--- Log opened Tue Jan 17 00:00:17 2023
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00:49 < real_or_random> > Exercise for the reader: why is there only one subgroup of size 13, despite the y^2=x^3+2 order being divisible by 13^2 ?
00:49 < real_or_random> The group order n of y^2=x^3+2 is odd, so the group is cyclic. By the fundamental theorem of cyclic groups, it has at most one subgroup for every divisor d of n. 
01:24 < real_or_random>  * - SECP256K1_CHECKMEM_CHECK(p, len):
01:24 < real_or_random>  *   - Assert or otherwise fail in case the len-byte memory block pointed to by p is
01:24 < real_or_random>  *     not considered entirely defined.
01:25 < real_or_random> sipa:   this asserts for msan but just prints an error message for valgrind. I think the behavior is good, just checking if printing an error is what you had in mind when you wrote "otherwise fail" 
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05:42 < sipa> @real_or_random Not every odd-ordered group is cyclic (in particular, the order-49 group discussed before is not).
05:47 < hebasto> prime-ordered is cyclic
05:48 < sipa> @hebasto That's correct!
05:53 < real_or_random> sipa: IIRC every odd-ordered full elliptic curve group (not subgroup) is cyclic
05:55 < sipa> Oh that may be true.
05:56 < sipa> In any case, your overall point is correct. The order-13^2 subgroup is isomorphic to C_169 (integer addition mod 169). The order-7^2 subgroups is not C_49, but  C_7 * C_7.
05:57 < real_or_random> no, I'm wrong, hm
05:57 < real_or_random> well wikipedia says this
05:58 < real_or_random> "The set of points E(Fq) is a finite abelian group. It is always cyclic or the product of two cyclic groups, depending whether q is even or odd. " but no further explanation
05:58 < real_or_random> and note q here is the field size, not group order
05:59 < real_or_random> sipa: did you see my code question above?
06:01 < sipa> The first part is correct: finite elliptic curves are either cyclic or the product of two cyclic groups.
06:02 < sipa> @real_or_random Yeah, I saw. If it's easy to do, perhaps we want to make it also assert if the checkmem fails.
06:07 < real_or_random> I think it will be easy, we can just CHECK the result. But I think not aborting is good. valgrind is totally able to continue at this point. I'd rather clarify the comment 
06:24 < roconnor> I want to argue that *_debruijn should return -1 indicating error when passed 0, but I realize that is like arguing about how many angels can dance on the head of a pin.
06:35 < real_or_random> lol
06:36 < real_or_random> you may want to read https://stackoverflow.com/questions/19527897/how-undefined-are-builtin-ctz0-or-builtin-clz0
06:43 < roconnor> I recall seeing that.
06:43 < roconnor> we could call __builtin_unreachable or whatever it is called instead.
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07:37 < hebasto> all recent build system changes, including `noverify_tests` and `ctime_tests`, have been incorporated into the CMake-based build system -- https://github.com/bitcoin-core/secp256k1/pull/1113#issuecomment-1385613193
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07:43 < real_or_random> > we could call __builtin_unreachable or whatever it is called instead.
07:43 < real_or_random> or better CHECK, it's probably a bug then :P
07:51 < real_or_random> hebasto: great... we should discuss how to proceed with this, i.e., if we want to merge now or if better doc of the options is a blocker  
07:51 < hebasto> next irc meeting?
07:51 < real_or_random> yes sounds good
07:52 < hebasto> cool
07:52 < real_or_random> I don't think the docs are a blocker... I mean without the docs cmake is not really helpful but that doesn't need to dictate the order in which we do things. if the PR is ready, I think we should try to get it n
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--- Log closed Wed Jan 18 00:00:18 2023