Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                           July 13, 2020
Expires: January 14, 2021

                      STIR Certificate Delegation


   The Secure Telephone Identity Revisited (STIR) certificate profile
   provides a way to attest authority over telephone numbers and related
   identifiers for the purpose of preventing telephone number spoofing.
   This specification details how that authority can be delegated from a
   parent certificate to a subordinate certificate.  This supports a
   number of use cases, including those where service providers grant
   credentials to enterprises or other customers capable of signing
   calls with STIR.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 14, 2021.

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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Delegation of STIR Certificates . . . . . . . . . . . . . . .   4
     4.1.  Scope of Delegation . . . . . . . . . . . . . . . . . . .   5
   5.  Authentication Services Signing with Delegate Certificates  .   6
   6.  Verification Service Behavior for Delegate Certificate
       Signatures  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acquiring Multiple Certificates in STIR . . . . . . . . . . .   7
   8.  Certification Authorities and Service Providers . . . . . . .   8
     8.1.  ACME and Delegation . . . . . . . . . . . . . . . . . . .   8
     8.2.  Handling Multiple Certificates  . . . . . . . . . . . . .   9
   9.  Alternative Solutions . . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     14.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The STIR problem statement [RFC7340] reviews the difficulties facing
   the telephone network that are enabled by impersonation, including
   various forms of robocalling, voicemail hacking, and swatting.  One
   of the most important components of a system to prevent impersonation
   is the implementation of credentials which identify the parties who
   control telephone numbers.  The STIR certificates [RFC8226]
   specification describes a credential system based on [X.509] version
   3 certificates in accordance with [RFC5280] for that purpose.  Those
   credentials can then be used by STIR authentication services
   [RFC8224] to sign PASSporT objects [RFC8225] carried in SIP [RFC3261]

   [RFC8226] specifies an extension to X.509 that defines a Telephony
   Number (TN) Authorization List that may be included by certification
   authorities (CAs) in certificates.  This extension provides
   additional information that relying parties can use when validating
   transactions with the certificate.  When a SIP request, for example,
   arrives at a terminating administrative domain, the calling number

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   attested by the SIP request can be compared to the TN Authorization
   List of the certificate that signed the PASSporT to determine if the
   caller is authorized to use that calling number.

   Initial deployment of [RFC8226] has focused on the use of Service
   Provider Codes (SPCs) to attest the scope of authority of a
   certificate.  Typically, these codes are internal telephone network
   identifiers such as the Operating Company Numbers (OCNs) assigned to
   carriers in the United States.  However, these network identifiers
   are effectively unavailable to non-carrier entities, and this has
   raised questions about how such entities might best participate in
   STIR, when needed.  Additionally, a carrier may sometimes operate
   numbers that are formally assigned to another carrier.  [RFC8226]
   gave an overview of a certificate enrollment model based on
   "delegation," whereby the holder of certificate might allocate a
   subset of that certificate's authority to another party.  This
   specification details how delegation of authority works for STIR

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

3.  Motivation

   The most pressing need for delegation in STIR arises in a set of use
   cases where callers want to use a particular calling number, but for
   whatever reason, their outbound calls will not pass through the
   authentication service of the service provider that controls that
   numbering resource.

   One example would be an enterprise that places outbound calls through
   a set of service providers, for each call choosing a provider based
   on a least-cost routing algorithm or similar local policy.  The
   enterprise was assigned a calling number by a particular service
   provider, but some calls originating from that number will go out
   through other service providers.

   A user might also roam from their usual service provider to a
   different network or administrative domain, for various reasons.
   Most "legitimate spoofing" examples are of this form: where a user
   wants to be able to use the main call-back number for their business
   as a calling party number, even when the user is away from the

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   These sorts of use cases could be addressed if the carrier who
   controls the numbering resource were able to delegate a credential
   that could be used to sign calls regardless of which network or
   administrative domain handles the outbound routing for the call.  In
   the absence of something like a delegation mechanism, outbound
   carriers may be forced to sign calls with credentials that do not
   cover the originating number in question.  Unfortunately, that
   practice would be difficult to distinguish from malicious spoofing,
   and if it becomes widespread, it could erode trust in STIR overall.

4.  Delegation of STIR Certificates

   STIR delegate certificates are certificates containing a TNAuthList
   object that have been signed with the private key of a parent
   certificate that itself contains a TNAuthList object.  The parent
   certificate needs contain a basic constraints extension with the cA
   boolean set to "true", indicating that the subject can sign
   certificates.  Every STIR delegate certificate identifies its parent
   certificate with a standard [RFC5280] Authority Key Identifier

   The authority bestowed on the holder of the delegate certificate by
   the parent certificate is recorded in the delegate certificate's
   TNAuthList.  Because STIR certificates use the TNAuthList object
   rather than the Subject Name for indicating the scope of their
   authority, traditional [RFC5280] name constraints are not directly
   applicable to STIR.  In a manner similar to the RPKI [RFC6480]
   "encompassing" semantics, each delegate certificate must have a
   TNAuthList scope that is equal to or a subset of its parent
   certificate's scope: it must be "encompassed."  For example, a parent
   certificate with a TNAuthList that attested authority for the
   numbering range +1-212-555-1000 through 1999 could issue a
   certificate to one delegate attesting authority for the range
   +1-212-555-1500 through 1599, and to another delegate a certificate
   for the individual number +1-212-555-1824.

   Delegate certificates may also contain a basic constraints extension
   with the cA boolean set to "true", indicating that they can sign
   subordinate certificates for further delegates.  In the STIR
   ecosystem, CA certificates may be used to sign PASSporTs; this
   removes the need for creating a redundant end-entity certificate with
   an identical TNAuthList to its parent, though if for operational or
   security reasons certificate holders wish to do so, they may.

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4.1.  Scope of Delegation

   STIR certificates may have a TNAuthList containing one or more SPCs,
   one or more telephone number ranges, or both.  When delegating from a
   STIR certificate, a child certificate may inherit from its parent
   either of the above.  Depending on the sort of numbering resources
   that a delegate has been assigned, various syntaxes can be used to
   capture the delegated resource.

   Some non-carrier entities may be assigned large and complex
   allocations of telephone numbers, which may be only partially
   contiguous or entirely disparate.  Allocations may also change
   frequently, in minor or significant ways.  These resources may be so
   complex, dynamic, or extensive that listing them in a certificate is
   prohibitively difficult.  Section 10.1 of [RFC8226] describes one
   potential way to address this, including the TNAuthList (specified in
   [RFC8226]) in the certificate by-reference rather than by value,
   where a URL in the certificate points to a secure, dynamically-
   updated list of the telephone numbers in the scope of authority of a
   certificate.  For entities that are carriers in all but name, another
   alternative is the allocation of an SPC; this yields much the same
   property, as the SPC is effectively a pointer to an external database
   which dynamically tracks the numbers associated with the SPC.  Either
   of these approaches may make sense for a given deployment.

   Other non-carrier entities may have straightforward telephone number
   assignments, such as enterprises receiving a set of thousand blocks
   from a carrier that may be kept for years or decades.  Particular
   freephone numbers may also have a long-term association with an
   enterprise and its brand.  For these sorts of assignments, assigning
   an SPC may seem like overkill, and using the TN ranges of the
   TNAuthList (by-value) is sufficient.

   Whichever approach is taken to representing the delegated resource,
   there are fundamental trade-offs regarding when and where in the
   architecture a delegation is validated: that is, when the delegated
   TNAuthList is checked to be "encompassed" by the TNAuthList of its
   parent.  This might be performed at the time the delegate certificate
   is issued, or at the time that a verification service receives an
   inbound call, or potentially both.  It is generally desirable to
   offload as much of this as possible to the certification process, as
   verification occurs during call setup and thus additional network
   dips could lead to perceptible delay, whereas certification happens
   outside of call processing as a largely administrative function.
   Ideally, if a delegate certificate can supply a by-value TN range,
   then a verification service could ascertain that an attested calling
   party number is within the scope of the provided certificate without
   requiring any additional network dip.  In practice, verification

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   services may already incorporate network queries into their
   processing (for example, to deference the "x5u" field of a PASSporT)
   that could piggyback any additional information needed by the
   verification service.

   Note that the permission semantics of the [RFC8226] TNAuthList are
   additive: that is, the scope of a certificate is the superset of all
   of the SPCs and telephone number ranges enumerated in the TNAuthList.
   As SPCs themselves are effectively pointers to a set of telephone
   number ranges, and a telephone number may belong to more than one
   SPC, this may introduce some redundancy to the set of telephone
   numbers specified as the scope of a certificate.  The presence of one
   or more SPCs and one or more sets of telephone number ranges should
   similarly be treated additively, even if the telephone number ranges
   turn out to be redundant to the scope of an SPC.

5.  Authentication Services Signing with Delegate Certificates

   Authentication service behavior for delegate certificates is little
   changed from [RFC8224] STIR behavior.  The same checks are performed
   by the authentication service, comparing the calling party number
   attested in call signaling with the scope of the authority of the
   signing certificate.  Authentication services SHOULD NOT use a
   delegate certificate without validating that its scope of authority
   is encompassed by that of its parent certificate, and if that
   certificate has a own parent, the entire certification path SHOULD be

   This delegation architecture does not require that a non-carrier
   entity act as its own authentication service.  That function may be
   performed by any authentication service that holds the private key
   corresponding to the delegate certificate, including one run by an
   outbound service provider, a third party in an enterprise's outbound
   call path, or in the SIP User Agent itself.

   Note that authentication services creating a PASSporT for a call
   signed with a delegate certificate MUST provide an "x5u" link
   corresponding to the entire certification path, rather than just the
   delegate certificate used to sign the call, as described in
   Section 7.

6.  Verification Service Behavior for Delegate Certificate Signatures

   The responsibility of a verification service validating PASSporTs
   signed with delegate certificates, while largely following baseline
   [RFC8224] and [RFC8225], requires some additional procedures.  When
   the verification service dereferences the "x5u" parameter, it will
   acquire a certificate list rather than a single certificate.  It MUST

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   then validate all of the credentials in the list, identifying the
   parent certificate for each delegate through its Authority Key
   Identifier extension.

   While ordinarily, relying parties have significant latitude in
   certification path construction when validating a certification path,
   STIR assumes a more rigid hierarchical subordination model, rather
   than one where relying parties may want to derive their own
   certification path to particular trust anchors.  If the certificates
   acquired from the "x5u" element of a PASSporT do not lead to an
   anchor that the verification service trusts, it treats the validation
   no differently than it would when a non-delegated certificate was
   issued by an untrusted root; in SIP, it MAY return a 437 "Unsupported
   Credential" response if the call should be failed for lack of a valid
   Identity header.

7.  Acquiring Multiple Certificates in STIR

   PASSporT [RFC8225] uses the "x5u" element to convey the URL where
   verification services can acquire the certificate used to sign a
   PASSporT.  This value is mirrored by the "info" parameter of the
   Identity header when a PASSporT is conveyed via SIP.  Commonly, this
   is an HTTPS URI.

   When a STIR delegate certificate is used to sign a PASSporT, the
   "x5u" element in the PASSporT will contain a URI indicating where a
   certificate list is available.  While baseline JWS also supports an
   "x5c" element specifically for certificate chains, in operational
   practice, certification path are already being delivered in the STIR
   environment via the "x5u" element, so this specification recommends
   continuing to use "x5u".  That list will be a concatenation of PEM-
   encoded certificates of the type "application/pem-certificate-chain"
   defined in [RFC8555].  The certificate path [RFC5280] ordering MUST
   be organized from the trust anchor towards the signer.  The list
   begins with the certificate used to sign the PASSporT, followed by
   its parent, and then any subsequent grandparents, great-grandparents,
   and so on.  The key identifier in the Authority Key Identifier
   extension in the first certificate MUST appear in the Subject Key
   Identifier extension in the second certificate.  The key identifier
   pairing MUST match in this way throughout the entire chain of
   certificates.  Note that ACME [RFC8555] requires the first element in
   a pem-certificate-chain to be an end-entity certificate; however,
   STIR relaxes this requirement, because CA certificates are permitted
   to sign PASSporTs, so for STIR, the first element in a pem-
   certificate-chain used for STIR MAY be a CA certificate.

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8.  Certification Authorities and Service Providers

   Once a telephone service provider has received a CA certificate
   attesting their numbering resources, they may delegate resources from
   it as they see fit.  Note that the allocation to a service provider
   of a certificate with a basic constraints extension with the cA
   boolean set to "true" does not require that a service provider act as
   a certification authority itself; serving as a certification
   authority is a function requiring specialized expertise and
   infrastructure.  A third-party certification authority, including the
   same one that issued the service provider its parent certificate,
   could act as the CA that issues delegate certificates for the service
   provider, if the necessary business relationships permit it.  A
   service provider might in this case act as a Token Authority (see
   Section 8.1) granting its customers permissions to receive
   certificates from the CA.

   Note that if the same CA that issued the parent certificate is also
   issuing a delegate certificate, it may be possible to shorten the
   certification path, which reduces the work required of verification
   services.  The trade-off here is that if the CA simply issued a non-
   delegate certificate (whose parent is the CA's trust anchor) with the
   proper TNAuthList value, relying parties might not be able to
   ascertain which service provider owned those telephone numbers,
   information which might be used to make an authorization decision on
   the terminating side.  However, some additional object in the
   certificate outside of the TNAuthList could preserve that
   information; this is a potential area for future work.

   All CAs must detail in their practices and policies a requirement to
   validate that the "encompassing" of a delegate certificate by its
   parent.  Note that this requires that CAs have access to the
   necessary industry databases to ascertain whether, for example, a
   particular telephone number is encompassed by an SPC.  Alternatively,
   a CA may acquire an Authority Token that affirms that a delegation is
   in the proper scope.  Exactly what operational practices this entails
   may vary in different national telephone administrations, and are
   thus left to the CP/CPS [RFC3647].

8.1.  ACME and Delegation

   STIR deployments commonly use ACME [RFC8555] for certificate
   acquisition, and it is anticipated that delegate certificates as well
   will be acquired through an ACME interface.  An entity can acquire a
   certificate from a particular CA by requesting an Authority Token
   [I-D.ietf-acme-authority-token] from the parent with the desired
   TNAuthList [I-D.ietf-acme-authority-token-tnauthlist] object.  Note

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   that if the client intends to do further subdelegation of its own, it
   should request a token with the "ca" Authority Token flag set.

   The entity then presents that Authority Token to a CA to acquire a
   STIR delegate certificate.  ACME returns an "application/pem-
   certificate-chain" object with suitable for publishing as an HTTPS
   resource for retrieval with the PASSporT "x5u" mechanism as discussed
   in Section 7.  If the CSR presented to the ACME server is for a
   certificate with the cA boolean set to "true", then the ACME server
   makes a policy decision to determine whether or not it is appropriate
   to issue that certificate to the requesting entity.  That policy
   decision will be reflected by the "ca" flag in the Authority Token.

   Service providers that want the capability to rapidly revoke
   delegated certificates can rely on the ACME STAR [I-D.ietf-acme-star]
   mechanism to automate the process of short-term certificate expiry.

8.2.  Handling Multiple Certificates

   In some deployments, non-carrier entities may receive telephone
   numbers from several different carriers.  This could lead to
   enterprises needing to maintain a sort of STIR keyring, with
   different certificates delegated to them from different providers,
   potentially issued by different CAs, which they choose between when
   signing a call.  This could be the case regardless of which syntax is
   used in the TNAuthList to represent the scope of the delegation (see
   Section 4.1).

   For a small number of certificates, this is probably not a
   significant burden.  For cases where it becomes burdensome, a few
   potential approaches exist.  A delegate certificate could be cross-
   certified with another delegate certificate via an Authority
   Information Access field containing the URL of a Certificate
   Authority Issuer, so that a signer would only need to sign with a
   single certificate to inherit the privileges of the other
   certificate(s) it has cross-certified with.  In very complex
   delegation cases, it might make more sense to establish a bridge CA
   that cross-certifies with all of the certificates held by the
   enterprise, rather than requiring a mesh of cross-certification
   between a large number of certificates.  Again, this bridge CA
   function would likely be performed by some existing CA in the STIR

9.  Alternative Solutions

   At the time this specification was written, STIR was only starting to
   see deployment.  In some future environments, the policies that
   govern CAs may not permit them to issue intermediate certificates

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   with a TNAuthList object and a cA boolean set to "true" in the basic
   constraints certificate extension [RFC5280].  Similar problems in the
   web PKI space motivated the development of TLS subcerts
   [I-D.ietf-tls-subcerts], which substitutes a signed "delegated
   credential" token for a certificate for such environments.  A
   comparable mechanism could be developed for the STIR space, allowing
   STIR certificates to sign a data object which contains effectively
   the same data as the delegate certificate specified here, including a
   public key that could sign PASSporTs.  The TLS subcerts system has
   furthermore developed ways for the issuer of a delegated credential
   to revoke it, as well as exploring the potential interaction with
   ACME to issue short-lived certificates for temporary delegation.
   Specification of a mechanism similar to TLS subcerts for STIR is
   future work, and will be undertaken only if the market require it.

10.  IANA Considerations

   This document contains no actions for the IANA.

11.  Privacy Considerations

   Any STIR certificate that identifies a narrow range of telephone
   numbers potentially exposes information about the entities that are
   placing calls.  As this information is necessarily a superset of the
   calling party number that is openly signaled during call setup, the
   privacy risks associated with this mechanism are not substantially
   greater than baseline STIR.  See [RFC8224] for guidance on the use of
   anonymization mechanisms in STIR.

12.  Security Considerations

   This document is entirely about security.  For further information on
   certificate security and practices, see [RFC5280], in particular its
   Security Considerations.  Also see the Security Considerations of
   [RFC8226] for general guidance on the implications of the use of
   certificates in STIR.

13.  Acknowledgments

   We would like to thank Richard Barnes, Chris Wendt, Dave Hancock,
   Russ Housley, and Sean Turner for key input to the discussions
   leading to this document.

14.  References

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14.1.  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,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,

14.2.  Informative References

              Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
              Challenges Using an Authority Token", draft-ietf-acme-
              authority-token-05 (work in progress), March 2020.

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              Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
              "TNAuthList profile of ACME Authority Token", draft-ietf-
              acme-authority-token-tnauthlist-06 (work in progress),
              March 2020.

              Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
              Fossati, "Support for Short-Term, Automatically-Renewed
              (STAR) Certificates in Automated Certificate Management
              Environment (ACME)", draft-ietf-acme-star-11 (work in
              progress), October 2019.

              Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
              "Delegated Credentials for TLS", draft-ietf-tls-
              subcerts-09 (work in progress), June 2020.

   [RFC3647]  Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
              Wu, "Internet X.509 Public Key Infrastructure Certificate
              Policy and Certification Practices Framework", RFC 3647,
              DOI 10.17487/RFC3647, November 2003,

   [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,

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,

   [X.509]    ITU-T Recommendation X.509 (10/2012) | ISO/IEC 9594-8,
              "Information technology - Open Systems Interconnection -
              The Directory: Public-key and attribute certificate
              frameworks", 2012.

   [X.520]    ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-6,
              "Information technology - Open Systems Interconnection -
              The Directory: Selected Attribute Types", 2012.

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   [X.680]    ITU-T Recommendation X.680 (08/2015) | ISO/IEC 8824-1,
              "Information Technology - Abstract Syntax Notation One:
              Specification of basic notation".

   [X.681]    ITU-T Recommendation X.681 (08/2015) | ISO/IEC 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Information Object Specification".

   [X.682]    ITU-T Recommendation X.682 (08/2015) | ISO/IEC 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Constraint Specification".

   [X.683]    ITU-T Recommendation X.683 (08/2015) | ISO/IEC 8824-3,
              "Information Technology - Abstract Syntax Notation One:
              Parameterization of ASN.1 Specifications".

Author's Address

   Jon Peterson
   Neustar, Inc.


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