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 * [TAP 12: Improving keyid flexibility](tap12.md)
 * [TAP 14: Managing TUF Versions](tap14.md)
 * [TAP 15: Succinct hashed bin delegations](tap15.md)
+* [TAP 16: Snapshot Merkle Trees](tap16.md)
 
 ## Rejected
 
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+* TAP: 16
+* Title: Snapshot Merkle Trees
+* Version: 0
+* Last-Modified: 22/01/2021
+* Author: Marina Moore, Justin Cappos
+* Type: Standardization
+* Status: Draft
+* Content-Type: markdown
+* Created: 14/09/2020
+* +TUF-Version:
+* +Post-History:
+
+# Abstract
+
+Snapshot metadata for repositories with a high number of targets
+metadata files (through significant use of delegations), can become
+prohibitively large. Due to the need to download the snapshot file on every
+update cycle, very large snapshot metadata files can become a significant
+overhead for TUF clients.
+
+This TAP proposes a method for reducing the size of snapshot metadata a client
+must download without significantly weakening the security properties of TUF.
+
+
+# Motivation
+
+For very large repositories, the snapshot metadata file could get very large.
+This snapshot metadata file must be downloaded on every update cycle, and so
+could significantly impact the metadata overhead. For example, if a repository
+has 50,000,000 targets metadata files, the snapshot metadata will be about
+380,000,000 bytes (https://docs.google.com/spreadsheets/d/18iwWnWvAAZ4In33EWJBgdAWVFE720B_z0eQlB4FpjNc/edit?ts=5ed7d6f4#gid=0).
+For this reason, it is necessary to create a more scalable solution for snapshot
+metadata that does not significantly impact the security properties of TUF.
+
+We designed a new approach to snapshot that improves scalability while
+achieving similar security properties to the existing snapshot metadata.
+Using this new approach, a repository with 50,000,000 targets metadata files
+would only require the user to download about 800 bytes of snapshot metadata (https://docs.google.com/spreadsheets/d/18iwWnWvAAZ4In33EWJBgdAWVFE720B_z0eQlB4FpjNc/edit?ts=5ed7d6f4#gid=924553486).
+
+
+# Rationale
+
+Snapshot metadata provides a consistent view of the repository in order to
+protect against mix-and-match attacks and rollback attacks. In order to provide
+these protections, snapshot metadata is responsible for keeping track of the
+version number of each targets metadata file, ensuring that all targets downloaded are
+from the same snapshot, and ensuring that no targets metadata file decreases its version
+number (except in the case of fast forward attack recovery). Any new solution
+we develop must provide these same protections.
+
+A snapshot Merkle tree manages version information for each targets metadata
+file by including this information in a leaf node for each targets metadata
+file. By using a Merkle tree to store these nodes,
+this proposal can cryptographically verify that different targets are from the
+same snapshot by ensuring that the Merkle tree roots match. Due to the
+properties of secure hash functions, any two leaves of a Merkle tree with the
+same root are from the same tree.
+
+In order to prevent rollback attacks between Merkle trees, this proposal
+introduces third-party auditors. These auditors are responsible for downloading
+all nodes of each Merkle tree to ensure that no version numbers have decreased
+between generated trees. This achieves rollback protection without every client
+having to store the version information for every targets metadata file.
+
+# Specification
+
+This proposal replaces the single snapshot metadata file with a snapshot Merkle
+metadata file for each targets metadata file. The repository generates these
+snapshot Merkle metadata files by building a Merkle tree using all targets
+metadata files and storing the path to each targets metadata file in the
+snapshot Merkle metadata. The root of this Merkle tree is stored in timestamp
+metadata to allow for client verification. The client uses the path stored in
+the snapshot Merkle metadata for a targets metadata file, along
+with the root of the Merkle tree, to ensure that metadata is from the given
+Merkle tree. The details of these files and procedures are described in
+this section.
+
+![Diagram of snapshot Merkle tree](merkletap-1.jpg)
+
+## Merkle tree generation
+
+When the repository generates snapshot metadata, instead of putting the version
+information for all targets metadata files into a single file, it instead uses the version
+information to generate a Merkle tree.  Each targets metadata file's version information forms
+a leaf of the tree, then these leaves are used to build a Merkle tree. The
+internal nodes of a Merkle tree contain the hash of their child nodes. The exact
+algorithm for generating this Merkle tree (ie the order of nodes in the hash,
+how version information is encoded, etc.), is left to the implementer, but this
+algorithm should be documented in a [POUF](https://github.com/theupdateframework/taps/blob/master/tap11.md)
+so that implementations can be
+compatible and correctly verify Merkle tree data. However, all implementations
+should meet the following requirements:
+* Leaf nodes must be unique. A unique identifier of the targets metadata – such as the
+filepath, filename, or the hash of the content – must be included in the leaf data to ensure that no two leaf
+node hashes are the same.
+* The tree must be a Merkle tree. Each internal node must contain a hash that
+includes both child nodes.
+
+Once the Merkle tree is generated, the repository must create a snapshot Merkle
+metadata file for each targets metadata file. This file must contain the leaf contents and
+the path to the root of the Merkle tree. This path must contain the hashes of
+nodes needed to reconstruct the tree during verification, including the leaf's
+sibling (see diagram).
+In addition the path should contain direction information so that the client
+will know whether each listed node is a left or right sibling when reconstructing the
+tree.
+
+This information will be included in the following metadata format:
+```
+{ “leaf_contents”: {METAFILES},
+  “merkle_path”: {INDEX:HASH}
+  “path_directions”:{INDEX:DIR}
+}
+```
+
+Where `METAFILES` is the version information as defined for snapshot metadata,
+`INDEX` provides the ordering of nodes, `HASH` is the hash of the sibling node,
+and `DIR` indicates whether the given node is a left or right sibling.
+
+In addition, the following optional field will be added to timestamp metadata.
+If this field is included, the client should use snapshot Merkle metadata to
+verify updates instead:
+
+```
+("merkle_root": ROOT_HASH)
+```
+
+Where `ROOT_HASH` is the hash of the Merkle tree's root node.
+
+Note that snapshot Merkle metadata files do not need to be signed by a snapshot
+key because the path information will be verified based on the Merkle root
+provided in timestamp. Removing these signatures will provide additional space
+savings for clients.
+
+Previous versions of snapshot Merkle metadata files using the current timestamp
+key must remain available to clients and auditors. The repository may store
+snapshot Merkle metadata files using consistent snapshots to facilitate
+access to previous Merkle trees.
+
+## Merkle tree verification
+
+If a client sees the `merkle_root` field in timestamp metadata, they will use
+the snapshot Merkle metadata to check version information. If this field is
+present, the client will download the snapshot Merkle metadata file only for
+the targets metadata the client is attempting to update. The client will verify the
+snapshot Merkle metadata file by reconstructing the Merkle tree and comparing
+the computed root hash to the hash provided in timestamp metadata. If the
+hashes do not match, the snapshot Merkle metadata is invalid. Otherwise, the
+client will use the version information in the verified snapshot Merkle
+metadata to proceed with the update.
+
+For additional rollback protection, the client may download previous versions
+of the snapshot Merkle metadata for the given targets metadata file. The client
+should perform this check immediately after verifying the current Merkle tree. After verifying
+these files, the client should compare the version information in the previous
+Merkle trees to the information in the current Merkle tree to ensure that the
+version numbers have never decreased. In order to allow for fast forward attack
+recovery (discussed further in Security Analysis), the client should only
+download previous versions whose root hashes were signed for with the same timestamp key.
+
+## Auditing Merkle trees
+
+In order to ensure the validity of all targets metadata version information in the
+Merkle tree, third-party auditors should validate the entire tree each time it
+is updated. Auditors should download every snapshot Merkle file, verify the
+paths, check the root hash against the hash provided in timestamp metadata,
+and ensure that the version information has not decreased for each leaf.
+Alternatively, the repository may provide auditors with information about the
+contents and ordering of leaf nodes so that the auditors can more efficiently
+verify the entire tree.
+
+An auditor should validate all versions of the Merkle tree signed by the
+current timestamp key. For fast-forward attack recovery, the auditor should
+not check for a rollback attack after the timestamp key
+has been replaced. This means that all new auditors should check the Merkle
+trees signed with the current timestamp keys before attesting to the validity
+of the current Merkle tree.
+
+## Client interaction with auditors
+
+Clients must ensure that snapshot Merkle trees have been verified by an auditor.
+To do so, implementations may use a few different mechanisms:
+
+* Auditors may provide an additional signature for timestamp metadata that
+indicates that they have verified the contents of the Merkle tree whose root
+is in that timestamp file. Using this signature, clients can check whether a
+particular third party has approved the Merkle tree. To use this mechanism,
+the auditor's key should be included in the root metadata.
+
+* Auditors may host a list of verified Merkle roots for a given repository,
+signed by the auditor's key. Clients may be configured with the auditor's key,
+or get it from the root metadata.
+
+* Clients may use a secure API to verify that a given Merkle root has been
+verified by an auditor. This API should provide compromise resilience similar to
+TUF's root metadata.
+
+## Garbage collection
+
+When a threshold of timestamp keys are revoked and replaced, the repository no
+longer needs to store snapshot Merkle files signed by the previous timestamp
+keys. Replacing the timestamp keys is an opportunity for fast forward attack
+recovery, and so all version information from before the replacement is no
+longer valid. At this point, the repository may garbage collect all snapshot
+Merkle metadata files.
+
+# Security Analysis
+
+This proposal impacts the snapshot metadata, so this section will discuss the
+attacks that are mitigated by snapshot metadata in TUF.
+
+## Rollback attack
+
+A rollback attack provides a client with an old, previously valid view of
+the repository. Using this attack, an attacker could convince a client to
+install a version from before a security patch was released.
+
+TUF currently protects against rollback attacks by checking the current time
+signed by timestamp and ensuring that no version information provided by
+snapshot has decreased since the last update. With both of these protections,
+a client that has a copy of trusted metadata is secure against a rollback
+attack to any version released
+before the previous update cycle, even if the timestamp and snapshot keys
+are compromised.
+
+Using snapshot Merkle trees, rollback attacks are prevented by both the
+client verification and by third party auditors. If no keys are compromised,
+the timestamp keys protect against a rollback attack by ensuring a valid
+snapshot Merkle tree. If the timestamp key is compromised, the client
+verification of previous Merkle trees provides rollback protection for the
+individual targets metadata files that are verified. However, if the attacker
+controls the repository and timestamp keys, they may provide malicious previous
+Merkle trees. For full rollback protection, clients rely on third party
+auditors. Third party auditors store the previous version of
+all metadata, and will detect when the version number decreases in a new
+Merkle tree. As long as the client checks for an auditor’s verification, the
+client will not install the rolled-back version of the target.
+
+In summary, without auditors, a client is vulnerable to rollback attacks when an attacker
+controls the timestamp key. With auditors, the client has the same rollback
+protection as the existing TUF specification.
+
+## Fast forward attack
+
+If an attacker is able to compromise the timestamp key, they may arbitrarily
+increase the version number of a target in the snapshot Merkle metadata. If
+they increase it to a sufficiently large number (say the maximum integer value),
+the client will not accept any future version of the target as the version
+number will be below the previous version.
+
+In the current specification, repositories can recover from a fast forward
+attack by replacing a threshold of timestamp keys. If the client sees that
+a threshold of timestamp keys were replaced, it deletes the currently trusted
+version information.
+
+Snapshot Merkle trees also reset snapshot information after a replacement of
+a threshold of timestamp keys in order to recover from fast forward attacks.
+Auditors and clients should not check version information from before a
+timestamp key replacement when verifying the Merkle tree.
+
+Thus, fast forward attack recovery with snapshot Merkle trees is the same
+as in the existing specification, but must be performed by both clients and
+auditors.
+
+## Mix and match attack
+
+In a mix and match attack, an attacker combines images from the current
+snapshot with images from other snapshots, potentially introducing
+vulnerabilities.
+
+Currently, TUF protects against mix and match attacks by providing a snapshot
+metadata file that contains all targets metadata files available on the
+repository. Therefore, a mix and match attack is only possible in an
+attacker is able to compromise the timestamp and snapshot keys to create
+a malicious snapshot metadata file.
+
+A snapshot Merkle tree prevents mix and match attacks by ensuring that all
+targets files installed come from the same snapshot Merkle tree. If all targets
+have version information in the same snapshot Merkle tree, the properties of
+secure hash functions ensure that these versions were part of the same snapshot.
+As in the existing specification, a mix and match attack would be possible
+if an attacker was able to replace the snapshot Merkle tree using compromised
+timestamp keys.
+
+Snapshot Merkle trees provide the same protection against mix and match attacks
+as the existing specification.
+
+
+# Backwards Compatibility
+
+This TAP is not backwards compatible. The following table describes
+compatibility for clients and repositories.
+
+| Parties that support snapshot Merkle trees | Result |
+| ------------------------------------------ | ------ |
+| Client and repository support this TAP | Client and repository are compatible |
+| Client supports this TAP, but repository does not | Client and repository are compatible. The timestamp metadata provided by the repository will never contain the `merkle_root` field, and so the client will not look for snapshot Merkle metadata. |
+| Repository supports this TAP, but client does not | Client and repository are not compatible. If the repository uses snapshot Merkle metadata, the client will not recognise the `merkle_root` field as valid. |
+| Neither client nor repository supports this TAP | Client and repository are compatible |
+
+# Augmented Reference Implementation
+
+https://github.com/theupdateframework/tuf/pull/1113/
+TODO: auditor implementation
+
+# Copyright
+
+This document has been placed in the public domain.