Add MASTG-TEST-0231 for weak encryption modes on Android

tests-beta/android/MASVS-CRYPTO/MASTG-TEST-0231.md

@@ -0,0 +1,52 @@
+---
+title: Weak Encryption Modes
+platform: android
+id: MASTG-TEST-0231
+type: [static, dynamic]
+weakness: MASWE-0020
+---
+
+## Overview
+
+To test for the [use of weak encryption modes](../../../Document/0x04g-Testing-Cryptography.md#weak-block-cipher-mode) in Android apps, we need to focus on methods from cryptographic frameworks and libraries that are used to configure and apply encryption modes.
+
+In Android development, the `Cipher` class from the Java Cryptography Architecture (JCA) is the primary API that allows you to specify the encryption mode for cryptographic operations. [`Cipher.getInstance`](https://developer.android.com/reference/javax/crypto/Cipher#getInstance(java.lang.String)) defines the transformation string, which includes the encryption algorithm, mode of operation, and padding scheme. The general format is `"Algorithm/Mode/Padding"`. For example:
+
+```kotlin
+Cipher.getInstance("AES/ECB/PKCS5Padding")
+```
+
+In this test we're going to focus on symmetric encryption modes such as [ECB (Electronic Codebook)](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_codebook_(ECB)) operate deterministically, dividing plaintext into blocks and encrypting them separately, which reveals patterns in the ciphertext. This makes it vulnerable to attacks like [known-plaintext attacks](https://en.wikipedia.org/wiki/Known-plaintext_attack) and [chosen-plaintext attacks](https://en.wikipedia.org/wiki/Chosen-plaintext_attack).
+
+For example, the following transformations are all [considered vulnerable](https://support.google.com/faqs/answer/10046138?hl=en):
+
+- "AES" (uses AES/ECB mode by default)
+- "AES/ECB/NoPadding"
+- "AES/ECB/PKCS5Padding"
+- "AES/ECB/ISO10126Padding"
+
+You can learn more about ECB and other modes in [NIST SP 800-38A - Recommendation for Block Cipher Modes of Operation: Methods and Techniques](https://csrc.nist.gov/pubs/sp/800/38/a/final). Also check the [Decision to Revise NIST SP 800-38A, Recommendation for Block Cipher Modes of Operation: Methods and Techniques](https://csrc.nist.gov/news/2023/decision-to-revise-nist-sp-800-38a) and [NIST IR 8459 Report on the Block Cipher Modes of Operation in the NIST SP 800-38 Series](https://nvlpubs.nist.gov/nistpubs/ir/2024/NIST.IR.8459.pdf) for the latest information.
+
+**Out of Scope**: Asymmetric encryption modes like RSA are out of scope for this test because they don't use block modes like ECB.
+
+In the transformation strings like `"RSA/ECB/OAEPPadding"` or `"RSA/ECB/PKCS1Padding"`, the inclusion of `ECB` in this context is misleading. Unlike symmetric ciphers, **RSA doesn't operate in block modes like ECB**. The `ECB` designation is a [placeholder in some cryptographic APIs](https://github.com/openjdk/jdk/blob/680ac2cebecf93e5924a441a5de6918cd7adf118/src/java.base/share/classes/com/sun/crypto/provider/RSACipher.java#L126) and doesn't imply that RSA uses ECB mode. Understanding these nuances helps prevent false positives.
+
+## Steps
+
+1. Run @MASTG-TECH-0014 with a tool such as @MASTG-TOOL-0110 on the app binary, or use @MASTG-TECH-0033 (dynamic analysis) with a tool like @MASTG-TOOL-0001, and look for cryptographic functions specifying the encryption mode to insecure modes.
+
+## Observation
+
+The output should contain a list of locations where insecure or deprecated encryption modes are used in cryptographic operations.
+
+## Evaluation
+
+The test case fails if any insecure encryption modes are identified in the app.
+
+## Mitigation
+
+Replace insecure encryption modes with secure block cipher modes such as [AES-GCM or AES-CCM](https://csrc.nist.gov/pubs/sp/800/38/d/final) which are authenticated encryption modes that provide confidentiality, integrity, and authenticity.
+
+We recommend avoiding CBC, which while being more secure than ECB, improper implementation, especially incorrect padding, can lead to vulnerabilities such as padding oracle attacks.
+
+For comprehensive guidance on implementing secure encryption modes in Android, refer to the official Android Developers documentation on [Cryptography](https://developer.android.com/privacy-and-security/cryptography).