Propose TP18 - Message Schema for Supporting Message-Level Encryption (…

…#6)

tp018/README.md

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+# TP18 Message Schema for Supporting Message-Level Encryption
+
+```yaml
+TP: 18
+Title: TP18 - Message Schema for Supporting Message-Level Encryption
+Authors: Henry Tsai (@thehenrytsai)
+Comments URI: https://github.com/TBD54566975/technical-proposals/discussions/5
+Status: Accepted
+Created: March 29, 2023
+Updated: May 24, 2023
+```
+
+## Problem Statement
+
+The DWeb Node (DWN) message schema needs an update to support message-level encryption using key derivation scheme(s) proposed by [TP17](https://github.com/TBD54566975/technical-proposals/pull/4/files).
+
+## Proposal
+1. An "encrypted DWN message" refers to its data being encrypted, the message containing metadata itself is _not_ encrypted.
+
+1. A randomly generated symmetric key is used for encryption per DWN message.
+
+   This is enables data encryption at the most granular level in DWN.
+
+1. The symmetric key used for data encryption is encrypted using an asymmetric public key derived through a scheme described in [TP17](https://github.com/TBD54566975/technical-proposals/pull/4/files).
+
+1. The encrypted symmetric key (ciphertext) is attached to the corresponding DWN message.
+
+1. The message schema supports attachment of multiple ciphertext of the _same_ symmetric key produced by encryption using different key derivation schemes.
+
+1. There may be other supported key derivation schemes beyond TP17 in the future.
+
+1. `dataCid` and `dataSize` are computed against the encrypted data.
+
+
+## Proposed Schema
+```json
+{
+  "recordId": "...",
+  "descriptor": {
+    "dataCid": "...", // computed against the encrypted data
+    "dataSize": 123, // computed against the encrypted data
+    ...
+  },
+  "authorization": { must contain encryptionCid },
+  "encryption": {
+    "algorithm": "A256CTR | etc", // algorithm used to encrypt the data
+    "initializationVector": "BASE64URL(Unique Initialization Vector)", // used in data encryption
+    // encryption of the SAME encryption key using one or more key derivation scheme
+    "keyEncryption":[{
+        "rootKeyId": "ID of the root asymmetric key used to derive the public key used for this key encryption",
+        "derivationScheme": "protocol | schema | etc",
+        "algorithm": "ECIES-ES256K | etc", // algorithm used to encrypt the symmetric key
+        "encryptedKey": "BASE64URL(Encrypted Data Encryption Key)",
+        // operational properties dependent on derivation scheme and/or key encryption algorithm:
+        "initializationVector": "BASE64URL(Unique Initialization Vector)", // used in ECIES
+        "messageAuthenticationCode": "BASE64URL(Message Authentication Code)", // used in ECIES
+        "ephemeralPublicKey": "Key in JWK format" // used in ECIES
+      },
+      ...
+    ]
+  }
+}
+
+```
+
+## Design Considerations
+1. The proposed schema takes inspiration from the General JWE JSON Serialization specification, however we are unable take a direct dependency on JWE because:
+
+   a. JWE requires the encrypted data to be a part of the JWE object, the DWN deliberately decouples the data from the message which only contains the metadata.
+
+   b. We could potentially encrypt the data encryption symmetric key in a JWE object as the `ciphertext`, but it means that we will be "double encrypting", coupled with use of ECIES, we will have a total 3 layers of symmetric key, which is an unnecessary overhead.
+
+
+1. The `algorithm` property adopts values registered in the [JOSE registry](https://www.iana.org/assignments/jose/jose.xhtml) when there is a direct match.
+
+1. The schema excludes the use of _Authentication Tag_ commonly used in symmetric encryption algorithm (e.g. `AES-GCM`) and specified in JWE, this is because the `dataCid` already serves the same purpose of message integrity verification. This is why `AES-CTR` is used instead.
+
+1. The message handler will determine whether a records is encrypted or not implicitly by checking the existence of `encryption` property. 
+
+1. The `authorization` property needs to contain the `encryptionCid` property so that a DWN cannot strip away the `encryption` property and still have the message considered to be valid, resulting in potentially irreversible corrupt state of a record if such message is replicated to all DWN instances of a DID.