STI Certificate Transparency
draft-wendt-stir-certificate-transparency-04
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Chris Wendt , Robert Śliwa , Alec Fenichel , Vinit Anil Gaikwad | ||
| Last updated | 2024-09-13 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-wendt-stir-certificate-transparency-04
Secure Telephone Identity Revisited C. Wendt
Internet-Draft R. Sliwa
Intended status: Standards Track Somos, Inc.
Expires: 15 March 2025 A. Fenichel
TransNexus
V. A. Gaikwad
Twilio
11 September 2024
STI Certificate Transparency
draft-wendt-stir-certificate-transparency-04
Abstract
This document describes a framework for the use of the Certificate
Transparency (CT) protocol for publicly logging the existence of
Secure Telephone Identity (STI) certificates as they are issued or
observed. This allows any interested party that is part of the STI
eco-system to audit STI certification authority (CA) activity and
audit both the issuance of suspect certificates and the certificate
logs themselves. The intent is for the establishment of a level of
trust in the STI eco-system that depends on the verification of
telephone numbers requiring and refusing to honor STI certificates
that do not appear in a established log. This effectively
establishes the precedent that STI CAs must add all issued
certificates to the logs and thus establishes unique association of
STI certificates to an authorized provider or assignee of a telephone
number resource. The primary role of CT in the STI ecosystem is for
verifiable trust in the avoidance of issuance of unauthorized
duplicate telephone number level delegate certificates or provider
level certificates. This provides a robust auditable mechanism for
the detection of unauthorized creation of certificate credentials for
illegitimate spoofing of telephone numbers or service provider codes
(SPC).
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://appliedbits.github.io/draft-wendt-stir-certificate-
transparency/draft-wendt-stir-certificate-transparency.html. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-wendt-stir-certificate-
transparency/.
Wendt, et al. Expires 15 March 2025 [Page 1]
Internet-Draft STI CT September 2024
Discussion of this document takes place on the Secure Telephone
Identity Revisited Working Group mailing list (mailto:stir@ietf.org),
which is archived at https://mailarchive.ietf.org/arch/browse/stir/.
Subscribe at https://www.ietf.org/mailman/listinfo/stir/.
Source for this draft and an issue tracker can be found at
https://github.com/appliedbits/draft-wendt-stir-certificate-
transparency.
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 15 March 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 5
3. The Use of Certificate Transparency for STI Certificates . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Authentication Service (AS) . . . . . . . . . . . . . . . 6
4.2. Certificate Transparency (CT) . . . . . . . . . . . . . . 6
Wendt, et al. Expires 15 March 2025 [Page 2]
Internet-Draft STI CT September 2024
4.3. Delegate Certificate . . . . . . . . . . . . . . . . . . 6
4.4. Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Merkle Tree . . . . . . . . . . . . . . . . . . . . . . . 6
4.6. Precertificate . . . . . . . . . . . . . . . . . . . . . 7
4.7. Signed Certificate Timestamp (SCT) . . . . . . . . . . . 7
4.8. STI Certification Authority (STI-CA) . . . . . . . . . . 7
4.9. STI Subordinate Certification Authority (STI-SCA) . . . . 7
4.10. Signed Tree Head (STH) . . . . . . . . . . . . . . . . . 7
4.11. TBSCertificate (To Be Signed Certificate) . . . . . . . . 7
4.12. Verification Service (VS) . . . . . . . . . . . . . . . . 8
5. STI Certificate Transparency Framework . . . . . . . . . . . 8
5.1. Log Entries . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Precertificate Submission . . . . . . . . . . . . . . . . 8
5.3. Log Entry Structure . . . . . . . . . . . . . . . . . . . 9
5.4. Structure of the Signed Certificate Timestamp (SCT) . . . 9
5.5. Merkle Tree Structure . . . . . . . . . . . . . . . . . . 10
5.6. Signed Tree Head (STH) . . . . . . . . . . . . . . . . . 10
6. STI-CT APIs . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Add Pre-certificate Chain to Log . . . . . . . . . . . . 11
6.2. Retrieve Latest Signed Tree Head . . . . . . . . . . . . 12
6.3. Retrieve Merkle Consistency Proof between Two Signed Tree
Heads . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.4. Retrieve Merkle Audit Proof from Log by Leaf Hash . . . . 13
6.5. Retrieve Entries from Log . . . . . . . . . . . . . . . . 14
6.6. Retrieve Accepted Root Certificates . . . . . . . . . . . 14
6.7. Retrieve Entry and Merkle Audit Proof . . . . . . . . . . 15
7. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Submitters (STI-CA/STI-SCA) . . . . . . . . . . . . . . . 15
7.2. AS/VS Clients . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Monitor . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.3.1. Monitor Workflow . . . . . . . . . . . . . . . . . . 16
7.4. Auditor . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.4.1. Auditor Functions . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Normative References . . . . . . . . . . . . . . . . . . . . 18
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Certificate Transparency (CT) aims to mitigate the problem of mis-
issued certificates by providing append-only logs of issued
certificates. The logs do not themselves prevent mis-issuance, but
ensure that interested parties (particularly those named in
legitimate certificates or certificate chains) can detect such mis-
issuance. [RFC6962] describes the core protocols and mechanisms for
use of CT for the purposes of public TLS server certificates
Wendt, et al. Expires 15 March 2025 [Page 3]
Internet-Draft STI CT September 2024
associated with a domain name as part of the public domain name
system (DNS). This document describes a conceptually similar
framework that directly borrows concepts like transparency receipts
in the form of SCTs and how they are used in certificates and its
specific use as part of the larger STIR framework for call
authentication. This framework is defined for the specific use with
both Secure Telephone Identity (STI) certificates [RFC8226] and
delegate certificates [RFC9060].
Telephone numbers (TNs) and their management and assignment by
telephone service providers and Responsible Organizations (RespOrgs)
for toll-free numbers share many similarities to the Domain Name
System (DNS) where there is a global uniqueness and established
association of telephone numbers to regulatory jurisdictions that
manage the allocation and assignment of telephone numbers under
country codes and a set of numeric digits for routing telephone calls
and messages over telephone networks. STI Certificates use a
TNAuthList extension defined in [RFC8226] to specifically associate
either telephone service providers or telephone numbers to the
issuance of STI certificates and certificate change that are intended
to represent the authorized right to use a telephone number. This
trusted association can be establish via mechanisms such as Authority
tokens for TNAuthList defined in [RFC9448]. Certificate transparency
and the concept of transparency is generally meant to provide a
publicly verifiable and auditable representation of the creation of
certificates in order to establish transparency and trust to
interested parties as part of a stir related eco-system.
There is three primary actors in the certificate transparency
framework. There is the STI Certification Authorities (CAs) that
submit all certificates to be issued to one or more transparency
append-only log services. The log services are network services that
implement the protocol operations for submissions of STI certificates
and subsequent queries. They are hosted by interested parties in the
STI ecosystem and can accept certificate log submissions from any
other CA participant. The second role is the monitors that play the
role of monitoring the CT logs to check for potential mis-issuance as
well as auditing of the log services. This role can be played by any
STI ecosystem participant interested in the trust of the ecosystem or
the integrity of the telephone number or provider level certificates
produced in the eco-system. CT provides a mechanism of a receipt or
Signed Certificate Timestamp (SCT) that is provided as a result of
submitting a certificate to the append-only log. The third actor
role in the certificate transparency framework is the eco-system
participants that can send and receive receipt(s) or SCT(s) to prove
and validate that a certificate was submitted to a log(s) and
optionally query the log directly for further validation.
Wendt, et al. Expires 15 March 2025 [Page 4]
Internet-Draft STI CT September 2024
The details that follow in this document will detail the specific
protocols and framework for Certificate Transparency associated with
STI certificates. Most of the details borrow many of the concepts of
certificate transparency defined in [RFC6962]}} used in web browser
and web PKI environments, but provides a specific framework designed
for STI certificates and their specific issuance and usage in a
telecommunications and telephone number dependent eco-system.
This general mechanism could also be used for transparently logging
other important stir related metadata associations perhaps via
JWTClaimConstraints defined in [RFC8226] and [RFC9118] or other ways
defined in potential future extensions of this document.
2. Conventions and Definitions
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.
3. The Use of Certificate Transparency for STI Certificates
CT log(s) contains certificate chains, which can be submitted by any
CA authorized in a STIR eco-system. It is expected that these CAs
will contribute all their newly issued certificates to one or more
logs. Note, in [RFC6962] it is possible for certificate holders to
directly contribute their own certificate chains or interested third
parties, however because in stir eco-systems that generally consist
of entities that are authorized to be assigned telephone number
resources, this does not seem to be a likely scenario. Generally,
many stir eco-systems have a controlled set of CAs that are
authorized to participate as valid trust anchors. It is required
that each chain ends with a trust anchor that is accepted by the log
which would include those authorized trust anchors or a subset of
them. When a chain is accepted by a log, a signed timestamp is
returned, which is later used to provide evidence to STIR
verification services (VS), defined in [RFC8224], that the chain has
been submitted. A VS can thus require that all certificates they
accept as valid are accompanied by signed timestamps.
Those concerned about mis-issuance of STIR certificates can monitor
the logs, asking them regularly for all new entries, and can thus
check whether the service provider codes or telephone numbers for
which they are responsible have had certificates issued that they did
not expect. What they do with this information, particularly when
they find that a mis-issuance has happened, is beyond the scope of
this document. However, broadly speaking, because many existing STI
Wendt, et al. Expires 15 March 2025 [Page 5]
Internet-Draft STI CT September 2024
ecosystems have a connection to regulated and industry environments
that govern the issuance of STI certificates, they can invoke
existing mechanisms for dealing with issues such as mis-issued
certificates, such as working with the CA to get the certificate
revoked or with maintainers of trust anchor lists to get the CA
removed.
4. Terminology
This section defines key terms used throughout the STI-CT framework
to ensure clarity and consistency.
4.1. Authentication Service (AS)
A service that signs the identity of a telephone call using Secure
Telephone Identity (STI) certificates, ensuring the authenticity of
the caller information. It ensures that STI Certificates contain
SCTs.
4.2. Certificate Transparency (CT)
A framework designed to provide an open and verifiable log of issued
certificates. It aims to detect and prevent the misuse or mis-
issuance of certificates by maintaining append-only logs that can be
audited by any interested party.
4.3. Delegate Certificate
A type of STI certificate that associates a specific telephone number
or a range of telephone numbers with a particular entity used to
delegate the right to use these numbers.
4.4. Log
An append-only, cryptographically verifiable structure used in
Certificate Transparency to record pre-certificate entries. Logs
accept submissions, generate Signed Certificate Timestamps (SCTs),
and maintain the integrity of the entries through a Merkle Tree
structure.
4.5. Merkle Tree
A cryptographic data structure used in logs to ensure the integrity
and consistency of the entries. It is built by hashing individual
log entries and combining them into a single root hash that
represents the state of the entire log.
Wendt, et al. Expires 15 March 2025 [Page 6]
Internet-Draft STI CT September 2024
4.6. Precertificate
A certificate issued by an CA that is intended to be submitted to a
Certificate Transparency log before the final certificate is issued.
The pre-certificate includes a special extension (the poison
extension) that prevents it from being used as a valid certificate on
its own.
4.7. Signed Certificate Timestamp (SCT)
A data structure provided by a Certificate Transparency log in
response to a pre-certificate submission. The SCT serves as a
promise from the log to include the submitted pre-certificate in the
log within a specified time frame (Maximum Merge Delay). It is
included in the final certificate to prove that it has been logged.
4.8. STI Certification Authority (STI-CA)
An entity responsible for issuing STI certificates in the Secure
Telephone Identity ecosystem. The CA can also issue pre-
certificates, which are submitted to CT logs before the final
certificate is issued.
4.9. STI Subordinate Certification Authority (STI-SCA)
An entity authorized by an CA to issue STI certificates under the
authority of the STI-CA. The STI-SCA can also issue pre-certificates
for submission to CT logs.
4.10. Signed Tree Head (STH)
A cryptographically signed data structure that represents the current
state of a Certificate Transparency log. It includes the root hash
of the Merkle Tree and the number of entries in the log, allowing
auditors to verify the integrity and consistency of the log.
4.11. TBSCertificate (To Be Signed Certificate)
A component of an X.509 certificate that contains all the information
about the certificate except the actual digital signature. The
TBSCertificate includes fields such as the version, serial number,
issuer, validity period, subject, and the subject's public key
information. This component is signed by the certificate authority
(CA) to create the final certificate. In the context of Certificate
Transparency, the TBSCertificate of a pre-certificate is submitted to
the log for inclusion.
Wendt, et al. Expires 15 March 2025 [Page 7]
Internet-Draft STI CT September 2024
4.12. Verification Service (VS)
A service that verifies the authenticity of a telephone call by
checking the validity of the PASSporT token, including verification
that certificate contains valid SCTs.
5. STI Certificate Transparency Framework
This section describes the format and operational procedures for logs
in the STI Certificate Transparency (CT) framework.
5.1. Log Entries
Logs in the STI CT framework are append-only structures that store
entries in a Merkle Tree and use SHA-256 for data hashing. The
entries consist of pre-certificates submitted by STI Certification
Authorities (STI-CAs) or Subordinate Certification Authorities (STI-
SCAs). The log entries help ensure that all issued STI certificates
can be audited for legitimacy.
5.2. Precertificate Submission
An STI-CA/STI-SCA submits a pre-certificate to a log before the
actual STI certificate is issued. The pre-certificate submission
must include all necessary intermediate certificates to validate the
chain up to an accepted root certificate. The root certificate may
be omitted from the submission.
When a pre-certificate is submitted:
* The log verifies the chain of the pre-certificate up to a trusted
root.
* If valid, the log generates and returns a Signed Certificate
Timestamp (SCT) to the submitter.
* The SCT serves as a promise from the log that the pre-certificate
will be included in the Merkle Tree within a defined Maximum Merge
Delay (MMD).
Logs must publish a list of accepted root certificates, which aligns
with those trusted in the STIR ecosystem. The inclusion of SCTs in
the actual STI certificates is critical, as Verification Services
(STI-VS) will only accept certificates that include valid SCTs.
Wendt, et al. Expires 15 March 2025 [Page 8]
Internet-Draft STI CT September 2024
5.3. Log Entry Structure
Each log entry consists of the following components:
struct {
PrecertChainEntry entry;
} LogEntry;
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert pre_certificate;
ASN.1Cert precertificate_chain<0..2^24-1>;
} PrecertChainEntry;
* pre_certificate: The pre-certificate submitted for auditing.
* precertificate_chain: A chain of certificates required to verify
the pre-certificate, including intermediate certificates but
excluding the root certificate.
Logs may impose a limit on the length of the certificate chain they
will accept. The log verifies the validity of the pre-certificate
chain up to an accepted root and, upon acceptance, stores the entire
chain for future auditing.
5.4. Structure of the Signed Certificate Timestamp (SCT)
The SCT is a data structure returned by the log when a pre-
certificate is accepted. It is structured as follows:
struct {
Version sct_version;
LogID id;
uint64 timestamp;
digitally-signed struct {
Version sct_version;
SignatureType signature_type = certificate_timestamp;
uint64 timestamp;
PreCert signed_entry;
CtExtensions extensions;
};
} SignedCertificateTimestamp;
* sct_version: The version of the SCT protocol, set to v1.
* id: The SHA-256 hash of the log's public key.
Wendt, et al. Expires 15 March 2025 [Page 9]
Internet-Draft STI CT September 2024
* timestamp: The timestamp of the SCT issuance.
* signed_entry: Contains the PreCert structure, which includes the
issuer's key hash and the TBSCertificate component of the pre-
certificate.
* extensions: Placeholder for future extensions.
The SCT is included in the final STI certificate, and VS services
will check the presence and validity of SCTs to verify the legitimacy
of the certificate.
5.5. Merkle Tree Structure
Logs use a Merkle Tree structure, with each leaf corresponding to a
MerkleTreeLeaf entry. The leaves are hashed to form the tree, which
is continuously updated as new entries are added.
struct {
Version version;
MerkleLeafType leaf_type;
TimestampedEntry timestamped_entry;
} MerkleTreeLeaf;
struct {
uint64 timestamp;
PreCert signed_entry;
CtExtensions extensions;
} TimestampedEntry;
* version: The protocol version, set to v1.
* leaf_type: The type of the leaf, set to timestamped_entry.
* timestamped_entry: Contains the timestamp and the pre-certificate
data.
The root hash of the Merkle Tree represents the state of the log at a
given time and can be used to verify the inclusion of specific
entries.
5.6. Signed Tree Head (STH)
The log periodically signs the root of the Merkle Tree, producing a
Signed Tree Head (STH), which ensures the integrity of the log over
time.
Wendt, et al. Expires 15 March 2025 [Page 10]
Internet-Draft STI CT September 2024
digitally-signed struct {
Version version;
SignatureType signature_type = tree_hash;
uint64 timestamp;
uint64 tree_size;
opaque sha256_root_hash[32];
} TreeHeadSignature;
* timestamp: The current time, ensuring it is more recent than the
most recent SCT.
* tree_size: The number of entries in the Merkle Tree.
* sha256_root_hash: The root hash of the Merkle Tree.
Logs must produce an STH within the Maximum Merge Delay (MMD) to
confirm that all SCTs issued have been incorporated into the Merkle
Tree. Auditors and monitors can use the STH to verify that the log
is operating correctly and that no entries have been tampered with.
6. STI-CT APIs
This section outlines the API operations that clients of the STI-CT
will use to interact with the logs. The APIs are designed to support
the submission and verification of pre-certificates (precerts) within
the STIR ecosystem. All operations are conducted over HTTPS and
utilize JSON for data exchange.
These APIs are based on RFC 6962, which defines the Certificate
Transparency protocol. The APIs are designed to be specific for STIR
ecosystem certificates.
6.1. Add Pre-certificate Chain to Log
Endpoint:
* POST https://<log server>/stict/v1/add-pre-chain
Inputs:
* chain: An array of base64-encoded certificates representing the
pre-certificate chain. The first element is the end-entity pre-
certificate (TBSCertificate), and subsequent elements are
certificates that chain up to a known root certificate.
Outputs:
Wendt, et al. Expires 15 March 2025 [Page 11]
Internet-Draft STI CT September 2024
* sct_version: The version of the Signed Certificate Timestamp (SCT)
structure, in decimal. For this version, it should always be 1.
* id: The log ID, base64 encoded. This is the identifier for the
log server that processed the submission.
* timestamp: The SCT timestamp, in decimal, representing the time
when the SCT was issued.
* extensions: An opaque field reserved for future use. Currently,
logs should set this to an empty string.
* signature: The SCT signature, base64 encoded, which is used to
verify the SCT.
This endpoint allows an STI-CA or STI-SCA to submit a pre-certificate
chain to the log for transparency. Upon submission, the log returns
an SCT, which should be included in the final STI certificate before
it is issued.
6.2. Retrieve Latest Signed Tree Head
Endpoint:
* GET https://<log server>/stict/v1/get-sth
Inputs:
* No inputs required.
Outputs:
* tree_size: The number of entries in the log, in decimal.
* timestamp: The timestamp of the latest Signed Tree Head (STH), in
decimal.
* sha256_root_hash: The SHA-256 hash of the root of the Merkle Tree,
base64 encoded.
* tree_head_signature: A signature of the STH data, ensuring its
integrity.
This endpoint retrieves the latest state of the log, represented by
the Signed Tree Head, which can be used by clients to verify that
entries have been properly incorporated into the log.
Wendt, et al. Expires 15 March 2025 [Page 12]
Internet-Draft STI CT September 2024
6.3. Retrieve Merkle Consistency Proof between Two Signed Tree Heads
Endpoint:
* GET https://<log server>/ct/v1/get-sth-consistency
Inputs:
* first: The tree size of the first STH, in decimal.
* second: The tree size of the second STH, in decimal.
Outputs:
* consistency: An array of base64-encoded Merkle Tree nodes that
provide a consistency proof between the two STHs.
This endpoint allows clients to verify that the log has been
consistently maintained over time, by checking the consistency proof
between two STHs.
6.4. Retrieve Merkle Audit Proof from Log by Leaf Hash
Endpoint:
* GET https://<log server>/stict/v1/get-proof-by-hash
Inputs:
* hash: A base64-encoded SHA-256 hash of the leaf.
* tree_size: The size of the tree for which the proof is desired, in
decimal.
Outputs:
* leaf_index: The 0-based index of the corresponding log entry.
* audit_path: An array of base64-encoded Merkle Tree nodes providing
the audit proof.
This endpoint retrieves a Merkle audit proof, which can be used to
verify that a particular entry is included in the log, ensuring that
the log is append-only and that the SCT is valid.
Wendt, et al. Expires 15 March 2025 [Page 13]
Internet-Draft STI CT September 2024
6.5. Retrieve Entries from Log
Endpoint:
* GET https://<log server>/stict/v1/get-entries
Inputs:
* start: The 0-based index of the first entry to retrieve, in
decimal.
* end: The 0-based index of the last entry to retrieve, in decimal.
Outputs:
* entries: An array of objects, each containing:
* leaf_input: The base64-encoded MerkleTreeLeaf structure.
* extra_data: The base64-encoded unsigned data related to the log
entry. For a pre-certificate entry, this will include the entire
PrecertChainEntry.
This endpoint allows clients to retrieve a range of entries from the
log, which can be used to verify the integrity of the log and audit
the inclusion of specific entries.
6.6. Retrieve Accepted Root Certificates
Endpoint:
* GET https://<log server>/stict/v1/get-roots
Inputs:
* No inputs required.
Outputs:
* certificates: An array of base64-encoded root certificates that
are accepted by the log.
This endpoint provides a list of root certificates that the log
accepts for verifying certificate chains.
Wendt, et al. Expires 15 March 2025 [Page 14]
Internet-Draft STI CT September 2024
6.7. Retrieve Entry and Merkle Audit Proof
Endpoint:
* GET https://<log server>/stict/v1/get-entry-and-proof
Inputs:
* leaf_index: The index of the desired entry, in decimal.
* tree_size: The size of the tree for which the proof is desired, in
decimal.
Outputs:
* leaf_input: The base64-encoded MerkleTreeLeaf structure.
* extra_data: The base64-encoded unsigned data related to the log
entry, similar to the output in get-entries.
* audit_path: An array of base64-encoded Merkle Tree nodes providing
the audit proof.
This endpoint is typically used for debugging or for in-depth
verification of specific entries in the log.
7. Clients
This section describes various roles clients of STI-CT perform. Any
inconsistency detected by clients could serve as evidence that a log
has not behaved correctly, and the signatures on the data structures
prevent the log from denying any misbehavior.
7.1. Submitters (STI-CA/STI-SCA)
Submitters in the STI-CT framework are typically STI Certification
Authorities (STI-CAs) or Subordinate Certification Authorities (STI-
SCAs). These entities submit pre-certificates to the log as
described in the APIs section. The returned Signed Certificate
Timestamp (SCT) can then be used to construct the final STI
certificate, which includes one or more SCTs.
7.2. AS/VS Clients
AS and VS services interact with SCTs and the underlying logs to
ensure the authenticity and validity of telephone calls.
Wendt, et al. Expires 15 March 2025 [Page 15]
Internet-Draft STI CT September 2024
* AS: The Authentication Service should use valid certificates that
contain SCT(s). The SCT(s) can be validated by computing the
signature input from the SCT data as well as the certificate and
verifying the signature using the corresponding log's public key.
AS MUST reject SCTs whose timestamps are in the future.
* VS: The Verification Service receives the signed PASSporT token
and verifies that the included SCTs in the certificate used to
sign the PASSporT. VS MUST reject Certificates that do not have
valid SCT(s) and fail PASSporT validation.
7.3. Monitor
Monitors in the STI-CT framework play a crucial role in maintaining
the integrity and trust of the ecosystem. They ensure that no
certificates are mis-issued, particularly concerning the TNAuthList
field, which lists the telephone numbers an entity is authorized to
use.
7.3.1. Monitor Workflow
1. Initialize Monitor: * Set up the Monitor to periodically query
the transparency logs for new entries. The Monitor must be
configured with the base URL of each log it intends to monitor.
* Configure the Monitor with a list of telephone numbers (TNs)
and associated entities to track.
2. Retrieve Latest STH: * The Monitor retrieves the latest Signed
Tree Head (STH) from each log to determine the current state of
the log. * API Call: GET https://<log server>/stict/v1/get-sth
3. Retrieve New Entries from Log: - Using the STH, the Monitor
retrieves new entries from the log that have been added since the
last known state. - API Call: GET https://<log server>/stict/v1/
get-entries?start=last_known_index&end=current_sth_index
4. Decode and Verify Certificates: - Decode each retrieved
certificate and verify its validity using the provided
certificate chain. Extract the entity name and TNAuthList from
the certificate.
5. Check for Mis-issuance: - Compare the TNAuthList and entity name
from the newly issued certificate with the Monitor's configured
list. Alarm if a certificate is issued in the name of a
different entity for the same TNs.
Wendt, et al. Expires 15 March 2025 [Page 16]
Internet-Draft STI CT September 2024
6. Alarm and Reporting: - If a mis-issuance is detected, raise an
alarm and log the details for further investigation. Notify
relevant stakeholders to rectify any confirmed mis-issuance.
7. Maintain State and Continuity: - Update the Monitor's last known
state with the current STH index to ensure continuity in
monitoring.
8. STH Verification and Consistency Check: - After retrieving a new
STH, verify the STH signature. - If not keeping all log entries,
fetch a consistency proof for the new STH with the previous STH
(GET https://<log server>/stict/v1/get-sth-consistency) and
verify it. - Go to Step 5 and repeat the process.
7.4. Auditor
Auditors are responsible for verifying the consistency and
correctness of the log, ensuring that the log behaves according to
the expected protocol. Auditors can operate as standalone services
or as part of another client role, such as a monitor or an VS.
7.4.1. Auditor Functions
1. STH Verification: - Auditors can fetch STHs periodically and
verify their signatures to ensure the log is maintaining its
integrity. - API Call: GET https://<log server>/stict/v1/get-sth
2. Consistency Proof Verification: - Auditors verify the consistency
of a log over time by requesting a consistency proof between two
STHs. - API Call: GET https://<log server>/stict/v1/get-sth-
consistency
3. Audit Proof Verification: - A certificate accompanied by an SCT
can be verified against any STH dated after the SCT timestamp +
the Maximum Merge Delay by requesting a Merkle audit proof. -
API Call: GET https://<log server>/stict/v1/get-proof-by-hash
4. Cross-Checking Logs: - Auditors can cross-check entries across
different logs by comparing SCTs and verifying that entries are
consistently logged across the ecosystem.
5. Error and Inconsistency Detection: - Any discrepancies or
failures in verification processes can be logged as evidence of
potential log misbehavior, and appropriate actions can be taken
based on the findings.
Wendt, et al. Expires 15 March 2025 [Page 17]
Internet-Draft STI CT September 2024
8. Security Considerations
TODO Security
9. IANA Considerations
None at this time.
10. 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/rfc/rfc2119>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/rfc/rfc6962>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[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,
<https://www.rfc-editor.org/rfc/rfc8224>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/rfc/rfc8226>.
[RFC9060] Peterson, J., "Secure Telephone Identity Revisited (STIR)
Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
September 2021, <https://www.rfc-editor.org/rfc/rfc9060>.
[RFC9118] Housley, R., "Enhanced JSON Web Token (JWT) Claim
Constraints for Secure Telephone Identity Revisited (STIR)
Certificates", RFC 9118, DOI 10.17487/RFC9118, August
2021, <https://www.rfc-editor.org/rfc/rfc9118>.
[RFC9448] Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
"TNAuthList Profile of Automated Certificate Management
Environment (ACME) Authority Token", RFC 9448,
DOI 10.17487/RFC9448, September 2023,
<https://www.rfc-editor.org/rfc/rfc9448>.
Wendt, et al. Expires 15 March 2025 [Page 18]
Internet-Draft STI CT September 2024
Acknowledgments
The authors would like to thank the authors and contributors to the
protocols and ideas around Certificate Transparency [RFC6962] which
sets the basis for the STI eco-system to adopt in a very straight
forward way, providing trust and transparency in the telephone number
world.
Authors' Addresses
Chris Wendt
Somos, Inc.
United States of America
Email: chris@appliedbits.com
Rob Sliwa
Somos, Inc.
United States of America
Email: robjsliwa@gmail.com
Alec Fenichel
TransNexus
United States of America
Email: alec.fenichel@transnexus.com
Vinit Anil Gaikwad
Twilio
United States of America
Email: vanilgaikwad@twilio.com
Wendt, et al. Expires 15 March 2025 [Page 19]