You’re just doing brute force, but with extra steps. It turns out that partial collisions are more common than you think, and it’s not particularly hard to find some.

Here, a 186-bits partial collision, found in less than two minutes on my CPU, by brute force :

sha256(cbfad45814d54d1d56d30de387d957ed3b50e06270ad6e4b897f4a0000000000) = 692207e28eb8dd3eb4f8fab938ea5103faa1060c3bbed204f564e10c65d06b33 sha256(cbfad45814d54d1d56d30de387d957ed3b50e06270ad6e4be8c33e0000000000) = 006347a21f7c9b3eb4fa52b75d0e5a03dbe556b579d6d2867d44c38c06546f6f

(in Python :

>>> hashlib.sha256(bytes.fromhex("cbfad45814d54d1d56d30de387d957ed3b50e06270ad6e4b897f4a0000000000")).hexdigest()

'692207e28eb8dd3eb4f8fab938ea5103faa1060c3bbed204f564e10c65d06b33'

>>> hashlib.sha256(bytes.fromhex("cbfad45814d54d1d56d30de387d957ed3b50e06270ad6e4be8c33e0000000000")).hexdigest()

'006347a21f7c9b3eb4fa52b75d0e5a03dbe556b579d6d2867d44c38c06546f6f'

>>> a = hashlib.sha256(bytes.fromhex("cbfad45814d54d1d56d30de387d957ed3b50e06270ad6e4b897f4a0000000000")).digest()

>>> b = hashlib.sha256(bytes.fromhex("cbfad45814d54d1d56d30de387d957ed3b50e06270ad6e4be8c33e0000000000")).digest()

>>> sum((x^y^0xff).bit_count() for (x, y) in zip(a, b))

186

)

Intuition pump : the expected number of equal bits for two random inputs is 128.