDS in the TypesOfCheck item.

The value for path validation to a trusted root, {type-arc 0}, can be
one of the following:

0  Valid
1  Not valid
2  Temporarily unknown
3  Unknown

The value for the revocation status, {type-arc 1}, can be one of the
following:

0  Good
1  Revoked
2  Temporarily unknown
3  Unknown

4.10 ReplyWantBack

The ReplyWantBack contains the responses to the client's WantBack item
in the request. It has the same form as the Extensions item, and the
OIDs in the ReplyWantBack item MUST match the OIDS in the WantBack
item.

The value for the certificate chain used to validate the certificate
in the request, {want-arc 1}, is a CertBundle item.

The value for the proof of revocation status, {want-arc 2}, is a
RevocationProof item.

4.11 RevocationProof

The RevocationProof item gives the client the proof that the server
used to check revocation. The structure of the RevocationProof item is
the same as an Extensions item. The OIDs in the RevocationProof item
are the same as those in the RevocationInfo item.

4.12 ResponseSignature

The ResponseSignature item is the signature of the TBSResponse item.
The ResponseSignature item contains a Name item that is the name of the
server, a SigningAlgorithm item that is the algorithm used to sign the
response, and a SignatureBits item that is the signature itself. The
ResponseSignature item may contain an optional KeyID item that
identifies the signing key. The ResponseSignature item may also contain
an optional CertBundle that represents a chain of certs for the key
used to sign the response.

The client SHOULD check the signature on every message it receives from
the server, and MUST check the signature on every message that includes
validation and/or revocation information on which the client will rely.
In order to check the signature, the client MUST know and rely on the
public signing key of the server. The client could have obtained the
server's public key through an out-of-band mechanism of direct trust or
through a certificate that chains to a root that the client trusts to
delegate this type of authority.


5. ASN.1 Syntax for SCVP

This section defines the syntax for SCVP messages. The semantics for
the messages are defined in sections 2, 3, and 4.

Note that the ASN.1 syntax here is purposely designed to ease the job
of "hand-coding" the messages while still making the syntax easy for
ASN.1 compilers to work with. It is also designed so that it is easy
for developers of other PKI protocols to reuse the syntax with minimal
changes.

SCVP DEFINITIONS EXPLICIT TAGS ::=

BEGIN

IMPORTS
    ; -- TO BE DONE. Will include Certificate, Name,
      --    AlgorithmIdentifier, and GeneralizedTime.

FullRequest ::= SEQUENCE {
    tbsRequest           TBSRequest,
    requestHash          OCTET STRING,
    requestSignature     Signature OPTIONAL
}

TBSRequest ::= SEQUENCE {
    version              INTEGER,
    query                Query,
    typesOfCheck         TypesOfCheck,
    wantBack             WantBack,
    requestNonce     [0] OCTET STRING OPTIONAL,
    extensions       [1] Extensions OPTIONAL
}

Query ::= CHOICE {
    certsQuery       [0] CertsQuery
}

CertsQuery ::= SEQUENCE {
    queriedCerts          SEQUENCE OF Cert,
    validityTime      [0] GeneralizedTime OPTIONAL,
    intermediateCerts [1] SEQUENCE OF Cert OPTIONAL,
    trustedRoots      [2] CertBundle OPTIONAL,
    revocationInfos   [3] Extensions OPTIONAL,
    usageID           [4] OBJECT IDENTIFIER OPTIONAL,
    configurationIdentifier [5] OBJECT IDENTIFIER OPTIONAL,
    extensions        [6] Extensions OPTIONAL
}

CertBundle ::= SEQUENCE OF CertItem

CertItem ::= CHOICE {
    fullCert      [0] Cert,
    certHash      [1] CertHash
}

Cert ::= CHOICE {
    pkixCert        [0] Certificate
}

CertHash ::= CHOICE {
    pkixCertHash    [0] OCTET STRING
}

TypesOfCheck ::= SEQUENCE SIZE (1..MAX) OF OBJECT IDENTIFIER

WantBack ::= SEQUENCE SIZE (1..MAX) OF OBJECT IDENTIFIER

Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension

Extension ::= SEQUENCE {
    ExtensionID          OBJECT IDENTIFIER,
    ExtensionCritical    BOOLEAN DEFAULT FALSE,
    ExtensionValue       OCTET STRING
}

Signature ::= SEQUENCE {
    signerName               Name,
    signatureAlgorithm       AlgorithmIdentifier,
    signatureBits            BIT STRING,
    keyID                [0] OCTET STRING OPTIONAL,
    certs                [1] CertBundle OPTIONAL
}

FullResponse ::= SEQUENCE {
    tbsResponse           TBSResponse,
    responseSignature     Signature
}

TBSResponse ::= SEQUENCE {
    version               INTEGER,
    producedAt            GeneralizedTime,
    responseStatus        ResponseStatus,
    requestHash           OCTET STRING,
    replyObjects      [0] ReplyObjects OPTIONAL,
    requestNonce      [1] RequestNonce OPTIONAL,
    extensions        [2] Extensions OPTIONAL
}

ResponseStatus ::= SEQUENCE {
    statusCode          INTEGER,
    errorMessage    [0] UTF8String OPTIONAL
}

ReplyObjects ::= CHOICE {
    certReplies     [0] SEQUENCE OF CertReply
}

CertReply ::= SEQUENCE {
    certHash              OCTET STRING,
    replyStatus           ReplyStatus,
    thisUpdate            GeneralizedTime,
    nextUpdate            GeneralizedTime,
    replyTypesOfCheck [0] Extensions OPTIONAL,
    replyWantBack     [1] Extensions OPTIONAL,
    extensions        [2] Extensions OPTIONAL
}

ReplyStatus ::= ENUMERATED {
    success                  (0),
    certTypeUnrecognized     (1),
    typeOfCheckUnrecognized  (2),
    wantBackUnrecognized     (3),
    certMalformed            (4),
    usageIDUnrecognized      (5),
    configInfoUnrecognized   (6),
    unauthorizedRequest      (7)
}

-- Need to include type-arc, want-arc, and revinfo-arc

END


6. Security Considerations

A client that trusts a server's responses for validation of
certificates inherently trusts that server as much as it would trust
its own validation software. This means that if an attacker compromises
a trusted SCVP server, the attacker can change the validation
processing for every client that relies on that server. Thus, an SCVP
server must be protected at least as well as the weakest root server
that the SCVP server trusts.

If a server does not check the RequestHash or the signature in the
optional RequestSignature item and the request contains certificates
that are not being validated, a man-in-the-middle attack could change
the values of the certificates and cause the server to give different
answers to a request.

If the client does not check the signature on the response, a
man-in-the-middle attack could fool the client into believing modified
responses from the server, or responses to questions the client did not
ask. This attack does not affect the usefulness of some responses (such
as a response that returns a certificate path that the client will
validate itself) but does affect things such as a validation response.

If the client does not include a RequestNonce item, or if the client
does not check that the RequestNonce in the reply matches that in the
request, an attacker can replay previous responses from the server.
This attack can also be mounted, even with signed requests, if the
server does not keep track of previous RequestNonce items.

If the server does not require some sort of authorization (such as
signed requests), an attacker can get the server to reply to arbitrary
requests. Such responses may give the attacker information about
weaknesses in the server or about the timeliness of the server's
checking. This information may be valuable for a future attack.


A. References

[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.

[OCSP] "PKIX Online Certificate Status Protocol (OCSP)", RFC 2560.

[OpenPGP] "OpenPGP Message Format", RFC 2440.

[PKIX] "PKIX Certificate and CRL Profile", RFC 2459.

[SHA-1] "Secure Hash Standard", NIST FIPS publication 180-1, April
1995.

[UTF8] "UTF-8, a transformation format of ISO 10646", RFC 2279.


B. Acknowledgments

The lively debate in the PKIX Working Group also had a significant
impact on the types of items described in this protocol. Denis Pinkas
suggested some additional requirements for the protocol, and Mike Myers
helped point out sections that needed clarification.


C. Changes Between Versions of This Document

C.1 Differences between -00 and -01

1: Rewrote to both narrow focus and to explain the goals more fully.

1.1: Removed second paragraph.

2: Removed the discussion of the two syntaxes.

3: Reorganized the section to put the Extensions items after the
CertsQuery items. The section numbers below are from the -00 draft.
Throughout the section, made RequestHash mandatory instead of optional.
Added RevocationInfo item. Changed CertID to CertHash throughout.
Fixed the names of the parts of the signature to match the text.

3.1: Split the item into a TBSRequest followed by the hash and/or
signature. Changed the order of the extensions item so that all the
optional items were together. Changed CertsQuery into Query. Added the
ValidityTime item.

3.3: Redefined Extension to be Extensions to be more similar to
Extensions in PKIX. Other wording changes.

3.5: Gave more explanation for the ExtensionCritical bit, and made
the values boolean. Note that this item may disappear, depending
on discussion of the open issue on it.

3.7: Changed CertsQuery into Query and described the one defined
instance as CertsQuery. Moved the TypesOfCheck and WantBack from the
Query and up one level to the TBSRequest.

3.9: Removed OpenPGP cert, but allowed for it to be added back in the
future.

3.10: Removed OpenPGP cert hash, but allowed for it to be added back in
the future.

3.11 Made TypesOfCheck OIDs.

3.12: Made WantBack OIDs. Removed the public key and the names.

3.10: Added sentence about when a client might include a CertHash item
in the TrustedRoots.

3.13: Clarified use of IntermediateCerts

3.18: Added wording that the RequestHash should not be used for
authentication.

3.19: Changed wording to make it clear that RequestSignature was needed
only for authentication of the client.

3.23: Clarified purpose of KeyID.

4: The section numbers below are from the -00 draft. Throughout the
section, made returning the RequestHash mandatory because it is now
mandatory in the request.

4.1: Split the item into a TBSResponse followed by the hash and/or
signature. Made ResponseSignature mandatory. Made the items returned in
the form of Extensions to match the fact that TypesOfCheck and WantBack
are now sequences of OIDs.

4.3: Made the status code a single number.

4.4 Removed the subject names and public keys. Added NextUpdate.

4.10: Clarified that CertSubject for PKIX certs must contain both the
subject name and the subjectAltName.

4.13: Made ResponseSignature mandatory; this might be changed back to
optional for some types of responses in a future revision of the spec.
Added a discussion of how the client can get the server's signing key.

Old 5: Removed tiny syntax, renumbered old 6 to 5.

5: Added note about semantics in 2-4.
Split FullRequest into FullRequest and TBSRequest.
Moved the extensions item in FullRequest.
Changed the certsQuery to Query.
Move TypesOfCheck and WantBack up to TBSRequest.
Made TypesOfCheck and WantBack SEQUENCE of OIDs.
Added ValidityTime.
Changed "CertID" to "CertHash".
Made the status code a single number.
Added reminder in CertItem about full certs.
Changed order of Signature items.
Split FullResponse into FullResponse and TBSResponse.
Added ReplyTypesOfChecks and ReplyWantBack items.
Added Extensions item and sub-items.

7: Updated to reflect mandatory RequestHash and ResponseSignature.
Added explicit words about compromise of the SCVP server. Removed the
first paragraph because it was confusing and will be fixed in later
versions of the draft.

A: Added reference to OCSP.

D: Updated.


D. MIME Registrations

D.1 Registration for application/scvp-request [TBD]

D.2 Registration for application/scvp-response [TBD]


E. Author Contact Information

Ambarish Malpani
ValiCert
1215 Terra Bella Ave.
Mountain View, CA 94043-1833
ambarish@valicert.com

Paul Hoffman
VPN Consortium
127 Segre Place
Santa Cruz, CA  95060 USA
paul.hoffman@vpnc.org

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