Update poly.go

crystals-kyber/poly.go

@@ -186,7 +186,7 @@ func polyToMsg(p Poly) []byte {
 	return msg
 }
 
-//compress packs a polynomial into a byte array using d bits per coefficient
+//compress packs a polynomial into a byte array using d bits per coefficient - fixed against https://kyberslash.cr.yp.to/faq.html (cases d=4,5 only for now)
 func (p *Poly) compress(d int) []byte {
 	c := make([]byte, n*d/8)
 	switch d {
@@ -229,18 +229,17 @@ func (p *Poly) compress(d int) []byte {
 
 	case 5:
 		var t [8]uint16
-		var d0 uint32 /* accumulation value for fixing KyberSlash2 */
+		var d5 uint32 /* accumulation value for fixing KyberSlash2 */
 		id := 0
 		for i := 0; i < n/8; i++ {
 			for j := 0; j < 8; j++ {
 				/* t[j] = uint16(((uint32(p[8*i+j])<<5)+uint32(q)/2)/
 					uint32(q)) & ((1 << 5) - 1) */
-				t[j] = uint16(p[8*i+j])
-				d0 = uint32(t[j] << 5)
-				d0 += 1664
-				d0 *= 40318
-				d0 >>= 27
-				t[j] = uint16(d0 & 0x1f)
+				d5 = uint32(p[8*i+j] << 5)
+				d5 += 1664
+				d5 *= 40318
+				d5 >>= 27
+				t[j] = uint16(d5 & 0x1f)
 			}
 			c[id] = byte(t[0]) | byte(t[1]<<5)
 			c[id+1] = byte(t[1]>>3) | byte(t[2]<<2) | byte(t[3]<<7)