]> Blockchain Commons

wolf@wolfmcnally.com

Blockchain Commons

christophera@lifewithalacrity.com

Applications and Real-Time Network Working Group Internet-Draft The envelope protocol specifies a structured format for hierarchical binary data focused on the ability to transmit it in a privacy-focused way. Envelopes are designed to facilitate "smart documents" and have a number of unique features including: easy representation of a variety of semantic structures, a built-in Merkle-like digest tree, deterministic representation using CBOR, and the ability for the holder of a document to selectively encrypt or elide specific parts of a document without invalidating the document structure including the digest tree, or any cryptographic signatures that rely on it. Discussion Venues Source for this draft and an issue tracker can be found at .

Introduction Gordian Envelope was designed with two key goals in mind: to be Structure-Ready, allowing for the reliable and interoperable storage of information; and to be Privacy-Ready, ensuring that transmission of that data can occur in a privacy-protecting manner.

• Structure-Ready. Gordian Envelope is designed as a Smart Document, meant to store information about a subject. More than that, it's a meta-document that can contain or refer to other documents. It can support multiple data structures, from simple hierarchies to labeled property graphs, semantic triples, and other forms of structured graphs. Though its fundamental structure is a tree, it can be used to create Directed Acyclic Graphs (DAGs) through references within or between Envelopes.
• Privacy-Ready. Gordian Envelope protects the privacy of its data through progressive trust, allowing for holders to minimally disclose information by using elision or encryption, and then to optionally increase that disclosure over time. The fact that a holder can control data revelation, not just an issuer, creates a new level of privacy for all stakeholders. The progressive trust in Gordian Envelopes is accomplished through hashing of all elements, which creates foundational support for cryptographic functions such as signing and encryption, without actually defining which cryptographic functions must be used.

The following architectural decisions support these goals:

• Structured Merkle Tree. A variant of the Merkle Tree structure is created by hashing the elements in the Envelope into a tree of digests. (In this "structured Merkle Tree", all nodes contain both semantic content and digests, rather than semantic content being limited to leaves.)
• Deterministic Representation. There is only one way to encode any semantic representation within a Gordian Envelope. This is accomplished through the use of Deterministic CBOR ("dCBOR") and the sorting of the Envelope's assertions into a lexicographic order. Any Envelope that doesn't follow these strict rules can be rejected; as a result, there's no need to worry about different people adding the assertions in a different order or at different times: if two Envelopes contain the same data, they will be encoded the same way.

Elision Support

• Elision of All Elements. Gordian Envelopes innately support elision for any part of its data, including subjects, predicates and objects of assertions, assertions as a whole, and envelopes as a whole.
• Elision, Encryption, and Compression. Elision can be used for a variety of purposes including redaction (removing information), encryption (enciphering information), and compression (removing duplicate information). (In addition to "compression by elision", envelopes also support binary-level compression.)
• Holder-initiated Elision. Elision can be performed by the Holder of a Gordian Envelope, not just the Issuer.
• Granular Holder Control. Elision can not only be performed by any Holder, but also for any data, allowing each entity to elide data as is appropriate for the management of their personal (or business) risk.
• Progressive Trust. The elision mechanics in Gordian Envelopes allow for progressive trust, where increasing amounts of data are revealed over time, and can be combined with encryption to escrow data to later be revealed.
• Consistent Hashing. Even when elided, encrypted, or compressed, digests for those parts of the Gordian Envelope remain the same.

Privacy Support

• Proof of Inclusion. As an alternative to presenting elided structures, proofs of inclusion can be included in top-level digests.
• Herd Privacy. Proofs of inclusion allow for herd privacy where all members of a class can share data such as a VC or DID without revealing individual information.
• Non-Correlation. Encrypted Gordian Envelope data can optionally be made less correlatable with the addition of salt.

Authentication Support

• Symmetric Key Permits. Gordian Envelopes can be locked ("closed") using a symmetric key.
• SSKR Permits. Gordian Envelopes can alternatively be locked ("closed") using a symmetric key sharded with Shamir's Secret Sharing, with the shares stored with copies of the Envelope, and the whole enveloped thus openable if copies of the Envelope with a quorum of different shares are gathered.
• Public Key Permits. Gordian Envelopes can alternatively be locked ("closed") with a public key and then be opened with the associated private key, or vice versa.
• Multiple Permits. Gordian Envelopes can simultaneously be locked ("closed") via a variety of means and then openable by any appropriate individual method, with different methods likely held by different people.

Future Looking

• Data Storage. The initial inspiration for Gordian Envelopes was for secure data storage.
• Credentials & Presentations. The usage of Gordian Envelope signing techniques allows for the creation of credentials and the ability to present them to different verifiers in different ways.
• Distributed or Decentralized Identifiers. Self-Certifying Identifiers (SCIDs) can be created and shared with peers, certified by a trust authority, or registered on blockchain.
• Future Techniques. Beyond its technical specifics, Gordian Envelopes still allows for cl-sigs, bbs+, and other privacy-preserving techniques such as zk-proofs, differential privacy, etc.
• Cryptography Agnostic. Generally, the Gordian Envelope architecture is cryptography agnostic, allowing it to work with everything from older algorithms with silicon support through more modern algorithms suited to blockchains and to future zk-proof or quantum-attack resistant cryptographic choices. These choices are made in sets via ciphersuites.

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 when, and only when, they appear in all capitals, as shown here. This specification makes use of the following terminology:

byte

Used in its now-customary sense as a synonym for "octet".

element

An envelope is a tree of elements, each of which is itself an envelope.

image

The source data from which a cryptographic digest is calculated.

Envelope Format Specification This section is normative, and specifies the binary format of envelopes in terms of its CBOR components and their sequencing. The formal language used is the Concise Data Definition Language (CDDL) . To be considered a well-formed envelope, a sequence of bytes MUST be well-formed deterministic CBOR and MUST conform to the specifications in this section.

Top Level An envelope is a tagged enumerated type with eight cases. Five of these cases have no children:

• leaf
• known-value
• encrypted
• elided
• compressed

Three of these cases, encrypted, elided, and compressed, "declare" their digest, i.e., they actually encode their digest in the envelope serialization. For all other cases, their digest is implicit in the data itself and may be computed and cached by implementations when an envelope is deserialized. The other three cases have one or more children:

• The node case has a child for its subject and an additional child for each of its assertions.
• The wrapped-envelope case has exactly one child: the envelope that has been wrapped.
• The assertion case has exactly two children: the predicate and the object.

Cases Without Children

Leaf Case Format A leaf case is used when the envelope contains only user-defined CBOR content. It is tagged using #6.24, per section 3.4.5.1, "Encoded CBOR Data Item". To preserve deterministic encoding, developers using the envelope format MUST specify where tags MUST or MUST NOT be used to identify the type of CBOR within leaf elements. In cases where simple CBOR values like numbers or UTF-8 strings are encoded, no additional tagging may be necessary because positionality within the envelope is sufficient to imply the type without ambiguity. For example, if a structure representing a person specifies that it MAY have a firstName predicate with a string object, there is no need for an additional tag within the object leaf element: it would be a coding error to place anything but a string in that position. But where developers are specifying a compound CBOR structure with a specified layout for inclusion in an envelope, especially one that may be used in a plurality of positions (for example a CBOR array of alias first names), they SHOULD specify a tag, and specify where it MUST or MUST NOT be used.

Known Value Case Format A known-value case is used to specify an unsigned integer in a namespace of well-known values. Known values are frequently used as predicates. For example, any envelope can be used as a predicate in an assertion, but many predicates are commonly used, e.g., verifiedBy for signatures; hence it is desirable to keep common predicates short.

Encrypted Case Format An encrypted case is used for an envelope that has been encrypted using an Authenticated Encryption with Associated Data (AEAD), and where the digest of the plaintext is declared by the encrypted structure's Additional Authenticated Data (AAD) field. This subsection specifies the construct used in the current reference implementation and is informative. For encrypted, the reference implementation uses the definition in "UR Type Definition for Secure Messages" and we repeat the salient specification here. This format specifies the use of "ChaCha20 and Poly1305 for IETF Protocols" as described in . When used with envelopes, the encrypted construct aad (additional authenticated data) field contains the digest of the plaintext, authenticating the declared digest using the Poly1305 MAC.

Compressed Case Format A compressed CBOR-encoded envelope. Implemented using the raw DEFLATE compression format. The following obtains the equivalent configuration of the encoder: If the payload is too small to compress using DEFLATE, the uncompressed payload is placed in the compressedData field and the length of that field MUST be the same as the uncompressedSize field. Due to fixed overhead, the compressed form of very small envelopes may be larger than their uncompressed form.

Elided Case Format An elided case is used as a placeholder for an element that has been elided and its digest, produced by a cryptographic hash algorithm, is left as a placeholder. For digest, the SHA-256 cryptographic hash function is used to generate a 32 byte digest.

Cases With Children

Node Case Format A node case is encoded as a CBOR array: indeed, it is the only envelope-content case that uses a bare array, and is therefore recognizable by its form. A node case MUST be used when one or more assertions are present on the envelope. It MUST NOT be present when there is not at least one assertion. The first element of the array is the envelope's subject, Followed by one or more assertion-elements, each of which MUST either be an assertion or an obscured-assertion, which is one of the encrypted, compressed, or elided transformations of that assertion. The assertion elements MUST appear in ascending lexicographic order by their digest. The array MUST NOT contain any assertion elements with identical digests. The assertion-element envelopes in the node case array MUST, when unelided/uncompressed/unencrypted be found to be actual assertion case envelopes, or it is a coding error.

Wrapped Envelope Case Format A wrapped-envelope case is used where an envelope, including all its assertions, should be treated as a single element, e.g. for the purpose of signing.

Assertion Case Format An assertion case is used for each of the assertions in an envelope. It is encoded as a CBOR array with exactly two elements in order:

1. the envelope representing the predicate of the assertion, followed by
2. the envelope representing the object of the assertion.

Computing the Digest Tree This section specifies how the digests for each of the envelope cases are computed, and is normative. The examples in this section may be used as test vectors. Each of the eight enumerated envelope cases produces an image which is used as input to a cryptographic hash function to produce a digest of its contents. The overall digest of an envelope is the digest of its specific case. In this and subsequent sections:

• digest(image) is the SHA-256 hash function that produces a 32-byte digest.
• The .digest attribute is the digest of the named element computed as specified herein.
• The || operator represents the concatenation of byte sequences.

Leaf Case Digest Calculation The leaf case consists of any CBOR object. The envelope image is the CBOR serialization of that object:

Example The CBOR serialization of the plaintext string "Hello" (not including the quotes) is 6548656C6C6F. The following command line calculates the SHA-256 sum of this sequence: Using the envelope command line tool , we create an envelope with this string as the subject and display the envelope's digest. The digest below matches the one above.

Known Value Case Digest Calculation The envelope image of the known-value case is the CBOR serialization of the unsigned integer value of the value tagged with #6.202, as specified in the Known Value Case Format section above.

Example The known value verifiedBy in CBOR diagnostic notation is 202(3), which in hex is D8CA03. The SHA-256 sum of this sequence is: Using the envelope command line tool , we create an envelope with this known value as the subject and display the envelope's digest. The digest below matches the one above.

Encrypted Case Digest Calculation The encrypted case declares its digest to be the digest of plaintext before encryption. The declaration is made using a MAC, and when decrypting an element, the implementation MUST compare the digest of the decrypted element to the declared digest and flag an error if they do not match.

Example If we create the envelope from the leaf example above, encrypt it, and then request its digest: ...we see that its digest is the same as its plaintext form:

Compressed Case Digest Calcultation The compressed case declares its digest to be the digest of the uncompressed envelope.

Example If we create the envelope from the leaf example above, compress it, and then request its digest: ...we see that its digest is the same as its uncompressed form:

Elided Case Digest Calculation The elided case declares its digest to be the digest of the envelope for which it is a placeholder.

Example If we create the envelope from the leaf example above, elide it, and then request its digest: ...we see that its digest is the same as its unelided form:

Node Case Digest Calculation The envelope image of the node case is the concatenation of the digest of its subject and the digests of its assertions sorted in ascending lexicographic order. With a node case, there MUST always be at least one assertion.

Example We create four separate envelopes and display their digests: We combine the envelopes into a single envelope with three assertions: Note that in the envelope notation representation above, the assertions are sorted alphabetically, with "knows": "Edward" coming last. But internally, the three assertions are ordered by digest in ascending lexicographic order, with "Carol" coming first because its digest starting with 4012caf2 is the lowest, as in the tree formatted display below: To replicate this, we make a list of digests, starting with the subject, and then each assertion's digest in ascending lexicographic order: We then calculate the SHA-256 digest of the concatenation of these four digests. Note that this is the same digest as the composite envelope's digest:

Wrapped Envelope Case Digest Calculation The envelope image of the wrapped-envelope case is the digest of the wrapped envelope:

Example As above, we note the digest of a leaf envelope is the digest of its CBOR: Now we note that the digest of a wrapped envelope is the digest of the wrapped envelope's digest:

Assertion Case Digest Calculation The envelope image of the assertion case is the concatenation of the digests of the assertion's predicate and object in that order:

Example We create an assertion from two separate envelopes and display their digests: To replicate this, we make a list of the predicate digest and the object digest, in that order: We then calculate the SHA-256 digest of the concatenation of these two digests. Note that this is the same digest as the composite envelope's digest:

Envelope Hierarchy This section is informative, and describes envelopes from the perspective of their hierarchical structure and the various ways they can be formatted. Notionally an envelope can be thought of as a subject and one or more predicate-object pairs called assertions: A concrete example of this might be: The notional concept of envelope is useful, but not technically accurate because envelope is implemented structurally as an enumerated type consisting of eight cases. This allows actual envelope instances to be more flexible, for example a "bare assertion" consisting of a predicate-object pair with no subject, which is useful in some situations: More common is the opposite case: a subject with no assertions: In the examples above, there are five distinct "positions" of elements, each of which is itself an envelope and which therefore produces its own digest:

1. envelope
2. subject
3. assertion
4. predicate
5. object

The examples above are printed in "envelope notation," which is designed to make the semantic content of envelopes human-readable, but it doesn't show the actual digests associated with each of the positions. To see the structure more completely, we can display every element of the envelope in Tree Notation: We can also show the digest tree graphically using Mermaid : For easy recognition, envelope trees and Mermaid diagrams only show the first four bytes of each digest, but internally all digests are 32 bytes. From the above envelope and its tree, we make the following observations:

• The envelope is a node case, which holds the overall envelope digest.
• The subject "Alice" has its own digest.
• Each of the three assertions has their own digests
• The predicate and object of each assertion each have their own digests.
• The assertions appear in the structure in ascending lexicographic order by digest, which is distinct from envelope notation, where they appear sorted alphabeticaly.

The following subsections present each of the eight enumerated envelope cases in five different output formats:

• Envelope Notation
• Envelope Tree
• Mermaid
• CBOR Diagnostic Notation
• CBOR hex

These examples may be used as test vectors. In addition, each subsection starts with the envelope command line needed to generate the envelope being formatted.

Leaf Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Known Value Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Encrypted Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Compressed Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Elided Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Node Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Wrapped Envelope Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Assertion Case

Envelope CLI Command Line

Envelope Notation

Tree

Mermaid

CBOR Diagnostic Notation

CBOR Hex

Known Values This section is informative. Known values are a specific case of an envelope that defines a namespace consisting of single unsigned integers. The expectation is that the most common and widely useful predicates will be assigned in this namespace, but known values may be used in any position in an envelope. Most of the examples in this document use UTF-8 strings as predicates, but in real-world applications, the same predicate may be used many times in a document and across a body of knowledge. Since the size of an envelope is proportionate to the size of its content, a predicate made using a string like a human-readable sentence or a URL could take up a great deal of space in a typical envelope. Even emplacing the digest of a known structure takes 32 bytes. Known values provide a way to compactly represent predicates and other common values in as few as three bytes. Other CBOR tags can be used to define completely separate namespaces if desired, but the reference implementation and its tools recognize specific known values and their human-readable names. Custom ontologies such as Web Ontology Language or Friend of a Friend may someday be represented as ranges of integers in this known space, or be defined in their own namespaces. A specification for a standard minimal ontology of known values is TBD. The following table lists all the known values currently defined in the reference implementation . This list is currently informative, but all these known values have been used in the reference implementation for various examples and test vectors. Note that a work-in-progress specification for remote procedure calls using envelope has been assigned a namespace starting at 100.

Value	Name	Used as	Description
1	id	predicate	A domain-unique identifier of some kind.
2	isA	predicate	A domain-specific type identifier.
3	verifiedBy	predicate	A signature on the digest of the subject, verifiable with the signer's public key.
4	note	predicate	A human-readable informative note.
5	hasRecipient	predicate	A sealed message encrypting to a specific recipient the ephemeral encryption key that was used to encrypt the subject.
6	sskrShare	predicate	A single SSKR share of the ephemeral encryption key that was used to encrypt the subject.
7	controller	predicate	A domain-unique identifier of the party that controls the contents of this document.
8	publicKeys	predicate	A "public key base" consisting of the information needed to encrypt messages to a party or verify messages signed by them.
9	dereferenceVia	predicate	A domain-unique Pointer such as a URL indicating from where the elided envelope subject can be recovered.
10	entity	predicate	A document representing an entity of interest in the current context.
11	hasName	predicate	The human-readable name of the subject.
12	language	predicate	The ISO 639 code for the human natural language used to write the subject.
13	issuer	predicate	A domain-unique identifier of the document's issuing entity.
14	holder	predicate	A domain-unique identifier of the document's holder, i.e., the entity to which the document pertains.
15	salt	predicate	A block of random data used to deliberately perturb the digest tree for the purpose of decorrelation.
16	date	predicate	A timestamp, e.g., the time at which a remote procedure call request was signed.
100	body	predicate	RPC: The body of a function call. The object is the function identifier and the assertions on the object are the function parameters.
101	result	predicate	RPC: A result of a successful function call. The object is the returned value.
102	error	predicate	RPC: A result of an unsuccessful function call. The object is a message or other diagnostic state.
103	ok	object	RPC: The object of a result predicate for a successful remote procedure call that has no other return value.
104	processing	object	RPC: The object of a result predicate where a function call is accepted for processing and has not yet produced a result or error.

Existence Proofs This section is informative. Because each element of an envelope provides a unique digest, and because changing an element in an envelope changes the digest of all elements upwards towards its root, the structure of an envelope is comparable to a merkle tree . In a Merkle Tree, all semantically significant information is carried by the tree's leaves (for example, the transactions in a block of Bitcoin transactions), while the internal nodes of the tree are nothing but digests computed from combinations of pairs of lower nodes, all the way up to the root of the tree (the "Merkle root".) In an envelope, every digest references some semantically significant content: it could reference the subject of the envelope, or one of the assertions in the envelope, or at the predicate or object of a given assertion. Of course, those elements are all envelopes themselves, and thus potentially the root of their own subtree. In a Merkle tree, the minimum subset of digests necessary to confirm that a specific leaf node (the "target") must be present is called a "Merkle proof." For envelopes, an analogous proof would be a transformation of the envelope that is entirely elided but preserves the structure necessary to reveal the target. As an example, we produce an envelope representing a simple FOAF style graph: We then elide the entire envelope, leaving only the root-level digest. This digest is a cryptographic commitment to the envelope's contents. A third party, having received this commitment, can then request proof that the envelope contains a particular assertion, called the target. The holder can then produce a proof, which is an elided form of the original document that contains a minimum spanning set of digests, including the target. Note that the proof:

1. has the same root digest as the commitment,
2. includes the digest of the knows-Bob assertion: 55560bdf,
3. includes only the other digests necessary to calculate the digest tree from the target back to the root, without revealing any additional information about the envelope.

Criteria 3 was met when the proof was produced. Criteria 1 and 2 are checked by the command line tool when confirming the proof:

Reference Implementation This section is informative. The current reference implementation of envelope is written in Swift and is part of the Blockchain Commons Secure Components Framework . The envelope command line tool is also written in Swift.

Future Proofing This section is informative. Because envelope is a specification for documents that may persist indefinitely, it is a design goal of this specification that later implementation versions are able to parse envelopes produced by earlier versions. Furthermore, later implementations should be able to compose new envelopes using older envelopes as components. The authors considered adding a version number to every envelope, but deemed this unnecessary as any code that parses later envelopes can determine what features are required from the CBOR structure alone. The general migration strategy is that the specific structure of envelopes defined in the first general release of this specification is the baseline, and later specifications may incrementally add structural features such as envelope cases, new tags, or support for new structures or algorithms, but are generally expected to maintain backward compatibility. An example of addition would be to add an additional supported method of encryption. The encrypted specification CDDL is a CBOR array with either three or four elements: For the sake of this example, we assume the new method to be supported has all the same fields but needs to be processed differently. In this case, the first element of the array could become an optional integer: If present, the first field specifies the later encryption method. If absent, the original encryption method is specified. For low-numbered versions, the storage cost of specifying a later version is one byte, and backward compatibility is preserved.

Commitment to the Hash Algorithm For changes that are more sweeping, like supporting a different hash algorithm to produce the merkle tree digests, it would be necessary to use a different top-level CBOR tag to represent the envelope itself. Currently the envelope tag is #6.200, and the choice of digest algorithm in our reference implementation is SHA-256. If this version were officially released and a future version of Gordian Envelope was also released that supported (for example) BLAKE3, it will need to have a different tag. However, a problem for interoperability of these two distinct formats then arises in the choice of whether a particular envelope is encoded assuming SHA-256 or BLAKE3. Whenever there is a choice about two or more ways to encode particular data, this violates the determinism requirement that Gordian Envelopes are designed to uphold. In other words, an envelope encoding certain information using SHA-256 will not, in general, be structurally identical to the same information encoded in an envelope using BLAKE3. For instance, they will both have different root digests, and simply knowing which algorithm produced each one will not help you know whether they have equivalent content. Three envelope cases actually encode their digest in the binary stream: ELIDED, COMPRESSED, and ENCRYPTED. If an envelope doesn't any of these cases, then you could choose to decode the envelope with either algorithm, but if it does use either of these cases then the envelope will still decode, but attempting to decrypt or unelide its contents will result in mismatched digests. This is why the envelope itself needs to declare the hashing algorithm used using its top-level CBOR tag, and why the choice of which hash algorithm to commit to should be carefully considered.

Security Considerations This section is informative unless noted otherwise.

Structural Considerations

CBOR Considerations Generally, this document inherits the security considerations of CBOR . Though CBOR has limited web usage, it has received strong usage in hardware, resulting in a mature specification.

Cryptographic Considerations

Inherited Considerations Generally, this document inherits the security considerations of the cryptographic constructs it uses such as IETF-ChaCha20-Poly1305 and SHA-256 .

Choice of Cryptographic Primitives (No Set Curve) Though envelope recommends the use of certain cryptographic algorithms, most are not required (with the exception of SHA-256 usage, noted below). In particular, envelope has no required curve. Different choices will obviously result in different security considerations.

Validation Requirements Unlike HTML, envelope is intended to be conservative in both what it sends and what it accepts. This means that receivers of envelope-based documents should carefully validate them. Any deviation from the validation requirements of this specification MUST result in the rejection of the entire envelope. Even after validation, envelope contents should be treated with due skepticism.

Signature Considerations This specification allows the signing of envelopes that are partially (or even entirely) elided. There may be use cases for this, such as when multiple users are each signing partially elided envelopes that will then be united. However, it's generally a dangerous practice. Our own tools require overrides to allow it. Other developers should take care to warn users of the dangers of signing elided envelopes.

Hashing

Choice of SHA-256 Hash Primitive Envelope uses the SHA-256 digest algorithm , which is regarded as reliable and widely supported by many implementations in both software and hardware.

Well-Known Digests Because they are short unsigned integers, well-known values produce well-known digests. Elided envelopes may, in some cases, inadvertently reveal information by transmitting digests that may be correlated to known information. Envelopes can be salted by adding assertions that contain random data to perturb the digest tree, hence decorrelating it from any known values.

Digest Trees Existence proofs include the minimal set of digests that are necessary to calculate the digest tree from the target to the root, but may themselves leak information about the contents of the envelope due to the other digests that must be included in the spanning set. Designers of envelope-based formats should anticipate such attacks and use decorrelation mechanisms like salting where necessary.

A Tree, Not a List Envelope makes use of a digest tree instead of a digest list to allow this sort of minimal revelation. This decision may also have advantages in scaling. However, there should be further investigation of the limitations of digest trees regarding scaling, particularly for the scaling of large, elided structures. There should also be careful consideration of the best practices needed for the creation of deeply nested envelopes, for the usage of sub-envelopes created at different times, and for other technical details related to the use of a potentially broad digest tree, as such best practices do not currently exist.

Salts Specifics for the size and usage of salt are not included in this specifications. There are also no requirements for whether salts should be revealed or can be elided. Careful attention may be required for these factors to ensure that they don't accidentally introduce vulnerabilities into usage.

Collisions Digest trees tend to make it harder to create collisions than the use of a raw hash function. If attackers manage to find a collision for a digest, they can only replace one node (and its children), so the impact is limited, especially since finding collisions higher in a digest tree grows increasingly difficult because the collision must be a concatenation of multiple digests. This should generally reduce issues with collisions: finding collisions that fit a digest tree tends to be harder than finding regular collisions. But, the issue should always be considered.

Leaf-Node Attacks Envelope's digest tree is proof against the leaf-node weakness of Bitcoin that can affect SPVs because its predicates are an unordered set, serialized in increasing lexicographic order by digest, with no possibility for duplication and thus fully deterministic ordering of the tree. See the leaf-node attack at .

Forgery Attacks on Unbalanced Trees Envelopes should be proof against a known forgery attack against Bitcoin because of their different construction, in which all tree nodes contain semantically important data and duplicate assertions are not allowed. See the forgery attack here: .

Elision

Duplication of Claims Support for elision allows for the possibility of contradictory claims where one is kept hidden at any time. So, for example, an envelope could contain contradictory predictions of election results and only reveal the one that matches the actual results. As a result, revealed material should be carefully assessed for this possibility when elided material also exists.

Additional Specification Creation Creators of specifications for envelope-based documents should give due consideration to security implications that are outside the scope of this specification to anticipate or avert. One example would be the number and type of assertions allowed in a particular document, and whether additional assertions (metadata) are allowed on those assertions.

IANA Considerations

CBOR Tags This section proposes a number of IANA allocated specific CBOR tags . This document requests that IANA reserve the assigned tags listed below for use by envelope and associated specifications.

code point	semantics
#6.200	envelope
#6.201	assertion
#6.202	known-value
#6.203	wrapped-envelope
#6.204	digest
#6.205	encrypted
#6.206	compressed

In addition, at this time work in progress is also using tags 207-212, 300-323, and 400-410. We may apply for some of these code points in the future. Points of contact:

• Christopher Allen christophera@blockchaincommons.com
• Wolf McNally wolf@wolfmcnally.com

Media Type The proposed media type for envelope is application/envelope+cbor.

• Type name: application
• Subtype name: envelope+cbor
• Required parameters: n/a
• Optional parameters: n/a
• Encoding considerations: binary
• Security considerations: See the previous section of this document
• Interoperability considerations: n/a
• Published specification: This document
• Applications that use this media type: None yet, but it is expected that this format will be deployed in protocols and applications.
• Additional information:
  • Magic number(s): n/a
  • File extension(s): .envelope
  • Macintosh file type code(s): n/a
• Person & email address to contact for further information:
  • Christopher Allen christophera@blockchaincommons.com
  • Wolf McNally wolf@wolfmcnally.com
• Intended usage: COMMON
• Restrictions on usage: none
• Author:
  • Wolf McNally wolf@wolfmcnally.com
• Change controller:
  • The IESG iesg@ietf.org

Appendix: Why CBOR? The Concise Binary Object Representation, or CBOR, was chosen as the foundational data structure envelopes for a variety of reasons. These include:

1. IETF Standardization. CBOR is a mature open international IETF standard .
2. IANA Registration. CBOR is further standardized by the registration of common data type tags through IANA .
3. Fully Extensible. Beyond that, CBOR is entirely extensible with any data types desired, such as our own listing of UR tags .
4. Self-describing Descriptions. CBOR-encoded data is self-describing, so there are no requirements for pre-defined schemas nor more complex descriptions such as those found in ASN.1 .
5. Constraint Friendly. CBOR is built to be frugal with CPU and memory, so it works well in constrained environments such as on cryptographic silicon chips.
6. Unambiguous Encoding. Our use of Deterministic CBOR, combined with our own specification rules, such as the sorting of Envelopes by hash, results in a singular, unambiguous encoding.
7. Multiple Implementations. Implementation are available in a variety of languages .
8. Compact Implementations. Compactness of encoding and decoding is one of CBOR's core goals; implementations are built on headers or snippets of code, and do not require any external tools.

Also see a comparison to Protocol Buffers , a comparison to Flatbuffers , and a comparison to other binary formats .

References Normative References The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. This document defines the ChaCha20 stream cipher as well as the use of the Poly1305 authenticator, both as stand-alone algorithms and as a "combined mode", or Authenticated Encryption with Associated Data (AEAD) algorithm. RFC 7539, the predecessor of this document, was meant to serve as a stable reference and an implementation guide. It was a product of the Crypto Forum Research Group (CFRG). This document merges the errata filed against RFC 7539 and adds a little text to the Security Considerations section. This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. This specification defines a lossless compressed data format that compresses data using a combination of the LZ77 algorithm and Huffman coding, with efficiency comparable to the best currently available general-purpose compression methods. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. n.d. n.d. Federal Information Processing Standard, FIPS n.d. n.d. n.d. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Acknowledgments TODO acknowledge.