UnsafeBufferPointer/UnsafeMutableBufferPointer可能是您现在需要的。我试着把你的代码翻译成Swift。（但代码没有经过很好的测试。

func XOR(inputData: NSMutableData, withKey key: NSData) {
    let b = UnsafeMutableBufferPointer<UInt8>(start:
        UnsafeMutablePointer(inputData.mutableBytes), count: inputData.length)

    let k = UnsafeBufferPointer<UInt8>(start:
        UnsafePointer(key.bytes), count: key.length)

    for i in 0..<inputData.length {
        b[i] ^= k[i % key.length]
    }
}

func checkSum(data: NSData) -> Int {
    let b = UnsafeBufferPointer<UInt8>(start:
        UnsafePointer(data.bytes), count: data.length)

    var sum = 0
    for i in 0..<data.length {
        sum += Int(b[i])
    }
    return sum & 0xff
}

Swift 3更新：

public extension Data {

    public mutating func xor(key: Data) {
        for i in 0..<self.count {
            self[i] ^= key[i % key.count]
        }
    }

    public func checkSum() -> Int {
        return self.map { Int($0) }.reduce(0, +) & 0xff
    }
}

您还可以创建另一个函数：xored(key: Data) -> Data。
然后你可以链接这些操作符：xored(key).checksum()

Swift支持运算符重载，让你可以轻松做let xorData = data1 ^ data2。我写了一个扩展，用于非相似大小的数据到xor。

extension Data {
    static func ^ (left: Data, right: Data) -> Data {
        if left.count != right.count {
            NSLog("Warning! XOR operands are not equal. left = \(left), right = \(right)")
        }

        var result: Data = Data()
        var smaller: Data, bigger: Data
        if left.count <= right.count {
            smaller = left
            bigger = right
        } else {
            smaller = right
            bigger = left
        }

        let bs:[UInt8] = Array(smaller)
        let bb:[UInt8] = Array (bigger)
        var br = [UInt8] ()
        for i in 0..<bs.count {
            br.append(bs[i] ^ bb[i])
        }
        for j in bs.count..<bb.count {
            br.append(bb[j])
        }
        result = Data(br)
        return result
    }
}

Swift 3更新：

func xor(data: Data, with key: Data) -> Data {
    var xorData = data

    xorData.withUnsafeMutableBytes { (start: UnsafeMutablePointer<UInt8>) -> Void in
        key.withUnsafeBytes { (keyStart: UnsafePointer<UInt8>) -> Void in
            let b = UnsafeMutableBufferPointer<UInt8>(start: start, count: xorData.count)

            let k = UnsafeBufferPointer<UInt8>(start: keyStart, count: data.count)
            let length = data.count

            for i in 0..<xorData.count {
                b[i] ^= k[i % length]
            }
        }
    }

    return xorData
}

Swift可以很容易地将Data对象视为字节数组，您可以使用zip和map的组合进行操作，从而避免了for循环的需要。这导致了更简洁和更可读的代码：

extension Data {
    static func ^ (lhs: Data, rhs: Data) -> Data {
        Data(zip(Array(lhs), Array(rhs)).map { $0 ^ $1 })
    }
}