Imported from pax-k/pax-ctf-agent (.opencode/skills/mobile-weak-crypto/SKILL.md). Install upstream with npx skills add pax-k/pax-ctf-agent --skill mobile-weak-crypto. Copyright stays with the author (MIT).

Mobile Weak Cryptography

What Is Broken and Why

Mobile apps frequently implement cryptography incorrectly: using broken algorithms (DES, RC4), insecure modes (ECB), static/reused IVs, hardcoded keys embedded in source or resources, or non-cryptographic RNGs for key generation. Android's java.util.Random is not a CSPRNG. iOS's arc4random() is acceptable but older code uses rand(). ECB mode leaks plaintext patterns in ciphertext. Hardcoded keys are extractable via static analysis of the APK or IPA in seconds.

Key Signals

• Cipher.getInstance("AES/ECB/NoPadding") or Cipher.getInstance("DES/...") in Android code
• kCCAlgorithmDES, kCCAlgorithmRC4 in iOS CommonCrypto calls
• Hardcoded hex/base64 strings adjacent to crypto API calls
• new Random() or Math.random() used to generate keys or IVs
• arc4random() replaced by rand() or custom PRNG in security-critical iOS code
• Static byte arrays used as IV: byte[] iv = {0,0,0,0,...}
• SecretKeySpec initialized directly from a string literal: new SecretKeySpec("hardcoded".getBytes(), "AES")
• RSA without OAEP: Cipher.getInstance("RSA/ECB/PKCS1Padding")
• Key sizes below 128-bit (AES), 2048-bit (RSA), 256-bit (EC)

Methodology

1. Static analysis — Android: Decompile APK with jadx or apktool; search for Cipher.getInstance, SecretKeySpec, MessageDigest, SecureRandom, Random()
2. Static analysis — iOS: Extract IPA; search Swift/ObjC source or binary strings for CCCrypt, kCCAlgorithm, SecKey, hardcoded key strings
3. Identify algorithm strings — grep for "DES", "ECB", "RC4", "MD5", "SHA-1" in decompiled code
4. Locate hardcoded key material — search for 16/32-byte hex strings, base64-encoded strings near crypto calls
5. Check IV handling — look for static IV byte arrays or IVs derived from non-random sources
6. Check key storage — verify keys are in Android Keystore / iOS Secure Enclave, not in SharedPreferences or NSUserDefaults
7. Dynamic analysis — use Frida to hook Cipher.doFinal() / CCCrypt() and log key, IV, and plaintext at runtime
8. Verify RNG — check that SecureRandom (Android) and SecRandomCopyBytes (iOS) are used for all security-sensitive randomness

Payloads & Tools

# jadx search for weak algorithms
grep -r "ECB\|DES\|RC4\|MD5\|SHA-1\|new Random()" jadx-output/

# Frida — Android: hook Cipher.doFinal and log key+plaintext
Java.use("javax.crypto.Cipher").doFinal.overload("[B").implementation = function(b) {
  var key = this.getParameters(); console.log("Key:", key); return this.doFinal(b); };

# Frida — iOS: hook CCCrypt to log algorithm and key
Interceptor.attach(Module.findExportByName("libSystem.B.dylib", "CCCrypt"), {
  onEnter: function(args) {
    console.log("alg:", args[0], "key:", hexdump(args[3], {length: args[4].toInt32()})); }});

# MobSF static analysis (Docker)
docker run -it -p 8000:8000 opensecurity/mobile-security-framework-mobsf
# Upload APK/IPA via web UI — check Crypto section of report

# semgrep rules for Android crypto
semgrep --config p/owasp-top-ten android-source/

Bypass Techniques

• String obfuscation — keys obfuscated via XOR or base64 in strings.xml; decode with CyberChef
• Split key reconstruction — key split across multiple constants assembled at runtime; trace with Frida
• Native library crypto — crypto implemented in JNI .so; use Frida to hook CCCrypt/AES_encrypt in native code
• ProGuard obfuscation — class/method names mangled; search by API signature rather than name

Exploitation Scenarios

Scenario 1 — Hardcoded AES Key Setup: App encrypts local SQLite DB with SecretKeySpec("SuperSecret1234".getBytes(), "AES"). → Trigger: Attacker decompiles APK, extracts key string. → Impact: Decrypts all user data without authentication.

Scenario 2 — ECB Mode Pattern Leak Setup: App encrypts user profile images with AES-ECB. → Trigger: Attacker observes ciphertext blocks; identical plaintext blocks produce identical ciphertext. → Impact: Partial plaintext recovery and pattern detection in encrypted files.

Scenario 3 — Predictable IV Leads to Decryption Setup: App uses new Random(System.currentTimeMillis()).nextBytes(iv) as IV for AES-CBC. → Trigger: Attacker knows approximate encryption time (from file timestamp). → Impact: Brute-forces seed space (~ms precision), recovers IV, decrypts ciphertext.

False Positives

• MD5/SHA-1 used for non-security purposes (cache key hashing, file checksums for integrity only)
• Random used for UI animations or non-security feature (retry delay jitter)
• Hardcoded strings that look like keys but are test vectors or example data in comments
• ECB mode used for single-block encryption where semantic security is irrelevant (e.g., 16-byte fixed-format ID)

Fix Patterns

// Android — correct AES-GCM with Android Keystore
val keyGen = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, "AndroidKeyStore")
keyGen.init(KeyGenParameterSpec.Builder("myKey",
    KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT)
    .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
    .setKeySize(256).build())
val secretKey = keyGen.generateKey()
val cipher = Cipher.getInstance("AES/GCM/NoPadding")
cipher.init(Cipher.ENCRYPT_MODE, secretKey) // IV auto-generated

// iOS — AES-GCM via CryptoKit (iOS 13+)
import CryptoKit
let key = SymmetricKey(size: .bits256)
let sealedBox = try AES.GCM.seal(plaintext, using: key)
// Store key in Secure Enclave or Keychain, never in UserDefaults

• Never hardcode keys — generate at first launch and store in Android Keystore / iOS Secure Enclave
• Use AES-GCM or AES-CBC with random IV; never reuse IV with the same key
• Prefer authenticated encryption (AEAD) to detect tampering
• Use SecureRandom (Android) / SecRandomCopyBytes (iOS) for all security-sensitive randomness

Related Skills

[[mobile-insecure-storage]] and weak crypto are tightly paired: data stored in SQLite or SharedPreferences with a hardcoded AES key is effectively the same as storing it in plaintext. [[mobile-auth-bypass]] is often enabled by weak crypto — if biometric authentication relies on a poorly keyed token rather than a hardware-bound key, cryptographic bypass is possible. The predictable IV and weak PRNG issues here parallel the session token predictability analysis in [[session-fixation]] and [[cookie-attacks]] on the web side.