iOS 可以用SecRandomCopyBytes函数生成密码学安全的伪随机数，下面是使用示例：

// Swift
import Foundation
func securityRandom(length: Int) -> (Data, OSStatus) {
    var data = Data(count: length)
    var local = data
    let result = local.withUnsafeMutableBytes { mutableBytes in
        SecRandomCopyBytes(kSecRandomDefault, data.count, mutableBytes)
    }
    return (local, result)
}
let (data, result) = securityRandom(length: 16)
if (result == errSecSuccess) {
    let str = data.base64EncodedString()
    print(str)
}

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密码学基础之伪随机数

参考文献

SecRandomCopyBytes