Ktor client IOS and self-signed certificate - ios

Question

I'm using Ktor and Kotlin/native in iOS in an iOS app that accesses an internal dev server. The dev server uses a certificate issued by an internal CA which is not publicly trusted.
When trying to access the server with the following code :
internal suspend fun performHttp(url : String)
{
// URL is a self signed HTTPS: request
val client = HttpClient(Ios)
val response = client.get<String>(url)
println(response)
}
it throws the following exception :
TIC SSL Trust Error [32:0x281956dc0]: 3:0
esri2[470:136341] NSURLSession/NSURLConnection HTTP load failed (kCFStreamErrorDomainSSL, -9807)
esri2[470:136341] Task <F3CC4C40-0231-4E58-97F3-F457D5A18BB0>.<1> HTTP load failed (error code: -1202 [3:-9807])
esri2[470:136417] Task <F3CC4C40-0231-4E58-97F3-F457D5A18BB0>.<1> finished with error - code: -1202
esri2[470:136211] Task <F3CC4C40-0231-4E58-97F3-F457D5A18BB0>.<1> load failed with error Error Domain=NSURLErrorDomain Code=-1202 "The certificate for this server is invalid. You might be connecting to a server that is pretending to be “server1.internal.lan” which could put your confidential information at risk." UserInfo={NSLocalizedRecoverySuggestion=Would you like to connect to the server anyway?, _kCFStreamErrorDomainKey=3, NSErrorPeerCertificateChainKey=(
"<cert(0x12b094e00) s: server1.internal.lan i: Internal-Issuing-CA2>",
How do I convince Ktor that it should access this URL, or ignore untrusted certs? Yes, I know that one should not ignore untrusted certs, but this is a lab test.

Answer 1

Ktor iOS engine offers the ability to configure the underlying NSURLSession with the help of IosClientEngineConfig.kt.
With it you can configure (amongst other things) a ChallengeHandler by setting the block for handleChallenge in the config like this:
val client = HttpClient(Ios) {
engine {
handleChallenge(TrustSelfSignedCertificate())
}
}
Then you need to implement a class in Kotlin something like this:
internal data class TrustSelfSignedCertificate internal constructor(
private val validateTrust: Boolean = true
) : ChallengeHandler {
override fun invoke(
session: NSURLSession,
task: NSURLSessionTask,
challenge: NSURLAuthenticationChallenge,
completionHandler: (NSURLSessionAuthChallengeDisposition, NSURLCredential?) -> Unit
) {
val hostname = challenge.protectionSpace.host
val serverTrust = challenge.protectionSpace.serverTrust
var result: SecTrustResultType = 0u
memScoped {
val nativeResult = alloc<SecTrustResultTypeVar>()
nativeResult.value = result
SecTrustEvaluate(serverTrust!!, nativeResult.ptr)
}
val serverCertificate = SecTrustGetCertificateAtIndex(serverTrust, 0)
val serverCertificateData = SecCertificateCopyData(serverCertificate)
val data = CFDataGetBytePtr(serverCertificateData)
val size = CFDataGetLength(serverCertificateData)
val cert1 = NSData.dataWithBytes(data, size.toULong())
val pathToCert = NSBundle.mainBundle.pathForResource("myOwnCert", "cer")
val localCertificate: NSData = NSData.dataWithContentsOfFile(pathToCert!!)!!
if (localCertificate == cert1) {
completionHandler(
NSURLSessionAuthChallengeUseCredential,
NSURLCredential.create(serverTrust)
)
} else {
completionHandler(NSURLSessionAuthChallengeCancelAuthenticationChallenge, null)
}
}
}
Also, don't forget to put you certificate as a file "myOwnCert.cer" into you iOS project (maybe on the top-level).
NOTE
Ktor with iOS engine does not respect/use NSApptransportSecurity.
The code is based on this answers.
With the help of this blog-post.