I was looking at the protocol constants using:
and noticed these two fields:
"proof_of_work_nonce_size": 8, "nonce_length": 32,
What are they used for?
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You will find the explanation to both these constants in the documentation of Tezos proof of stake algorithm.
They relate to two different nonces with different purposes.
A nonce is used to prevent the chain from being spammed with blocks:
proof_of_work_nonce: a nonce used to pass a low-difficulty proof-of-work for the block, as a spam prevention measure.
In the section on Random seeds, you can read about nonces (whose length is
nonce_length) using the on-chain random number generation used to select baking slots. In sum, in each cycle the algorithm assigns baking slots randomly using a based on random seed calculated a set of pre-commited "nonces" from bakers from previous cycles:
The random seed for cycle n is a 256-bit number generated at the very end of cycle n-1 from nonces to which delegates commit during cycle n-2. One out of every BLOCKS_PER_COMMITMENT = 32 blocks can contain a commitment. There are therefore at most BLOCKS_PER_CYCLE / BLOCKS_PER_COMMITMENT = 128 commitments. A commitment is the hash of a nonce. The commitment is generated by the baker who produces the block and is included in the block header.