Transport Layer Security (TLS) Extension: Validation Request
draft-segers-tls-cert-validation-ext-02
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| Last updated | 2022-06-28 | ||
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draft-segers-tls-cert-validation-ext-02
TLS R. Segers
Internet-Draft Federal Aviation Administration
Intended status: Standards Track A. Kopman
Expires: 30 December 2022 Concepts Beyond
28 June 2022
Transport Layer Security (TLS) Extension: Validation Request
draft-segers-tls-cert-validation-ext-02
Abstract
This document describes the Path Validation extension to the
Transport Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) protocols.
The Path Validation Extension provides a new protocol for TLS/DTLS
allowing inclusion of certificate path validation information in the
TLS/DTLS handshake.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on 30 December 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Validation Request Extension . . . . . . . . . . . . . . . . 4
3. Path Validation Request . . . . . . . . . . . . . . . . . . . 5
3.1. SCVP Validation Request . . . . . . . . . . . . . . . . . 6
3.1.1. Responder URIs . . . . . . . . . . . . . . . . . . . 6
3.1.2. Trust Anchors . . . . . . . . . . . . . . . . . . . . 6
3.1.3. Validation Extensions . . . . . . . . . . . . . . . . 7
3.1.4. TLS Server CVRequest . . . . . . . . . . . . . . . . 7
4. Path Validation Response . . . . . . . . . . . . . . . . . . 12
4.1. SCVP Validation Response . . . . . . . . . . . . . . . . 12
4.1.1. SCVP Response Processing by TLS Server . . . . . . . 13
4.1.2. SCVP Response Processing by TLS Client . . . . . . . 14
4.2. Path Validation Cache . . . . . . . . . . . . . . . . . . 14
5. Error Alerts . . . . . . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6.1. Reference for TLS Alerts and ExtensionTypes . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7.1. Support for Extension . . . . . . . . . . . . . . . . . . 16
7.2. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 16
7.3. Extension Modifications . . . . . . . . . . . . . . . . . 16
7.4. Unrelated Path Validation Response . . . . . . . . . . . 17
7.5. Trust Anchor Maintenance . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
This document describes an extension to TLS 1.3 [RFC8446] and DTLS
1.3 [RFC9147] for the inclusion of certificate path validation
information in the TLS/DTLS handshake. Specifically, this extension
covers the use of the Server-based Certificate Validation Protocol
(SCVP) [RFC5055] for path validation. However, the extension is
designed to allow for expansion to other path validation protocols.
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This extension is defined for TLS and DTLS protocols. For
convenience, the protocol will be referred to as TLS for the rest of
the document. DTLS will only be specifically mentioned in cases
where the protocols differ.
The TLS standard specifies that certificates should always be
verified to ensure proper signing by a trusted Certificate Authority
(CA) in Part Appendix C.2 of TLS 1.3 [RFC8446]. The establishment of
trust requires construction and validation of a trust path from the
end-entity certificate to a trust anchor. This validation can be a
complex process of chaining certificates, validating revocation
information, and enforcing organizational policies. Therefore,
constrained clients may wish to delegate certificate path
construction and validation to a trusted server. Additionally, to
ensure that policies are consistently enforced throughout an
ecosystem, centralization of certificate validation may be needed.
Protocols such as Server-based Certificate Validation Protocol (SCVP)
allow simplification of client implementations and consistent
application of validation policies by delegating validation to a
server.
The extension described here allows a TLS client to request that the
TLS server return the certificate path validation corresponding to
its certificate. If the server supports this extension, it performs
the appropriate certificate validation queries and returns it to the
client. The server returns the path validation as an extension to
the Certificate message. Since path building and validation has been
performed, the server can return only the end-entity certificate to
be used for authentication, and does not need to return any
supporting certificates in the chain. This further reduces the
bandwidth consumption. The server can use a previously cached
validation response, but it will need to retrieve it periodically as
described in Section 4.2. The client then examines the returned
validation response and the response signature using a local trust
anchor.
TLS clients and servers MAY use the extension described in this
document. The extension is designed to be backwards compatible,
meaning that TLS clients that support the extension can talk to TLS
servers that do not support the extension, and vice versa.
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 RFC 2119 [RFC2119].
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2. Validation Request Extension
A new extension type (validation_request (TBD)) is added to the
extensions used in the client hello and certificate handshake
messages. The extension type is specified as follows.
enum {
validation_request(TBD), (65535)
} ExtensionType;
To indicate their desire to receive certificate path validation
information, TLS clients MAY include an extension of type
validation_request in the (extended) client hello. The
extension_data field of the validation_request extension MUST contain
a PathValidationRequest. The PathValidationRequest MUST consist of a
PathValidationType and Request as defined in Section 3.
Servers that receive a client hello containing the validation_request
extension MAY return a suitable certificate path validation response
to the client along with their certificate.
Servers return a certificate path validation response along with
their certificate by adding the validation_request extension to the
extension block of the TLS server certificate. The Certificate
message containing the TLS server end-entity certificate SHOULD
contain the extension with extension_type validation_request and
extension_data of PathValidation. Severs that send the
PathValidation extension data MUST have received a validation_request
in the extended client hello.
Servers that send the PathValidation extension SHOULD only include
the end-entity authentication certificate in the Certificate message.
The server MAY include supporting certificates. The client MAY
ignore supporting certificates if the PathValidation is found to be
satisfactory.
Clients that receive a PathValidation extension without sending a
validation_request extension MUST abort the connection.
Servers that do not support (or are not configured to enable the use
of) this extension SHOULD NOT include the validation_request
extension in the Certificate message.
A server MAY also choose not to send a PathValidation extension, even
if has received a validation_request extension in the client hello
message.
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If the client sent a validation_request in the client hello extension
but did not receive a validation_request extension in the server
Certificate message MAY choose to use alternative means to validate
the server certificate or MAY choose to abort the connection.
Clients requesting a certificate path validation and receiving
validation_request Certificate extension MUST check the
PathValidation message and abort the handshake if the response is not
satisfactory with bad_certificate_validation_response (TBD) alert.
This alert is always fatal.
The PathValidationRequest and PathValidation types are further
defined in Section 3 and Section 4 of this document.
3. Path Validation Request
Deployment of Public Key Infrastructure (PKI) enabled applications
can be simplified by delegating path validation processing to a
server. Additionally, for organizations wishing to centralize
administration of validation policies, delegation to a server ensures
consistent policy validation across clients in an ecosystem.
Constrained clients wishing to delegate path validation also face
challenges such as bandwidth and latency limitations. This extension
allows for such information to be sent in the TLS handshake, saving
roundtrips and resources.
To indicate their desire to receive certificate validation
information, TLS clients MAY include an extension of type
validation_request in the extended client hello. ClientHello
handshake messages containing the validation_request extension SHALL
contain a PathValidationRequest in the extension_data field of this
extension. The PathValidationRequest is defined as follows:
struct {
PathValidationType path_validation_type;
select (path_validation_type) {
case scvp: SCVPValidationRequest;
} request;
} PathValidationRequest;
enum { scvp(1), (255) } PathValidationType;
The PathValidationRequest is defined for type Server-based
Certificate Validation Protocol (SCVP) however is designed to be
extendable to other protocols.
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3.1. SCVP Validation Request
SCVP provides a standards-based client-server protocol for delegated
path validation defined in RFC 5055 [RFC5055]. Clients wishing to
request the use of SCVP path validation MAY include the
path_validation extension in the client hello handshake with the
PathValidationType of scvp (1) in the PathValidationRequest. If the
PathValidationType is set to scvp, the Request SHALL be an
SCVPValidationRequest.
The SCVPValidationRequest consists of three optional lists: a list of
SCVP responder URIs, a list of trust anchors, and a list of
validation extensions.
struct {
ResponderURIs responder_uri_list<0..2^16-1>;
TrustAnchors trust_anchor_list<0..2^16-1>;
ValidationExtensions validation_extensions_list<0..2^16-1>;
} SCVPValidationRequest;
3.1.1. Responder URIs
The ResponderURIs provides a list of SCVP responders that the client
trusts. A zero-length responder_uri_list sequence has special
meaning that the responders are implicitly known to the server, e.g.
by prior arrangement. The ResponderURIs list is in the client's
preferred order. The TLS server SHOULD process the responder URI
list in order and return a response from the first reachable URI with
an acceptable response. What constitutes an acceptable response is
discussed in Section 4.1.1. The ResponderURIs list is represented as
a DER encoded SEQUENCE OF ASN.1 IA5String objects.
3.1.2. Trust Anchors
Zero or more trust anchors MAY be provided in the
SCVPValidationRequest to specify the trust anchors at which the
certification path must terminate if the path is to be considered
valid. The TrustAnchors type is an ASN.1 SEQUENCE OF PKCReference.
The SCVP Server usage is defined in Section 3.2.4.7 of RFC 5055
[RFC5055]. If a TLS server receives a SCVPValidationRequest which
contains TrustAnchors it SHOULD include the TrustAnchors in the SCVP
Request Validation Policy. A non-zero length TrustAnchors sequence
combined with a zero length Responder URI sequence indicates that the
TLS client wishes to use the TLS server's default SCVP Responder to
construct a certification path which terminates at a specified
certificate. If a non-zero length TrustAnchors is provided and the
TLS Client includes the validation_policy validation extension as
defined in Section 3.1.3, the ValidationPolicy TrustAnchors MUST be
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equivalent to the SCVPValidationRequest TrustAnchors.
As defined in RFC 5055, the trust anchor PKCReference MAY be either
an SCVPCertID or a Certificate. To minimize the size of the
SCVPValidationRequest, TrustAnchors SHOULD be included by SCVPCertID.
3.1.3. Validation Extensions
This document defines nine optional ValidationExtensionTypes. These
validation extensions allow the client to specify values in the CV
Request as described in Section 3.1.4. Inclusion of the validation
extensions will increase the size of the request and response.
Therefore, the extensions should be included only when necessary.
The validation_extension_data values are DER-encoded ASN.1 types that
can be directly mapped to the SCVP CVRequest as defined by RFC 5055
[RFC5055].
struct {
ValidationExtensionType validation_extension_type;
select (validation_extension_type {
case want_back: WantBack;
case validation_policy: ValidationPolicy;
case cached_response: BOOLEAN;
case query_extensions: Extensions;
case request_nonce: OCTET STRING;
case request_extensions: Extensions;
case signature_algorithm: AlgorithmIdentifier;
case hash_algorithm: OBJECT IDENTIFIER;
case requestor_text: UTF8String (SIZE (1..256));
} validation_extension_data;
} ValidationExtension
3.1.4. TLS Server CVRequest
Servers that receive a client hello containing the validation_request
extension MAY return a suitable path validation response to the
client along with their certificate. If SCVP is requested, the TLS
Server SHOULD use the information contained in the extension when
selecting an SCVP responder and Trust Anchor. The TLS server SHOULD
map the validation extension types to the SCVP request as follows.
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want_back : CVRequest.query.wantBack
validation_policy : CVRequest.query.validationPolicy
cached_response : CVRequest.query.responseFlags.cachedResponse
query_extensions : CVRequest.query.queryExtensions
request_nonce : CVRequest.requestNonce
request_extensions : CVRequest.requestExtensions
signature_algorithm : CVRequest.signatureAlgorithm
hash_algorithm : CVRequest.hashAlgorithm
requestor_text : CVRequest.requestorText
Conforming TLS Servers MUST construct a CVRequest with a
cvRequestVersion and query.
3.1.4.1. cvRequestVersion
TLS Servers conforming to RFC 5055 [RFC5055] MUST set the value of
the cvRequestVersion item to one (1).
3.1.4.2. Query
The query item is defined in Section 3.2 of RFC 5055 [RFC5055]
TLS Servers conforming to RFC 5055 [RFC5055] SHALL include the query
item. For processing the TLS Validation Request the TLS server SHALL
include the queriedCerts, certChecks and validationPolicy fields, MAY
include wantBack, responseFlags, intermediateCerts, revInfos, and
queryExtension fields, SHOULD NOT include producedAt and
serverContextInfo fields, and MUST NOT include the validationTime
field.
The TLS Server MUST populate the CertReferences item. The
CertReferences sequence MUST be of length one and specify the
server's X.509v3 TLS Certificate. This certificate MUST correspond
to the TLS Certificate sent by the TLS server to the TLS client in
the Certificate handshake message.
The TLS server MUST populate the CertChecks item. The item MUST be
set to id-stc-build-status-checked-pkc-path (id-stc 3).
The TLS Server SHOULD NOT include want back unless specified by the
TLS Client in the validation extension want_back item. If the TLS
Server receives a SCVPValidationRequest with a want_back validation
extension, the TLS Server MAY set the wantBack item in the CVRequest
to the value of the want_back. Want backs can significantly increase
the size of the CVResponse and should be used only when specifically
required.
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The TLS Server MUST include the ValidationPolicy item in the
CVRequest query item as follows:
If the TLS Client specified a validation_policy in the validation
extensions:
* The TLS Server SHOULD include that ValidationPolicy in the
CVRequest.
If the TLS Client did not specify a validation_policy
* If the ResponderURIs is a zero-length list, indicating that the
TLS server should query a pre-configured SCVP responder, the TLS
Server SHOULD set the ValidationPolicy validationPolRef to either
a pre-configured ValidationPolicy or the default validation policy
OID id-svp-defaultValPolicy (id-svp 1).
* If the ResponderURIs is a non-zero-length list, indicating that
the TLS server should query a client specified SCVP responder, the
TLS Server SHOULD set the ValidationPolicy validationPolRef item
to the default validation policy OID id-svp-defaultValPolicy (id-
svp 1).
* If the TrustAnchors is a non-zero length list the TLS Server MUST
include the TLS Client provided TrustAnchors in the
ValidationPolicy TrustAnchors.
The TLS Server MAY only include the ResponseFlags item in the
CVRequest if requesting non-default values. If default values are
used for all flags, the responseFlags item MUST NOT be included in
the request. To enable to TLS client to trust the CVResponse, the
TLS Server MUST use the default value for the protectResponse flag
(TRUE). To minimize the size of the CVResponse, the TLS Server
SHOULD use the default values for the response flags
fullRequestInResponse (FALSE) and responseValidationPolicyByRef
(TRUE). If the TLS Client included the cached_response validation
extension with a value of FALSE, the TLS Server SHOULD NOT use its
cache as described in Section 4.1.2 and MAY include the
cachedResponse set to FALSE in the CVRequest responseFlags item. If
the TLS Server sets the cachedResponse flag to FALSE the
request_nonce MUST be set. If the TLS Server received the
request_nonce validation extension, the requestNonce in the CVRequest
MAY be set to the value of the request_nonce extension. Otherwise,
the TLS Server MUST generate and set a requestNonce in the CVRequest.
The TLS Server SHOULD NOT include the serverContextInfo item in the
CVRequest.
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The TLS Server MUST NOT include the validationTime item in the
CVRequest.
The TLS Server MAY include the intermediateCerts item in the CVRequst
to help the SCVP server create a valid certification path as defined
in Section 3.2.8 of RFC 5055 [RFC5055].
The TLS Server MAY include the revInfos item in the CV Request which
MAY be used by the SCVP Server when validating certification paths as
defined in Section 3.2.9 of RFC 5055 [RFC5055].
The TLS Server SHOULD NOT include the producedAt item in the
CVRequest.
The TLS Server MAY include the queryExtensions item in the CVRequest
to extend the query. If the TLS Server receives a
SCVPValidationRequest with a query_extensions validation extension,
the TLS Server MAY set the queryExtensions item in the CVRequest to
the value of the query_extension.
3.1.4.3. requestorRef
The TLS Server SHOULD NOT set the requestorRef item in the CVRequest.
3.1.4.4. requestNonce
The TLS Server MAY include the requestNonce item in the CVRequest to
indicate a preference for a non-cached response. If the TLS Server
receives a SCVPValidationRequest with a request_nonce validation
extension, the TLS Server MAY set the requestNonce item in the
CVRequest to the value of the request_nonce validation extension. If
the TLS Server sets the cachedResponse response flag to FALSE but did
not receive the request_nonce validation extension the TLS Server
MUST generate and set a requestNonce in the CVRequest.
3.1.4.5. requestorName
The TLS Server SHOULD NOT include the requestorName item in the
CVRequest.
3.1.4.6. responderName
The TLS Server SHOULD NOT include the responderName item in the
CVRequest.
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3.1.4.7. requestExtensions
The TLS Server MAY include the requestExtensions item in the
CVRequest to extend the request. If the TLS Server receives a
SCVPValidationRequest with a request_extensions validation extension,
the TLS Server MAY set the requestExtensions item in the CVRequest to
the value of the request_extension.
3.1.4.8. signatureAlgorithm
The TLS Server SHOULD NOT include the signatureAlgorithm item in the
CVRequest unless specified by the TLS Client in the
signature_algorithm validation extension. If the TLS Server receives
a SCVPValidationRequest with a signature_algorithm validation
extension, the TLS server MAY set the signatureAlgorithm item in the
CVRequest to the value of the signature_algorithm.
To keep the size of the request and response small, it is recommended
that community define the signature algorithm rather than using the
signature_algorithm extension.
3.1.4.9. hashAlgorithm
The TLS Server SHOULD NOT include the hashAlgorithm item in the
CVRequest unless specified by the TLS Client in the hash_algorithm
validation extension. If the TLS Server receives a
SCVPValidationRequest with a hash_algorithm validation extension, the
TLS Server MAY set the hashAlgorithm item in the CVRequest to the
value of the hash_algorithm.
To keep the size of the request and response small, it is recommended
that community define the hash algorithm rather than using the
hash_algorithm extension.
3.1.4.10. requestorText
The TLS Server SHOULD NOT include the requestorText item in the
CVRequest unless specified by the TLS Client in the requestor_text
validation extension. If the TLS Server receives a
SCVPValidationRequest with a requestor_text validation extension, the
TLS Server MAY set the requestorText item in the CVRequest to the
value of the requestor_text.
To keep the size of the request and response small, it is recommended
that the requestor text be used only for debugging purposes.
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4. Path Validation Response
Servers that receive a client hello containing the validation_request
extension MAY return a suitable path validation response to the
client along with their certificate by sending a PathValidation as an
extension to the Certificate message. Like the
PathValidationRequest, the ValidationResponse is defined for type
Server-Based Certificate Validation Protocol (SCVP) however is
designed to be extendable to other protocols.
The PathValidation response is defined as follows:
struct {
PathValidationType path_validation_type;
select (path_validation_type) {
case scvp: SCVPResponse;
} response;
} PathValidation;
enum { scvp(1), (255) } PathValidationType;
TLS Servers send the PathValidation response to the client in the
extension_data of the validation_request extension to the TLS server
end-entity certificate in the Certificate message. The
PathValidation data conveys whether the certificate path was
successfully built and validated. Therefore, in most cases, it is
unnecessary for the TLS server to include supporting certificates in
the Certificates message.
4.1. SCVP Validation Response
If a TLS server returns a PathValidation message in response to a
PathValidationRequest of type scvp, that PathValidation message MUST
be of type scvp and contain a SCVPResponse as follows.
struct {
opaque signed_cv_response
ValidationExtensions validation_extensions_list<0..2^16-1>;
} SCVPResponse;
A signed_cv_response contains a complete, DER-encoded CMS SignedData
object as defined in RFC 5652 [RFC5652] with an EncapsulatedContent
of type CVResponse as defined in RFC 5055 [RFC5055]. Only one SCVP
response may be sent.
The TLS Server SHOULD include the list of validation extensions from
the SCVPValidationRequest that were used in the CVRequest to indicate
to the TLS Client which validation extensions were honored. If the
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TLS Server did not use a validation extension in the CVRequest, it
MUST NOT be included in the SCVPResponse. The SCVPResponse MUST NOT
include validation extensions that were not present in the
SCVPValidationRequest.
Section 9 of RFC 5055 [RFC5055] asserts that clients MUST verify that
the response matches their original request and outlines the steps
necessary to perform this verification. For this extension, the
client responsibility of divided between the TLS server and the TLS
client. Certain values are only known by the TLS server whereas
other values require verification at the final end-point, the TLS
client. The following two sections specify the verification of the
SCVP response at the TLS server Section 4.1.1 and at the TLS client
Section 4.1.2.
4.1.1. SCVP Response Processing by TLS Server
The TLS Server MUST verify the response from the SCVP server. If the
TLS Server finds the response unacceptable, it MAY query another SCVP
server (from the ResponderURIs or a pre-configured list) or MAY send
a bad_path_validation_response alert notifying to close the
connection.
The TLS server MUST verify that the response is a protected response
consisting of a CVResponse encapsulated in CMS SignedData.
The TLS server SHOULD verify that the SignedData Message Digest is a
hash of the received CVResponse.
The TLS server MAY verify the certificate of the SCVP responder used
for signing the response. In some environments, it may be left to
the TLS client to validate.
As an SCVP client, the TLS server MUST process the CVResponse as
defined by RFC 5055 [RFC5055].
The TLS server SHOULD verify the responseStatus code. If the code
does not indicate okay(0), the TLS server MAY choose to query another
SCVP server from the Responder URIs.
As stated in RFC 5055, the requestRef item allows the SCVP client to
determine that the request was not maliciously altered. The TLS
Server creates the CVRequest and is therefore the only place where
the full CVRequst is known. The TLS Server SHOULD compare the
returned requestRef to the CVRequest.
If the TLS server generated a requestNonce it SHOULD verify that the
requestNonce in the response matches the value in the request.
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4.1.2. SCVP Response Processing by TLS Client
On receipt of a SCVPResponse, the TLS client MUST verify that the
response indicates a successful path validation and can be trusted.
The TLS Client MUST verify that the CVResponse is encapsulated in a
CMS SignedData object and validate the digital signature on the
response to ensure that the expected SCVP server generated the
response. The TLS Client MUST verify that the SignedData Message
Digest matches a hash of the received CVResponse.
The CVResponse CertReply item MUST contain a single certificate
matching the TLS Server certificate sent in the TLS Certificate
Handshake message. If the CertReply does not meet this requirement
the TLS client MUST abort the connection with a
bad_path_validation_response.
The TLS Client MUST verify that the CVResponse indicates success. A
CVResponse is successful if: the responseStatus is CVStatusCode
okay(0) and the CertReply item containing the TLS Server's
certificate has a replyStatus of success (0). If these conditions
are not met the TLS client MUST abort the connection with a
bad_path_validation_response.
The TLS Client should check the SCVPResponse validation_extensions
against the validation_extensions sent in the SCVPValidationRequest.
If the SCVPResponse validation_extensions list does not match the
list of sent validation_extensions, the TLS client MAY abort the
connection.
If the client set validation extensions in the SCVPValidationRequest,
the TLS client SHOULD verify that the CVResponse appropriately
reflects those validation extensions. For example, if the
request_nonce validation extension was set the client SHOULD verify
that the CVResponse respNonce contains the same value.
4.2. Path Validation Cache
To improve performance and survive path validation service outages
the TLS server MAY cache PathValidation responses. On receipt of a
client hello with a validation_request extension the TLS Server MAY
check a local cache for a PathValidation response matching the TLS
client's settings. If a matching response is found the server MAY
use this response rather than generating a fresh response.
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If the validation_request is of type scvp, the server should check if
the client has set validation extensions before returning a cached
response. If the cached_response validation extension is set to
FALSE or the request_nonce validation extension is set. The TLS
server SHOULD NOT return a cached response.
The TLS server SHOULD place a time limit on cached responses and
generate a fresh PathValidation response after that time has elapsed.
To improve performance, the TLS server MAY proactively refresh cached
responses before the cache time limit has been reached. Path
validation servers may also perform caching to optimize response
times. Timestamps may be included in the path validation response
that MAY be used by the TLS server to determine when a fresh response
will be available from the path validation server. In the case of
SCVP, this information can optionally be communicated in the
nextUpdate wantBack value.
5. Error Alerts
On receipt of a PathValidation response the TLS client MUST validate
that the response indicates a successful path validation as described
in Section 4. If the PathValidation response does not indicate that
the server certificate was successfully validated the TLS client MUST
abort the connection with a bad_path_validation_response as follows.
enum {
bad_path_validation_response(TBD),
(255)
} AlertDescription;
6. IANA Considerations
IANA considerations for TLS extensions and the creation of a registry
are covered in Section 11 of RFC 8446 [RFC8446].
6.1. Reference for TLS Alerts and ExtensionTypes
The following values in the TLS Alert Registry have been updated to
reference this document:
TBD bad_path_validation_response
The following ExtensionType values have been updated to reference
this document:
TBD validation_request
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7. Security Considerations
General security considerations for TLS extensions are covered in TLS
1.3 RFC 8446 [RFC8446].
For security considerations specific to the Cryptographic Message
Syntax message formats, see RFC 5652 [RFC5652]. For security
considerations specific to the process of PKI certification path
validation, see RFC 5280 [RFC5280]. For security considerations
specific to SCVP, see RFC 5055 [RFC5055].
This section summarizes some of the more important security aspects
specific to the TLS validation_request extension, though there are
many security-relevant details in the remainder of this document.
7.1. Support for Extension
If a client requests a path validation response, it must consider
that an attacker's server could (and probably would) pretend not to
support the extension. In this case, a client that requires path
validation of certificates SHOULD either contact the validation
server directly or abort the handshake.
7.2. Replay Attacks
Use of the optional SCVP cached response flag and request nonce items
by either the TLS client in the validation extensions or by the TLS
server may improve security against attacks that attempt to replay
SCVP responses. However, use of these properties must be balanced
with the performance impact of requiring generation of a fresh SCVP
response.
7.3. Extension Modifications
Values in the client hello validation_request extension and
PathValidationRequest are passed between the TLS client to the TLS
server unprotected. This makes the values vulnerable to
modification. An attacker might try to influence the handshake
exchange in multiple ways including to increase latency, cause
parties to abort the connection or to create trust in an untrusted
server.
The SCVP type of this extension is made further vulnerable by the
inclusion of validation extensions in the SCVPValidationRequest.
These validation extensions have been included to support
flexibility. However, to mitigate the vulnerability to modification,
domains should consider limiting use of validation extensions and
instead use preconfigured domain specific values. TLS client and
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server verification of values returned in the signed CVResponse as
described in Section 4.1 should also be used to protect against these
attacks and detect attempts to modify these values
7.4. Unrelated Path Validation Response
The received PathValidation response could contain information
unrelated to the request. A path for an end-entity certificate other
than the TLS server certificate could be returned. The first
certificate in the certificate path could not match any of the client
provided trust anchors. Or the SCVP responder signing the response
could be unknown to the client. If any such unrelated PathValidation
response is received, it MUST be discarded and the TLS client MAY
choose to use alternative means to validate the server certificate or
MAY choose to abort the connection.
7.5. Trust Anchor Maintenance
The TLS client relies on a locally known trust anchor to verify the
signed PathValidation response. The trust anchor may change or
expire periodically. TLS clients using this specification MUST
implement a secure mechanism to keep their trust anchors up to date.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, DOI 10.17487/RFC5055, December 2007,
<https://www.rfc-editor.org/info/rfc5055>.
[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, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
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[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
Authors' Addresses
Robert Segers
Federal Aviation Administration
800 Independence Ave. SW
Washington, DC 20591
United States of America
Email: Robert.Segers@faa.gov
Ashley Kopman
Concepts Beyond
1155 F St NW
Washington, DC 20004
United States of America
Email: akopman@conceptsbeyond.com
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