sha3: fix cSHAKE initialization for extremely large N ana and or S

While both impractical and unlikely, the multiplication could overflow
on 32-bit architectures.

The 64-bit architecture case is unaffected by both the maximum length
of Go slices being too small to trigger the overflow (everything except
s390), and it being safe to assume no machine has more than 2 EiB of
memory.

Fixes golang/go#66232

sha3/shake.go

@@ -85,9 +85,9 @@ func newCShake(N, S []byte, rate, outputLen int, dsbyte byte) ShakeHash {
 
 	// leftEncode returns max 9 bytes
 	c.initBlock = make([]byte, 0, 9*2+len(N)+len(S))
-	c.initBlock = append(c.initBlock, leftEncode(uint64(len(N)*8))...)
+	c.initBlock = append(c.initBlock, leftEncode(uint64(len(N))*8)...)
 	c.initBlock = append(c.initBlock, N...)
-	c.initBlock = append(c.initBlock, leftEncode(uint64(len(S)*8))...)
+	c.initBlock = append(c.initBlock, leftEncode(uint64(len(S))*8)...)
 	c.initBlock = append(c.initBlock, S...)
 	c.Write(bytepad(c.initBlock, c.rate))
 	return &c