Network Working Group S. Jovancevic Internet-Draft SKGO, IKT Support Intended status: Informational 5 August 2026 Expires: 6 February 2027 Verifiable Data Access Contract (VDAC) draft-jovancevic-vdac-03 Abstract This document specifies the Verifiable Data Access Contract (VDAC), a protocol for cryptographically verifiable bilateral agreement between a content publisher and an automated agent regarding the terms of programmatic data access. VDAC defines the mechanism by which a site issues an access offer, an agent accepts that offer, both parties sign the resulting contract, and per-request references bind individual interactions to agreed terms. VDAC is the protocol-layer realization of the bilateral commitment principle introduced in Section 6.6 of the Verifiable Identity Claims and Delegation Model [VICDM] and operates as a companion specification to the Signed Agent Identity Protocol [SAIP]. This document defines mechanism, not content: VDAC verifies the existence and integrity of an agreement; the substance of what is agreed remains entirely between the contracting parties. 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 6 February 2027. Copyright Notice Copyright (c) 2026 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. Table of Contents 1. Introduction 1.1. Relationship to VICDM 1.2. The Emerging Architectural Stack 1.3. Relationship to SAIP and ALOI 1.3.1. Identity Mechanism Requirements 1.4. Relationship to webbotauth Work 1.5. Protocol Scope and Non-Goals 2. Terminology 3. Conceptual Overview 3.1. The Progression from Anonymous to Contracted 3.2. Programmable Bilateral Consent 3.3. Contracted Web Semantics 3.4. Separation of Identity Trust and Behavior Trust 3.5. Mechanism, Not Content 4. Site Offer Document 4.1. Publication Location 4.2. Offer Document Format 4.3. Multiple Concurrent Offers 4.4. Discovery and Capability Advertisement 5. Agent Acceptance Document 5.1. Acceptance Format 5.2. Submission and Server Response 6. Contract Document 6.1. Contract Format 6.2. Contract Identifier Derivation 6.3. Contract Storage Obligations 6.4. Delegated Contracting and Infrastructure Scale 7. Per-Request Contract Reference 8. Mutual Tracking Obligation 8.1. Site-Side Log Entry Format 8.2. Agent-Side Log Entry Format 8.3. End-User Privacy Boundaries 8.4. Log Retention 9. Reconciliation and Dispute Evidence 9.1. Reconciliation Manifest Format 10. Violation Detection and Sanctions 10.1 Violation Notice Format 10.2 Sanction Progression 10.3 Self-Reporting (Good Faith Mechanism) 11. Contract Termination 11.1 Termination Conditions 11.2 Termination Notice Format 11.3 Post-Termination Obligations 12. Relationship to ALOI and Contract Formation 12.1. How a Contract Is Formed 12.2. Sequence of an Exchange 12.3. Revision Limit 13. Use Case Profiles (Informative) 13.1. AI Training Data Access 13.2. Content Retrieval and Inference Access 13.3. Monitoring and Health-Check Access 14. Security Considerations 15. Privacy Considerations 16. IANA Considerations 17. References 17.1. Normative References 17.2. Informative References Appendix A. Document History Author's Address 1. Introduction The Verifiable Data Access Contract (VDAC) provides a cryptographic mechanism by which a content publisher (the Site) and an automated client (the Agent) can establish a verifiable bilateral agreement regarding the terms of programmatic data access. Modern content publishers face a recurring problem: automated agents -- AI training crawlers, retrieval systems, monitoring services, IoT telemetry consumers -- access publisher resources at scale, often outside the scope envisioned by the publisher. Existing mechanisms address this only partially: o robots.txt declares what a site permits, but is unilateral, advisory, and cryptographically unverified. o Terms-of-service URLs declare legal terms, but bind no party cryptographically and are difficult to enforce per-request. o Anonymous attestation models (such as those developed in the IETF webbotauth working group) verify that a bot is vouched for, but do not establish any agreement between bot and site about what the bot will do. o Identity attestation models (such as [SAIP]) verify who the bot is, but make no statement about agreed terms of access. VDAC fills the gap between identity verification and behavioral agreement. Once a Site and Agent have both signed a VDAC contract, each subsequent request from the Agent references the contract cryptographically, and both parties retain non-repudiable logs of the actual interaction. Discrepancies between intent and behavior become provable rather than asserted. Conceptually, VDAC performs two functions. First, it notarizes: it provides the bilateral, cryptographic act by which an already- reached agreement becomes sealed and mutually attested. Second, it safeguards: once sealed, the contract is bound to every subsequent interaction, so that what was agreed remains provably in force over the entire life of the contract. VDAC is, in this sense, the final link in the chain to a concluded agreement -- it does not negotiate or author the agreement, but it is the mechanism by which the agreement becomes sealed and is then kept faithful to itself. 1.1. Relationship to VICDM This document develops the bilateral agreement mechanism originally introduced in Section 6.6 of [VICDM]. VDAC is extracted into a standalone specification to enable focused review and independent deployment. VDAC implements the fifth principle of the VICDM model: Agreed intent MUST be honored bilaterally. An Agent operating under a valid VDAC contract is classified as Class 4 (Contracted Agent) under the VICDM trust model, receiving the highest trust level in that classification. The two documents are companion specifications: [VICDM] provides the conceptual framework and identity classification within which VDAC operates; this document defines the wire protocol, document formats, and operational mechanics. 1.2. The Emerging Architectural Stack The automated web ecosystem naturally separates into distinct functional layers. Each layer is a responsibility, and one or more protocols may fulfill that responsibility: +==================+=====================+======================+ | Layer | Responsibility | Protocol(s) | +==================+=====================+======================+ | Identity | Establishing a | SAIP (full identity | | | verifiable agent | framework), or | | | identity | VICDM DNS-based | | | | verification | +------------------+---------------------+----------------------+ | Framework | Trust | VICDM | | | classification and | | | | delegation | | +------------------+---------------------+----------------------+ | Intent | Unilateral intent | ALOI (defined in | | | declaration | VICDM) | +------------------+---------------------+----------------------+ | Agreement | Bilateral | VDAC | | | operational | | | | agreement | | +------------------+---------------------+----------------------+ The Identity layer is deliberately shown with two protocol options. SAIP and VICDM both provide identity verification, by different mechanisms: SAIP is a complete identity and signature framework with per-request key rotation and related operational features, while VICDM additionally defines a lighter-weight DNS-based verification suitable for agents that do not need the full SAIP machinery. A protocol at a higher layer -- such as VDAC -- consumes "an identity", not "SAIP specifically". This is what makes the stack composable: each layer can be satisfied by whichever protocol fits the deployment. This separation is fundamental to modern automated network interactions because identity trust does not equate to behavioral trust. Cryptographic identity alone cannot establish operational accountability. VDAC introduces an explicit, operational layer where identity answers "who you are", and VDAC answers "whether you honored what was agreed". 1.3. Relationship to SAIP and ALOI VDAC requires that every participating Agent possess a verified identity: a cryptographically verifiable public key bound to the Agent and discoverable by the Site. VDAC does NOT mandate any single identity mechanism. The Signed Agent Identity Protocol [SAIP] is the RECOMMENDED identity mechanism for VDAC, as it provides a complete identity and signature framework designed for automated agents. However, SAIP is not required. An Agent MAY establish its identity through any of the following: o SAIP -- the RECOMMENDED full identity framework. o The DNS-based identity verification described in [VICDM] Section 5, which publishes an Agent public key in DNS without requiring the full SAIP machinery. This is a lighter-weight option suitable for Agents that do not need per-request key rotation or the other operational features of SAIP. o Any equivalent mechanism that provides a verifiable Agent public key, provided both Site and Agent can rely on it. VDAC and SAIP are complementary, not interdependent. VDAC defines the bilateral contract layer; how the Agent proves its identity is a separate concern that VDAC deliberately leaves open. Where this document refers to an Agent's signing key, that key is whatever public key the chosen identity mechanism establishes. VDAC is complementary to the Agent Letter of Intent (ALOI) defined in Section 6.5 of [VICDM]: o ALOI is unilateral: the Agent declares its intent in DNS, and any Site may observe and enforce that declaration. o VDAC is bilateral: a specific Site and a specific Agent enter into a mutually signed agreement that supersedes general ALOI declarations for interactions under the contract. An Agent MAY operate under ALOI alone (Intent-Declaring mode), under VDAC with a specific Site (Contracted Agent mode), or under both simultaneously. When in conflict, the more specific commitment governs: VDAC terms supersede ALOI declarations for requests under the contract; ALOI continues to apply outside any VDAC contract. 1.3.1. Identity Mechanism Requirements VDAC is identity-mechanism-agnostic. The protocol does not privilege any particular identity system. Any identity mechanism MAY be used as the Identity layer for VDAC, provided it satisfies all of the following requirements: o It MUST yield a public key that is cryptographically bound to the Agent. o It MUST allow the Site to verify that public key independently, without contacting the Agent out-of-band. o It MUST produce per-request signatures that can cover the contract-id and contract-hash parameters defined in Section 7, so that each request is bound to a specific contract version. Any mechanism meeting these three requirements is suitable as the VDAC Identity layer. SAIP and the VICDM DNS-based verification are two such mechanisms specified within this document family. They are RECOMMENDED for convenience and for interoperability across independent implementations, but they are in no way privileged by the protocol itself. A VDAC implementation that relies on neither SAIP nor VICDM is fully conformant, provided its chosen identity mechanism meets the requirements above. Implementers MAY therefore use existing identity infrastructure -- including mechanisms not authored within this document family, and including organization-internal identity systems -- wherever that infrastructure satisfies the stated requirements. The Identity layer is a replaceable component, not a fixed dependency. This is a deliberate design property: it keeps VDAC composable and avoids binding adopters to any single identity ecosystem. The agent.id_method field in the Acceptance Document (Section 5.1) names the identity mechanism actually in use, allowing the Site to apply the correct verification procedure. 1.4. Relationship to webbotauth Work The IETF webbotauth working group is developing mechanisms for anonymous bot attestation [WEBBOTAUTH-ARCH]. VDAC is complementary to that work: o Anonymous attestation addresses the privacy use case: rate limiting and abuse prevention without per-instance disclosure. o VDAC addresses the bilateral accountability use case: explicit mutual agreement on terms of access, with cryptographic evidence retained by both parties. A deployment MAY support anonymous attestation for general traffic and require VDAC for specific access tiers (e.g., bulk data retrieval, AI training, premium API endpoints). The two mechanisms address different threat models and need not be mutually exclusive. 1.5. Protocol Scope and Non-Goals VDAC defines the mechanism for verifiable bilateral agreement. It addresses: o Offer issuance by Sites. o Acceptance by Agents. o Cryptographic binding of both parties to agreed terms. o Per-request contract reference. o Mutual tracking and audit obligations. o Dispute evidence format. o Contract termination. VDAC explicitly does NOT address: o Pricing, payment, or commercial settlement mechanisms. o Legal enforcement, arbitration, or jurisdiction. o Tax, regulatory, or compliance matters. o Open-ended negotiation. VDAC permits each party a single revision (Section 12.3); anything beyond that takes place out of band and outside this protocol. These elements are private between the contracting parties or addressed by other documents. VDAC provides only the mechanism for verifiable agreement. 2. Terminology 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. Site: The party publishing content and offering access terms. Identified by a DNS domain. Agent: The automated client seeking access. Holds a verified identity providing a cryptographically verifiable public key (see Section 1.3). Offer Document: A signed JSON object describing access terms. Ordinarily issued by the Site. Acceptance Document: A signed JSON object produced by the Agent indicating intent to operate under a specific Offer. Contract Document: The mutually signed JSON object representing the final bilateral agreement. Contains both the Offer and the Acceptance, plus signatures from both parties. Proposal: Any document put forward by one party for the other's consideration: an Offer, a revision of an Offer, or an ALOI observed by a Site. Revision: A proposal that modifies terms previously proposed by the other party. An opening proposal is not a revision. See Section 12.3. Contract Identifier: A unique value identifying a specific Contract Document, used as a per-request reference. Contract Hash: SHA-256 hash of the canonical Contract Document, used to bind individual requests cryptographically to a specific contract version. 3. Conceptual Overview 3.1. The Progression from Anonymous to Contracted VDAC sits at the top of the VICDM interaction progression: +======================+============+============+============+ | Interaction Mode | Identity | ALOI | VDAC | +======================+============+============+============+ | Anonymous | No | No | No | +----------------------+------------+------------+------------+ | Identified | Yes | No | No | +----------------------+------------+------------+------------+ | Intent-Declaring | Yes | Yes | No | +----------------------+------------+------------+------------+ | Contracted Agent | Yes | Yes opt. | Yes | +----------------------+------------+------------+------------+ Each level is opt-in. No Agent is compelled to enter a VDAC contract; no Site is compelled to issue a VDAC Offer. Where bilateral terms make sense, the mechanism is available. 3.2. Programmable Bilateral Consent VDAC introduces a transition from static legal declarations to machine-verifiable operational consent. Traditional mechanisms are unsuited for programmatic execution: robots.txt is advisory only, Terms of Service documents are legally descriptive but unparseable by systems, and API keys grant entry without tracking ongoing behavioral obligations. Under the Programmable Bilateral Consent model: 1. Both parties cryptographically bind themselves to operational scopes. 2. The operational scope is explicitly machine-readable. 3. Violations become algorithmically provable rather than merely asserted. 3.3. Contracted Web Semantics VDAC establishes a structural shift in web interaction philosophy. The historical web model relies on anonymous, unilateral, advisory, and loosely enforceable interactions. VDAC formalizes mutually authenticated interactions, cryptographically verifiable obligations, and bilateral accountability. This introduces a transition from advisory web semantics toward verifiable bilateral interaction semantics. 3.4. Separation of Identity Trust and Behavior Trust A critical distinction within this architecture is the complete decoupling of identity verification from behavioral tracking: o Identity Trust establishes who the Agent is, whether the identity is cryptographically valid, and whether delegation roots are intact. This is provided by an identity mechanism such as SAIP or the DNS-based verification of [VICDM]. o Behavior Trust (VDAC) verifies whether the Agent and Site have honored the underlying agreement over time, and preserves independent mathematical logs. Because a valid identity does not automatically guarantee compliant behavior, VDAC serves as the explicit operational accountability layer above whatever identity mechanism is in use. 3.5. Mechanism, Not Content VDAC is a protocol primitive. It cryptographically establishes that: o The Site offered specific terms at a specific time. o The Agent accepted those specific terms at a specific time. o Both parties bound themselves to the contract by signature. o Each subsequent request was made under the claimed contract. o Both parties retained matching logs of the interaction. What the terms ARE -- whether they permit AI training, restrict to ten requests per minute, require attribution, exclude personal data paths, or impose specific structural limits -- is outside the scope of this document. VDAC verifies that an agreement exists and is honored. The wisdom or commercial value of the agreement is for the contracting parties to determine. This separation makes VDAC policy-source-agnostic. The terms of a contract MAY incorporate a resolved policy outcome produced by an external preference or policy system -- for example, the AIPREF framework, robots.txt, or a Site's internal policy. VDAC does not depend on, and does not privilege, any particular such system. Where an ALOI carries policy compliance content (see [VICDM] Section 6.5.1.1), a Site that responds to that ALOI with an Offer MAY adopt that content as one of the Offer's terms. It is then a clause that both parties have signed. VDAC binds the clause; it does not interpret it. No AIPREF-specific, robots-specific, or other source-specific machinery is required in this document, because the policy source is named by whichever layer produced the term, and the term is carried, not parsed, by VDAC. The same separation governs the grant_ref member defined in Section 4.2. A grant_ref names an externally defined procedure by which a term can be resolved and verified. VDAC defines the extension point and the integrity of the reference; it does not define, endorse, or require any particular procedure, and the type tokens registered under Section 16 are equally available to any specification that registers one. A VDAC implementation that carries no grant_ref is fully conformant. 4. Site Offer Document 4.1. Publication Location A Site that wishes to enable VDAC-based access SHOULD publish an Offer Document at a well-known location, per [RFC8615]: https:///.well-known/vdac-offer When multiple Offers are published, they MAY be available at versioned paths: https:///.well-known/vdac-offer/ The Offer Document MUST be served over TLS. Servers MAY include the Offer Document inline in API responses where convenient, but the well-known URL remains the authoritative source. An Offer need not be published to be valid. A Site MAY also issue an Offer directly to a specific Agent in response to a prior proposal, as described in Section 12. Such an Offer is not published at the well-known location and carries an in_response_to member identifying what it answers. 4.2. Offer Document Format The Offer Document is a JSON object signed by the issuing party's identity key: { "offer_id": "premium-ai-training-v1", "site": { "domain": "site.domain", "pubkey": "pX023...[Truncated Base64URL]" }, "in_response_to": { "scheme": "aloi", "ref": "Base64URLValue...[Truncated]" }, "valid_from": 1779369600, "valid_until": 1795132800, "terms": { "scope": ["/api/v1/public/*", "/articles/archived/*"], "exclusions": ["/api/v1/public/users/*", "/articles/premium/*"], "rate_limit": { "window_seconds": 60, "requests_per_window": 120, "burst_allowance": 15, "max_concurrent_connections": 4, "bandwidth_cap_bytes_per_day": 1073741824 }, "obligations": ["attribution_required", "no_resell"], "custom_terms_uri": "https://site.domain/legal/tos-ai.html", "custom_terms_hash": "47DEQpj8HBSa...[Truncated Base64URL]", "grant_ref": { "type": "verifiable-grant", "uri": "https://grants.example/g/8817", "digest": "sha-256=:X48E9qOo...[Truncated]:" } }, "offer_sig": "SigVal...[Truncated Base64URL]" } Field semantics: offer_id Unique identifier issued by the Site. MUST be unique within the scope of the Site for the Offer's validity period. A revision issued by the Agent (Section 12.2) carries the offer_id of the Offer it revises; Agents do not mint offer_id values of their own. site.domain The DNS domain of the Site, matching the host portion of the publication URL where the Offer is published. In a revision issued by the Agent, this member continues to identify the Site whose Offer is being revised. site.pubkey The Site's Ed25519 public key. in_response_to OPTIONAL. Present when this document answers a proposal made by the other party. Absent when this document opens the exchange. in_response_to.scheme A string naming the kind of proposal being answered, so that the receiving party knows which layer produced it. Defined values: "aloi" (an Agent Letter of Intent per [VICDM] Section 6.5) and "vdac-offer" (an Offer Document, including one issued as a revision). Other implementation-defined strings MAY be used, following the same convention as agent.id_method (Section 5.1). in_response_to.ref An opaque reference to the proposal being answered, as supplied by the layer that produced that proposal. VDAC does not parse, resolve, or interpret this value; it records which proposal was answered so that the sequence of an exchange is verifiable. For scheme "vdac-offer" the value is the SHA-256 hash of the canonicalized document, Base64URL- encoded. valid_from Unix timestamp before which the Offer MUST NOT be accepted. valid_until Unix timestamp after which the Offer MUST NOT be accepted. terms Substantive content of the Offer. terms.scope An array of URL path matchers defining permitted endpoints. Pattern matching follows standard glob/wildcard semantics relative to the site root path. terms.exclusions An array of URL path matchers defining explicit blocks within the permitted scope. Exclusions MUST override scope definitions when paths overlap. terms.rate_limit Stated operational performance constraints. Parsers MUST support the primitive integer definitions for windows, requests, concurrency, and optional daily bandwidth caps. terms.obligations Free-form list of obligations the Agent agrees to fulfill if accepting. The protocol does not parse these strings natively. terms.custom_terms_uri An optional URI pointing to additional contract terms that do not fit within JSON. terms.custom_terms_hash REQUIRED when custom_terms_uri is present, otherwise absent. SHA-256 hash of the octets retrieved from custom_terms_uri at Offer issuance time, Base64URL-encoded. A party that retrieves the referenced document and computes a different value MUST treat the terms as unavailable rather than substituted. terms.grant_ref OPTIONAL. A reference to an externally defined grant, licence, or authorization whose resolution procedure is specified outside this document. VDAC carries the reference and guarantees its integrity; it does not resolve or interpret it. A Contract containing no grant_ref is fully conformant. terms.grant_ref.type A type token naming the procedure by which the reference is resolved and verified. The token MUST be registered in the VDAC Grant Reference Types registry (Section 16). The normative procedure for a given token is named by that registry entry, not by this document. terms.grant_ref.uri A URI at which the referenced grant is retrievable. terms.grant_ref.digest An integrity digest of the representation retrievable at uri, expressed as a Digest field value per [RFC9530]. A party that retrieves the grant and computes a different digest MUST treat the grant as unavailable rather than substituted. offer_sig Ed25519 signature over the canonicalized offer object excluding the signature field itself, produced by the party issuing the document. Canonicalization follows [RFC8785]. The Digest field encoding of [RFC9530] is used at the grant_ref boundary only, because that is where VDAC hands off to a representation it did not produce and whose retrieval is an HTTP concern. Hashes internal to VDAC's own notarial operations -- offer_hash, contract_hash, custom_terms_hash, log_summary_hash -- are plain SHA-256 values encoded as Base64URL per [RFC4648] Section 5, without padding, as defined where each is introduced. 4.3. Multiple Concurrent Offers A Site MAY publish multiple Offers concurrently representing different access tiers. Each Offer MUST have a distinct offer_id. The set of currently valid Offers MAY be enumerated at: https:///.well-known/vdac-offer-index 4.4. Discovery and Capability Advertisement To minimize the application layer overhead of fetching offer indexes proactively, implementations SHOULD leverage discovery mechanisms. 4.4.1. HTTP Capability Advertisement Sites MAY advertise contract capabilities during standard HTTP interactions by including a VDAC-Advertise header in responses: VDAC-Advertise: offer="https://site.domain/.well-known/vdac-offer", types="ai-training, retrieval", index="https://site.domain/.well-known/vdac-offer-index" 4.4.2. DNS-Based Discovery Agents wishing to evaluate a domain's VDAC enforcement posture before initiating application-layer handshakes MAY query the zone's DNS records. The VDAC pointer is constructed as a TXT record located at a dedicated subdomain string: _vdac.site.domain. IN TXT "v=vdac1; index=https://site.domain/.well-known/vdac-offer-index" 5. Agent Acceptance Document 5.1. Acceptance Format An Agent that wishes to accept a Site Offer constructs an Acceptance Document: { "contract_id": "Z21h...[Truncated Base64URL]", "offer_hash": "SHA256Val...[Truncated Base64URL]", "agent": { "agent_id": "id=master-agent-identity", "id_method": "saip", "pubkey": "aF912...[Truncated Base64URL]", "vendor": "vendor-domain.com", "delegation_allowed": true }, "accepted_at": 1779370000, "expires_at": 1795132800, "agent_sig": "SigVal...[Truncated Base64URL]" } Field semantics: contract_id The Contract Identifier derived per Section 6.2. The Agent computes this value at acceptance time. offer_hash SHA-256 hash of the canonicalized Offer Document. This binds the Acceptance to a specific version of the Offer. agent.agent_id The Agent's identity string, as established by the chosen identity mechanism (Section 1.3). For SAIP, this is the id= value per [SAIP]. agent.id_method The identity mechanism in use. Defined values: "saip" (SAIP identity), "vicdm-dns" (DNS-based verification per [VICDM] Section 5), or an implementation-defined string for other mechanisms. agent.pubkey The Agent's public key. MUST match the key discoverable through the identity mechanism named in id_method (for SAIP, the key in the Agent's DNS _saip record; for vicdm-dns, the key published per [VICDM] Section 5). agent.vendor The DNS domain of the vendor operating the Agent. agent.delegation_allowed A boolean flag declaring whether the contract intends to utilize distributed worker nodes authenticated via VICDM delegation structures. accepted_at Unix timestamp at which the Acceptance was constructed. MUST be within the Offer's validity period. expires_at Unix timestamp at which the resulting Contract expires. MUST NOT be later than the Offer's valid_until. agent_sig Ed25519 signature over the canonicalized acceptance object excluding the signature field. 5.2. Submission and Server Response The Agent submits the Acceptance Document to the Site via HTTP POST: POST https:///.well-known/vdac-accept Content-Type: application/json The request body is the Acceptance Document. If validation succeeds, the Site returns the complete signed Contract Document (Section 6) with HTTP status 201 (Created). The same endpoint is used to submit a revision (Section 12.2). The request body is then an Offer Document carrying an in_response_to member and signed by the Agent. A revision does not form a Contract; the Site responds per Section 12. If validation fails, the Site returns HTTP status 400 (Bad Request) with structured error codes: `offer_not_found`, `offer_expired`, `expiry_exceeds`, `signature_invalid`, `identity_unverified`, `duplicate_contract`, or `revision_limit_exceeded`. expiry_exceeds indicates that the expires_at value in the Acceptance is later than the Offer's valid_until. revision_limit_exceeded indicates that the submitting party has already issued its one permitted revision (Section 12.3). 6. Contract Document 6.1. Contract Format The Contract Document is the canonical record of the bilateral agreement: { "contract_id": "DerivedID-Value", "offer": { ... complete Offer Document ... }, "acceptance": { ... complete Acceptance Document ... }, "prior_exchange": [ { "scheme": "aloi", "ref": "Base64URLValue..." }, { "scheme": "vdac-offer", "ref": "Base64URLValue..." } ], "site_sig": "SigVal...[Truncated Base64URL]", "agent_sig": "SigVal...[Truncated Base64URL]" } The site_sig and agent_sig fields are Ed25519 signatures computed over the canonicalized concatenation of the offer, acceptance, and prior_exchange members (in that order), per [RFC8785]. The prior_exchange member lists, in chronological order, every proposal that preceded and was superseded by the Offer contained in this Contract. Each entry has the same structure as the in_response_to member defined in Section 4.2. The list is empty when the Offer opened the exchange and was accepted without revision. Because prior_exchange is covered by both signatures, either party -- or a third party examining the Contract -- can verify that the revision limit of Section 12.3 was observed, without retaining the superseded documents themselves. 6.2. Contract Identifier Derivation The contract_id is deterministically derived to allow either party to compute it independently: contract_id = Base64URL(SHA256( offer_hash || agent.agent_id || accepted_at )) Where || denotes concatenation of the UTF-8 octets of each value, with a single ASCII null byte (0x00) inserted between consecutive values as a strict delimiter. offer_hash, rather than offer_id, is used so that two Offers sharing an offer_id but differing in terms cannot produce the same Contract Identifier. 6.3. Contract Storage Obligations Both parties MUST retain identical copies of the Contract Document until the contract's expires_at timestamp (Section 5.1) has passed, plus the log retention period (Section 8.4). The Contract Document contains the complete Offer and Acceptance, and therefore the full text of the agreed terms. No party needs to resolve any external source to determine what was agreed. Where a term references material outside the Contract -- through custom_terms_uri or grant_ref -- the Contract retains the reference and its digest, so that the integrity of the referenced material remains verifiable even if that material later changes or becomes unavailable. 6.4. Delegated Contracting and Infrastructure Scale For large-scale, enterprise, and distributed systems where scraping or API consumption is handled by clustered pools of machines, executing unique contracts per ephemeral node is unfeasible. VDAC addresses this by integrating with the [VICDM] delegation mechanism. If `agent.delegation_allowed` is true in the signed contract, application requests operating under the contract_id MAY be signed by distinct, ephemeral operational public keys. The Agent node MUST include the matching umbrella corporate `Delegation Token` within the request protocol metadata, proving that the ephemeral node's key is a legitimate downstream delegate of the contract's `agent.pubkey`. 7. Per-Request Contract Reference Once a Contract is established, the Agent includes a contract reference in every signed request to the Site: contract-id=; contract-hash= Where contract_hash is the SHA-256 hash of the canonicalized Contract Document, encoded as Base64URL. The contract-id and contract-hash parameters MUST be covered by the request signature produced by the Agent's identity mechanism, so that each request is cryptographically bound to a specific contract version. When SAIP is the identity mechanism, these parameters MUST be included in the SAIP canonical signature string per [SAIP] Section 6.4.2. When another identity mechanism is used, the parameters MUST be covered by that mechanism's request signature in an equivalent manner. 8. Mutual Tracking Obligation VDAC requires mutual logging by both parties. Tracking is an integral part of the contract, not an optional feature. In both log entry formats defined below, ts is the request timestamp asserted by the Agent and covered by the request signature of the identity mechanism in use. It is not the local clock reading of either party. Both parties therefore record the same value for the same request, and reconciliation (Section 9) is not sensitive to clock skew between them. 8.1. Site-Side Log Entry Format { "contract_id": "DerivedID-Value", "ts": 1779371050, "endpoint": "/articles/archived/news-item", "method": "GET", "bytes_sent": 45120, "status_code": 200, "agent_sig": "SigVal...[Truncated Base64URL]", "site_log_sig": "SigVal...[Truncated Base64URL]" } 8.2. Agent-Side Log Entry Format { "contract_id": "DerivedID-Value", "ts": 1779371050, "endpoint": "/articles/archived/news-item", "method": "GET", "agent_sig": "SigVal...[Truncated Base64URL]", "response_hash": "SHA256Val...[Truncated Base64URL]", "bytes_received": 45120, "agent_log_sig": "SigVal...[Truncated Base64URL]" } agent_sig is the request signature the Agent produced for the request, as recorded by both parties. It is the value on which reconciliation is computed (Section 9.1). Because it is produced by the Agent and verified by the Site, it is the only per-request value both parties are guaranteed to hold identically. 8.3. End-User Privacy Boundaries This section addresses a critical constraint: VDAC tracking MUST NOT become a vector for surveillance of end-users whose interactions are mediated by a contracted Agent. The following constraints apply to all VDAC log entries: 1. End-user identifying information MUST NOT be included in VDAC log entries. This includes, but is not limited to: end-user IP addresses, user-agent strings of human users whose interaction is proxied by the Agent, end-user authentication tokens, session identifiers, and personally identifiable URL parameters. 2. The endpoint field MUST be normalized to exclude query parameters that could identify end-users. Implementations MUST log only the path component, or a redacted form of the query string where retention of certain parameters is operationally necessary. 3. Response content MUST NOT be logged in raw form. Only a hash (response_hash) is logged. The hash provides verification capability without retaining the content itself. 8.4. Log Retention Both parties retain logs for the duration specified in the contract terms. The RECOMMENDED minimum retention is 90 days from the request timestamp. Logs older than the retention period SHOULD be securely deleted. 9. Reconciliation and Dispute Evidence Periodic reconciliation between Site and Agent logs is RECOMMENDED. To facilitate rapid, automated cross-checking without mass data transfer, parties utilize a structural summary document. 9.1. Reconciliation Manifest Format When performing an audit checkpoint, a party compiles state records into a canonical Manifest object: { "contract_id": "DerivedID-Value", "period_start": 1779369600, "period_end": 1779456000, "total_requests": 14500, "total_bytes": 48291045, "log_summary_hash": "SHA256Val...[Truncated Base64URL]", "manifest_sig": "SigVal...[Truncated Base64URL]" } The log_summary_hash is derived as follows: 1. Select every log entry under this contract_id whose ts falls in the interval [period_start, period_end). ts is the Agent-asserted request timestamp defined in Section 8, so both parties select the same set. 2. Take the agent_sig value of each selected entry, as the Base64URL string recorded in the log. 3. Sort the values in ascending order by the octet values of those strings. Sorting on the signature rather than on the timestamp is deliberate: it makes the ordering independent of any clock and total even when several requests carry the same ts. 4. Concatenate the sorted values, each followed by a single line feed octet (0x0A), including after the last. 5. log_summary_hash = Base64URL(SHA256(that octet sequence)). total_requests MUST equal the number of entries selected in step 1. If both local calculations match the log_summary_hash, the historical block is mutually verified. If hashes diverge, systems enter detailed reconciliation by repeating the procedure over narrower sub-intervals of the period to isolate the precise mismatched request signatures. 10. Violation Detection and Sanctions 10.1. Violation Notice Format { "contract_id": "DerivedID-Value", "violation": "rate_limit_exceeded", "evidence_ref": "ManifestHashOrLogID", "detected_at": 1779372000, "site_sig": "SigVal..." } The notice MAY be delivered via HTTP response header on the offending request: VDAC-Violation: 10.2. Sanction Progression Sanctions for VDAC violations SHOULD follow the same progressive model defined for ALOI in [VICDM] Section 6.5.7: +=================+============+================================+ | Violation Count | Sanction | Action | +=================+============+================================+ | 1 | Warning | Notice only; no rate change. | +-----------------+------------+--------------------------------+ | 2-3 | Throttle | Reduce to minimal rate. | +-----------------+------------+--------------------------------+ | 4-5 | Downgrade | Trust downgrade. | +-----------------+------------+--------------------------------+ | 6-10 | Block | Reject contract requests. | +-----------------+------------+--------------------------------+ | >10 | Termination| Terminate contract per | | | | Section 11. | +-----------------+------------+--------------------------------+ 10.3. Self-Reporting (Good Faith Mechanism) An Agent that detects an unintentional violation of contract terms MAY self-report the violation before the Site detects it, per the Good Faith Mechanism defined in [VICDM] Section 6.5.8. Self-reporting under VDAC uses the same signal format as ALOI self-reporting: VDAC-Self-Report: contract-id=; violation=; ts=; self-sig= 11. Contract Termination 11.1. Termination Conditions A VDAC contract terminates in any of the following conditions: o Natural expiry: reaching the contract's expires_at timestamp. o Mutual termination: both parties signing a termination notice. o Unilateral termination by either party, with signed notice. o Material breach: severe violation as defined in contract terms. 11.2. Termination Notice Format { "contract_id": "DerivedID-Value", "terminated_by": "site", "reason": "material_breach", "effective_at": 1779373000, "evidence_ref": "ViolationNoticeHash", "terminator_sig": "SigVal..." } Defined reason codes: `natural_expiry`, `mutual_consent`, `site_initiated`, `agent_initiated`, `material_breach`, or `offer_revoked`. 11.3. Post-Termination Obligations After termination, the Agent MUST cease using the contract_id in requests. Both parties retain logged evidence for the contract's log retention period. The Agent reverts to its prior interaction mode (Anonymous, Identified, or Intent-Declaring per the VICDM progression). 12. Relationship to ALOI and Contract Formation ALOI and VDAC are complementary. ALOI, defined in [VICDM] Section 6.5, is a unilateral declaration by the Agent. VDAC is a bilateral agreement in which both parties are bound. An Agent MAY operate under both simultaneously. ALOI defines the Agent's general operational scope across all interactions; VDAC defines specific terms for one particular Site. When in conflict, the more specific commitment governs: VDAC terms supersede ALOI declarations for requests made under the contract, while ALOI continues to apply outside any VDAC contract. 12.1. How a Contract Is Formed A VDAC Contract is always the same object: an Offer and an Acceptance, each signed by its author, together constituting a dual-signed agreement (Section 6). Every Contract is formed by the same final two acts: the Site issues an Offer, and the Agent signs an Acceptance of that Offer. This is invariant. It follows from the structure of the Contract Document itself, which contains exactly one Offer and exactly one Acceptance. What may differ is what precedes those two acts. Two entry points are defined: Entry 1 -- Site-initiated. The Site publishes one or more Offers (Section 4). The Agent selects an Offer and either accepts it (Section 5) or answers with a revision (Section 12.3). Entry 2 -- Agent-initiated, via ALOI. The Agent has published an ALOI: a declaration of what it intends to do, per [VICDM] Section 6.5. A Site observing that ALOI MAY respond with an Offer that identifies the ALOI in its in_response_to member. An ALOI is not an offer of access. It states what the Agent intends to do; it does not and cannot state what the Site will provide. A Site that finds an ALOI acceptable expresses that by issuing an Offer whose terms adopt the ALOI's declared boundaries; a Site that finds it partly acceptable issues an Offer whose terms adjust them. Either way, the Site states its own side of the arrangement for the first time, and no agreement exists until the Agent signs an Acceptance in return. A Site MAY also decline, by issuing a signed refusal, or remain silent. Silence forms no agreement and MUST NOT be interpreted as acceptance. In both cases the ALOI remains in force as a unilateral declaration under [VICDM]. Both entry points converge on an identical Contract Document. Neither forms a Contract by the action of one party alone. 12.2. Sequence of an Exchange The following sequences are the longest permitted under the revision limit of Section 12.3. Shorter sequences are the common case. Entry 1 -- Site-initiated: Site Offer opening Agent Offer (in_response_to) Agent's revision Site Offer (in_response_to) Site's revision Agent Acceptance Contract formed Entry 2 -- Agent-initiated: Agent ALOI opening [VICDM] Site Offer (in_response_to: aloi) Site's first terms Agent Offer (in_response_to) Agent's revision Site Offer (in_response_to) Site's revision Agent Acceptance Contract formed In Entry 2 the Site's first Offer is not a revision. There was no Offer to revise; an ALOI states intent, not access terms. The Site is stating access terms for the first time, whether or not it adopts the Agent's declared boundaries unchanged. A revision issued by the Agent has the Offer Document structure (Section 4.2) and is signed by the Agent rather than the Site. It is a proposal, not an Offer the Agent could publish: an Agent does not publish Offers, and such a document has no meaning outside the exchange that produced it. The Site answers it either by issuing an Offer of its own -- adopting the Agent's terms unchanged, or adjusting them once -- or by refusing. Submission of Offers and revisions between the parties uses the endpoint defined in Section 5.2. 12.3. Revision Limit Each party MAY revise at most once in a given exchange. A revision is a proposal that modifies terms previously proposed by the other party. An opening proposal is not a revision: neither an ALOI, nor a Site's first Offer, modifies anything. A party that has already issued one revision MUST respond to any subsequent proposal with either an Acceptance or a refusal. It MUST NOT issue a further revision under this protocol. A party receiving a further revision from a party that has exhausted its allowance MUST treat the exchange as terminated and MUST NOT form a Contract from it. The prior_exchange member of the Contract (Section 6.1) records the sequence, so that the limit is verifiable after the fact rather than merely asserted. This limit is deliberate: o It captures the common real-world case, in which one party accepts the substance of a proposal but adjusts one or two terms. o It gives each party the same allowance regardless of who opened. A party is not penalized for having made the first move. o It avoids an unbounded exchange, which would introduce open-ended state, replay surface, and a denial-of-service vector, and would exceed the intent of a contract-sealing mechanism. Multi-round negotiation, where genuinely required, takes place out of band, outside this protocol; VDAC is then used only to seal the agreement ultimately reached. 13. Use Case Profiles (Informative) 13.1. AI Training Data Access An AI model vendor wishes to access bulk content from a publisher Site for training purposes. The structural `rate_limit` block maps a macro configuration (e.g., aggregate daily byte caps, low multi-concurrency connections) alongside obligations like `no_resell` and explicit content path exclusions. 13.2. Content Retrieval and Inference Access A retrieval-augmented AI assistant accesses individual articles in real time to answer user queries. Configuration profiles employ lower window boundaries, higher burst thresholds, and shorter termination spans. Under this profile, the privacy constraints of Section 8.3 are strictly critical. 13.3. Monitoring and Health-Check Access A third-party monitoring service accesses Site endpoints to verify availability. The profile establishes lightweight logging structures with simple token structures and predictable heartbeat windows. 14. Security Considerations 14.1. Signature Forgery The integrity of VDAC depends entirely on the security of the Ed25519 signature scheme [RFC8032] and the secrecy of private keys. Agent private keys SHOULD be protected by hardware-backed key storage (TPM, HSM, or Secure Enclave). When SAIP is the identity mechanism, Agent keys are protected per [SAIP] key protection requirements. 14.2. Replay of Acceptance Resubmission of the same Acceptance Document produces an identical contract_id (per Section 6.2). The Site MUST detect duplicate contract_id values and reject duplicate Acceptance submissions. 14.3. Offer Substitution The offer_hash field in the Acceptance Document binds the Acceptance to a specific Offer version. A Site cannot substitute different terms after Acceptance without invalidating agent_sig. 14.4. Exhaustion of the Exchange The revision limit of Section 12.3 bounds an exchange to a fixed number of documents, which limits the state a party must retain for an exchange that has not yet produced a Contract. A party that receives proposals beyond the limit MUST terminate the exchange rather than continue it, and SHOULD apply ordinary rate limiting to repeated exchange attempts from the same counterparty. 14.5. Integrity of External References A term may reference material outside the Contract through custom_terms_uri or grant_ref. In both cases the Contract retains a digest of the referenced representation, so that substitution of that material after signature is detectable. A party that cannot retrieve the material, or that computes a different digest, MUST treat the referenced terms as unavailable rather than assume the current representation is the agreed one. 15. Privacy Considerations See Section 8.3 (End-User Privacy Boundaries) for the core privacy requirements applicable to VDAC tracking. Because reconciliation logs map hashes and paths rather than raw states, they protect end-user activity while preserving behavioral audit trails. 16. IANA Considerations This document requests IANA to register the following well-known URIs per [RFC8615]: `vdac-offer`, `vdac-offer-index`, `vdac-accept`, and `vdac-contract`. This document requests IANA to register the following HTTP header fields per [RFC3864]: `VDAC-Violation` and `VDAC-Self-Report`. This document requests IANA to create the VDAC Termination Reason Codes registry. This document requests IANA to create the VDAC Grant Reference Types registry. Registry name: VDAC Grant Reference Types Registration policy: Specification Required Fields per entry: Type token; Resolution procedure (a normative reference to the specification defining how a reference of this type is resolved and verified); Change controller; Reference. The resolution procedure for a type token is defined by the specification named in its registry entry. This document defines the extension point and the integrity of the reference; it does not define any resolution procedure and does not privilege any registered token over another. Initial contents: +====================+========================================+ | Type token | Resolution procedure | +====================+========================================+ | verifiable-grant | draft-wallace-aipref-grant-binding | +--------------------+----------------------------------------+ An implementation encountering an unregistered type token, or a registered token whose procedure it does not implement, MUST treat the grant as unresolvable. It MUST NOT infer a resolution procedure from the uri member. 17. References 17.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, May 2017. [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, January 2017. [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006. [RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers (URIs)", RFC 8615, May 2019. [RFC8785] Rundgren, A., "JSON Canonicalization Scheme (JCS)", RFC 8785, June 2020. [RFC9530] Polli, R. and L. Pardue, "Digest Fields", RFC 9530, February 2024. [RFC3864] Klyne, G., "Registration Procedures for Message Header Fields", RFC 3864, September 2004. [VICDM] Jovancevic, S., "Verifiable Identity Claims and Delegation Model (VICDM)", Work in Progress, Internet- Draft, draft-jovancevic-vicdm-09, August 2026. [SAIP] Jovancevic, S., "SAIP: Signed Agent Identity Protocol", Work in Progress, Internet-Draft, draft-jovancevic-saip-08, April 2026. 17.2. Informative References [WEBBOTAUTH-ARCH] Meunier, T. and S. Major, "HTTP Message Signatures for automated traffic Architecture", Work in Progress, Internet-Draft, draft-meunier-web-bot-auth-architecture, March 2026. Appendix A. Document History The Verifiable Data Access Contract (VDAC) was originally defined in Section 6.6 of draft-jovancevic-vicdm-05. It was extracted into this standalone specification to enable focused review and independent versioning. Changes in draft-jovancevic-vdac-03: o Corrected contract formation. An ALOI is a declaration of the Agent's intent, not an offer of access; a Site's acceptance of an ALOI therefore cannot by itself constitute a Contract, because the Site's own terms have not been stated. Section 12 now defines two entry points, both converging on the same final acts: a Site Offer and an Agent Acceptance. The ALOI is an entry point, not a parallel path to a Contract. o Replaced the single-round counter-offer rule with a revision limit stated per party: each party may revise at most once, and an opening proposal is not a revision. This gives both parties the same allowance regardless of who opened. o Moved in_response_to from Section 12.3 into the Offer Document (Section 4.2), and defined it as an opaque reference carrying a scheme name. VDAC records which proposal was answered without parsing it. o Added prior_exchange to the Contract Document (Section 6.1), so that the revision limit is verifiable rather than merely asserted. o Added the OPTIONAL terms.grant_ref member (Section 4.2) and the VDAC Grant Reference Types registry (Section 16). The normative resolution procedure for a type token lives in its registry entry, not in this document. Added [RFC9530] for the grant_ref digest; hashes internal to VDAC's own operations are unchanged. o Changed the contract_id derivation (Section 6.2) to use offer_hash in place of offer_id, so that two Offers sharing an offer_id but differing in terms cannot produce the same Contract Identifier. o Corrected Section 6.3, which required retention "for the duration specified in terms.duration"; no such field exists. Retention is now tied to expires_at. o Corrected the Section 5.2 error code duration_exceeds, which named the same non-existent field; added revision_limit_exceeded. o Added a field semantics entry for terms.custom_terms_hash, which was previously described only inside the entry for custom_terms_uri, and made its encoding consistent with every other hash in the document. o Defined the meaning of ts in log entries (Section 8) as the Agent-asserted request timestamp, so that both parties record the same value. o Added agent_sig to the Agent-side log entry (Section 8.2). Without it the two log formats shared no per-request value and the reconciliation of Section 9 could not be computed. o Defined the derivation of log_summary_hash (Section 9.1), which was previously named but never specified. o Added Sections 14.4 and 14.5 to Security Considerations, covering exhaustion of an exchange and the integrity of external references. o Added [RFC4648], which the document had relied on throughout without citing; updated [VICDM] to draft-jovancevic-vicdm-09. Changes in draft-jovancevic-vdac-02: o Added notarization-and-safeguarding framing to the Introduction: VDAC notarizes (seals an already-reached agreement) and then safeguards it (binds it to every subsequent interaction). o Added a policy-source-agnostic paragraph to Section 3.5, stating that contract terms MAY incorporate a resolved policy outcome from any external system (AIPREF, robots.txt, internal policy) and that VDAC carries such terms without parsing them. No source-specific machinery is required. o Expanded Section 12 into "Relationship to ALOI and Contract Formation": added Section 12.1 (Three Paths to a Contract -- Site-initiated, Agent-initiated via ALOI, and negotiated), Section 12.2 (the ALOI-to-VDAC transition with three explicit outcomes), and Section 12.3 (single-round counter-offer). o Editorial: corrected line wrapping in Section 12. Changes in draft-jovancevic-vdac-01: o Made the identity layer mechanism-independent. SAIP is now RECOMMENDED rather than mandatory. Section 1.3 rewritten to permit SAIP, VICDM DNS-based verification, or any equivalent identity mechanism providing a verifiable Agent public key. o Added Section 1.3.1 (Identity Mechanism Requirements) defining the three properties any identity mechanism must satisfy to serve as the VDAC Identity layer, and stating explicitly that no identity system is privileged by the protocol. o Reorganized the Section 1.2 architectural stack table by functional layer rather than by protocol, showing that the Identity layer can be satisfied by SAIP or by VICDM DNS-based verification. o Renamed the agent.saip_id field to agent.agent_id and added an agent.id_method field naming the identity mechanism in use. o Section 7 (Per-Request Contract Reference) generalized so that contract parameters are covered by whatever request signature the chosen identity mechanism provides. o Editorial: corrected line wrapping in Sections 3.4, 14, 15. Changes relative to VICDM Section 6.6 (in draft -00): o Expanded Privacy Considerations with explicit Section 8.3 boundaries. o Added Section 4.4 Capability Discovery architectures (HTTP/DNS). o Formalized structural standard fields for rate_limit JSON. o Introduced Section 6.4 Enterprise Infrastructure Key Delegation. o Added Section 9.1 Fast Reconciliation Manifest hashes. Author's Address Srecko Jovancevic SKGO, IKT Support Makedonska 22 11000 Belgrade Serbia Email: srecko.jovancevic@skgo.org Email: srecko.jovancevic@gmail.com URI: https://github.com/sreckojovancevic