Issuance and Presentation Exchange Protocol

Terminology

Presentation Exchange: An exchange that provides disclosure of one or more ACDCs between a Discloser and a Disclosee.

A presentation exchange is the process by which authenticatable information may be exchanged between two parties, namely, the Discloser and Disclosee.

ACDC: Type of data as issuance concretely defined by the ACDC specification .
Discloser: An ACDC in a disclosure is disclosed by the Discloser.
Disclosee: An ACDC in a disclosure is disclosed to the Disclosee.
Issuer: An ACDC is issued by the Issuer. The Issuer identifier (AID) appears in the top level of the ACDC.
Issuee: An ACDC is optionally issued to the Issuee. When present, the Issuee identifier (AID) appears at the top level of the attribute section or in the attribute list at the top level of the attribute aggregate section of the ACDC.

Each ACDC MUST have an Issuer and MAY have an Issuee. The set of ACDCs so disclosed in a presentation exchange MUST be chained. This set of chained ACDCs define a directed acyclic graph (DAG) that MUST have at least one vertex and MAY have zero or more edges pointing to other vertices. Each ACDC itself defines a graph fragment consisting of one vertex and zero or more directed edges. Each directed edge contained in an ACDC points to a vertex contained in another ACDC. The ACDC that contains the origin vertex of the DAG is called the origin or primary ACDC of the presentation exchange. The disclosure performed by a presentation exchange MAY be graduated and MAY be contractually protected.

Issuance Exchange: A special case of a presentation exchange where the Discloser is the Issuer of the origin (Primary) ACDC of the DAG formed by the set of chained ACDCs so disclosed.

In an issuance exchange, when the origin ACDC has an Issuee, the Disclosee MAY also be the origin ACDC's Issuee.

Chain-Link Confidentiality Disclosures via Presentations Exchanges may be contractually protected by Chain-Link Confidentiality (i.e a Chain-Link Confidential disclosure). The chaining in this case is different from the chaining described above between Issuances in a DAG of chained Issuances. Chain-link confidentiality, in contrast, chains together a sequence of Disclosees. Each Disclosee in the sequence in turn is the Discloser to the next Disclosee. The terms-of-use of the original disclosure as applied to the original Disclosee MUST be applied by each subsequent Discloser to each subsequent Disclosee via each of the subsequent disclosures (presentation exchanges). These terms-of-use typically constrain disclosure to only approved parties, i.e. imbue the chain of disclosures with some degree of confidentiality. These terms-of-use are meant to contractually protect the data rights of the original Issuer or Issuee of the data being disclosed.

Exchange Protocol

Discloser	Disclosee	Initiate	Contents	Description
	`apply`	Y	schema or its SAID, attribute field label list, signature on `apply` or its SAID	schema SAID is type of ACDC, optional label list for selective disclosure, CESR-Proof signature
`spurn`		N		rejects `apply`
`offer`		Y	metadata ACDC or its SAID, signature on `offer` or its SAID	includes schema or its SAID, other partial disclosures, selective disclosure label list, CESR-Proof signature
	`spurn`	N		rejects `offer`
	`agree`	N	signature on `offer` or its SAID	CESR-Proof signature
`spurn`		N		rejects `agree`
`grant`		Y	full or selective disclosure ACDC, signature on `grant` or its SAID	includes attribute values, CESR-Proof signature
	`admit`	N	signature on `grant` or its SAID	CESR-Proof signature

Discussion All the variants of an ACDC are various degrees of expansion of the compact variant. Therefore, an Issuer commitment via a signature to any variant of ACDC (compact, full, etc) makes a cryptographic commitment to the top-level section fields shared by all variants of that ACDC because the value of a top level section field is either the SAD or the SAID of the SAD of the associated section. Both a SAD and its SAID, when signed, each provide a verifiable commitment to the SAD. In the former the signature verification is directly agains the SAD itself. In the latter, the SAID as digest must first be verified against its SAD and then the signature on the SAID may be verified. This indirect verifiablity assumes that the cryptographic strength of the SAID digest is equivalent to the cryptographic strength of the signature used to sign it. To clarify, because all variants share the same top level structure as the compact variant, then a signature on any variant may be used to verify the Issuer's committment to any other variant either directly or indirectly, in whole or in part on a top-level section by top-level section basis. This cross-variant Issuer commitment verifiability is an essential property that supports graduated disclosure by the Disclosee of any or all variants wether it be full, compact, metadata, partial, selective, bulk issued, or contractually protected. To elaborate, the SAID of a given variant is useful even when it is not the SAID of the variant the Issuer signed because during graduated disclosure the Discloser MAY choose to sign that given variant to fullfill a given step in an IPEX graduated disclosure transaction. The Discloser thereby can make a verifiable disclosure in a given step of the SAD of a given variant that fulfills a commitment made in a prior step via its signature on merely the SAID of the SAD of the variant so disclosed. For example, the Metadata variant of an ACDC will have a different SAID than the Compact variant because some of the top-level field values may be empty in the Metadata variant. One can think of the The metadata variant as a partial manifest that only includes those top level sections that the Discloser is committing to disclose in order to induce the Disclosee to agree to the contractual terms of use when disclosed. The IPEX transaction is between the Discloser and Disclosee, who both may make non-repudiable commitments via signing to each other. Typically this means that the Discloser will eventually need to fulfull its commitment with a proof of disclosure to the Disclosee. This proof may be satisfied with either directly against the Discloser's signature on the the actual disclosed SAD or indirectly agaisnt the Discloser's signature on the SAID of the actual disclosed SAD. In addition, the Disclosee will typically require a proof of issuance via a non-repudiable signature by the Issuer on a variant of the disclosed SAD that is verifiable (directly or indirectly) against the variant that is the disclosed SAD. To summarize, when the Issuer commits to the composed schema of an ACDC it is committing to all the variants so composed. As described above, the top level field values in the compact variant enable verification against a disclosure of any of the other Issuer committed variants because they all share the same top level structure. This applies even to the metadata variant in spite of it only providing values for some top level sections and not others. The verifiablity of a top level section is separable. Consequently, the IPEX protocol must specify how a validator does validation of any variant in a graduated disclosure. To restate there are two proofs that a Discloser must provide. The first is proof of issuance and the second is proof of disclosure. In the former, the Discloser provide the variant via its SAD that was actually signed (as SAD or SAID of SAD) by the Issuer in order for the Disclosee to verify authentic issuance via the signature on that variant. In the latter, the Discloser must disclose any other Issuer enabled (via schema composition) variants that the Discloser offered to disclose as part of the graduated disclosure process.

IPEX Validation The goal is to define a validation process (set of rules) that works for all variants of an ACDC and for all types of graduated disclosure of that ACDC. For example, in the bulk issuance of an ACDC, the Issuer only signs the blinded SAID of the SAD that is the Compact variant of the ACDC not the SAD itself. This enable a Discloser to make a proof of inclusion of the ACDC in a bulk issuance set by unblinding the signature on the blinded SAID without leaking correlation to anything but the blinded SAID itself. To clarify, the Disclosee can verify the signature on the SAID without to prove set inclusion with needing the disclosure of any other information about the ACDC. Issuer signing of the SAID not the SAD also has the side benefit of minimizing the computation of large numbers of bulk issued signatures.

Issuer Signing Rules The Issuer MUST provide a signature on the SAID of the most compact variant defined by the schema of the ACDC. When more than one variant is defined by the schema via the oneOf composition operator for any top-level field, the most compact variant MUST appear as the first entry in the oneOf list. When only one variant of each top-level field is defined by the schema, that variant is therefore by defintion the most compact variant. The different variants of an ACDC form a hash tree (using SAIDs) that is analogous to a Merkle Tree. Signing the top-level SAID of the compact version of the ACDC is equivalent to signing the Merkle Root of a Merkle Tree. Different variants of an ACDC (SADs with SAIDs) correspond to different paths through a Merkle tree. The process of verifying that a SAD via its SAID of a section is included in a schema authorized variant down from the top-level SAID is equivalent to a Merkle Tree proof of inclusion along a path in the Merkel Tree down from its Root. This allows a single signature to provide proof of Issuance of the presentation of any schema authorized variants of the ACDC. An Issuer MAY provide signatures of the SAIDS of other variants, as well as signatures of the SADs of other variants. Proof of issuance is provided by disclosing the SAID of the most compact variant and the signature by the Issuer on that SAID. Proof of disclosure is provided by disclosing the SAD of the most compact variant and then recursively disclosing the nested SADs of each of the top level sections of the most compact variant as needed for the promised disclosure. Thus for any disclosed variant of an ACDC, the Disclosee need only verify only one proof of issuance as defined above and may need to verify a different proof of disclosure for each disclosed variant as defined above.