X25519.cs_fst: def of bend-collections-laws-crypto@1.0.0.0
Import: import bend-collections-laws-crypto@1.0.0.0/spec/crypto/curve25519/x25519.bend as X25519
Lines 78-85 of bend-collections-laws-crypto@1.0.0.0/spec/crypto/curve25519/x25519.bend: https://jend.leeeet.dev/read.txt?path=bend-collections-laws-crypto@1.0.0.0/spec/crypto/curve25519/x25519.bend&start_line=78&end_line=85

import Base

# cswap(swap, a, b), RFC 7748: (a, b) when swap is 0, (b, a) when it is 1;
# cs_fst and cs_snd are its two components
def cs_fst(swap: Nat, a: Nat, b: Nat) -> Nat:
  match swap:
    case 0n:
      a
    case 1n+q:
      b

Uses (1):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=Nat