Android MSAL: Fix Redirect URI Error With '+' SHA-1

by Alex Johnson 52 views

Are you encountering a frustrating MSAL Android redirect URI failure on Android 14 and above, specifically when your app's signing certificate's Base64 SHA-1 hash contains a plus (+) character? You're not alone! This issue, often manifesting as a MsalClientException stating that "The redirect URI in the configuration file doesn't match with the one generated with package name and signature hash," can be a real head-scratcher. Let's dive deep into why this happens and, more importantly, how you can resolve it, ensuring a smooth authentication flow for your Android users.

Understanding the Root Cause: The '+' Character Conundrum

The core of the problem lies in how Android handles URI encoding and how the Microsoft Authentication Library (MSAL) for Android interacts with it. When your Android app is signed with a certificate whose Base64-encoded SHA-1 hash includes a + character, a peculiar transformation occurs. The + character, which is a valid character in Base64 encoding, is interpreted and decoded as a space within the URI path by the Android system. This seemingly small change has significant implications for MSAL's redirect URI validation process. MSAL expects a precise match between the redirect URI configured in your msal_config.json file and the one it generates based on your app's package name and signature hash. When the + is decoded into a space, the generated URI no longer matches the registered one, leading to the validation failure.

For instance, imagine your expected redirect URI is msauth://<your.package.name>/+/lRvk.... Due to the Android decoding, MSAL receives msauth://<your.package.name>/ /lRvk... (note the space where the + should be). This discrepancy, even a single character difference, is enough for MSAL to throw the MsalClientException. This issue is particularly noticeable on newer Android versions like Android 14 (API level 34) and beyond, though it might have been present in earlier versions with varying degrees of impact. The environment where this was observed includes devices like the Google Pixel running Android 16 with MSAL version 7.0.1, highlighting that this is a contemporary concern for developers targeting recent Android platforms.

Reproducing the Issue: A Step-by-Step Guide

To effectively troubleshoot and confirm this behavior in your own development environment, follow these steps:

  1. Sign your Android app with a certificate whose Base64 SHA-1 contains a '+' character. This is the crucial first step. If your current signing certificate doesn't have a + in its Base64 SHA-1 hash, you might need to generate a new debug or release keystore for testing purposes. You can obtain your app's SHA-1 hash using the keytool command (e.g., keytool -list -v -keystore your_keystore.jks | findstr SHA1). Ensure you're looking at the Base64 encoded SHA-1. If it contains +, proceed to the next step.

  2. Register your Android platform in Azure Entra ID (formerly Azure AD). Navigate to your application registration in the Azure portal. Under 'Authentication,' add an Android platform. You'll need to provide your app's package name and the SHA-1 fingerprint of your signing certificate.

  3. Use the generated redirect URI. Azure Entra ID will generate a recommended redirect URI based on your input, typically in the format msauth://<package.name>/<base64-encoded-sha1>. If your SHA-1 has a +, this URI will include it. Copy this URI precisely.

  4. Configure the same redirect URI in your msal_config.json file. Ensure that the redirectUri property in your MSAL configuration JSON precisely matches the URI generated by Azure Entra ID, including the + character.

  5. Call acquireToken(...) from your Android app. Initiate the authentication flow using MSAL's acquireToken method. Observe the logs for MsalClientException or any other errors related to redirect URI validation. You should see the error message indicating a mismatch between the expected and received redirect URIs, directly stemming from the + to space conversion.

By following these steps, you can reliably reproduce the issue and confirm that the + character in the Base64 SHA-1 hash is indeed the culprit behind the MSAL redirect URI validation failure on Android 14+.

The Technical Breakdown: URI Encoding and MSAL's Logic

Let's delve a bit deeper into the technical nuances of this problem. The Uniform Resource Identifier (URI) specification has certain reserved characters that perform specific functions within a URI. The + character, in the context of a URI's query string, is typically reserved to represent a space character (often used in application/x-www-form-urlencoded data). While the redirect URI provided to MSAL might not strictly be a query string, the Android system's URL parsing and handling mechanisms can sometimes apply similar decoding rules, especially when dealing with parameters within a path segment. The msauth:// scheme itself is designed to facilitate inter-app communication for authentication, and it expects specific formats.

MSAL for Android generates a redirect URI that includes your application's package name and the Base64-encoded SHA-1 fingerprint of your signing certificate. This generated URI is crucial for Azure Entra ID to identify and securely redirect back to your application after successful authentication. The signature hash acts as a verifiable proof of identity, ensuring that the redirect is indeed coming back to the legitimate application.

When the Base64 SHA-1 hash contains a + character, and Android's URI processing decodes this + into a space, the resulting string passed to MSAL for validation looks different from what was originally registered. MSAL's validation logic performs a strict, character-by-character comparison. It compares the URI it expects (based on your configuration) with the URI it receives (after the Android system has potentially modified it). If the + was decoded into a space, these two URIs will not match, leading to the MsalClientException.

Example Scenario:

  • Your App's Base64 SHA-1: +/lRvk...
  • Redirect URI Registered in Azure Entra ID: msauth://com.example.myapp/+/lRvk...
  • msal_config.json: "redirectUri": "msauth://com.example.myapp/+/lRvk..."
  • Android System Processing: The + in the SHA-1 part of the redirect URI gets decoded to a space.
  • URI MSAL Compares Against: msauth://com.example.myapp/ lRvk... (notice the space)
  • MSAL Validation Failure: The "+/lRvk..." from config does not equal " lRvk..." received.

This highlights how a seemingly minor character can break the entire authentication flow due to the interplay between Base64 encoding, URI parsing, and strict validation routines within the authentication library and the Android OS. Understanding this specific decoding behavior is key to finding the correct workaround.

The Solution: A Multi-pronged Approach

Fortunately, there are effective ways to overcome this + character issue. The most robust solution involves adapting your redirect URI registration and configuration to account for this Android-specific behavior. Here are the recommended strategies:

  1. Modify the Redirect URI in Azure Entra ID and msal_config.json: This is the most direct and often the most effective solution. Instead of registering the redirect URI with the literal + character, you should register it with the URL-encoded equivalent, which is %2B. The + character, when it needs to be part of a URI path segment and not represent a space, should be encoded.

    • Original (problematic) SHA-1: +/lRvk...
    • Original Redirect URI: msauth://<package.name>/+/lRvk...
    • Corrected Redirect URI: msauth://<package.name>/%2BlRvk...

    You need to make this change in two key places:

    • Azure Entra ID: Go to your application registration, navigate to 'Authentication,' and edit the Android platform's redirect URI. Replace the + with %2B.
    • msal_config.json: Update the redirectUri value in your msal_config.json file to use %2B instead of +.

    When MSAL generates the URI on the device, it will use the + (which Android then often decodes to a space), but this is not the part being validated. MSAL's validation logic is comparing the configured redirect URI with the one it generates internally. If you configure %2B, and the system decodes it correctly when parsing the URI later, or if MSAL's internal generation correctly uses %2B when it detects the problematic SHA-1, the match should occur. Correction: The more common and effective approach is to register %2B in Azure AD and your msal_config.json. MSAL then correctly handles the comparison or generation. The key is consistency.

  2. Consider Using a Different Redirect URI Format: While msauth:// is the standard for Android, MSAL also supports custom RedirectUri schemes. If modifying the existing msauth:// URI proves problematic or you wish to explore alternatives, you can define a custom scheme. For example, you could use myapp://auth. When registering this custom scheme in Azure Entra ID and your msal_config.json, ensure it adheres to URI standards and avoids characters that might be ambiguously decoded.

    • msal_config.json: "redirectUri": "myapp://auth"
    • Azure Entra ID: Register myapp://auth as the Android redirect URI.

    This approach bypasses the msauth:// scheme's specific handling and relies on your custom scheme, which you control entirely. However, this requires careful implementation to ensure security and proper handling within your app.

  3. Update MSAL for Android: Ensure you are using the latest stable version of MSAL for Android. The MSAL team is actively working on improving compatibility and addressing platform-specific issues. While version 7.0.1 was mentioned, it's always a good practice to check for newer releases (e.g., 7.1.0, 7.2.0, etc.) as they might contain direct fixes or improved handling for such encoding/decoding discrepancies. Check the MSAL Android release notes for any relevant updates.

  4. Alternative SHA-1 Generation (Advanced/Less Recommended): In some very niche scenarios, developers might consider re-signing their application with a certificate that does not have a + character in its Base64 SHA-1 hash. This is generally not recommended for production environments as it involves managing multiple signing keys and can complicate the signing process. However, for specific testing or legacy situations, it might be a temporary workaround. It's far better to address the redirect URI configuration directly.

Recommendation:

The most recommended and straightforward solution is to use the URL-encoded form (%2B) for the + character in your redirect URIs, both in your Azure Entra ID application registration and your msal_config.json file. This approach directly addresses the decoding issue without requiring significant architectural changes.

Verifying the Fix: A Final Check

After implementing the suggested solution (preferably using %2B), it's crucial to verify that the redirect URI validation now passes successfully. Re-run the steps outlined in the