“ base64url”在两个方面与标准base64编码不同：
- 索引62和63使用不同的字符（
-
并_
代替+
和/
） - 无需强制填充
=
字符以使字符串长度为四的倍数。
（请比较https://en.wikipedia.org/wiki/base64#Variants_summary_table）。
这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}例：
let base64url = "hJQWHABDBjoPHorYF5xghQ"let base64 = base64urlTobase64(base64url: base64url)print(base64) // hJQWHABDBjoPHorYF5xghQ==if let data = Data(base64Enpred: 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}
