Does macros(`MCL_MAX_FP_BIT_SIZE`, `MCL_MAX_BIT_SIZE`, etc.) actually have impact on efficiency?

[xfap]

Beginner for this repo.

A little confused about the role of macros MCL_MAX_FP_BIT_SIZE, MCL_MAX_BIT_SIZE, etc. Seems they are only for checking bounds or being consistent with C.

My final aim here: want to make a more unified C++ wrapper upon libmcl for pairing & ecc curve instead of including different header file for different macro.

[herumi]

Sorry, it's confusing for historical reasons and backward compatibility. What parameters of the pairing do you want to use? If you want to use only BLS12_381, make MCL_MAX_BIT_SIZE=384 lib/libmcl.a and use it with -DMCL_MAX_BIT_SIZE=384 and "including bls12_381.hpp".

[herumi]

• MCL_MAX_BIT_SIZE is the maximum bit length of the characteristic of a finite field that libmcl can handle.
  • mcl supports multiple different characteristic fields.
• MCL_MAX_FP_BIT_SIZE is the maximum bit length defined mcl::bn::Fp for the defined field of an elliptic curve for a pairing.
• MCL_MAX_FR_BIT_SIZE is the maximum bit length defined mcl::bn::Fr for the group order of an elliptic curve for a pairing.

For BLS12_381, sizeof(Fp) = 384/8, sizeof(Fr) = 256/8.

[xfap]

@herumi Thanks. Sorry for further some more questions. I see an example for multi instances for pairing in sample/multi.cpp. If we can use this method to have all pairing instance?

BTW, notice api.md only describes four kinds of pairing curves BN254 BN_SNARK1 BN381_1 BLS12_381, and in include/mcl/curve_type.h have already params for all curve declared by enum. So if the lib could actually support these pairings?

[herumi]

Would you like to use all pairing instances simultaneously? Could you tell me the reason? I don't recommend sample/multi.cpp because it's a special usage.

We can compute all pairings defined in the enum, but mcl supports only BLS12-381 for the standard hash-to-G1/G2 functions defined in IRTF.

[xfap]

Cause I want to make a universal wrapper for different paring libs, so pairing curve switching needs happen over the wrapper(where requiring support of pairing instances simultaneously), but not over the underlying lib's building args.

[herumi]

I don't think mcl suits that purpose, though you can make two or three instances by sample/multi.cpp. The parameters other than BLS12_381 are not standard, but what kind of application are you thinking of?

[xfap]

Wanner be a part for a more user-friendly crypto lib. AND another question from building error about MCL_MAX_BIT_SIZE, may you could help? When setting building args -DMCL_MAX_BIT_SIZE=1024, and initCurve will fail for curves like:

• (MCL_SECP256K1)mcl::initCurve< mcl::EcT<mcl::FpT<::mcl::FpTag, 256>>,mcl::FpT<::mcl::ZnTag, 256> >(...)
• (MCL_SECP521R1)mcl::initCurve< mcl::EcT<mcl::FpT<::mcl::FpTag, 521>>,mcl::FpT<::mcl::ZnTag, 521> >(...);
• or others.

[herumi]

Wanner be a part for a more user-friendly crypto lib.

mcl basically assumes single instance pairing, so could you use the other library?

-DMCL_MAX_BIT_SIZE=1024 Now mcl does not support the size. The max is 512.

MCL_SECP521R1 and MCL_SECP521R1 are not parameters for pairing.

[xfap]

Yes, not for pairing, I set MCL_MAX_BIT_SIZE for purpose of using ecc MCL_SECP521R1. But it seems not working right. BTW, I'm working for a universal wrapper for ecc & pairing, not just pairing.

mcl basically assumes single instance pairing, so could you use the other library?

OK... maybe that's the way and I should just include only BLS12_381 except others pairing for libmcl.

[xfap]

Now mcl does not support the size. The max is 512.

wait...max is 512? but how could we instantiate curveMCL_SECP521R1?

[herumi]

Yes, not for pairing, I set MCL_MAX_BIT_SIZE for purpose of using ecc MCL_SECP521R1. But it seems not working right.

The old version supported MCL_SECP521R1, but I gave up on it now. I'll fix it later if I can afford it.

[xfap]

OK, very thanks for your helpful answers!

[herumi]

@xfap

requiring support of pairing instances simultaneously

By the way, in what situations do you need different pairings for multiple threads?