Code snippet for setting up an Azure Notification Hub installation (*not* registration) in Kotlin using Retrofit

AzureRestApiService.kt

interface RestApiService {
    companion object {
        const val PUSH_PLATFORM = "gcm"
    }

    @PUT
    fun installPush(@Url endpoint: String,
                    @Header("Authorization") authorization: String, 
                    @Body request: PushInstallationRequest): Observable<ResponseBody>

    @Keep data class PushInstallationRequest(
            val installationId: String,
            val pushChannel: String,
            val templates: Map<String, PushTemplate>,
            val platform: String = PUSH_PLATFORM
    ) {
        @Keep data class PushTemplate(val body: String)
        constructor(installationId: String, pushChannel: String, templateName: String, 
                    template: String, platform: String = PUSH_PLATFORM)
            : this(installationId, pushChannel, mapOf(templateName to PushTemplate(template)), platform)
    }
}

fun RestApiService.installPush(fcmToken: String, installationId: String, templateName: String, template: String) 
        = Observable.fromCallable {
    val endpoint = hubBaseEndpoint.buildUpon()
            .scheme("https")
            .appendPath(BuildConfig.HUB_NAME)
            .appendPath("installations")
            .appendPath(installationId)
            .appendQueryParameter("api-version", "2016-07")
            .toString()
    val sasToken = getSasToken(endpoint, 60)
    val request = RestApiService.PushInstallationRequest(installationId, fcmToken, templateName, template)
    installPush(endpoint, sasToken, request)
}.flatMap { it }

private val hubConnectionParams by lazy {
    BuildConfig.HUB_LISTEN_CONNECTION.split(";").associate { it.split("=", limit = 2).let { kv -> kv[0] to kv[1] } }
}
private val hubBaseEndpoint by lazy { Uri.parse(hubConnectionParams["Endpoint"])!! }
private val hubSharedAccessKeyName by lazy { hubConnectionParams["SharedAccessKeyName"]!! }
private val hubSharedAccessKey by lazy { hubConnectionParams["SharedAccessKey"]!! }

private fun getSasToken(uri: String, minUntilExpire: Long): String {
    // Encode target Uri
    val targetUri = Uri.encode(uri).toLowerCase()
    // Set expiry date. 'minUntilExpire' from now (in seconds)
    val expiry = (Instant.now() + Duration.standardMinutes(minUntilExpire)).millis / 1000
    // Create signature of target uri & expiry date
    val signature = Uri.encode(Base64.encodeToString("$targetUri\n$expiry".toHMac256(hubSharedAccessKey), Base64.NO_WRAP))
    return "SharedAccessSignature sr=$targetUri&sig=$signature&se=$expiry&skn=$hubSharedAccessKeyName"
}

private fun String.toHMac256(keyString: String): ByteArray {
    val encoder = Mac.getInstance("HmacSHA256")
    val key = SecretKeySpec(keyString.toByteArray(Charsets.UTF_8), "HmacSHA256")
    encoder.init(key)
    return encoder.doFinal(toByteArray(Charset.defaultCharset()))
}