i believe i just did something no one has ever done before: i wrote a constant-time galois field implementation on a 6502 chipset, which not only does not have a constant-time hardware multiply instruction, but does not have a multiply instruction at all
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the 6510 chips in commodore 64s run at ~1MHz depending on whether it's the PAL or NTSC version, and a field element in this 434-bit prime field takes 56 bytes, so multiplying two field elements takes roughly 20,944 cycles or ~21ms assuming page boundaries aren't crossed
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i'm not sure how many field element operations i'm going to need to walk the isogeny graph yet, but i feel pretty confident that this is going to be the slowest post-quantum cryptographic implementation in existence, and quite possibly just straight up slowest crypto in the world
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this amusingly means that i will hold the title for implementing both the fastest and slowest elliptic curve related cryptographic implementations in the world

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I had to employ a similar trick in sodium_compat: https://github.com/paragonie/sodium_compat/blob/3b953109fdfc821c1979bc829c8b7421721fef82/src/Core/Util.php#L507-L584 … This ended up becoming extremely useful for supporting 32-bit machines. (PHP integers are always signed, and when they overflow they become floats, which screws up cryptography.)
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Getting crypto to run in places that cryptographers traditionally did not expect it to run is "in" this decade, IMHO
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I can't help but think there has to be a better way than pushing the status to the stack and pulling it again. Maybe use a second add operation to set a byte to all ones or all zeros depending on the carry.
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