CryptoSwift/Sources/CryptoSwift/PKCS/PKCS7Padding.swift

//
//  PKCS7PAdding.swift
//  CryptoSwift
//
//  Copyright (C) 2014-2017 Marcin Krzyżanowski <marcin@krzyzanowskim.com>
//  This software is provided 'as-is', without any express or implied warranty.
//
//  In no event will the authors be held liable for any damages arising from the use of this software.
//
//  Permission is granted to anyone to use this software for any purpose,including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:
//
//  - The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation is required.
//  - Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
//  - This notice may not be removed or altered from any source or binary distribution.
//

//  PKCS is a group of public-key cryptography standards devised
//  and published by RSA Security Inc, starting in the early 1990s.
//

public struct PKCS7Padding: Padding {

    public enum Error: Swift.Error {
        case invalidPaddingValue
    }

    public init() {
    }

    public func add(to bytes: Array<UInt8>, blockSize: Int) -> Array<UInt8> {
        let padding = UInt8(blockSize - (bytes.count % blockSize))
        var withPadding = bytes
        if padding == 0 {
            // If the original data is a multiple of N bytes, then an extra block of bytes with value N is added.
            for _ in 0..<blockSize {
                withPadding += Array<UInt8>(arrayLiteral:UInt8(blockSize))
            }
        } else {
            // The value of each added byte is the number of bytes that are added
            for _ in 0..<padding {
                withPadding += Array<UInt8>(arrayLiteral:UInt8(padding))
            }
        }
        return withPadding
    }

    public func remove(from bytes: Array<UInt8>, blockSize _: Int?) -> Array<UInt8> {
        guard !bytes.isEmpty, let lastByte = bytes.last else {
            return bytes
        }

        assert(!bytes.isEmpty, "Need bytes to remove padding")

        let padding = Int(lastByte) // last byte
        let finalLength = bytes.count - padding

        if finalLength < 0 {
            return bytes
        }

        if padding >= 1 {
            return Array(bytes[0..<finalLength])
        }
        return bytes
    }
}