Megolm doc: nitpicking question about ratchet

[dest4]

Hi,
I do not fully understand the following sentence in the Megolm doc.

This scheme allows the ratchet to be advanced an arbitrary amount forwards while needing at most 1023 hash computations.

If I understood correctly the way the ratchet advances, the worst case scenario would be to advance from i=0 to i=2^32 - 1, assuming that i is a 32-bit counter that cannot overflow (can it?).

• 255 forwards would be needed to go from i=0 to i=2^32 - 2^24
• 255 forwards would be needed to go from i=2^32 - 2^24 to i=2^32 - 2^16
• 255 forwards would be needed to go from i=2^32 - 2^16 to i=2^32 - 2^8
• 255 forwards would be needed to go from i=2^32 - 2^8 to i=2^32 - 1

That makes 1020 hash computations, not 1023. Am I wrong?
Not that it's particularly important, but I am afraid to have an erroneous understanding of the whole thing.

[ara4n]

it sounds to me like a mental arithmetic thinko; the difference between 4*(2^8 - 1) and (4*2^8) - 1. @richvdh, does this sound right?

[richvdh]

@richvdh, does this sound right?

yes.

[dest4]

OK thanks for the clarification

[ara4n]

this got fixed back in sept.

[ara4n]

...except it didn't.

[ara4n]

now fixed in https://gitlab.matrix.org/matrix-org/olm/commit/214908ace54604da669b015cfda7c0496918dfc0