Add protocol fuzzers for OSS-Fuzz integration

fuzzing/CMakeLists.txt

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+# Telegram Desktop Protocol Fuzzers
+#
+# Standalone fuzzer build system for OSS-Fuzz integration.
+# These fuzzers don't require the full tdesktop build.
+#
+# Usage:
+#   cmake -B build .
+#   cmake --build build -j$(nproc)
+#
+# Requirements:
+#   - Clang compiler with libFuzzer support
+#   - AddressSanitizer, UndefinedBehaviorSanitizer
+
+cmake_minimum_required(VERSION 3.16)
+project(telegram_fuzzers CXX)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+# Verify Clang compiler (required for libFuzzer)
+if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    message(WARNING "Fuzzing requires Clang compiler for libFuzzer support")
+    message(WARNING "Set CMAKE_CXX_COMPILER=clang++ to use fuzzers")
+    message(WARNING "Skipping fuzzer build")
+    return()
+endif()
+
+# Fuzzing flags for standalone build
+set(FUZZER_FLAGS
+    -fsanitize=fuzzer,address,undefined
+    -g
+    -O1
+)
+
+# List of all fuzzers
+set(FUZZERS
+    # MTProto protocol layers
+    mtproto_v0_fuzzer
+    mtproto_v1_obfuscated_fuzzer
+    mtproto_vd_padded_fuzzer
+    tl_serialization_fuzzer
+    aes_ctr_obfuscation_fuzzer
+    # Private message encryption
+    aes_ige_encryption_fuzzer
+    message_key_derivation_fuzzer
+    auth_key_management_fuzzer
+)
+
+# Build each fuzzer as standalone binary
+foreach(FUZZER ${FUZZERS})
+    add_executable(${FUZZER} ${FUZZER}.cpp)
+    target_compile_options(${FUZZER} PRIVATE ${FUZZER_FLAGS})
+    target_link_options(${FUZZER} PRIVATE ${FUZZER_FLAGS})
+    set_target_properties(${FUZZER}
+        PROPERTIES
+        RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/fuzzers"
+    )
+endforeach()
+
+message(STATUS "Telegram Protocol Fuzzers configured")
+message(STATUS "  Compiler: ${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}")
+message(STATUS "  Fuzzers: ${FUZZERS}")
+message(STATUS "  Output: ${CMAKE_BINARY_DIR}/fuzzers/")
+
+# Note: These fuzzers are standalone and don't link against tdesktop
+# For OSS-Fuzz integration, see oss-fuzz/build.sh

fuzzing/INTEGRATION.md

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+# Integration Guide - Telegram Protocol Fuzzers
+
+This document explains how these fuzzers integrate with Telegram Desktop and Google OSS-Fuzz.
+
+## Overview
+
+These fuzzers are **standalone** - they don't require linking against the full Telegram Desktop codebase. Instead, they contain simplified implementations of protocol parsing and encryption logic extracted from tdesktop source files.
+
+## Why Standalone?
+
+1. **Fast Build** - Compiles in ~5 seconds (vs hours for full tdesktop)
+2. **No Dependencies** - Works without Qt, OpenSSL, or other tdesktop dependencies
+3. **OSS-Fuzz Ready** - Simple Docker build process
+4. **Portable** - Runs locally and in OSS-Fuzz without modification
+
+## File Structure in tdesktop
+
+```
+tdesktop/
+├── Telegram/              # Main application code
+│   └── SourceFiles/
+│       └── mtproto/       # Protocol implementation (fuzzing targets)
+├── cmake/                 # Build system
+├── docs/                  # Documentation
+└── fuzzing/              # ← This directory
+    ├── *.cpp             # 8 fuzzer sources
+    ├── CMakeLists.txt    # Standalone build
+    ├── README.md         # Documentation
+    ├── INTEGRATION.md    # This file
+    └── oss-fuzz/         # OSS-Fuzz files
+```
+
+## Fuzzing Targets
+
+These fuzzers test protocol implementations from:
+
+### `Telegram/SourceFiles/mtproto/mtproto_auth_key.cpp`
+- **auth_key_management_fuzzer** → Tests `AuthKey` class
+- **message_key_derivation_fuzzer** → Tests `prepareAES()` and `prepareAES_oldmtp()`
+- **aes_ige_encryption_fuzzer** → Tests `aesIgeEncryptRaw()` and `aesIgeDecryptRaw()`
+
+### `Telegram/SourceFiles/mtproto/connection_tcp.cpp`
+- **mtproto_v0_fuzzer** → Tests MTProto v0 packet parsing
+- **mtproto_v1_obfuscated_fuzzer** → Tests obfuscated handshake
+- **mtproto_vd_padded_fuzzer** → Tests padded protocol
+- **aes_ctr_obfuscation_fuzzer** → Tests connection obfuscation
+
+### TL Serialization
+- **tl_serialization_fuzzer** → Tests Type Language binary format
+
+## Local Testing
+
+### Build Fuzzers
+
+```bash
+cd fuzzing
+cmake -B build .
+cmake --build build -j$(nproc)
+```
+
+**Requirements:**
+- Clang compiler
+- libFuzzer support
+
+### Run Single Fuzzer
+
+```bash
+./build/fuzzers/mtproto_v0_fuzzer -max_total_time=60
+```
+
+### Run All Fuzzers
+
+```bash
+# Test each fuzzer for 60 seconds
+for fuzzer in build/fuzzers/*_fuzzer; do
+    echo "Testing $fuzzer..."
+    $fuzzer -max_total_time=60
+done
+```
+
+## Integration with tdesktop Build System
+
+These fuzzers are **optional** and don't affect the main tdesktop build. They can be:
+
+1. **Standalone** - Built independently using their own CMakeLists.txt
+2. **Optional** - Only built when explicitly requested
+3. **Skipped** - If Clang is not available
+
+### Option 1: Keep Separate (Recommended)
+
+Fuzzers remain in `fuzzing/` directory with their own build system:
+
+```cmake
+# fuzzing/CMakeLists.txt (already provided)
+# Builds only when explicitly invoked
+```
+
+Build:
+```bash
+cd fuzzing
+cmake -B build . && cmake --build build
+```
+
+### Option 2: Add to Root CMakeLists.txt (Optional)
+
+If tdesktop maintainers want to optionally include fuzzers:
+
+```cmake
+# In root CMakeLists.txt
+option(BUILD_FUZZERS "Build protocol fuzzers" OFF)
+
+if (BUILD_FUZZERS)
+    if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        add_subdirectory(fuzzing)
+    else()
+        message(STATUS "Skipping fuzzers (requires Clang)")
+    endif()
+endif()
+```
+
+Build with fuzzers:
+```bash
+cmake -B build -DBUILD_FUZZERS=ON .
+cmake --build build
+```
+
+## OSS-Fuzz Integration
+
+### Directory Structure in OSS-Fuzz
+
+```
+oss-fuzz/
+└── projects/
+    └── telegram/
+        ├── project.yaml      # Project config
+        ├── Dockerfile        # Docker build environment
+        ├── build.sh          # Build script
+        └── *.cpp            # Fuzzer sources (copied from tdesktop/fuzzing/)
+```
+
+### Build Process
+
+1. **Clone tdesktop**:
+   ```dockerfile
+   RUN git clone --depth 1 https://github.com/telegramdesktop/tdesktop $SRC/tdesktop
+   ```
+
+2. **Copy fuzzer sources**:
+   ```dockerfile
+   COPY *.cpp $SRC/telegram_fuzzers/
+   ```
+
+3. **Build fuzzers**:
+   ```bash
+   cd $SRC/telegram_fuzzers
+   for fuzzer in *.cpp; do
+       $CXX $CXXFLAGS -std=c++20 -c $fuzzer
+       $CXX $CXXFLAGS $LIB_FUZZING_ENGINE ${fuzzer%.cpp}.o -o $OUT/${fuzzer%.cpp}
+   done
+   ```
+
+### Why Not Link Against tdesktop?
+
+The tdesktop build is complex:
+- Requires Qt 6
+- Multiple dependencies (OpenSSL, FFmpeg, etc.)
+- Platform-specific code
+- Long build time (hours)
+
+Standalone fuzzers:
+- ✅ Build in ~5 seconds
+- ✅ Zero dependencies
+- ✅ Simple Docker image
+- ✅ Works on all platforms
+
+## Testing in OSS-Fuzz
+
+From oss-fuzz repository:
+
+```bash
+# Build Docker image
+python infra/helper.py build_image telegram
+
+# Build fuzzers
+python infra/helper.py build_fuzzers telegram
+
+# Test a fuzzer
+python infra/helper.py run_fuzzer telegram mtproto_v0_fuzzer
+
+# Check build quality
+python infra/helper.py check_build telegram
+```
+
+## Maintenance
+
+### Adding New Fuzzers
+
+1. Create `new_fuzzer.cpp` in `fuzzing/`
+2. Add to `FUZZERS` list in `CMakeLists.txt`
+3. Update `oss-fuzz/build.sh` (fuzzers auto-detected from `*.cpp`)
+4. Test locally and in OSS-Fuzz
+
+### Updating Fuzzers
+
+When tdesktop protocol code changes:
+
+1. Review changes in `Telegram/SourceFiles/mtproto/`
+2. Update corresponding fuzzer if protocol changed
+3. Test locally
+4. OSS-Fuzz will automatically pick up changes after PR merge
+
+## CI/CD Integration
+
+### GitHub Actions (Optional)
+
+Add fuzzer testing to tdesktop CI:
+
+```yaml
+name: Fuzzer Tests
+
+on: [push, pull_request]
+
+jobs:
+  fuzzers:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install Clang
+        run: sudo apt-get install -y clang
+
+      - name: Build Fuzzers
+        run: |
+          cd fuzzing
+          cmake -B build -DCMAKE_CXX_COMPILER=clang++ .
+          cmake --build build -j$(nproc)
+
+      - name: Test Fuzzers (Quick)
+        run: |
+          for fuzzer in fuzzing/build/fuzzers/*_fuzzer; do
+            echo "Testing $fuzzer..."
+            $fuzzer -max_total_time=10 -max_len=4096
+          done
+```
+
+## Security Considerations
+
+### Bug Reporting
+
+When OSS-Fuzz finds issues:
+1. Private report sent to configured contacts
+2. 90-day disclosure deadline
+3. Fix, test, release
+4. Public disclosure
+
+### Coverage Improvements
+
+Monitor OSS-Fuzz coverage reports to identify:
+- Uncovered code paths
+- Edge cases not tested
+- New fuzzing opportunities
+
+## References
+
+- **OSS-Fuzz Docs**: https://google.github.io/oss-fuzz/
+- **libFuzzer Tutorial**: https://github.com/google/fuzzing/blob/master/tutorial/libFuzzerTutorial.md
+- **Telegram Protocol**: https://core.telegram.org/mtproto
+- **tdesktop Source**: https://github.com/telegramdesktop/tdesktop
+
+## Questions?
+
+For questions about:
+- **Fuzzer Implementation**: See source code comments
+- **OSS-Fuzz Integration**: See `oss-fuzz/README.md`
+- **Local Testing**: See `README.md`
+
+---
+
+**Maintainer**: Vahagn Vardanian (@vah13)
+**Purpose**: Continuous security testing via Google OSS-Fuzz
+**Status**: Production ready