swift - 在 Swift 中反向 CRC-32 NSData

Question

我认为答案的组成部分就在那里，但我对这些东西的理解还不够好，而且我在转换方面遇到了很大的困难。

这是我找到的 NSData 到 crc32 的实现:

https://github.com/krzyzanowskim/CryptoSwift/blob/643bbabd6781b6e226f18815dba616cf6a08629b/CryptoSwift/CRC.swift

不幸的是，我需要一个有反转的版本:

这个线程似乎最接近:Reversing CRC32

我已经使用它的逻辑来创建生成两个 CRC 表的逻辑:

func crcTables() -> (forwardTable: [UInt32], reverseTable: [UInt32]) {
    let poly: UInt32 = 0xedb88320;

    var forwardTable: [UInt32] = []
    var reverseTable: [UInt32] = []

    var forward: UInt32 = 0
    var reverse: UInt32 = 0
    for i in 0...UInt32(UInt8.max) {
        forward = i
        reverse = i << (3 * 8)

        for _ in 1...8 {
            if (forward & 1) == 1 {
                forward = (forward >> 1) ^ poly
            } else {
                forward >>= 1
            }

            if (reverse & 0x80000000) != 0 {
                reverse = ((reverse ^ poly) << 1) | 1
            } else {
                reverse <<= 1
            }
        }

        forwardTable.append(forward)
        reverseTable.append(reverse)
    }

    return (forwardTable, reverseTable)
}

但是，现在我非常困惑如何获取数据并创建反向 crc32:

func reverseCRC(data: NSData) -> UInt32 {
    var bytes = [UInt8](count: data.length, repeatedValue: 0)
    data.getBytes(&bytes, length:data.length * sizeof(UInt8))


    return 0
}

更新

通过各种搜索，我想出了这个:

func reverseCRC32WithData(data: NSData, wantedCRC: UInt32 = 0) -> UInt32 {
    var reversedCRC = wantedCRC

    var bytes = [UInt8](count: data.length, repeatedValue: 0)
    data.getBytes(&bytes, length:data.length * sizeof(UInt8))

    // Drop trailing 1 if exists
    let bufferLength = (data.length >> 1) << 1

    for i in 0.stride(to: bufferLength + 4, by: sizeof(UInt16)) {
        var word: UInt16 = 0x0000
        if i < bufferLength {
            let b1 = bytes[bufferLength - 2 - i]
            let b2 = bytes[bufferLength - 1 - i]
            word = (UInt16(b1) << 8) | UInt16(b2)
        } else {
            word = 0xffff
        }

        reversedCRC = ((reversedCRC << 8) & 0xffffffff) ^ ReverseTable[Int((reversedCRC >> 24) & 0xff)] ^ (UInt32(word) >> 8) & 0xff
        reversedCRC = ((reversedCRC << 8) & 0xffffffff) ^ ReverseTable[Int((reversedCRC >> 24) & 0xff)] ^ (UInt32(word) & 0xff)
    }

    return reversedCRC
}

但是，我并不真正理解它(尤其是最后两行)，而且我不确定如何测试它的准确性。如果有人可以朝这个方向提供帮助。

目标

我有一个固件更新，我正在通过蓝牙传输到硬件，一旦固件更新的所有数据都已传输，我将以以下形式提交验证有效负载:

Reversed CRC 32 (Big Endian)

Answer 1

我刚从 Apple Document 找到这个答案,

就我而言，它对我有用，

let CRC32_UInt32 = CRC32.checksum(bytes: inputBytes) // 73DECCF1
let reverse_CRC32_UInt32 = CFSwapInt32(CRC32_UInt32)
let reverse_crc32 = String(reverse_CRC32_UInt32, radix: 16, uppercase: true) // F1CCDE73

CRC32算法如下:

class CRC32 {
        
    static var table: [UInt32] = {
        (0...255).map { i -> UInt32 in
            (0..<8).reduce(UInt32(i), { c, _ in
                (c % 2 == 0) ? (c >> 1) : (0xEDB88320 ^ (c >> 1))
            })
        }
    }()

    static func checksum(bytes: [UInt8]) -> UInt32 {
        return ~(bytes.reduce(~UInt32(0), { crc, byte in
            (crc >> 8) ^ table[(Int(crc) ^ Int(byte)) & 0xFF]
        }))
    }
}

希望这对任何需要的人都有帮助😀