From: Joann Mõndresku <joann@cernodile.com>
Date: Sun, 12 May 2024 18:55:14 +0000 (+0300)
Subject: Protobuf API post: Added CA instructions and webserver config
X-Git-Url: https://git.based.quest/?p=web-hugo.git;a=commitdiff_plain;h=refs%2Fheads%2Fmaster

Protobuf API post: Added CA instructions and webserver config
---

diff --git a/content/posts/reverse-engineering-a-mobile-app-protobuf-api.md b/content/posts/reverse-engineering-a-mobile-app-protobuf-api.md
index 5c94bd1..ce06d43 100644
--- a/content/posts/reverse-engineering-a-mobile-app-protobuf-api.md
+++ b/content/posts/reverse-engineering-a-mobile-app-protobuf-api.md
@@ -95,6 +95,8 @@ When I jumped to it, I saw an exactly adjacent string to it which could give mor
 Interesting, `www.auxbrain.com`. If we jump to its XREF, we get a garbled function, but what it seems to be doing is setting up
 certain global values.
 
+## The smoke-test
+
 So we have a potential API endpoint, let's put it to the test. We're not going to recompile anything yet or do any byte-patching,
 let's try a quick smoke-test. Ensure your phone is rooted and you have a variant of Xposed Framework installed (I used LSPosed).
 We will need to unarm the SSL pinning present in most apps, including this one, I used [io.github.tehcneko.sslunpinning](https://github.com/Xposed-Modules-Repo/io.github.tehcneko.sslunpinning) module.
@@ -104,7 +106,40 @@ Next, install [AdAway app from F-Droid](https://f-droid.org/packages/org.adaway/
 Inside AdAway, add a redirection rule for the address we just found and point it to an IP address in your LAN that will run the API server.
 
 Generate a self-signed certificate authority and a certificate signed by it and run a webserver with both HTTP and HTTPS on the API server machine.
-Import the self-signed CA to your phone's truststore. Once all of that is done, run the app for first time.
+```
+# Create an ext file containing the Subject Alternative Name (SAN)
+# DNS.1 should correspond to the API endpoint of the app (more info near end of article if you plan on changing)
+cat > auxbrain.ext << EOF
+authorityKeyIdentifier=keyid,issuer
+basicConstraints=CA:FALSE
+keyUsage = digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment
+subjectAltName = @alt_names
+
+[alt_names]
+DNS.1 = www.auxbrain.com
+EOF
+
+# Create your own Certificate Authority
+openssl genrsa -des3 -out myCA.key 2048
+openssl req -x509 -new -nodes -key myCA.key -sha256 -days 1825 -out myCA.pem
+# Create a CSR and lets have the new CA sign it
+openssl req -new -key auxbrain.key -out auxbrain.csr -nodes
+openssl x509 -req -in auxbrain.csr -CA myCA.pem -CAkey myCA.key -CAcreateserial -out auxbrain.crt -days 825 -sha256 -extfile auxbrain.ext
+# You now have myCA.pem - the public certificate of your root CA, auxbrain.key - the private key for your webserver, auxbrain.pem - the public cert for your webserver.
+```
+
+Use the generated `auxbrain.pem` and `auxbrain.key` files for your webserver SSL/TLS configuration. For nginx, append following values to your server directive:
+```
+listen 443 ssl;
+ssl_certificate /path/to/auxbrain.pem;
+ssl_certificate_key /path/to/auxbrain.key;
+ssl_session_cache shared:SSL:1m;
+ssl_session_timeout 5m;
+ssl_ciphers  HIGH:!aNULL:!MD5;
+ssl_prefer_server_ciphers on;
+```
+
+Import the self-signed CA (myCA.pem) to your phone's truststore (Check under your phone's Security/Encryption settings). Once all of that is done, run the app for first time.
 
 ```
 192.168.1.212 - - [...] "POST /ei/first_contact HTTP/1.1" 404 0 "-"