分两个步骤：
1.从字符串创建md5数据
2.将md5数据隐蔽为十六进制字符串

Swift 2.0：

func md5(string string: String) -> String {
    var digest = [UInt8](count: Int(CC_MD5_DIGEST_LENGTH), repeatedValue: 0)
    if let data = string.dataUsingEncoding(NSUTF8StringEncoding) {
        CC_MD5(data.bytes, CC_LONG(data.length), &digest)
    }

    var digestHex = ""
    for index in 0..<Int(CC_MD5_DIGEST_LENGTH) {
        digestHex += String(format: "%02x", digest[index])
    }

    return digestHex
}

//Test:
let digest = md5(string:"Hello")
print("digest: \(digest)")

输出：

摘要：8b1a9953c4611296a827abf8c47804d7

Swift 3.0：

func MD5(string: String) -> Data {
    let messageData = string.data(using:.utf8)!
    var digestData = Data(count: Int(CC_MD5_DIGEST_LENGTH))

    _ = digestData.withUnsafeMutableBytes {digestBytes in
        messageData.withUnsafeBytes {messageBytes in
            CC_MD5(messageBytes, CC_LONG(messageData.count), digestBytes)
        }
    }

    return digestData
}

//Test:
let md5Data = MD5(string:"Hello")

let md5Hex =  md5Data.map { String(format: "%02hhx", $0) }.joined()
print("md5Hex: \(md5Hex)")

let md5Base64 = md5Data.base64EncodedString()
print("md5Base64: \(md5Base64)")

输出：

md5Hex：8b1a9953c4611296a827abf8c47804d7
md5Base64：ixqZU8RhEpaoJ6v4xHgE1w ==

Swift 5.0：

import Foundation
import var CommonCrypto.CC_MD5_DIGEST_LENGTH
import func CommonCrypto.CC_MD5
import typealias CommonCrypto.CC_LONG

func MD5(string: String) -> Data {
        let length = Int(CC_MD5_DIGEST_LENGTH)
        let messageData = string.data(using:.utf8)!
        var digestData = Data(count: length)

        _ = digestData.withUnsafeMutableBytes { digestBytes -> UInt8 in
            messageData.withUnsafeBytes { messageBytes -> UInt8 in
                if let messageBytesBaseAddress = messageBytes.baseAddress, let digestBytesBlindMemory = digestBytes.bindMemory(to: UInt8.self).baseAddress {
                    let messageLength = CC_LONG(messageData.count)
                    CC_MD5(messageBytesBaseAddress, messageLength, digestBytesBlindMemory)
                }
                return 0
            }
        }
        return digestData
    }

//Test:
let md5Data = MD5(string:"Hello")

let md5Hex =  md5Data.map { String(format: "%02hhx", $0) }.joined()
print("md5Hex: \(md5Hex)")

let md5Base64 = md5Data.base64EncodedString()
print("md5Base64: \(md5Base64)")

输出：

md5Hex：8b1a9953c4611296a827abf8c47804d7
md5Base64：ixqZU8RhEpaoJ6v4xHgE1w ==

注意：
#import <CommonCrypto/CommonCrypto.h>必须添加到Bridging-Header文件中

有关如何创建Bridging-Header的信息

通常，不应将MD​​5用于新工作，SHA256是当前的最佳实践。

不推荐使用的文档部分中的示例：

MD2，MD4，MD5，SHA1，SHA224，SHA256，SHA384，SHA512（Swift 3+）

这些函数将使用八种密码哈希算法之一来哈希String或Data输入。

name参数将哈希函数名称指定为String
支持的函数是MD2，MD4，MD5，SHA1，SHA224，SHA256，SHA384和SHA512 a此示例需要通用加密
必须在项目中具有桥接头： #import <CommonCrypto/CommonCrypto.h>
添加安全性.framework到项目。

此函数采用哈希名称和要哈希的String并返回一个Data：

name：作为字符串的哈希函数的名称  
string：要散列的字符串  
返回：哈希结果为Data



func hash(name:String, string:String) -> Data? {
    let data = string.data(using:.utf8)!
    return hash(name:name, data:data)
}

例子：

let clearString = "clearData0123456"
let clearData   = clearString.data(using:.utf8)!
print("clearString: \(clearString)")
print("clearData: \(clearData as NSData)")

let hashSHA256 = hash(name:"SHA256", string:clearString)
print("hashSHA256: \(hashSHA256! as NSData)")

let hashMD5 = hash(name:"MD5", data:clearData)
print("hashMD5: \(hashMD5! as NSData)")

输出：

clearString: clearData0123456
clearData: <636c6561 72446174 61303132 33343536>

hashSHA256: <aabc766b 6b357564 e41f4f91 2d494bcc bfa16924 b574abbd ba9e3e9d a0c8920a>
hashMD5: <4df665f7 b94aea69 695b0e7b baf9e9d6>