Routing V1 HTTP Delegated Routing Reader Privacy Upgrade

This specification details the implementation of a new HTTP API for Privacy Preserving Delegated Content Routing provider lookups. It represents an expansion of the HTTP Delegated Routing API, embracing its formats and design principles.

1. API Specification

1.1 Magic Values

All salts below are 64-bytes long and represent a string padded with \x00.

SALT_DOUBLEHASH: The string value CR_DOUBLEHASH, where each if the 13 characters are represented by their byte value. The remainder of the 64 bytes is filled with null bytes represented by \x00. This results in 51 null bytes after the CR_DOUBLEHASH string. The following illustrates its corresponding byte frame diagram:
```
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| C | R | _ | D | O | U | B | L | E | H | A | S | H | \x00...\x00 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
&lt;---------------------------- 64 Bytes ---------------------------&gt;
```
For reference, the following snippet represents the hex dump of the above, where each character of CR_DOUBLEHASH is represented by its ASCII hexadecimal equivalent, and the null bytes are represented by "00":
```
43 52 5F 44 4F 55 42 4C 45 48 41 53 48 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
```
SALT_ENCRYPTIONKEY: The string value CR_ENCRYPTIONKEY, where each if the 15 characters are represented by their byte value. The remainder of the 64 bytes is filled with null bytes represented by \x00. This results in 49 null bytes after the CR_ENCRYPTIONKEY string. The following illustrates its corresponding byte frame diagram:
```
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| C | R | _ | E | N | C | R | Y | P | T | I | O | N | K | E | Y |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| \x00...\x00 |
+---+---+---+
&lt;---------------------------- 64 Bytes ---------------------------&gt;
```
For reference, the following snippet represents the hex dump of the above, where each character of CR_ENCRYPTIONKEY is represented by its ASCII hexadecimal equivalent, and the null bytes are represented by "00":
```
43 52 5F 45 4E 43 52 59 50 54 49 4F 4E 4B 45 59 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
```

These magic values are utilized to compute distinct digests from identical values for varying purposes. For instance, a hash of a Multihash employed for lookups should differ from the one used for key derivation, despite originating from the same value. To achieve this, the Multihash is concatenated with different magic values before applying the hash function: SALT_DOUBLEHASH for lookups and SALT_ENCRYPTIONKEY for key derivation as elaborated in the Glossary.

1.2 Glossary

enc refers to AESGCM encryption. The notation enc(passphrase, nonce, payload) will be used henceforth in this specification.
hash denotes SHA256 hashing.
|| signifies concatenation of two values.
deriveKey pertains to the derivation of a 32-byte encryption key from a passphrase, performed as hash(SALT_ENCRYPTIONKEY || passphrase).
CID stands for Content IDentifier.
MH refers to the Multihash contained in a CID. It corresponds to the hash function's digest over certain content.
HASH2 is a second hash over the multihash. Second Hashes must follow the Multihash format with SHA2_256 codec. The digest must be calculated as hash(SALT_DOUBLEHASH || MH).
ProviderRecord is a JSON with Provider Record as described in the HTTP Delegated Routing Specification.
ProviderRecordKey is a concatenation of peerID || contextID. Explicit encoding lengths are unnecessary as they are inherently encoded as part of the multihash format. Max contextID length is 64 bytes.
EncProviderRecordKey is Nonce || enc(deriveKey(multihash), Nonce, ProviderRecordKey). Max EncProviderRecordKey is 200 bytes.
HashProviderRecordKey is a hash over ProviderRecordKey, calculated as hash(SALT_DOUBLEHASH || ProviderRecordKey).
Metadata are free-form bytes that can represent such information such as IPNI metadata. Max Metadata length is 1024 bytes.
EncMetadata is Nonce || enc(deriveKey(ProviderRecordKey), Nonce, Metadata). Max EncMetadata length is 2000 bytes.

Maximum allowed lengths may change without incrementing the API version. Such fields as contextID or Metadata are free-form bytes and their maximum lengths can be altered in the underlying protocols.

1.3 API

1.3.1 `GET /routing/v1/encrypted/providers/{HASH2}`

1.3.1.1 Response codes

200 (OK): the response body contains one or more records
404 (Not Found): must be returned if no matching records are found
422 (Unprocessable Entity): request does not conform to schema or semantic constraints

1.3.1.2 Response Body

{
    "EncProviderRecordKeys": [
        "EBxdYDhd.....",
        "IOknr9DK....."
    ]
}

Where:

EncProviderRecordKeys is a list of base64 encoded EncProviderRecordKey;

1.3.2 `GET /routing/v1/encrypted/metadata/{HashProviderRecordKey}`

1.3.2.1 Response codes

200 (OK): the response body contains one record
404 (Not Found): must be returned if no matching records are found
422 (Unprocessable Entity): request does not conform to schema or semantic constraints

1.3.2.2 Response Body

{
    "EncMetadata": "EBxdYDhd....."
}