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mbedtls-SGX: a TLS stack in SGX

mbedtls-SGX is a port of mbedtls (previously PolarSSL) to Intel-SGX. mbedtls-SGX aims to preserve all of the features of mbedtls. With mbedtls-SGX, you can

  • use a wide array of cryptographic primitives (hash, RSA, ECC, AES, etc) in SGX.
  • build SGX-secured tls clients and servers -- even OS cannot access session secrets.
  • enjoy the awesome documentation and clean API of mbedtls.

In addition, mbedtls-SGX comes with examples to help you get started. Note that certain functionality is lost due to limitations of SGX. Read on for details.

Usage and Examples

mbedtls-SGX is a static enclave library. General steps of using mbedtls-SGX in your project are:

  • compile and install mbedtls-SGX (see below)
  • include trusted/mbedtls_sgx.edl in your enclave's EDL file.
  • make sure your compiler can find the headers in include.
  • link libmbedtls_sgx_u.a to the untrusted part of your application
  • link libmbedtls_sgx_t.a to your enclave. Note that mbedtls-SGX needs to be linked in the same group with other SGX standard libs. Your Makefile (or CMakeLists.txt) needs something like
-Wl,--start-group  -lmbedtls_sgx_t -lsgx_tstdc -lsgx_tcxx -l$(Crypto_Library_Name) -l$(Service_Library_Name) -Wl,--end-group

Build

git clone https://github.com/bl4ck5un/mbedtls-SGX && cd mbedtls-SGX
mkdir build && cd build
cmake ..
make -j && make install

Include the resultant mbedtls_SGX-2.6.0 as part of your project.

mbedtls_SGX-2.6.0
├── include
│   └── mbedtls
└── lib
    ├── libmbedtls_SGX_t.a
    ├── libmbedtls_SGX_u.a
    └── mbedtls_SGX.edl

Examples

To compile examples, run cmake with -DCOMPILE_EXAMPLES=YES

cmake .. -DCOMPILE_EXAMPLES=YES
make -j

Three examples will be built

  • s_client: a simple TLS client (by default it connects to google.com:443, dumps the HTML page and exits)
  • s_server: a simple TLS server. You can play with it by openssl s_client localhost:4433.
  • m_server: a multi-threaded TLS server, also listening at localhost:4433 by default.

Missing features and workarounds

Due to SGX's contraints, some features have been turned off.

  • The lack of trusted wall-clock time. SGX provides trusted relative timer but not an absolute one. This affects checking expired certificates. A workaround is to maintain an internal clock and calibrate it frequently.
  • No access to file systems: mbedtls-SGX can not load CA files from file systems. To work this around, you need to hardcode root CAs as part of the enclave program. See example/enclave/ca_bundle.h for an example.

License

mbedtls-SGX is open source under Apache 2.0. See LICENSE for more details.