ANIMA Working Group | K. Watsen |
Internet-Draft | Juniper Networks |
Intended status: Standards Track | M. Richardson |
Expires: January 4, 2018 | Sandelman Software |
M. Pritikin | |
Cisco Systems | |
T. Eckert | |
Huawei | |
July 3, 2017 |
Voucher Profile for Bootstrapping Protocols
draft-ietf-anima-voucher-04
This document defines a strategy to securely assign a pledge to an owner, using an artifact signed, directly or indirectly, by the pledge's manufacturer. This artifact is known as a "voucher".
The voucher artifact is a YANG-defined JSON document that has (by default) been signed using a PKCS#7 structure. The voucher artifact is normally generated by the pledge's manufacturer or delegate (i.e. the Manufacturer Authorized Signing Authority).
This document only defines the voucher artifact, leaving it to other documents to describe specialized protocols for accessing it.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This document defines a strategy to securely assign a pledge to an owner, using an artifact signed, directly or indirectly, by the pledge's manufacturer or delegate, i.e. the Manufacturer Authorized Signing Authority (MASA). This artifact is known as the voucher.
The voucher artifact is a JSON document, conforming to a data model described by YANG [RFC7950], that has (by default) been signed using a PKCS#7 structure.
A voucher may be useful in several contexts, but the driving motivation herein is to support secure bootstrapping mechanisms. Assigning ownership is important to bootstrapping mechanisms so that the pledge can authenticate the network that's trying to take control of it.
The lifetimes of vouchers may vary. In some bootstrapping protocols the vouchers may be ephemeral, whereas in others the vouchers may be potentially long-lived. In order to support the second category of vouchers, this document recommends using short-life vouchers with programatic renewal, enabling the MASA to communicate the ongoing validity of vouchers.
This document only defines the voucher artifact, leaving it to other documents to describe specialized protocols for accessing it. Some bootstrapping protocols using the voucher artifact defined in this draft include: [I-D.ietf-netconf-zerotouch], [I-D.ietf-6tisch-dtsecurity-secure-join], and [I-D.ietf-anima-bootstrapping-keyinfra]).
This document uses the following terms (sorted by name):
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
A simplified graphical representation of the data models is used in this document. The meaning of the symbols in these diagrams is as follows:
A voucher is a cryptographically protected statement to the Pledge device authorizing a zero-touch "imprint" on the Join Registrar of the domain. The specific information a voucher provides is influenced by the bootstrapping use case.
The voucher can impart the following information to the Join Registrar and Pledge:
A number of bootstrapping scenarios can be met using differing combinations of this information. All scenarios address the primary threat of a Man-in-The-Middle Registrar gaining control over the Pledge device. The following combinations are "types" of vouchers:
|Assertion |Registrar ID | Validity | Voucher |Log-|Veri- |Trust |CN-ID or| RTC | Nonce | Name | ged| fied |Anchor |DNS-ID | | | ---------------------------------------------------------| Audit | X | | X | | | X | -------------|----|-------|-------|--------|-----|-------| Nonceless | X | | X | | X | | Audit | | | | | | | -------------|----|-------|-------|--------|-----|-------| Owner Audit | X | X | X | | X | X | -------------|----|-------|-------|--------|-----|-------| Owner ID | | X | X | X | X | | -------------|----|-------|----------------|-----|-------| Bearer | X | | wildcard | optional | out-of-scope | | | | | -------------|----|-------|----------------|-------------| NOTE: All voucher types include a 'Pledge ID serial number' (Not shown for space reasons)
The voucher's primary purpose is to securely assign a pledge to an owner. The voucher informs the pledge which entity it should consider to be its owner.
The voucher is signing structure that MUST contain JSON-encoded content conforming to the voucher-artifact YANG data schema of the YANG module specified in Section 6.3.
Unless otherwise signaled (outside of the voucher artifact), the signing structure is by default a PKCS#7 SignedData structure, as specified by Section 9.1 of [RFC2315], encoded using ASN.1 distinguished encoding rules (DER), as specified in ITU-T X.690.
The PKCS#7 structure MUST also contain a 'signerInfo' structure, as described in Section 9.1 of [RFC2315], containing the signature generated over the content using a private key trusted by the recipient. Normally the recipient is the pledge and the signer is the MASA. A possible other use use could be as a "signed voucher request" format originating from pledge or registrar toward the MASA. Within this document the signer is assumed to be the MASA.
The PKCS#7 structure SHOULD also contain all of the certificates leading up to and including the signer's trust anchor certificate known to the recipient.
The PKCS#7 structure MAY also contain revocation objects for any intermediate CAs between the voucher-issuer and the trust anchor known to the recipient.
Methods of signaling alternative signature methods are out-of-scope of this document, but documents that leverage vouchers can provide this signaling. For example they might instruct that JWS signing is the signature method in their work. Documents describing the use of alternative signature methods for voucher artifacts need to encode the same information as described above for PKCS#7 or else describe why the encoded information may differ.
The following tree diagram (Section 4) illustrates a high-level view of a voucher document. Each field in the voucher is fully described by the YANG module provided in Section 6.3. Please review this YANG module for a detailed description of the voucher format.
module: ietf-voucher yang-data: voucher-artifact +---- voucher +---- created-on yang:date-and-time +---- expires-on? yang:date-and-time +---- assertion enumeration +---- serial-number string +---- idevid-issuer? binary +---- pinned-domain-cert binary +---- domain-cert-revocation-checks? boolean +---- nonce? binary +---- last-renewal-date? yang:date-and-time +---- prior-signed-voucher? binary
This section provides a couple voucher examples for illustration purposes.
The following example illustrates an ephemeral voucher (uses a nonce). The MASA generated this voucher using the 'logged' assertion type, knowing that it would be suitable for the pledge making the request.
{ "ietf-voucher:voucher": { "created-on": "2016-10-07T19:31:42Z", "assertion": "logged", "serial-number": "JADA123456789", "idevid-issuer": "base64-encoded Authority Key Identifier", "pinned-domain-cert": "base64-encoded X.509 DER", "nonce": "base64-encoded octet string" } }
The following example illustrates a non-ephemeral voucher (no nonce). While the voucher itself expires after two weeks, it presumably can be renewed for up to a year later. The MASA generated this voucher using the 'verified' assertion type, which should satisfy all pledges.
{ "ietf-voucher:voucher": { "created-on": "2016-10-07T19:31:42Z", "expires-on": "2016-10-21T19:31:42Z", "assertion": "verified", "serial-number": "JADA123456789", "idevid-issuer": "base64-encoded Authority Key Identifier", "pinned-domain-cert": "base64-encoded X.509 DER", "domain-cert-revocation-checks": "true", "last-renewal-date": "2017-10-07T19:31:42Z" } }
<CODE BEGINS> file "ietf-voucher@2017-07-03.yang" module ietf-voucher { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-voucher"; prefix "vch"; import ietf-yang-types { prefix yang; reference "RFC 6991: Common YANG Data Types"; } import ietf-restconf { prefix rc; description "This import statement is only present to access the yang-data extension defined in RFC 8040."; reference "RFC 8040: RESTCONF Protocol"; } organization "IETF ANIMA Working Group"; contact "WG Web: <http://tools.ietf.org/wg/anima/> WG List: <mailto:anima@ietf.org> Author: Kent Watsen <mailto:kwatsen@juniper.net> Author: Max Pritikin <mailto:pritikin@cisco.com> Author: Michael Richardson <mailto:mcr+ietf@sandelman.ca> Author: Toerless Eckert <mailto:tte+ietf@cs.fau.de>"; description "This module defines the format for a voucher, which is produced by a pledge's manufacturer or delegate (MASA) to securely assign one or more pledges to an 'owner', so that the pledges may establish a secure connection to the owner's network infrastructure. The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in the module text are to be interpreted as described in RFC 2119."; revision "2017-07-03" { description "Initial version"; reference "RFC XXXX: Voucher Profile for Bootstrapping Protocols"; } rc:yang-data voucher-artifact { // YANG data template for a voucher. uses voucher-artifact-grouping; } grouping voucher-artifact-grouping { description "Grouping for the voucher-artifact to allow reuse/extensions in future work."; container voucher { description "A voucher that can be used to assign one or more pledges to an owner."; leaf created-on { type yang:date-and-time; mandatory true; description "A value indicating the date this voucher was created. This node is optional because its primary purpose is for human consumption. However, when present, pledges that have reliable clocks SHOULD ensure that this created-on value is not greater than the current time."; } leaf expires-on { type yang:date-and-time; must "not(../nonce)"; description "A value indicating when this voucher expires. The node is optional as not all pledges support expirations, such as pledges lacking a reliable clock. If this field exists, then the the pledges MUST ensure that the expires-on time has not yet passed. A pledge without an accurate clock cannot meet this requirement. The expires-on value MUST NOT exceed the expiration date of any of the listed 'pinned-domain-cert' certificates."; } leaf assertion { type enumeration { enum verified { description "Indicates that the ownership has been positively verified by the MASA (e.g., through sales channel integration)."; } enum logged { description "Indicates that this ownership assignment has been logged into a database maintained by the MASA, after first verifying that there has not been a previous claim in the database for the same pledge (voucher transparency)."; } enum proximity { description "Indicates that this assertion is made based on the proximity of the signer as determined by local network information. This is useful for a pledge device to indicate it 'sees' a specific registrar on a TLS connection, or for a registrar to indicate it 'sees' a pledge."; } } mandatory true; description "The assertion is a statement from the MASA regarding how the owner was verified. This statement enables pledges to support more detailed policy checks. Pledges MUST ensure that the assertion provided is acceptable before processing the voucher."; } leaf serial-number { type string; mandatory true; description "The serial number of the hardware. When processing a voucher, a pledge MUST ensure that its serial number matches this value. If no match occurs, then the pledge MUST NOT process this voucher."; } leaf idevid-issuer { type binary; description "The RFC5280 4.2.1.1 Authority Key Identifier OCTET STRING from the pledge's IDevID certificate. Optional since some serial-numbers are already unique within the scope of a MASA. Inclusion of the statistically unique key identifier ensures statistically unique identification of the hardware. When processing a voucher, a pledge MUST ensure that its IDevID Authority Key Identifier matches this value. If no match occurs, then the pledge MUST NOT process this voucher. When issuing a voucher, the MASA MUST ensure that this field is populated for serial numbers that are not otherwise unique within the scope of the MASA."; } leaf pinned-domain-cert { type binary; mandatory true; description "An X.509 v3 certificate structure as specified by RFC 5280, Section 4 encoded using the ASN.1 distinguished encoding rules (DER), as specified in ITU-T X.690. This certificate is used by a pledge to trust a public key infrastructure, in order to verify a domain certificate supplied to the pledge separately by the bootstrapping protocol. The domain certificate MUST have this certificate somewhere in its chain of certificates. This certificate MAY be an end-entity certificate, including a self-signed entity."; reference "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile. ITU-T X.690: Information technology – ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)."; } leaf domain-cert-revocation-checks { type boolean; must "../expires-on"; description "A processing instruction to the pledge that it MUST verify the revocation status for the domain certificate. This instruction is only available for vouchers that expire. If this field is not set, then normal PKIX behaviour applies to validation of the domain certificate."; } leaf nonce { type binary { length "8..32"; } must "not(../expires-on)"; description "A value that can be used by a pledge in some bootstrapping protocols to enable anti-replay protection. This node is optional because it is not used by all bootstrapping protocols. When present, the pledge MUST compare the provided nonce value with another value that the pledge randomly generated and sent to a bootstrap server in an earlier bootstrapping message. If the values do not match, then the pledge MUST NOT process this voucher."; } leaf last-renewal-date { type yang:date-and-time; must "../expires-on"; description "The date that the MASA projects to be the last date it will renew a voucher on. This field is merely informative, it is not processed by pledges. Circumstances may occur after a voucher is generated that may alter a voucher's validity period. For instance, a vendor may associate validity periods with support contracts, which may be terminated or extended over time."; } leaf prior-signed-voucher { type binary; description "If it is necessary to change a voucher, or re-sign and forward a voucher that was previously provided along a protocol path, then the previously signed voucher SHOULD be included in this field. For example, a pledge might sign a proximity voucher, which an intermediate registrar then re-signs to make its own 'proximity' assertion. This is a simple mechanism for a chain of trusted parties to change a voucher, while maintaining the prior signature information. The pledge MUST ignore all prior voucher information when accepting a voucher for imprinting. Other parties MAY examine the prior signed voucher information for the purposes of policy decisions. For example this information could be useful to a MASA to determine that both pledge and registrar agree on proximity assertions. The MASA SHOULD remove all 'prior-signed-voucher' information when signing a voucher for imprinting so as to minimize the final voucher size."; } } // end voucher } // end voucher-grouping } <CODE ENDS>
The lifetimes of vouchers may vary. In some bootstrapping protocols, the vouchers may be created and consumed immediately whereas, in other bootstrapping solutions, there may be a significant delay between when a voucher is created and when it is consumed. In cases when there is a delay, there is a need for the pledge to ensure that the assertions made when the voucher was created are still valid when it is consumed.
A revocation artifact is generally used to verify the continued validity of an assertion such as a PKIX certificate, web token, or a "voucher". With this approach, a potentially long-lived assertion is paired with a reasonably fresh revocation status check to ensure that the assertion is still valid. However, this approach increases solution complexity, as it introduces the need for additional protocols and code paths to distribute and process the revocations.
Addressing the short-comings of revocations, this document recommends instead the use of lightweight renewals of short-lived non-revocable vouchers. That is, rather than issue a long-lived voucher, the expectation is for the MASA to instead issue a short-lived voucher along with a promise (reflected in the 'last-renewal-date' field) to re-issue the voucher again when needed. Importantly, while issuing the initial voucher may incur heavyweight verification checks (are you who you say you are? does the pledge actually belong to you?), re-issuing the voucher should be a lightweight process, as it ostensibly only updates the voucher's validatity period. With this approach, there is only the one artifact, and only one code path is needed to process it, without any possibility for a pledge to choose to skip the revocation status check because, for instance, the OCSP Responder is not reachable.
While this document recommends issuing short-lived vouchers, the voucher artifact does not restrict the ability to create a long-lived vouchers, if required, however no revocation method is described.
Note that a voucher may be signed by a chain of intermediate CAs leading up to the trust anchor certificate known by the pledge. Even though the voucher itself is not revocable, it may still be revoked, per se, if one of the intermediate CA certificates is revoked.
The solution described herein originally enabled a single voucher to apply to many pledges, using lists of regular expressions to represent ranges of serial numbers. However, it was determined that blocking the renewal of a voucher that applied to many devices would be excessive when only the ownership for a single pledge needed to be blocked. Thus, the voucher format now only supports a single serial-number to be listed.
An attacker could use an expired voucher to gain control over a device that has no understand of time.
To defend against this there are three things: devices are required to verify that the expires-on field has not yet passed. Devices without access to time can use nonces to get ephermal vouchers. Thirdly, vouchers without expiration times may be used, which will appear in the audit log, informing the security decision.
This document defines a voucher format that contains time values for expirations, which require an accurate clock in order to be processed correctly. Vendors planning on issuing vouchers with expiration values must ensure devices have an accurate clock when shipped from manufacturing facilities, and take steps to prevent clock tampering. If it is not possible to ensure clock accuracy then vouchers with expirations should not be issued.
A voucher is signed by a CA, that may itself be signed by a chain of CAs leading to a trust anchor known to a pledge. Revocation checking of the intermediate certificates may be difficult in some scenarios. The voucher format supports the existing PKIX revocation information distribution within the limits of the current PKI technology (a PKCS7 structure can contain revocation objects as well), but pledges MAY accept vouchers without checking X.509 certificate revocation (when 'domain-cert-revocation-checks' is false). Without revocation checking, a compromized MASA keychain could be used to issue vouchers ad infinitum without recourse. For this reason, MASA implementations wanting to support such deployments SHOULD ensure that all the CA private keys used for signing the vouchers are protected by hardware security modules (HSMs).
If a domain certificate is compromised, then any outstanding vouchers for that domain could be used by the attacker. The domain administrator is clearly expected to initiate revocation of any domain identity certificates (as is normal in PKI solutions).
Similarly they are expected to contact the MASA to indicate that an outstanding (presumably short lifetime) voucher should be blocked from automated renewal. Protocols for voucher distribution are RECOMMENDED to check for revocation of any domain identity certificates before automated renewal of vouchers.
This document registers a URIs in the IETF XML registry [RFC3688]. Following the format in [RFC3688], the following registration is requested:
URI: urn:ietf:params:xml:ns:yang:ietf-voucher Registrant Contact: The ANIMA WG of the IETF. XML: N/A, the requested URI is an XML namespace.
This document registers a YANG module in the YANG Module Names registry [RFC6020]. Following the format defined in [RFC6020], the the following registration is requested:
name: ietf-voucher namespace: urn:ietf:params:xml:ns:yang:ietf-voucher prefix: vch reference: RFC XXXX
[RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
[RFC2315] | Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998. |
[RFC6020] | Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010. |
[RFC7950] | Bjorklund, M., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016. |
[RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
[I-D.ietf-6tisch-dtsecurity-secure-join] | Richardson, M., "6tisch Secure Join protocol", Internet-Draft draft-ietf-6tisch-dtsecurity-secure-join-01, February 2017. |
[I-D.ietf-anima-bootstrapping-keyinfra] | Pritikin, M., Richardson, M., Behringer, M., Bjarnason, S. and K. Watsen, "Bootstrapping Remote Secure Key Infrastructures (BRSKI)", Internet-Draft draft-ietf-anima-bootstrapping-keyinfra-06, May 2017. |
[I-D.ietf-netconf-zerotouch] | Watsen, K., Abrahamsson, M. and I. Farrer, "Zero Touch Provisioning for NETCONF or RESTCONF based Management", Internet-Draft draft-ietf-netconf-zerotouch-14, June 2017. |
[imprinting] | Wikipedia, "Wikipedia article: Imprinting", July 2015. |
[RFC3688] | Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004. |
[RFC7435] | Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014. |
[Stajano99theresurrecting] | Stajano, F. and R. Anderson, "The resurrecting duckling: security issues for ad-hoc wireless networks", 1999. |
The authors would like to thank for following for lively discussions on list and in the halls (ordered by last name): William Atwood, Toerless Eckert, Sheng Jiang.