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most smart contracts that rely on "randomness", will require said entropy to be in a number format. A practical way to extract entropy from a arbitrary payload is by using a hashing function. This means that it's intuitive have the entropy in bytes format inside a Tezos smart contract. There are no "direct" ways to convert arbitrary bytes to nat/int in Michelson, so someone proposed (thank you Rodrigo) to go with something like this:

def entropy_to_int(self, param):
    entropy = sp.bytes("0x00") 
    sp.result(sp.to_int(sp.unpack(sp.bytes("0x050a00000020")+sp.blake2b(entropy), sp.TBls12_381_fr).open_some()))

It looks like it's working as intended. However as this is very security relevant is there a security concern when going with this solution (i.e. skewed/biased distribution of the resulting numbers)?

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  • The trick is that it uses the representation of the Bls12_381_fr scalar field, so you bypass any issue with the encoding of nat. You should get a uniform distribution out of this, but I have only taken a cursory look.
    – Arthur B
    Feb 1 at 14:10
  • Bls12_381_fr does not support all 32 byte integers. (e.g: 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff will fail). You could just drop the latest byte and append 0x00. It is extremely unlikely to ever happen, but.
    – Rodrigo Quelhas
    Feb 1 at 17:39

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The next protocol Mumbai brings new features to Michelson. It will be possible to perform logical operations on bytes, including converting between bytes and integers.

Documentation

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