JWT Debugger — Swift source

// JWT base64url decoding helper — Swift port of the jwt tool.

import Foundation

enum B64urlError: Error { case invalidInput }

enum Jwt {
    // Decode a base64url segment (JWT header/payload) to text. Data's base64
    // decoder is strict: after the URL-safe mapping it returns nil for input
    // outside the standard alphabet, mapped here to the TS throw analogue.
    static func b64urlDecode(_ str: String) throws -> String {
        // 1. URL-safe alphabet -> standard base64 alphabet.
        var s = ""
        for c in str.unicodeScalars {
            switch c {
            case "-": s.unicodeScalars.append("+")
            case "_": s.unicodeScalars.append("/")
            default: s.unicodeScalars.append(c)
            }
        }

        // 2. Restore stripped '=' padding so the length is a multiple of 4.
        if s.unicodeScalars.count % 4 == 1 { throw B64urlError.invalidInput }
        s += String(repeating: "=", count: (4 - s.unicodeScalars.count % 4) % 4)

        // 3. Strict standard base64 -> bytes.
        guard let data = Data(base64Encoded: s) else { throw B64urlError.invalidInput }

        // 4. Bytes -> text, byte-faithful: each byte becomes the Unicode
        //    scalar of the same value (Latin-1), mirroring JS atob so the
        //    full 0x00-0xFF byte range round-trips (UTF-8 would drop bytes).
        return data.map { String(UnicodeScalar($0)) }.joined()
    }
}

// eyJhbGciOiJIUzI1NiJ9 decodes to {"alg":"HS256"} (main.swift: top-level try)
print(try Jwt.b64urlDecode("eyJhbGciOiJIUzI1NiJ9"))