// Copyright 2016 Andre Burgaud. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package padding provides functions for padding blocks of plain text in the
// context of block cipher mode of encryption like ECB or CBC.
package padding

import (
	"bytes"
	"errors"
)

// Padding interface defines functions Pad and Unpad implemented for PKCS #5 and
// PKCS #7 types of padding.
type Padding interface {
	Pad(p []byte) ([]byte, error)
	Unpad(p []byte) ([]byte, error)
}

// Padder struct embeds attributes necessary for the padding calculation
// (e.g. block size). It implements the Padding interface.
type Padder struct {
	blockSize int
}

// NewPkcs5Padding returns a PKCS5 padding type structure. The blocksize
// defaults to 8 bytes (64-bit).
// See https://tools.ietf.org/html/rfc2898 PKCS #5: Password-Based Cryptography.
// Specification Version 2.0
func NewPkcs5Padding() Padding {
	return &Padder{blockSize: 8}
}

// NewPkcs7Padding returns a PKCS7 padding type structure. The blocksize is
// passed as a parameter.
// See https://tools.ietf.org/html/rfc2315 PKCS #7: Cryptographic Message
// Syntax Version 1.5.
// For example the block size for AES is 16 bytes (128 bits).
func NewPkcs7Padding(blockSize int) Padding {
	return &Padder{blockSize: blockSize}
}

// Pad returns the byte array passed as a parameter padded with bytes such that
// the new byte array will be an exact multiple of the expected block size.
// For example, if the expected block size is 8 bytes (e.g. PKCS #5) and that
// the initial byte array is:
//
//	[]byte{0x0A, 0x0B, 0x0C, 0x0D}
//
// the returned array will be:
//
//	[]byte{0x0A, 0x0B, 0x0C, 0x0D, 0x04, 0x04, 0x04, 0x04}
//
// The value of each octet of the padding is the size of the padding. If the
// array passed as a parameter is already an exact multiple of the block size,
// the original array will be padded with a full block.
func (p *Padder) Pad(buf []byte) ([]byte, error) {
	bufLen := len(buf)
	padLen := p.blockSize - (bufLen % p.blockSize)
	padText := bytes.Repeat([]byte{byte(padLen)}, padLen)
	return append(buf, padText...), nil
}

// Unpad removes the padding of a given byte array, according to the same rules
// as described in the Pad function. For example if the byte array passed as a
// parameter is:
//
//	[]byte{0x0A, 0x0B, 0x0C, 0x0D, 0x04, 0x04, 0x04, 0x04}
//
// the returned array will be:
//
//	[]byte{0x0A, 0x0B, 0x0C, 0x0D}
func (p *Padder) Unpad(buf []byte) ([]byte, error) {
	bufLen := len(buf)
	if bufLen == 0 {
		return nil, errors.New("crypto/padding: invalid padding size")
	}

	pad := buf[bufLen-1]
	if pad == 0 {
		return nil, errors.New("crypto/padding: invalid last byte of padding")
	}

	padLen := int(pad)
	if padLen > bufLen || padLen > p.blockSize {
		return nil, errors.New("crypto/padding: invalid padding size")
	}

	for _, v := range buf[bufLen-padLen : bufLen-1] {
		if v != pad {
			return nil, errors.New("crypto/padding: invalid padding")
		}
	}

	return buf[:bufLen-padLen], nil
}
