The RPC GET /chains/<chain_id>/chain_id returns:

'the chain unique identifier'

How is this identifier computed?

  • 3
    Possible duplicate of How do I base58 encode the chain ID using Python?
    – Ezy
    Feb 19 '19 at 22:40
  • The answers in the "using Python" version of the question :( do not seem to answer the general question... So I will attempt to answer this one.
    – Tom
    Feb 27 '19 at 16:52

The chain_id is computed from the genesis block hash as follows.

First, in pseudocode:

    blake2b(msg = tezosB58CheckDecode('B', genesisBlockHash),
            size = 32)))

In detail:

  1. Take the genesis block hash. For example, in mainnet, this is "BLockGenesisGenesisGenesisGenesisGenesisf79b5d1CoW2".

  2. Base58Check-decode the block hash. The prefix bytes for blocks "B" are [1, 52] (in decimal), as you can see by doing git grep B\( in tezos.git. This gives us 0x8fcf233671b6a04fcf679d2a381c2544ea6c1ea29ba6157776ed8424c7ccd00b.

  3. Compute the BLAKE2B hash (size 32) of the block hash bytes. We get 0x7a06a7709ff405d1791d856c52a3c55246e03ec913599b813ec2977398afb3be. Take only the first four bytes, 0x7a06a770.

  4. Base58Check-encode these four bytes with the "Net" prefix [87, 82, 0] (git grep Net\(). We get "NetXdQprcVkpaWU".

For a protocol update test chain, I believe the 'genesis' block will be the block in the main chain from which the test chain was forked.

You can find this computation defined in lib_crypto/chain_id.ml and then used as Chain_id.of_block_hash in various places (e.g. lib_shell/state.ml, lib_shell/chain_validator.ml, lib_storage/context.ml).

  • thanks for your answer! could you provide if possible for reference a source for this information ?
    – Ezy
    Mar 1 '19 at 3:32

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