feat(branch-keys): add hv-2 spec

changes/2025-06-30_branch_keys_version_2/background.md

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+[//]: # "Copyright Amazon.com Inc. or its affiliates. All Rights Reserved."
+[//]: # "SPDX-License-Identifier: CC-BY-SA-4.0"
+
+# Customers should control encryption context
+
+# Definitions
+
+## Conventions used in this document
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
+"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
+interpreted as described in [RFC 2119](https://tools.ietf.org/html/rfc2119).
+
+## HV-1
+
+The Branch Key Store's Branch Keys are designated as `"hierarchy-version" : "1"`.
+
+This document proposes changes to theses Branch Keys;
+when HV-1 is written,
+we mean a Branch Key Item written by the Branch Key Store prior to this change, which will always describe it's self as `"hierarchy-version" : "1"`.
+
+## Branch Key's Cryptographic Material
+
+Cryptographic material (AES-256 bit key) generated and protected by KMS.
+
+## Branch Key's Properities
+
+These three values are stored on every
+Active,
+Version,
+and Beacon Key Item in the Branch Key Store's Storage.
+
+They are also present in the KMS Encryption Context.
+
+```json
+"kms-arn" : "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab",
+"create-time" : "2023-06-03T19:03:29.358Z",
+"hierarchy-version" : "1",
+```
+
+The Active has an additional field:
+
+```json
+"version": "branch:version:83eec007-5659-4554-bf11-699b90f41ac6"
+```
+
+## Branch Key's Location
+
+These two values are stored on every
+Active,
+Version,
+and Beacon Key Item in the Branch Key Store's Storage.
+
+For Active:
+
+```json
+"branch-key-id" : "bbb9baf1-03e6-4716-a586-6bf29995314b",
+"type" : "branch:ACTIVE"
+```
+
+For Version:
+
+```json
+"branch-key-id" : "bbb9baf1-03e6-4716-a586-6bf29995314b",
+"type": "branch:version:83eec007-5659-4554-bf11-699b90f41ac6"
+```
+
+For Beacon:
+
+```json
+"branch-key-id" : "bbb9baf1-03e6-4716-a586-6bf29995314b",
+"type": "beacon:ACTIVE"
+```
+
+However,
+the Logical Key Store Name is also included in the KMS Encryption Context.
+
+```json
+"branch-key-id" : "bbb9baf1-03e6-4716-a586-6bf29995314b",
+"type" : "branch:ACTIVE",
+"tablename": "KeyStore"
+```
+
+These three values describe where the Branch Key is stored; however, the tablename is a logical description, not a physical description.
+At this time, no cryptography binds the Branch Key to the physical table.
+
+```json
+"branch-key-id" : "bbb9baf1-03e6-4716-a586-6bf29995314b",
+"tablename": "KeyStore"
+```
+
+These two values label the Branch Key, seperating it from all other Branch Keys.
+
+## Branch Key's Encryption Context
+
+These values are determined by the Branch Key Creator
+or last Branch Key Mutator.
+
+In DynamoDB and in KMS Encryption Requests for HV-1,
+their keys are prefixed with `aws-crypto-ec:`.
+
+```json
+"aws-crypto-ec:department" : "admin"
+```
+
+However,
+when returned to the requesters of the Branch Key Store's
+`Get*Key*` operations,
+the prefix is removed.
+
+## Branch Key's Context
+
+The union of the Branch Key's:
+
+- Properities
+- Location (including logical key store name)
+- Encryption Context
+
+# Background
+
+The current Branch Key Store's Branch Keys use KMS Encryption Context
+to cryptographically bind the Branch Key's cryptographic material to
+the Branch Key's Context.
+
+This cryptographic binding mitigates a number of threats around
+storing the Branch Keys
+and is vital to the safe usage of the Hierarchal Keyring.
+
+However,
+it is not clear that this is the best use of KMS Encryption Context,
+as it interferes with customers ability to use KMS Key Policies to
+constrain Key Usage.
+
+Further more,
+Encryption Context evaluation can be customized by the calling principles authorization;
+KMS Key Grants can have restricted but unique conditions.
+
+Many potential Branch Key Customers are prevented from using
+the Hierarchy Keyring,
+as they have pre-existing Key Policies with their tenants
+that cannot be met if the KMS Encryption Context is transformed
+by the Branch Key Store.
+
+# Requirements
+
+1. Branch Key's Context be cryptographically bound to the Branch Key's Cryptographic Material
+
+2. Branch Key's Encryption Context, untransformed in any way, is the KMS Encryption Context
+
+3. Support for all of Behaviors of the current Branch Key Store (Create, Version, Get\*)
+
+# Out of Scope
+
+- Abstracting away from KMS
+- Supporting any Branch Key protection Scheme via the DB-ESDK, much like the DDBEC's MetaStore.
+
+## Why not Abstract away from KMS
+
+Over the past year,
+Crypto Tools has spent a significant amount of our time
+supporting services integrating with KMS for multi-tenant applications.
+
+While we MAY eventually want to support GCP or Azure,
+we MUST focus on the fasting growing customer base we have;
+AWS Services and Software-as-a-service providers integrating with KMS.
+
+## Why not the MetaStore approach?
+
+The MetaStore was the predecessor to the Branch Key Store;
+it is the "caching" solution for the legacy DynamoDB Encryption Client (DDBEC).
+The MetaStore used the DDBEC itself to protect the hierarchical material with KMS;
+this affords for some flexibility,
+as the MetaStore was an interface that exposes the full breadth of DDBEC functionality.
+This gives customers significant freedom on what data is bound to a Branch Key Item's material and how that binding is facilitated.
+
+However, our customers have complained that the DB-ESDK is complicated;
+using the DB-ESDK to protect the materials used by the DB-ESDK and the ESDK is NOT
+a step towards simplification.
+
+While such a feature MAY provide the greatest flexibility to
+our customers,
+it is not a simplification of the Hierarchy Keyring,
+but a complication to it.
+It also would make the DB-ESDK a dependency of the ESDK,
+and introduce a circular dependency between the MPL and the DB-ESDK.
+
+# Design Questions
+
+## 1 How can the Branch Key's Context be protected without using KMS EC?
+
+See [protect_branch_key_without_kms_ec.md](./protect_branch_key_without_kms_ec.md).
+
+## 2 How are we going to offer operation in a mixed mode?
+
+UPDATE: 2.1 was rejected.
+There will be no Policy (2.2).
+
+### 2.1 `hierarchy-version-policy`
+
+Much like [Key Commitment](https://docs.aws.amazon.com/encryption-sdk/latest/developer-guide/concepts.html#key-commitment),
+we could introduce a policy that
+would allow for mixed usage or restricting to one or the other.
+
+Something like `hierarchy-version-policy`:
+
+- ALLOW_V1
+- ALLOW_V2
+- ALLOW_V1_OR_V2
+- ANY
+
+This policy would be part of (all) Branch Key Store configuration.
+
+(all: Branch Key Store, any future Branch Key Store Usage client.)
+
+This allows our customers to
+restrict to one KMS Encryption Context
+experience or another.
+
+The effort to implement and test this is very low,
+relative to other code paths in this effort.
+
+### 2.2 Do Nothing
+
+UPDATE: We are going with Do Nothing
+
+There is no immediate customer demand for a policy like this.
+In general,
+the Branch Key in the table,
+in combination with KMS,
+the authority of it's treatment.
+
+However,
+given the low effort required to implement/support this,
+and the aid it gives our customers in having
+a consistent KMS Encryption Context experience,
+I suggest we implement the policy.
+
+### 2.3 Pros of 2.1
+
+**Update Readers, than Writers**:  
+To use HV-2,
+customers already using the H-Keyring will need
+to update their readers before they start creating
+or mutating Branch Keys to HV-2.
+
+i.e: They need to update their readers before they update their writers.
+
+This alone is justification for the policy.
+
+**Consistent KMS Encryption Context**:  
+The arguments for the HV-1 Encryption Context suggest
+users SHOULD be able to restrict their Branch Key Store clients
+to a HV-1 such that they know any IAM or KMS Key Policies
+they created that depend on HV-1's Encryption Context
+are still valid.
+
+### 2.4 Cons
+
+**Additional Development work and documentation**:  
+Nothing is free;
+we would need to implement, document, test, and maintain this policy.
+
+Still,
+the migration benefit justifies this cost.
+
+## 3 Branch Key Creation
+
+UPDATE: The team decided that the Branch Key Store would create and version both HV-1 and HV-2 BKs.
+
+UPDATE: The team changed their mind; the Branch Key Store WILL support HV-2 Branch Key Creation, via a flag.
+
+Customers MUST be able to chose which `hierarchy-version`
+new Branch Keys are created with.
+
+The question is what should the UX be.
+
+### 3.1 Specify the `hierarchy-version` at Creation
+
+Currently, our library has one Branch Key Creation operations:
+
+- `BranchKeyStore#CreateKey`
+
+To this operation,
+we add a flag that dictates the `hierarchy-version` to be created with.
+
+If that flag conflicts with `hierarchy-version-policy`,
+then FAIL (update: `hierarchy-version-policy` was rejected, see [section 2](#2-how-are-we-going-to-offer-operation-in-a-mixed-mode)).
+
+Otherwise, respect the flag.
+
+**Plumbing through `GenerateRandom`**:  
+UPDATE: `kms:GenerateDataKey` closed this negative consequence.
+
+### 3.2 Follow the Configuration
+
+This does not work if the configuration is `ALLOW\_V1\_OR_V2`;
+this is not recommended.
+
+### 3.3 New Operation or Client
+
+We could leave the old Branch Key Store alone,
+and introduce a new Branch Key Store V2.
+
+The UX advantage here is that we can introduce new error types
+without breaking customers.
+
+For example,
+the creation of the `hierarchy-version-policy` implies
+that we will need an error type for rejecting a Branch Key
+that does not match.
+
+Particularly if we went down the additional KMS approaches (1.2, 1.3),
+new error types will need to be created to represent
+failures from the additional KMS key.
+
+But we have committed to 1.4;
+this limits the failure modes/additional errors
+to the point that I do not think new client
+or operation is needed.
+
+### 3.4 Author's conclusion
+
+Unless someone can think of something else,
+3.1.
+
+## 4 Branch Key Versioning (Rotation)
+
+Branch key store can VersionKey for HV-1 or HV-2 based on schema version (equivalent to hierarchical version) of the branch key item.