基於密碼的金鑰派生校準（Swift 3）

確定當前平臺上特定延遲使用的 PRF 輪次數。

有幾個引數預設為不應對圓計數產生實質影響的代表值。

password Sample password.  
salt     Sample salt.  
msec     Targeted duration we want to achieve for a key derivation.

returns  The number of iterations to use for the desired processing time.

func pbkdf2SHA1Calibrate(password: String, salt: Data, msec: Int) -> UInt32 {
    let actualRoundCount: UInt32 = CCCalibratePBKDF(
        CCPBKDFAlgorithm(kCCPBKDF2),
        password.utf8.count,
        salt.count,
        CCPseudoRandomAlgorithm(kCCPRFHmacAlgSHA1),
        kCCKeySizeAES256,
        UInt32(msec));
    return actualRoundCount
}

用法示例：

let saltData       = Data(bytes: [0x73, 0x61, 0x6c, 0x74, 0x44, 0x61, 0x74, 0x61])
let passwordString = "password"
let delayMsec      = 100

let rounds = pbkdf2SHA1Calibrate(password:passwordString, salt:saltData, msec:delayMsec)
print("For \(delayMsec) msec delay, rounds: \(rounds)")

示例輸出：

For 100 msec delay, rounds: 93457