AASA - Apple App Site Association - Not working - apple-app-site-associate

Question

I have been having a long and frustrating experience trying to get AASA to work for webcredentials. My goal here is to allow usernames and passwords to be stored in the iOS keychain.
I did have this working on a root domain the other week but it is not sufficient for my scenario as I will explain. It didn't work for me straight away I have to say but it eventually started working after a clean build so I thought this was the issue then but now I am not so sure.
I am using Expo with EAS build. We have a multi-tenant application. From a single codebase we deploy to multiple apps in the store. All are on the same team ID but they are separate applications and use separate credentials, nothing is shared.
I am confident my apps textContentType of username and password on my TextFields is correct as this has not changed from when I managed to get it working originally and I have checked it countless times.
Expectation
For the "Save Password" prompt to be displayed after login. What I have noticed however is when going to store a password manually using "add password" via iCloudKeychain from the keyboard accessory this does accurately show the correct "TENANT_SUBDOMAIN.example.com". I find this confusing.
Goal Scenario
I am hosting a site on Netlify. I have it setup to support wildcard subdomains with a LetsEncrypt provisioned wildcard SSL certificate. I then have edge functions which change the content of my index.html and apple-app-site-association file dynamically based on the requested subdomain.
I have added the Associated Domains capability to my provisioning profile.
I am using the latest Expo 47 and EAS build. I have added in the appropriate associated domains configuration and I can see this when introspecting my entitlements under com.apple.developer.associated-domains and it is correct.
I am using TestFlight for testing. I am doing a --clean-build on EAS every time and I also increase the runtime version. I have also tried manually refreshing credentials outside of the build process which does this automatically. This must be using the correct provisioning profile otherwise I would get a build failure as the requested entitlements wouldn't match.
The AASA file is currently hosted just in the .well-known directory. I have tried using the root and also tried using both. There are no redirects taking place.
I am aware the AASA file is pulled on application installation and update. I repeatedly remove the apps and then reboot my phone in an attempt to reset any device caches.
The content-type of the file is application/json and I have confirmed this using developer tools in the browser.
There is no robots.txt or anything blocking the request from an infrastructure perspective. There are no additional firewalls or geo restricted access as I am just using plain Netlify to host this, nothing fancy.
I am confident the Team ID and bundle IDs are correct in the AASA file.
I remove the content-length header in the Edge function so it is correctly calculated by the network instead and I have confirmed this using curl.
When I check the file using https://app-site-association.cdn-apple.com/a/v1/site.example.com Apple has the correct file cached on it's CDN so I would expect it to work.
I added in an applinks section so I could use the Apple App Search API validation tool and the Branch.io AASA verification tool to verify correctness. Branch.io says the file is fine and Apple says it's fine also but because the App has not been deployed to the store yet I see Error no apps with domain entitlements. From what I can tell this is normal in development and makes sense as it uses the current released version of the app to verify the deep link configuration. So to me this means Apple can parse the file correctly.
When I stream my device console logs; on install I can see the AASA requesting the correct domains. I see no errors on swcd I just see the Beginning data task AASA-XXXX with the correct domains.
When I run Charles proxy on my phone with a verified SSL installation (also reinstalled a few times now) I do not see quite what I would expect - but the device logs seem to imply it is doing the correct thing. When I view the app-site-association... URL requests in Charles there is one per application install which is correct. The request is marked as Unknown and when I look at the request the host is shown but as you would expect from SSL I see no path. The info says METHOD: CONNECT with Error - Input Error: EOF. This is the only error I see, I am not sure if it is a red herring and something to do with Charles. Given the error as you expect there is no body in the request or response. It is worth noting in general testing I have no VPN enabled and I have do not have Private Relay enabled in my iOS settings.
When I perform a sysdiagnose I see the following at the timestamp in my console log in the swcutil_show.txt device log. This looks correct in comparison to other apps webcredentials and applinks services I see there and I see no errors:
Service: webcredentials
App ID: MYTEAMID.com.cf.example.b2c.ios
App Version: 1.0
App PI: <LSPersistentIdentifier 0x141816200> { v = 0, t = 0x8, u = 0x1e7c, db = 0094F7C4-3078-41A2-A33E-79D5A62C80A6, {length = 8, bytes = 0x7c1e000000000000} }
Domain: CORRECT_SUBDOMAIN.example.app
User Approval: unspecified
Site/Fmwk Approval: approved
Flags:
Last Checked: 2022-12-09 14:14:32 +0000
Next Check: 2022-12-14 14:03:00 +0000
Service: applinks
App ID: MYTEAMID.com.cf.example.b2c.ios
App Version: 1.0
App PI: <LSPersistentIdentifier 0x13fd38d00> { v = 0, t = 0x8, u = 0x219c, db = 0094F7C4-3078-41A2-A33E-79D5A62C80A6, {length = 8, bytes = 0x9c21000000000000} }
Domain: CORRECT_SUBDOMAIN.example.app
Patterns: {"/":"*"}
User Approval: unspecified
Site/Fmwk Approval: approved
Flags:
Last Checked: 2022-12-13 13:13:23 +0000
Next Check: 2022-12-18 13:01:51 + 0000
At end of file:
MYTEAMID.com.cf.example.b2c.ios: 8 bytes
(This seems correct for all apps)
Other Scenario
I have tried setting this up using an apex on another domain which hasn't been seen before by Apple. I have tried using a subdomain with a root domain serving the same content and I have tried the subdomain and root domain on their own. I have also tried not using the Edge functions and having static files but to no avail.
When I do this I ensure I wait for the Apple CDN to catch up and remove/add entries prior to deleting the apps, rebooting my device, and reinstalling to test.
AASA File
AASA content comes back with the correct payload and Content-Type: application/json and Content-Length headers, both from Apples CDN and the origin. When I had this somehow working in my initial test it was on a root domain and I did not have an applinks section, this was only added so I could use the verification tools for universal links.
I am not sending back different content or duplicated content and I block the www subdomain - I have also tried it with a www subdomain for the record.
{
"applinks": {
"details": [
{
"appIDs": [
"MYTEAMID.com.cf.example.b2c.ios"
],
"components": [
{
"#": "no_universal_links",
"exclude": true,
"comment": "Matches any URL with a fragment that equals no_universal_links and instructs the system not to open it as a universal link."
}
]
}
]
},
"webcredentials": {
"apps": [
"MYTEAMID.com.cf.example.b2c.ios"
]
}
}
I have also tried this with the older format:
{
"applinks": {
"apps": [],
"details": [
{
"appID": "MYTEAMID.com.cf.example.b2c.ios",
"paths": [
"*"
]
}
]
},
"webcredentials": {
"apps": [
"MYTEAMID.com.cf.example.b2c.ios"
]
}
}
associatedDomains iOS. expo config
associatedDomains: [
`webcredentials:${SUBDOMAIN}.example.app`,
`applinks:${SUBDOMAIN}.example.app`,
],
Help :)
I have been trying to get this to work for a long time now and I am completely out of ideas. If anybody has any suggestions I would really appreciate it. I am very confused how the devices request seems correct and the CDN content is correct but it is still not working. It's worth also reiterating that I need to have different subdomains for each tenant as the credentials must not be shared across apps so the keychain->domain association store must be different.
I am wondering if it's the LetsEncrypt wildcard SSL certificate but I wouldn't expect it to verify and for Apple to cache the file if this was the case. It seems very unlikely to me but it is the only thing I haven't tried at this point.
Many Thanks,
Mark