[0.17 backport] Update secp-zkp to 44db4d801fff3cd94105136cb443d603683baad2#654

Merged

instagibbs merged 2 commits intoElementsProject:elements-0.17from stevenroose:e17-bp-secp-zkp

Jun 7, 2019

+5,449-512

Commits on Jun 6, 2019