crypto-recon-apk-java

name	crypto-recon-apk-java
description	Specialist for finding ALL cryptography in decompiled Android APK — Java source (jadx) and Smali bytecode (apktool). Covers OkHttp interceptors, Retrofit, HMAC, AES, MD5, custom signing, ProGuard-obfuscated code.

Crypto Recon — APK Java/Smali Crypto Specialist

Companion Files

Read these alongside this SKILL.md for deep pattern catalogs:

• java-patterns.md — BouncyCastle, Tink, Guava, custom hex, ProGuard-survived patterns, BuildConfig extraction
• smali-patterns.md — Smali bytecode search commands, Smali → Python translation, ProGuard navigation

Purpose

Scan decompiled APK source (Java + Smali) for every cryptographic operation. Priority target: OkHttp/Retrofit interceptors that sign ALL API requests — equivalent of axios.interceptors in web.

Input

• output/[apk_name]/java/ — jadx decompiled Java (readable, preferred)
• output/[apk_name]/smali/ — apktool Smali (fallback when Java fails)
• output/[apk_name]/class_index.txt — all class names
• HAR test vectors from output/[apk_name]/har/ if available

Priority File List

Search these first — highest chance of crypto:

High priority class names:
  *Interceptor*, *SignInterceptor*, *AuthInterceptor*
  *Crypto*, *CryptoUtil*, *CryptoHelper*, *EncryptUtil*
  *Sign*, *Signer*, *SignUtil*, *SignHelper*
  *Security*, *SecurityUtil*, *SecurityManager*
  *Hash*, *MD5*, *SHA*, *HMAC*
  *ApiClient*, *HttpClient*, *NetworkUtil*
  *RequestUtil*, *RequestBuilder*, *RequestHelper*
  *Token*, *Auth*, *Session*

High priority package names:
  *.network.*, *.http.*, *.api.*
  *.utils.*, *.util.*, *.helper.*
  *.security.*, *.crypto.*

Run priority scan first:

python tools/apk_scan.py output/[apk_name]/ --priority

---

Java Pattern Catalog

1. OkHttp Interceptors (HIGHEST VALUE)

OkHttp interceptors intercept ALL requests — this is where signing lives.

// Pattern 1: implements Interceptor
public class SignInterceptor implements Interceptor {
    @Override
    public Response intercept(Chain chain) throws IOException {
        Request original = chain.request();
        // ← signing logic here
        return chain.proceed(signedRequest);
    }
}

// Pattern 2: Lambda interceptor
OkHttpClient client = new OkHttpClient.Builder()
    .addInterceptor(chain -> {
        Request request = chain.request();
        // ← signing logic here
        return chain.proceed(signed);
    })
    .build();

// Pattern 3: Header injection
Request signed = original.newBuilder()
    .header("X-Sign", generateSign(body))
    .header("X-Timestamp", String.valueOf(System.currentTimeMillis() / 1000))
    .header("X-Nonce", UUID.randomUUID().toString().replace("-", ""))
    .build();

Search patterns (Java):

implements Interceptor
addInterceptor(
addNetworkInterceptor(
chain.request()
chain.proceed(
newBuilder().*header(

2. Hash Functions

// MD5
MessageDigest md = MessageDigest.getInstance("MD5");
md.update(data.getBytes("UTF-8"));
byte[] hash = md.digest();
String hex = bytesToHex(hash).toUpperCase();  // ← check case and truncation

// SHA variants
MessageDigest.getInstance("SHA-1")
MessageDigest.getInstance("SHA-256")
MessageDigest.getInstance("SHA-512")

// Apache Commons (bundled in many apps)
DigestUtils.md5Hex(data)
DigestUtils.sha256Hex(data)
org.apache.commons.codec.digest.DigestUtils

// Custom hex conversion (trace this too)
private static String bytesToHex(byte[] bytes) {
    StringBuilder sb = new StringBuilder();
    for (byte b : bytes) {
        sb.append(String.format("%02x", b));  // or %02X for uppercase
    }
    return sb.toString();
}

3. HMAC

// Standard Java
Mac mac = Mac.getInstance("HmacMD5");      // or HmacSHA1/HmacSHA256/HmacSHA512
SecretKeySpec keySpec = new SecretKeySpec(secret.getBytes("UTF-8"), "HmacSHA256");
mac.init(keySpec);
byte[] result = mac.doFinal(data.getBytes("UTF-8"));

// Base64 encoded output
String sig = Base64.encodeToString(result, Base64.NO_WRAP);
// OR hex
String sig = bytesToHex(result).toUpperCase();

4. AES Encryption

// AES-CBC (most common)
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
SecretKeySpec keySpec = new SecretKeySpec(key.getBytes(), "AES");
IvParameterSpec ivSpec = new IvParameterSpec(iv.getBytes());
cipher.init(Cipher.ENCRYPT_MODE, keySpec, ivSpec);
byte[] encrypted = cipher.doFinal(plaintext.getBytes("UTF-8"));
String result = Base64.encodeToString(encrypted, Base64.NO_WRAP);

// AES-ECB (flag as security issue)
Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");

// AES-GCM (modern)
Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
GCMParameterSpec paramSpec = new GCMParameterSpec(128, iv);

Extract verbatim: key source, IV source, mode, padding, output encoding.

5. RSA

// Encrypt with public key
KeyFactory kf = KeyFactory.getInstance("RSA");
X509EncodedKeySpec keySpec = new X509EncodedKeySpec(Base64.decode(PUBLIC_KEY, Base64.DEFAULT));
PublicKey pubKey = kf.generatePublic(keySpec);
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pubKey);
byte[] encrypted = cipher.doFinal(data.getBytes());

// Hardcoded public key (CRITICAL — extract full PEM/base64 value)
private static final String PUBLIC_KEY = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...";

6. Base64 & Encoding

// Android Base64
Base64.encode(data, Base64.DEFAULT)
Base64.encode(data, Base64.NO_WRAP)       // no line breaks
Base64.encode(data, Base64.URL_SAFE)      // URL-safe variant
Base64.decode(str, Base64.DEFAULT)

// Java standard
java.util.Base64.getEncoder().encodeToString(data)
java.util.Base64.getDecoder().decode(str)

// Hex encoding
Integer.toHexString(b & 0xFF)
String.format("%02x", b)
String.format("%02X", b)                  // uppercase

7. Signing Flow Patterns

Sort + concat pattern (common in Chinese apps):

// Sort params alphabetically, concat, then hash
Map<String, String> params = new TreeMap<>(requestParams);  // TreeMap = sorted
StringBuilder sb = new StringBuilder();
for (Map.Entry<String, String> entry : params.entrySet()) {
    if (!entry.getKey().equals("sign") && entry.getValue() != null) {
        sb.append(entry.getKey()).append("=").append(entry.getValue()).append("&");
    }
}
String canonical = sb.toString();
String sign = MD5(canonical + secretKey).toUpperCase();

// OR JSON stringify approach
JSONObject sorted = new JSONObject(new TreeMap<>(paramsMap));
String json = sorted.toString();
String sign = MD5(json).toUpperCase().substring(0, 32);

Timestamp + nonce pattern:

long timestamp = System.currentTimeMillis() / 1000;  // unix seconds
String nonce = UUID.randomUUID().toString().replace("-", "");
// OR
String nonce = String.valueOf((int)(Math.random() * 1000000));

8. Certificate Pinning (important to note)

// OkHttp pinning
CertificatePinner pinner = new CertificatePinner.Builder()
    .add("api.example.com", "sha256/AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=")
    .build();

// TrustManager override (common bypass target)
TrustManager[] trustAllCerts = new TrustManager[]{
    new X509TrustManager() {
        public void checkClientTrusted(...) {}
        public void checkServerTrusted(...) {}
        public X509Certificate[] getAcceptedIssuers() { return new X509Certificate[]{}; }
    }
};

---

Smali Pattern Catalog

Smali is Dalvik bytecode — use when Java decompile fails or is obfuscated.

Smali Hash Patterns

# MD5
const-string v0, "MD5"
invoke-static {v0}, Ljava/security/MessageDigest;->getInstance(Ljava/lang/String;)Ljava/security/MessageDigest;
invoke-virtual {v0, v1}, Ljava/security/MessageDigest;->update([B)V
invoke-virtual {v0}, Ljava/security/MessageDigest;->digest()[B

# SHA-256
const-string v0, "SHA-256"
invoke-static {v0}, Ljava/security/MessageDigest;->getInstance(Ljava/lang/String;)Ljava/security/MessageDigest;

# HMAC
const-string v0, "HmacSHA256"
invoke-static {v0}, Ljavax/crypto/Mac;->getInstance(Ljava/lang/String;)Ljavax/crypto/Mac;
invoke-virtual {v0, v1}, Ljavax/crypto/Mac;->init(Ljava/security/Key;)V
invoke-virtual {v0}, Ljavax/crypto/Mac;->doFinal()[B

Smali Cipher Patterns

# AES
const-string v0, "AES/CBC/PKCS5Padding"
invoke-static {v0}, Ljavax/crypto/Cipher;->getInstance(Ljava/lang/String;)Ljavax/crypto/Cipher;
invoke-virtual {v0, v1, v2}, Ljavax/crypto/Cipher;->init(ILjava/security/Key;Ljava/security/spec/AlgorithmParameterSpec;)V
invoke-virtual {v0, v1}, Ljavax/crypto/Cipher;->doFinal([B)[B

# RSA
const-string v0, "RSA/ECB/PKCS1Padding"
invoke-static {v0}, Ljavax/crypto/Cipher;->getInstance(Ljava/lang/String;)Ljavax/crypto/Cipher;

Smali Base64 Patterns

# Android Base64
invoke-static {v0, v1}, Landroid/util/Base64;->encode([BI)[B
invoke-static {v0, v1}, Landroid/util/Base64;->decode([BI)[B
invoke-static {v0, v1}, Landroid/util/Base64;->encodeToString([BI)Ljava/lang/String;

Smali OkHttp Patterns

# Interceptor
invoke-virtual {v0, v1}, Lokhttp3/OkHttpClient$Builder;->addInterceptor(Lokhttp3/Interceptor;)Lokhttp3/OkHttpClient$Builder;

# Request builder
invoke-virtual {v0, v1, v2}, Lokhttp3/Request$Builder;->header(Ljava/lang/String;Ljava/lang/String;)Lokhttp3/Request$Builder;
invoke-virtual {v0, v1}, Lokhttp3/Request$Builder;->addHeader(Ljava/lang/String;Ljava/lang/String;)Lokhttp3/Request$Builder;

---

Analysis Steps

Step 1: Priority Class Scan

python tools/apk_scan.py output/[name]/ --priority

Lists all high-priority classes matching crypto/network patterns.

Step 2: Find Interceptors

Search for implements Interceptor in Java, or OkHttp patterns in Smali. Read the FULL interceptor class — this is the signing entry point.

Step 3: Trace the Sign Function

From the interceptor, trace backwards to find the signing function:

SignInterceptor.intercept()
  └── CryptoUtil.generateSign(params, timestamp)
        ├── sortParams(params)              → TreeMap / sorted keys
        ├── buildCanonical(sorted)          → concat / JSON
        └── MD5(canonical + secret).upper() → signature

Step 4: Extract Every Parameter

• What fields are included? (field names)
• What fields are excluded? (null check, skip list)
• Sort order? (TreeMap = alphabetical, vs insertion order)
• Canonical format? (key=value&, JSON, custom)
• Hash algorithm and output format?
• Timestamp format? (ms or seconds)
• Nonce format? (UUID, random int, hex)

Step 5: Find Hardcoded Secrets

// Look for:
private static final String SECRET = "..."
private static final String APP_KEY = "..."
static final String SIGN_KEY = "..."
BuildConfig.API_SECRET

Also check assets/ folder — config.json, app.properties often contain keys.

Step 6: Check String Obfuscation

ProGuard/R8 obfuscates class/method names but NOT string values. Search for crypto algorithm strings directly:

grep -r '"MD5"\|"HmacSHA256"\|"AES/CBC"\|"RSA"' output/[name]/smali/

---

Output Format

APK CRYPTO FINDINGS
===================

[APK1] OkHttp Signing Interceptor — HIGH
  File:    java/com/example/app/network/SignInterceptor.java
  Class:   SignInterceptor implements Interceptor
  Line:    45

  Signing flow:
    timestamp = System.currentTimeMillis() / 1000
    nonce     = UUID.randomUUID().toString().replace("-", "")
    body      = request.body() → string
    sorted    = new TreeMap<>(params)  ← alphabetical sort
    canonical = buildCanonical(sorted) ← key=val& format
    signature = MD5(canonical + SECRET_KEY).toUpperCase()[:32]
    → headers: X-Sign, X-Timestamp, X-Nonce

  SECRET_KEY: HARDCODED "x7k9mNpQ2rSvWyZ3"
  Callers:  OkHttpClient.Builder.addInterceptor (ApiClient.java:23)

[APK2] AES Password Encryption — HIGH
  File:    java/com/example/app/utils/CryptoUtil.java
  Class:   CryptoUtil.encryptPassword(String)
  Mode:    AES/CBC/PKCS5Padding
  Key:     SHA256(userId + DEVICE_ID)[:32]
  IV:      HARDCODED "1234567890123456"
  Output:  Base64.NO_WRAP

[APK3] RSA Public Key — HIGH
  File:    java/com/example/app/security/RsaUtil.java
  Mode:    RSA/ECB/PKCS1Padding
  Key:     HARDCODED in source (see below)
  -----BEGIN PUBLIC KEY-----
  MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...
  -----END PUBLIC KEY-----