When developing Android apps for sensitive industries like finance, security is paramount. One of the most critical aspects of securing communication between the app and the server is ensuring that TLS (Transport Layer Security) is implemented correctly. TLS is what keeps our data encrypted while being transferred over the internet, protecting users from attackers trying to intercept or tamper with the information.

In this blog, we’ll dive deep into Strong TLS Validation and how we can implement it in financial Android apps. This includes ensuring that the server we’re communicating with is legitimate and that the communication is safe and encrypted. I’ll walk you through the concept, why it’s so important, and how to integrate strong TLS validation into your Android financial app.

Let’s get started!

Why TLS Validation Matters in Financial Apps

When developing financial applications, we’re dealing with sensitive information like user credentials, financial transactions, and personal data. If an attacker can intercept or manipulate the communication between the app and the server, they could potentially steal money, data, or perform unauthorized actions. This makes it absolutely crucial to implement strong TLS validation to ensure that the communication is both confidential and authentic.

TLS ensures that the data sent from the client (our Android app) to the server is encrypted and cannot be read or altered by anyone in between. However, just encrypting the data isn’t enough. We also need to ensure that the app communicates with the right server (and not a malicious one) by verifying the server’s identity.

The Basics of TLS

Before we go into the code, let’s quickly recap what TLS does. TLS (formerly SSL) is a protocol used to secure data transmission over the internet. It ensures three key things:

1. Confidentiality – Encrypts data so that even if it’s intercepted, it’s unreadable.
2. Integrity – Ensures the data hasn’t been altered during transmission.
3. Authentication – Verifies the identity of the server (so we know we’re talking to the right server).

When we connect to a server over HTTPS (which uses TLS), the server sends its TLS certificate to prove its identity. The client (our Android app) then checks the validity of the certificate. If the certificate is valid, the communication is established securely.

But how do we ensure that the certificate is trusted and legitimate in our Android app? That’s where Strong TLS Validation comes in.

Strong TLS Validation Explaination

Strong TLS validation involves verifying the following:

1. Certificate Authenticity — Is the certificate issued by a trusted Certificate Authority (CA)?
2. Certificate Expiry — Is the certificate expired?
3. Certificate Revocation — Has the certificate been revoked by the CA?
4. Domain Validation — Does the domain match the one specified in the certificate?
5. Public Key Pinning — Is the public key of the server the same as the one expected by the app?

By performing these checks, we can ensure that the server we’re communicating with is authentic and that the connection is secure.

Implementing Strong TLS Validation in Android

Now that we understand the importance of strong TLS validation, let’s see how we can implement it in our Android financial app using Kotlin.

Enforcing HTTPS in Android

The first step in implementing TLS validation is ensuring that our app communicates over HTTPS rather than HTTP. HTTP is not encrypted, so it should never be used for sensitive communication.

In Android, we can enforce HTTPS by ensuring that all our URLs are prefixed with https://. We can also configure the app’s network security configuration to block insecure connections.

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config cleartext-traffic-permitted="false">
        <domain includeSubdomains="true">your-financial-app.com</domain>
    </domain-config>
</network-security-config>

This configuration blocks all cleartext (non-HTTPS) traffic while allowing traffic to the specified domain.

Validating Server Certificates with Custom Trust Manager

The next step is to implement certificate validation using a custom TrustManager. This is the core of our TLS validation, where we ensure that the server’s certificate is valid and trustworthy.

import android.util.Log
import java.security.cert.X509Certificate
import javax.net.ssl.X509TrustManager
import javax.net.ssl.SSLContext
import javax.net.ssl.TrustManagerFactory

class CustomTrustManager : X509TrustManager {
    override fun checkClientTrusted(chain: Array<out X509Certificate>?, authType: String?) {
        // Here, you can add additional client-side certificate validation if needed.
    }

    override fun checkServerTrusted(chain: Array<out X509Certificate>?, authType: String?) {
        // Validate the server certificate chain
        try {
            // Perform strong certificate validation here (e.g., certificate pinning, issuer validation)
            val cert = chain?.firstOrNull()
            val issuer = cert?.issuerDN?.name
            if (issuer != "CN=Your Trusted CA") {
                throw Exception("Untrusted certificate issuer: $issuer")
            }
            Log.d("TLS", "Server certificate is trusted.")
        } catch (e: Exception) {
            Log.e("TLS", "Certificate validation failed: ${e.message}")
            throw e
        }
    }

    override fun getAcceptedIssuers(): Array<X509Certificate>? {
        return null // Use default trust management for accepted issuers
    }
}

Here, we are checking the issuer of the server’s certificate. You can extend this to validate other aspects, like expiration, revocation, and more.

Configuring SSLContext

Next, we need to create an SSLContext that uses our custom TrustManager to enforce strong validation.

import javax.net.ssl.SSLContext
import javax.net.ssl.HttpsURLConnection
import java.security.NoSuchAlgorithmException
import java.security.KeyManagementException

fun setupSSLContext() {
    try {
        // Create an SSL context with our custom TrustManager
        val sslContext = SSLContext.getInstance("TLS")
        sslContext.init(null, arrayOf(CustomTrustManager()), null)

        // Set the default SSLSocketFactory to use our custom validation
        HttpsURLConnection.setDefaultSSLSocketFactory(sslContext.socketFactory)
    } catch (e: NoSuchAlgorithmException) {
        Log.e("TLS", "Error initializing SSLContext: ${e.message}")
    } catch (e: KeyManagementException) {
        Log.e("TLS", "Error initializing SSLContext: ${e.message}")
    }
}

This setupSSLContext function initializes an SSLContext with our custom TrustManager. It ensures that any HTTPS connection made by the app will undergo strong validation based on our rules.

Using Custom SSL Pinning in Android

One of the strongest techniques for ensuring the integrity of the server’s identity is SSL pinning. SSL pinning involves hardcoding the server’s certificate or public key in the app, ensuring that the app only trusts the specified server.

import okhttp3.*
import java.security.cert.CertificateFactory
import java.io.InputStream

class CustomSSLPinningInterceptor(private val certificateInputStream: InputStream) : Interceptor {

    override fun intercept(chain: Interceptor.Chain): Response {
        // Create an SSLContext using the custom certificate
        val cf = CertificateFactory.getInstance("X.509")
        val ca = cf.generateCertificate(certificateInputStream)

        // Creating a KeyStore that contains our certificate
        val keyStore = java.security.KeyStore.getInstance("PKCS12")
        keyStore.load(null, null)
        keyStore.setCertificateEntry("ca", ca)

        // Set up the TrustManager with our certificate
        val trustManagerFactory = javax.net.ssl.TrustManagerFactory.getInstance(javax.net.ssl.TrustManagerFactory.getDefaultAlgorithm())
        trustManagerFactory.init(keyStore)

        // Create an SSLContext
        val sslContext = javax.net.ssl.SSLContext.getInstance("TLS")
        sslContext.init(null, trustManagerFactory.trustManagers, java.security.SecureRandom())

        // Create a custom OkHttpClient with our SSLContext
        val sslSocketFactory = sslContext.socketFactory
        val client = OkHttpClient.Builder()
            .sslSocketFactory(sslSocketFactory, trustManagerFactory.trustManagers[0] as javax.net.ssl.X509TrustManager)
            .hostnameVerifier { _, _ -> true }  // Disable hostname verification for custom pinning
            .build()

        return client.newCall(chain.request()).execute()
    }
}

In this code,

• We first load the certificate that we want to pin (usually obtained from the server) into a KeyStore.
• We then create a TrustManagerFactory and set it up to use our custom certificate.
• The SSLContext is configured to only trust our specified certificate for secure communication.
• The OkHttpClient is then configured to use this custom SSL context, enforcing SSL pinning.

Using the Custom SSL Pinning Interceptor

Once we’ve created the custom SSL pinning interceptor, we need to attach it to our OkHttp client.

val certificateInputStream = assets.open("my_server_certificate.crt") // Load certificate from assets
val interceptor = CustomSSLPinningInterceptor(certificateInputStream)
val okHttpClient = OkHttpClient.Builder()
    .addInterceptor(interceptor)
    .build()

// Now, use this client for your network requests
val retrofit = Retrofit.Builder()
    .baseUrl("https://your-financial-app.com")
    .client(okHttpClient)
    .addConverterFactory(GsonConverterFactory.create())
    .build()

Host Name Verification

In addition to certificate pinning, it’s also important to perform proper hostname verification to ensure the server’s identity. Android’s default SSL handling does this for you, but when implementing custom SSL pinning, you should still verify the hostname manually.

val client = OkHttpClient.Builder()
    .hostnameVerifier { hostname, session ->
        // Manually verify the server's hostname
        hostname == "your-financial-app.com"  // Replace with your expected server hostname
    }
    .build()

Handling Expired or Invalid Certificates

Another crucial part of TLS validation is handling expired or invalid certificates. In production apps, certificates may expire, so it’s important to have a strategy in place for handling these cases. One approach is to implement fallback mechanisms, like showing a user-friendly error message or redirecting to a page explaining the issue.

try {
    val response = okHttpClient.newCall(request).execute()
    if (response.isSuccessful) {
        // Handle successful response
    } else {
        // Handle server-side error
    }
} catch (e: SSLHandshakeException) {
    // Handle SSL validation failure
    showError("Security certificate is invalid or expired. Please contact support.")
}

Public Key Pinning (Optional but Recommended)

For even more security, we can use Public Key Pinning to ensure that we’re always communicating with the expected server. This involves storing the server’s public key hash in the app and verifying that it matches the one in the server’s certificate.

import okhttp3.CertificatePinner
import okhttp3.OkHttpClient

fun createPinnedClient(): OkHttpClient {
    val certificatePinner = CertificatePinner.Builder()
        .add("your-financial-app.com", "sha256/AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=")
        .build()

    return OkHttpClient.Builder()
        .certificatePinner(certificatePinner)
        .build()
}

This ensures that the app only connects to the server with the specified public key. If the key doesn’t match, the connection will be blocked, preventing man-in-the-middle attacks.

So, by pinning the certificate, we are making sure that our app only trusts the exact server we’ve configured. Even if a malicious attacker tries to intercept the communication by presenting a forged certificate, the app will reject the connection since the server certificate doesn’t match the one it expects.

Best Practices and Testing

• Testing: Use tools like SSL Labs to test your server’s TLS configuration.
• Stay Updated: Regularly review TLS best practices and update your implementation to address emerging threats.
• Avoid Shortcuts: Never disable TLS checks in production, even during debugging.

Conclusion

Implementing strong TLS validation in financial Android apps is crucial to ensure the security and privacy of sensitive user data. By enforcing HTTPS, using custom TrustManagers, and even implementing certificate pinning, we can significantly reduce the risk of man-in-the-middle attacks and ensure that our app communicates only with trusted servers.

Remember, security is an ongoing process, and it’s essential to stay updated with the latest security best practices. With the steps I’ve outlined here, you’ll be on your way to making your financial Android app secure and trustworthy for your users.