Certificate Pinning (DI)

Overview

Public key pinning is provided using the Android Network Security Configuration and TrustKit. To use public key pinning, you can either create an Android network security configuration XML file or set a custom TrustManager implementation. The network security configuration is supported natively on Android Nougat (API Level 24) and newer. For versions between API Level 21 and 23 the Gini SDK relies on TrustKit. The custom TrustManager is supported on all Android versions. We recommend reading the Android Network Security Configuration guide and the TrustKit limitations for API Levels 21 to 23.

Configure pinning

The following sample configuration shows how to set the public key pin for two domains. The Gini Bank SDK’s default networking implementation uses by default pay-api.gini.net and user.gini.net. It should be saved under res/xml/network_security_config.xml:

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config>
        <trustkit-config
            disableDefaultReportUri="true"
            enforcePinning="true" />
        <domain includeSubdomains="false">pay-api.gini.net</domain>
        <pin-set>
            <!-- old *.gini.net public key-->
            <pin digest="SHA-256">cNzbGowA+LNeQ681yMm8ulHxXiGojHE8qAjI+M7bIxU=</pin>
            <!-- new *.gini.net public key, active from around June 2020 -->
            <pin digest="SHA-256">zEVdOCzXU8euGVuMJYPr3DUU/d1CaKevtr0dW0XzZNo=</pin>
        </pin-set>
        <domain-config>
            <trustkit-config
                disableDefaultReportUri="true"
                enforcePinning="true" />
            <domain includeSubdomains="false">user.gini.net</domain>
        </domain-config>
    </domain-config>
</network-security-config>

If you set different base urls when instantiating with the GiniCaptureDefaultNetworkService.Builder, then make sure you set matching domains in the network security configuration XML.

The above digests serve as an example only. You should always create the digest yourself from the Gini API’s public key and use that one (see Extract hash from pay-api.gini.net). If you received a digest from us then always validate it by comparing it to the digest you created from the public key (see Extract hash from public key). Failing to validate a digest may lead to security vulnerabilities.

TrustKit

The TrustKit configuration tag <trustkit-config> is required in order to disable TrustKit reporting and to enforce public key pinning. This is important because without it TrustKit won’t throw CertificateExceptions if the local public keys didn’t match any of the remote ones, effectively disabling pinning. The only downside of enforcing pinning is that two public key hashes are required. In the example above we create and used a “zero” key hash as a placeholder. Setting the same key hash twice won’t help since key hashes are stored in a set. Ideally, you should use a backup public key hash as the second one.

In your AndroidManifest.xml you need to set the android:networkSecurityConfig attribute on the <application> tag to point to the XML:

<?xml version="1.0" encoding="utf-8"?>
<manifest ...>
    ...
    <application android:networkSecurityConfig="@xml/network_security_config">
    ...
</manifest>

Enable pinning with a network security configuration

For the library to know about the XML, you need to set the XML resource id using the GiniCaptureDefaultNetworkService.Builder.setNetworkSecurityConfigResId() method:

val defaultNetworkService = GiniCaptureDefaultNetworkService.builder(context)
        .setClientCredentials("gini-client-id", "GiniClientSecret", "example.com")
        .setNetworkSecurityConfigResId(R.xml.network_security_config)
        .build();

Enable pinning with a custom TrustManager implementation

You can also take full control over which certificates to trust by passing your own TrustManager implementation to the GiniBankAPIBuilder#setTrustManager() method:

val defaultNetworkService = GiniCaptureDefaultNetworkService.builder(context)
        .setClientCredentials("gini-client-id", "GiniClientSecret", "example.com")
        .setTrustManager(yourTrustManager)
        .build();

Setting a custom TrustManager will override the network security configuration.

Extract hash from pay-api.gini.net

$ openssl s_client -servername pay-api.gini.net -connect pay-api.gini.net:443 | openssl x509 -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

Extract hash from public key

You can also extract the hash from a public key. The following example shows how to extract it from a public key named pay-api.gini.pub:

$ cat pay-api.gini.pub | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64