Lightweight C library for HTML5 websockets
JIT trust

JIT Trust logo


Most systems using openssl rely on a system trust bundle that openssl was compiled to load at library init. This is a bit expensive, since it instantiates over 120 CA X.509 certs, but most modern Linux systems don't really notice the permanent use of 1MB or so of heap from init, the advantage is client connections have all the trusted root certs available in memory to perform validation.

Using system trust bundles

For the kind of systems that choose mbedtls, they will typically either be burdened by or not even have enough ram to take this approach.

If the device only connects to endpoints that are signed by a specific CA, you can just prepare the connection with the known trusted CA, that's the approach the examples take. This method should still be used for critical connections to the cloud, for example provide the necessary CA cert in the Secure Streams policy, or at vhost creation time.

Using system trust bundles

However if you also have a browser type application that could connect anywhere, but you don't have heap spare to preload all the CAs, you need something like "JIT trust".

JIT trust overview

The basic approach is to connect to the server to retrieve its certificates, then study the certificates to determine the identity of the missing trusted cert we should be trying to validate with.

JIT Trust overview

We attempt to get the trusted cert from some local or remote store, and retry the connection having instantiated the missing CA cert as trusted for that connection, if it is one that we do actually trust. If it lies about what CA it needs to validate, or we do not trust the one it asks for, subsequent connections will fail.

If it asked for a trusted CA that we trust, and the relationship was valid, the tls negotiation should then complete successfully, and we can cache the CA cert and the host -> CA cert pre-trust requirement so future connections can work first time.

Subject Key Id and Authority Key Id

All of the certificates publish a unique-enough personal "Subject Key ID" or SKID blob. These are typically 20-byte hashes based on the cert public key.

When a server certificate is issued by the CA, an entry is made first in the certificate noting the SKID of the certificate that will be used to sign it, in an "Authority Key ID", or AKID, extension. The certificate is then signed by the parent certificate private key to prove it was issued by the real owner of the CA or intermediate certificate.

X.509 validation paths

Basically this AKID on a certificate is guiding the validator with information about which certificate it claims is next in the chain of trust leading back to a trusted CA. Lying about it doesn't help an attacker, because we're only using the AKID to get the CA certificate and then try to do the full signature check using it, if it's not really signed by the AKID cert it told, or anything else wrong, the actual validation will just fail.

A chain that terminates in a CA certificate is complete, and can undergo full validation using the tls library.

Converting the Mozilla trust bundle for JIT trust

Lws provides a bash script ./scripts/ that can fetch the latest Mozilla CA trust bundle for certs usable for tls validation, and convert it to three different forms to allow maintaining the trust bundle in different ways for different kinds of device to consume.

  • as a webroot directory, so you can server trusted DERs, with symlink indexes to the CA certs by SKID and issuer/serial
  • as an atomic binary blob, currently about 143KB, with structure at the start pointing to DER certs and indexes inside
  • a C-compiler friendly uint8_t array version of the blob, so it can be compiled into .rodata directly if necessary.

Currently there are 128 certs in the trust bundle, and the whole blob is about 143KB uncompressed.

Considerations about maintaining the trust blob

Mozilla update their trust bundle at intervals, and there have been at least three cases where they have removed or distrusted CAs from it by their own decision, because they have issued dangerous certificates, (like one for * that will validate anything at all). Certifacte owners may also revoke their own certificates for any reason and issue replacements.

The certs in the trust bundle expire, currently 10/128 will expire within 3 years and 50/128 over the next 10 years. So new and replacement certificates are also being added at intervals.

Part of using the trust bundle is building in some way to update what is trusted over the lifetime of the device, which may exceed 10 years.

Depending on the device, it may not be any problem to keep the trust blob in the firmware, and update the firmware ongoing every few months. So you could build it into the firmware using the C array include file (the minimal example takes this approach).

Another device may have difficulty updating the firmware outside of emergencies, it could keep the trust blob in a separate area and update it separately. Having it as a single blob makes it easy to fetch and update.

Finally constrained devices, say in ESP32 class, may not have space or desire to store the trust blob in the device at all, it could query a remote server on demand to check for any trusted CA matching a given AKID and retrieve and cache it in volatile ram. This would use the webroot produced by the script, via tls and a fixed CA cert outside this system.

Format of the JIT trust blob

The trust blob layout is currently

00: 54 42 4c 42 Magic "TBLB"
04: 00 01 MSB-first trust blob layout version
06: XX XX MSB-first count of certificates
08: XX XX XX XX MSB-first trust blob generation unix time
0c: XX XX XX XX MSB-first offset from blob start of cert length table
10: XX XX XX XX MSB-first offset from blob start of SKID length table
14: XX XX XX XX MSB-first offset from blob start of SKID table
18: XX XX XX XX MSB-first total blob length
1c: XX .. XX DER certs (start at +0x1c)
: XX .. XX DER cert length table (MSB-first 16-bit per cert)
: XX .. XX SKID length table (8-bit per cert)
: XX .. XX SKID table (variable per cert)

Enabling JIT Trust


Minimal example for JIT Trust

minimal-examples/http-client/minimal-http-client-jit-trust is built if JIT Trust is enabled at cmake and -DLWS_WITH_MINIMAL_EXAMPLES=1. This is based on minimal-http-client, except the loading of the system trust bundle is defeated, so by default it does not trust anything and cannot complete any tls connection. It includes the mozilla trust blob as a header file when built.

It tries to do an http client connection twice, the first time fails but JIT Trust determines which trusted CA cert is missing, retreives it from the trust blob and creates the necessary temporary vhost with the correct CA cert(s) trusted. On the next retry, the connection succeeds.

Processing of x509 AKID and SKIDs

We study each x509 cert sent by the server in turn. We parse out the SKID and AKID on each one and stash them (up to 4 deep).

After the initial validation fails due to lack of any trusted CA, lws has collected all the AKID and SKIDs that were in certs sent by the server. Since these may be sent in any order, may be malicious, and may even contain the (untrusted) root CA, they are sorted into a trust path using the AKID and SKID relationships.

To cover cross-signing and cases where the root cert(s) were wrongly sent by a misconfigured server, all of the AKIDs in the stash are queried against the trusted CA store. In cross-signing, multiple intermediates are provided with the same SKID, that all match the server certificate AKID parent. Since we might meet certificates that trust multiple valid CAs that can validate the certificate, we support up to three CA certs imported.

A user lws_system_ops handler performs the query, so it can consist of any kind of backing store or remote lookup. Helpers are provided to query the JIT trust mozilla blob, so the system helper is small in the typical case, just calling lws helpers.

The results (up to three CA certs to account for cross-signing scenarios) are collected and a 1hr TTL cache entry made for the hostname and the SKIDs of the matched CAs, if there is no existing JIT vhost with its tls context configured with the needed trusted CAs, one is created.

When the connection is retried, lws checks the cache for the hostname having a binding to an existing JIT vhost, if that exists the connection proceeds bound to that. If there is a cache entry but no JIT vhost, one is created using the information in the cache entry.

Efficiency considerations

From cold, the JIT Trust flow is

  1. A sacrificial connection is made to get the server certs
  2. Query the JIT Trust database for AKIDs mentioned in the certs (this may be done asynchronously)
  3. Create a temporary vhost with the appropriate trusted certs enabled in it, and add an entry in the cache for this hostname to the SKIDs of the CAs enabled on this temporary vhost
  4. Retry, querying the cache to bind the connection to the right temporary vhost

An lws_cache in heap is maintained so step 1 can be skipped while hostname-> SKID items exist in the cache. If the items expire or are evicted, it just means we have to do step 1 again.

For a short time, the vhost created in step 3 is allowed to exist when idle, ie when no connections are actively using it. In the case the vhost exists and the cache entry exists for the hostname, the connection can proceed successfully right away without steps 1 through 3.

APIs related to JIT Trust

Systems that support JIT trust define an lws_system_ops callback that does whatever the system needs to do for attempting to acquire a trusted cert with a specified SKID or issuer/serial.

int (*jit_trust_query)(struct lws_context *cx, const uint8_t *skid, size_t skid_len, void *got_opaque);

The ops handler doesn't have to find the trusted cert immediately before returning, it is OK starting the process and later if successful calling a helper lws_tls_jit_trust_got_cert_cb() with the got_opaque from the query. This will cache the CA cert so it's available at the next connection retry for preloading.

An helper suitable for ops->jit_trust_query using trust blob lookup in .rodata is provided in lws_tls_jit_trust_blob_queury_skid(), the callback above should be called with its results as shown in the minimal example.

Runtime tuning for JIT Trust

The context creation info struct has a couple of runtime-tunable settings related to JIT Trust.

.jitt_cache_max_footprint: default 0 means no limit, otherwise the hostname-> SKID cache is kept below this many bytes in heap, by evicting LRU entries.

.vh_idle_grace_ms: default 0 means 5000ms, otherwise sets the length of time a JIT Trust vhost is allowed to exist when it has no connections using it. Notice that, eg, h2 connections have their own grace period when they become idle, to optimize reuse, this period does not start until any h2 network connection bound to the vhost has really closed.

Considerations around http redirects

HTTP redirects are transactions that tell the client to go somewhere else to continue, typically a 301 response with a Location: header explaining where to go.

JIT Trust supports redirects to hosts with the same or different trust requirements, each step in the redirect is treated as a new connection that will fail, try to create a vhost with the right trust and work on the retry.

Lws rejects by default protocol downgrades (https -> http) on redirects, the example used a context option LCCSCF_ACCEPT_TLS_DOWNGRADE_REDIRECTS to override this.

Works out of the box on recent mbedtls and openssl

No modifications are needed to either tls library.

Compatibility Testing

A list of the top 100 sites each from the US and the ROW were combined to produce 156 unqiue domain names [1]

The Mbedtls build of JIT trust minimal example was run against each of these doing a GET on path / and restricted to h1 (--server xxx --h1). In some cases, the server at the base domain name is broken or down, as verified using as a second opinion. These domains only resolve properly using www. prefix.

In some cases the sites check the user agent and return a 4xx, these are taken as success for this test, since there was no problem at the tls layer.

site h1 h2 comment redirect-> www. then h1: timeout, h2: 403 forbidden... geolocated? TLS trust works fine. Continuously redirects to self... sends set-cookie that we don't return yet

||✕|✓|TLS trust works fine. But with or without www. server does not reply within 15s on h1, sends 403 OK on h2... Curl acts the same as we do, firefox works... geolocated?|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|www. served correctly, base domain is misconfigured with expired cert, confirmed with ssllabs + curl| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|www. served correctly, base domain is misconfigured with nonresponsive server, confirmed with ssllabs| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✕|✓|TLS trust works fine. Continuously redirects to self... www. same, curl acts same but OK if given -b -c, so akami cookie storage issue| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|OK with www., without www. TLS trust works fine but server does not reply, same with curl| ||✓|✓|| ||✓|✓|| ||✓|✓||| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✕|✓|TLS trust works fine. Needs www. else server doesn't respond in 15s, sends 403 on h2, Curl acts the same, geolocated?| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓|| ||✓|✓||


wget -O- | grep most-visited-websites-us | \
sed -E 's/class="column-2">/|/g' | tr '|' '\n' | \
sed 's/<.*//g' | grep -v Domain | grep -v Josh | sort | uniq