Preventing Man-in-the-Middle Attacks in iOS with SSL Pinning

Validating Digital Certificates

When you get a certificate from a CA, that certificate is part of a chain of trust, or a chain of certificates.

The number of certificates in the chain depends on the CA’s hierarchical structure. The two-tier hierarchy is the most common. An issuing CA signs the user’s certificate and a root CA signs the issuing CA’s certificate. The root CA is self-signed and the app must trust it at the end.

During a certificate validation, the app verifies:

• The date of evaluation, which must fall between the Valid From and Valid To fields of the certificate for the certificate to be valid.
• The digital signature, by finding the public key of the next issuing CA or intermediate CA. The process continues until it reaches the root certificate.

SSL Certificate Pinning Under the Hood

SSL Certificate Pinning, or pinning for short, is the process of associating a host with its certificate or public key. Once you know a host’s certificate or public key, you pin it to that host.

In other words, you configure the app to reject all but one or a few predefined certificates or public keys. Whenever the app connects to a server, it compares the server certificate with the pinned certificate(s) or public key(s). If and only if they match, the app trusts the server and establishes the connection.

You usually add a service’s certificate or public key at development time. In other words, your mobile app should include the digital certificate or the public key within your app’s bundle. This is the preferred method, since an attacker cannot taint the pin.

Why Do You Need SSL Certificate Pinning?

Usually, you delegate setting up and maintaining TLS sessions to iOS. This means that when the app tries to establish a connection, it doesn’t determine which certificates to trust and which not to. The app relies entirely on the certificates that the iOS Trust Store provides.

This method has a weakness, however: An attacker can generate a self-signed certificate and include it in the iOS Trust Store or hack a root CA certificate. This allows such an attacker to set up a man-in-the-middle attack and capture the transmitted data moving to and from your app.

Restricting the set of trusted certificates through pinning prevents attackers from analyzing the functionality of the app and the way it communicates with the server.

Types of SSL Certificate Pinning

If you want to implement pinning — which it seems you do, since you’re reading this tutorial — you can decide between two options:

• Pin the certificate: You can download the server’s certificate and bundle it into your app. At runtime, the app compares the server’s certificate to the one you’ve embedded.
• Pin the public key: You can retrieve the certificate’s public key and include it in your code as a string. At runtime, the app compares the certificate’s public key to the one hard-coded in your code.

Choosing between these two options depends on your needs and server configuration. If you choose the first option, you need to upload your app when your server rotates (changes) its certificate or it will stop working. If you choose the second option, it may violate key rotation policy because the public key doesn’t change.

Now that you have a solid grasp on how pinning works, it’s time to see what Alamofire 5 can do for you!

Pinning in Alamofire 5

Alamofire 5 supports the pinning of both the certificate and the public key. In particular, it provides two different classes, called respectively PinnedCertificatesTrustEvaluator and PublicKeysTrustEvaluator, which allow you to deal with these cases.

Storing The Certificate

To see Alamofire 5 in action, first you need to download the certificate from StackExchange.com.

Use OpenSSL to retrieve the certificate from the Stack Overflow server. More specifically, you’ll use the s_client command, which can connect to any server over SSL by specifying the server address and port 443.

Open a new Terminal and type cd followed by a space. Then, drag and drop the directory of the starter project that you downloaded in the Getting Started section and press Enter on your keyboard.

Still in the terminal window, type cd PinMyCert to move into your project’s root folder.

Next, copy and paste the following snippet:

openssl s_client -connect api.stackexchange.com:443 </dev/null

Once it completes, you'll receive a lot of data including a list of certificates. Each certificate in the chain has a Common Name (CN).

Certificate chain
 0 s:/C=US/ST=NY/L=New York/O=Stack Exchange, Inc./CN=*.stackexchange.com
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA
 1 s:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA

Below that, you can see the actual certificate you're interested in, which is the one where CN is *.stackexchange.com.

Server certificate
-----BEGIN CERTIFICATE-----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CWFjKWJNMAI6ZBAtlbgSNgRYxdcdleIhPLCzkzWysfltfiBmsmgz6VCoFR4KgJo8
Gd3MeTWojBthM10SLwIDAQABo4IDzDCCA8gwHwYDVR0jBBgwFoAUUWj/kK8CB3U8
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GCoubWV0YS5zdGFja2V4Y2hhbmdlLmNvbYIWbWV0YS5zdGFja2V4Y2hhbmdlLmNv
bYIQbWF0aG92ZXJmbG93Lm5ldIISKi5tYXRob3ZlcmZsb3cubmV0gg1hc2t1YnVu
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LmVtYWlsghJzdGFja292ZXJmbG93LmJsb2cwDgYDVR0PAQH/BAQDAgWgMB0GA1Ud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-----END CERTIFICATE-----
subject=/C=US/ST=NY/L=New York/O=Stack Exchange, Inc./CN=*.stackexchange.com
issuer=/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA

To copy the certificate into a file, use openssl again. Repeat the previous command and pass its output to openssl x509, specify DER encoding and output it to a new file named stackexchange.com.der:

openssl s_client -connect api.stackexchange.com:443 </dev/null \
  | openssl x509 -outform DER -out stackexchange.com.der

If you've followed the steps correctly, you should be able to see that certificate in the same folder of your project.