`cloudhead` on IRC was asking how to properly use
`BlockHeader::validate_pow()` on genesis (or similar) when the
pow_limit field isn't expressible as a compact target (and, thus,
does not actually represent the PoW limit/genesis target). We
swap it for the actual PoW limit by truncating the way a compact
encoding round-trip would.
Note that, in Bitcoin Core, the original value is only ever used
once in its original form:
```
if (bnNew > bnPowLimit) // Note: bnPowLimit is params.powLimit
bnNew = bnPowLimit;
return bnNew.GetCompact();
```
Thus, even if Core adopted our change, as long as there exist no
256-bit integer x which satisfies
`x <= powLimit && x > encoding_roundtrip_truncated(powLimit)` and
`enoding_roundtrip_truncated(x) != powLimit`, the change would have
no impact on consensus.
It is trivial to show that there are no values which are between
the new value
(0x00000000ffff0000000000000000000000000000000000000000000000000000)
and the original value
(0x00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff)
which can be encoded in compact form, but it is also critically, no
such values will encode to a compact form of anything different than
the new value as the encoding always truncates the low bits, never
rounding up.
Matt Corallo
Carl Dong
Dimitris Apostolou