Encountering invalid_client error when calling Apple authentication/revocation API

Hello,

I have implemented Sign in with Apple in my iOS app and am currently trying to implement the revocation feature. However, I keep encountering an invalid_client error when calling the Apple authentication/revocation API.

Here are the details of my configuration:

Team ID: HUGD2H952H
Client ID: com.puppylink.puppylinkapp
Key ID: KXSYK98424

I am using these details to generate a client secret with the JWT ES256 algorithm. Below is the code I am using on the backend server to generate the client secret:

 private fun makeClientSecret(): String {
        val now: ZonedDateTime = ZonedDateTime.now(ZoneOffset.UTC)
        val expirationTime: ZonedDateTime = now.plusMinutes(5) // Setting expiration time to 5 minutes

        return Jwts.builder()
            .setHeaderParam(JwsHeader.KEY_ID, appleProperties.keyId)
            .setHeaderParam("alg", "ES256")
            .setIssuer(appleProperties.teamId)
            .setIssuedAt(Date.from(now.toInstant()))
            .setExpiration(Date.from(expirationTime.toInstant()))
            .setAudience("https://appleid.apple.com")
            .setSubject(appleProperties.clientId)
            .signWith(getPrivateKey(), SignatureAlgorithm.ES256)
            .compact()
    }

    private fun getPrivateKey(): PrivateKey {
        val resource = ClassPathResource(appleProperties.privateKeyFile)
        val privateKey = String(Files.readAllBytes(Paths.get(resource.uri)))
        val pemReader: Reader = StringReader(privateKey)
        val pemParser = PEMParser(pemReader)
        val converter = JcaPEMKeyConverter()
        val keyInfo = pemParser.readObject() as PrivateKeyInfo
        return converter.getPrivateKey(keyInfo)
    }
}

Additionally, here is the code used to call the Apple authentication API from the backend server:

@Service
class AppleAuthService(
    private val appleProperties: AppleProperties,
) {
    private val logger = LoggerFactory.getLogger(javaClass)
    private val restTemplate = RestTemplate()

    fun getTokens(authorizationCode: String): TokenResponse {
        try {
            val clientSecret = makeClientSecret()

            val formData: MultiValueMap<String, String> = LinkedMultiValueMap()
            formData.add("client_id", appleProperties.clientId)
            formData.add("client_secret", clientSecret)
            formData.add("code", authorizationCode)
            formData.add("grant_type", "authorization_code")

            val headers = HttpHeaders()
            headers.contentType = MediaType.APPLICATION_FORM_URLENCODED

            val requestEntity = HttpEntity(formData, headers)

            val response =
                restTemplate.postForObject(
                    "https://appleid.apple.com/auth/token",
                    requestEntity,
                    TokenResponse::class.java,
                )

            return response ?: throw RuntimeException("Failed to retrieve tokens from Apple")
        } catch (ex: Exception) {
            logger.error("Error retrieving tokens: ", ex)
            throw ex
        }
    }


data class TokenResponse(
    val access_token: String,
    val expires_in: Long,
    val id_token: String,
    val refresh_token: String,
    val token_type: String,
)

Despite generating the client secret correctly, I am still receiving the invalid_client error when calling the API. Could you please help me identify the cause of this error and provide guidance on how to resolve it?

Thank you.