ID Token verification for RS256

IDTokenVerificationRS256.swift

import Foundation
import Security

/// JWT検証
/// - Parameters:
///   - rsaPublicKey: 公開鍵の SecKey
///   - signature: JWT のドット区切りの3番目
///   - headerAndPayload: JWT のドット区切りの1・2番目
/// - Returns: JWTの検証結果
func verifyJWT(rsaPublicKey: SecKey, signature: String, headerAndPayload: String) -> Bool {
    guard let signatureData = Data(base64URLEncoded: signature) else { return false }
    let headerAndPayloadData: Data = .init(headerAndPayload.utf8)
    var error: Unmanaged<CFError>?
    /// https://mtanriverdi.medium.com/how-to-decode-jwt-and-validate-the-signature-in-swift-97092bd654f7
    let result: Bool = SecKeyVerifySignature(
        rsaPublicKey,
        .rsaSignatureMessagePKCS1v15SHA256,
        headerAndPayloadData as CFData,
        signatureData as CFData,
        &error
    )
    if let error {
        print(#line, "Error verifying data: \(error.takeRetainedValue())")
        return false
    }
    return result
}

/// JWKからSecKeyを生成する処理
func createRSAPublicKey(n: String, e: String) -> SecKey? {
    guard let modulusData = Data(base64URLEncoded: n),
          let exponentData = Data(base64URLEncoded: e) else {
        print("Invalid JWK")
        return nil
    }

    var modulusBytes: [UInt8] = .init(modulusData)
    let exponentBytes: [UInt8] = .init(exponentData)
    if let prefix = modulusBytes.first, prefix != 0 {
        modulusBytes.insert(0, at: 0)
    }
    let modulusEncoded: [UInt8] = modulusBytes.encode(as: .integer)
    let exponentEncoded: [UInt8] = exponentBytes.encode(as: .integer)
    let sequenceEncoded: [UInt8] = (modulusEncoded + exponentEncoded).encode(as: .sequence)
    let keyData: Data = .init(sequenceEncoded)
    let keySize: Int = modulusData.count * 8

    // RSA 公開鍵の辞書を作成
    let attributes: [String: Any] = [
        kSecAttrKeyType as String: kSecAttrKeyTypeRSA,
        kSecAttrKeyClass as String: kSecAttrKeyClassPublic,
        kSecAttrKeySizeInBits as String: keySize,
        kSecAttrIsPermanent as String: false
    ]

    var error: Unmanaged<CFError>?
    let publicKey: SecKey? = SecKeyCreateWithData(
        keyData as CFData,
        attributes as CFDictionary,
        &error
    )
    if let error {
        print("Error creating public key: \(error.takeRetainedValue())")
    }
    return publicKey
}

private extension Data {
    init?(base64URLEncoded string: String) {
        self.init(
            base64Encoded: string.decodeBase64URL(),
            options: .ignoreUnknownCharacters
        )
    }
}

private extension String {
    func decodeBase64URL() -> String {
        let paddingEquals: String = .init(repeating: "=", count: (4 - count % 4) % 4)
        return (self + paddingEquals)
            .replacingOccurrences(of: "-", with: "+")
            .replacingOccurrences(of: "_", with: "/")
    }
}

enum ASN1Type {
    case sequence, integer
    var tag: UInt8 {
        switch self {
        case .sequence: 0x30
        case .integer: 0x02
        }
    }
}

extension [UInt8] {
    func encode(as type: ASN1Type) -> Self {
        [type.tag] + lengthField() + self
    }

    private func lengthField() -> Self {
        guard count >= 128 else { return [UInt8(count)] }

        // The number of bytes needed to encode count.
        let lengthBytesCount: UInt8 = .init(log2(Double(count)) / 8 + 1)

        // The first byte in the length field encoding the number of remaining bytes.
        let firstLengthFieldByte: UInt8 = 128 + lengthBytesCount

        var c: Int = count
        let lengthField: [UInt8] = (0..<lengthBytesCount).reduce(into: []) { result, _ in
            // Take the last 8 bits of count.
            let lengthByte: UInt8 = .init(c & 0xff)
            // Add them to the length field.
            result.append(lengthByte)
            // Delete the last 8 bits of count.
            c >>= 8
        } + [firstLengthFieldByte] // Include the first byte.

        return lengthField.reversed()
    }
}

let n: String = "wnfD2k6iOI8IdDTKPY4J6HFOT1nKor6v2xEZ9G2n1_KtPs5-5aC8W_SvRTzXF9Ym-BeoQI5mfHSbaYafbeEDaCSVpxXja1K8n7EAlpYVGydTHgL2NLHADb-Gtkkiv8Gw9sSyea_foPW_i2YknOIyBM4A2Sxqf9VPQTSTj5zJGFtRnyQYuuTprxqj9qgZfAAhrGCizsW8bm62nH2DYORQ10rwaiY9kL4gVOPrU39vaB80YX5a2N-TRzDCzHaKlo9vSBMzysFs1WFmb9VdOLuIae1I7h50KFUIDncxv7tGrVxnYBi_etNl989JmDtDzLnPK3u4AMFEGcha52Y2QwxQeQ"
let e: String = "AQAB"
/// JWT のドット区切りの3番目
let signature: String = "bzNpok6tybsHOicXvbP9Q97kKO14ei3B1DXlNa8LFiZj8rQJfnm_rATRlMFEGs1fsW5Av7srDy-2JjdEbQufHbYlUBXIJh7_sBwI_qU6NIYn2t8hcGpMnXoe2z0BtkP3CyvvTINRVxA6WwHv_Teh0nzxnaxmcOVm0ajLKT603Crtt4MNur_azADTxNxYafaQ5o7XOo9V0PMM0nVy6kqn-N3IjxBPNXqQapmxub6qzJcRsOyAjOyzK1hRAuxvX9vd9fAoBf4ycpbeTWIy7nQIeEU8kl2lTNSb9DBZrsVP7GzhFRdEMDIxctcBoqXDxBuYLuSXGlnMyfSYy0sU39VBtw"
/// JWT のドット区切りの1・2番目
let headerAndPayload: String = "eyJhbGciOiJSUzI1NiIsImtpZCI6ImRhMGI1ZDQyNDRjY2ZiNzViMjcwODQxNjI5NWYwNWQ1MThjYTY5MDMifQ.eyJpc3MiOiJhY2NvdW50cy5nb29nbGUuY29tIiwiYXRfaGFzaCI6Ijd2ajAzMklIQWdzMEdNUGxOUDFkV2ciLCJhdWQiOiI5MTQ2OTk1NzE0NS1qYjg0MnUwcmNnbTg3bTIyMDlhZWxiZnRzbDlwMzU3aS5hcHBzLmdvb2dsZXVzZXJjb250ZW50LmNvbSIsInN1YiI6IjExNzQ0MjQ1MDQ0MzI0NDAzNTk1NSIsImVtYWlsX3ZlcmlmaWVkIjp0cnVlLCJhenAiOiI5MTQ2OTk1NzE0NS1qYjg0MnUwcmNnbTg3bTIyMDlhZWxiZnRzbDlwMzU3aS5hcHBzLmdvb2dsZXVzZXJjb250ZW50LmNvbSIsImVtYWlsIjoiYm9idW5kZXJzb25AZ21haWwuY29tIiwiaWF0IjoxNDQzNzY4NzcxLCJleHAiOjE0NDM3NzIzNzF9"

if let publicKey = createRSAPublicKey(n: n, e: e) {
    let result: Bool = verifyJWT(
        rsaPublicKey: publicKey,
        signature: signature,
        headerAndPayload: headerAndPayload
    )
    print(#line, result)
}