clarify DeclareMiningJob

Author: plebhash

06-Job-Declaration-Protocol.md

@@ -80,7 +80,11 @@ Notably, if the pool intends to change the space it requires for coinbase transa
 
 ### 6.3.4 `DeclareMiningJob` (Client -> Server)
 
-A request sent by the Job Declarator that proposes a selected set of transactions to the upstream (pool) node.
+A request sent by JDC that proposes a selected set of transactions to JDS.
+
+[`SipHash`](https://www.aumasson.jp/siphash/siphash.pdf) is used for short txids as a strategy to reduce bandwidth consumption, since including full txids (transaction data hash) would make the message body excessively big.
+
+More specifically, the `SipHash 2-4` variant is used. Aside from the preimage (which is the full txid), keys `k0` and `k1` are used to avoid collisions, which is the main purpose of `tx_short_hash_nonce`.
 
 | Field Name                  | Data Type             | Description                                                                                                                                                                                                                                                                                                                                                                                                                  |
 | --------------------------- | --------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
@@ -91,7 +95,7 @@ A request sent by the Job Declarator that proposes a selected set of transaction
 | coinbase_tx_suffix         | B0_64K                 | Up to 8 bytes (not including the length byte) which are to be placed at the beginning of the coinbase field in the coinbase transaction                                                                                                                                                                                                                                                                                      |
 | tx_short_hash_nonce         | U64                   | A unique nonce used to ensure tx_short_hash collisions are uncorrelated across the network                                                                                                                                                                                                                                                                                                                                   |
 | tx_short_hash_list          | SEQ0_64K[SHORT_TX_ID] | Sequence of SHORT_TX_IDs. Inputs to the SipHash functions are transaction hashes from the mempool. Secret keys k0, k1 are derived from the first two little-endian 64-bit integers from the SHA256(tx_short_hash_nonce), respectively (see bip-0152 for more information). Upstream node checks the list against its mempool. Does not include the coinbase transaction (as there is no corresponding full data for it yet). |
-| tx_hash_list_hash           | U256                  | Hash of the full sequence of SHA256(transaction_data) contained in the transaction_hash_list                                                                                                                                                                                                                                                                                                                                 |
+| tx_hash_list_hash           | U256                  | SHA256 hash of the list of full txids, concatenated in the same sequence as they are declared in the_short_hash_list                                                                                                                                                                                                                                                                                                         |
 | excess_data                 | B0_64K                | Extra data which the Pool may require to validate the work (as defined in the Template Distribution Protocol)                                                                                                                                                                                                                                                                                                                |
 
 ### 6.3.5 `DeclareMiningJob.Success` (Server -> Client)