You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: block.go
    path of file: ./repos/gccrs/libgo/go/crypto/des
    
    the code of the file until where you have to start completion: // Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package des

import (
	"encoding/binary"
	"sync"
)

func cryptBlock(subkeys []uint64, dst, src []byte, decrypt bool) {
	b := binary.BigEnd
    
func initFeistelBox() {
	for s := range sBoxes {
		for i := 0; i < 4; i++ {
			for j := 0; j < 16; j++ {
				f := uint64(sBoxes[s][i][j]) << (4 * (7 - uint(s)))
				f = permuteBlock(f, permutationFunction[:])

				// Row is determined by the 1st and 6th bit.
				// Column is the middle four bits.
				row := uint8(((i & 2) << 4) | i&1)
				col := uint8(j << 1)
				t := row | col

				// The rotation was performed in the feistel rounds, being factored out and now mixed into the feistelBox.
				f = (f << 1) | (f >> 31)

				feistelBox[s][t] = uint32(f)
			}
		}
	}
}