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What would be the max number of bits that hashing hardware specifically designed for bitcoin mining would ever need to hash per operation? For example:

block header = 80 bytes = 640 bits
after first hash = 256 bits

To find merkle root: 256(per txn) x 2 = 512 bits

So is 640 bits the max input message size the hashing hardware needs to handle?

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    Note that 1. finding the Merkle root only needs to be done once per 4 billion hash attempts. 2. Most of the time just one branch needs to be calclated. 3. The main loop in SHA-256 calculation always works on a 2048-bit message schedule, so the initial message size might not have a great significance on hardware design. – Meni Rosenfeld Jun 25 '14 at 7:03
  • SHA-256 operates on a 512 bit message block. So I believe the hashing hardware can compute the initial block header hash with just two blocks. – Joe M Jun 25 '14 at 19:51

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