How can I decode JWT (JSON web token) token in Swift?

If you are okay with using a library i would suggest this https://github.com/auth0/JWTDecode.swift

and then import the library import JWTDecode and execute.

let jwt = try decode(jwt: token)

Since you didn’t want to include this library i brought out the needed parts to make it work.

func decode(jwtToken jwt: String) -> [String: Any] {
  let segments = jwt.components(separatedBy: ".")
  return decodeJWTPart(segments[1]) ?? [:]
}

func base64UrlDecode(_ value: String) -> Data? {
  var base64 = value
    .replacingOccurrences(of: "-", with: "+")
    .replacingOccurrences(of: "_", with: "https://stackoverflow.com/")

  let length = Double(base64.lengthOfBytes(using: String.Encoding.utf8))
  let requiredLength = 4 * ceil(length / 4.0)
  let paddingLength = requiredLength - length
  if paddingLength > 0 {
    let padding = "".padding(toLength: Int(paddingLength), withPad: "=", startingAt: 0)
    base64 = base64 + padding
  }
  return Data(base64Encoded: base64, options: .ignoreUnknownCharacters)
}

func decodeJWTPart(_ value: String) -> [String: Any]? {
  guard let bodyData = base64UrlDecode(value),
    let json = try? JSONSerialization.jsonObject(with: bodyData, options: []), let payload = json as? [String: Any] else {
      return nil
  }

  return payload
}

Call it like this:

decode(jwtToken: TOKEN)