const barrettShift = 16 // log₂(2¹⁶) quotient := (uint32(b) * barrettMultiplier) >> barrettShift remainder := uint32(b) - quotient*5 return fieldSubToMontgomery(2, remainder), true case 4: // Return z = 4 − b, which maps from // // b = ( 8, 7, 6, 5, 4, 3, 2, 1, 0 ) // // to // // z = ( −4, -3, -2, -1, 0, 1, 2, 3, 4 ) // if b > 8 { return 0, false } return fieldSubToMontgomery(4, uint32(b)), true default: panic("internal error: unsupported η") } } // power2Round implements Power2Round from FIPS 204....