"base64url"与标准Base64编码在两个方面不同：

• 索引62和63使用不同的字符（-和_代替+和/）
• 不需要用=字符填充字符串，使字符串长度为4的倍数。

（请比较https：//en.wikipedia.org/wiki/Base64#variants_summary_table）。
下面是一个可能的转换函数：

func base64urlToBase64(base64url: String) -> String {
    var base64 = base64url
        .replacingOccurrences(of: "-", with: "+")
        .replacingOccurrences(of: "_", with: "/")
    if base64.characters.count % 4 != 0 {
        base64.append(String(repeating: "=", count: 4 - base64.characters.count % 4))
    }
    return base64
}

示例：

let base64url = "hJQWHABDBjoPHorYF5xghQ"
let base64 = base64urlToBase64(base64url: base64url)
print(base64) // hJQWHABDBjoPHorYF5xghQ==

if let data = Data(base64Encoded: base64) {
    print(data as NSData) // <8494161c 0043063a 0f1e8ad8 179c6085>
}

为了完整起见，这将是相反的转换：

func base64ToBase64url(base64: String) -> String {
    let base64url = base64
        .replacingOccurrences(of: "+", with: "-")
        .replacingOccurrences(of: "/", with: "_")
        .replacingOccurrences(of: "=", with: "")
    return base64url
}

• • Swift 4的更新：**

func base64urlToBase64(base64url: String) -> String {
    var base64 = base64url
        .replacingOccurrences(of: "-", with: "+")
        .replacingOccurrences(of: "_", with: "/")
    if base64.count % 4 != 0 {
        base64.append(String(repeating: "=", count: 4 - base64.count % 4))
    }
    return base64
}

以下是Martin在Swift 4扩展中发布的内容的清理版本：

import Foundation

/// Extension for making base64 representations of `Data` safe for
/// transmitting via URL query parameters
extension Data {

    /// Instantiates data by decoding a base64url string into base64
    ///
    /// - Parameter string: A base64url encoded string
    init?(base64URLEncoded string: String) {
        self.init(base64Encoded: string.toggleBase64URLSafe(on: false))
    }

    /// Encodes the string into a base64url safe representation
    ///
    /// - Returns: A string that is base64 encoded but made safe for passing
    ///            in as a query parameter into a URL string
    func base64URLEncodedString() -> String {
        return self.base64EncodedString().toggleBase64URLSafe(on: true)
    }

}

extension String {

    /// Encodes or decodes into a base64url safe representation
    ///
    /// - Parameter on: Whether or not the string should be made safe for URL strings
    /// - Returns: if `on`, then a base64url string; if `off` then a base64 string
    func toggleBase64URLSafe(on: Bool) -> String {
        if on {
            // Make base64 string safe for passing into URL query params
            let base64url = self.replacingOccurrences(of: "/", with: "_")
                .replacingOccurrences(of: "+", with: "-")
                .replacingOccurrences(of: "=", with: "")
            return base64url
        } else {
            // Return to base64 encoding
            var base64 = self.replacingOccurrences(of: "_", with: "/")
                .replacingOccurrences(of: "-", with: "+")
            // Add any necessary padding with `=`
            if base64.count % 4 != 0 {
                base64.append(String(repeating: "=", count: 4 - base64.count % 4))
            }
            return base64
        }
    }

}

这是Base64 URL到Base64的目标c版本

• （NSString *）基本64 url到基本64：（NSString *）基本64 url {

NSString * base64 = [base64url stringByReplacingOccurrencesOfString:@"-" withString:@"+"];
base64 =  [base64 stringByReplacingOccurrencesOfString:@"_" withString:@"/"];
if(base64.length  % 4 != 0)
{
    base64 = [base64 stringByAppendingString:[self stringWithRepeatString:"=" times:(4 - base64.length % 4)]];
}
return  base64;

}

• （NSString*）字符串与重复字符串：（char*）个字符乘以：（unsigned int）重复次数;{

unsigned long stringLength = strlen(characters);
unsigned long repeatStringLength = stringLength * repetitions + 1;
char repeatString[repeatStringLength];

for (unsigned int i = 0; i < repetitions; i++) {
    unsigned int pointerPosition = i * repetitions;
    memcpy(repeatString + pointerPosition, characters, stringLength);
}

repeatString[repeatStringLength - 1] = 0;
return  [NSString stringWithCString:repeatString encoding:NSASCIIStringEncoding];

}