1# Porting from OpenSSL to BoringSSL 2 3BoringSSL is an OpenSSL derivative and is mostly source-compatible, for the 4subset of OpenSSL retained. Libraries ideally need little to no changes for 5BoringSSL support, provided they do not use removed APIs. In general, see if the 6library compiles and, on failure, consult the documentation in the header files 7and see if problematic features can be removed. 8 9In some cases, BoringSSL-specific code may be necessary. In that case, the 10`OPENSSL_IS_BORINGSSL` preprocessor macro may be used in `#ifdef`s. This macro 11should also be used in lieu of the presence of any particular function to detect 12OpenSSL vs BoringSSL in configure scripts, etc., where those are necessary. 13 14For convenience, BoringSSL defines upstream's `OPENSSL_NO_*` feature macros 15corresponding to removed features. These may also be used to disable code which 16uses a removed feature. 17 18Note: BoringSSL does *not* have a stable API or ABI. It must be updated with its 19consumers. It is not suitable for, say, a system library in a traditional Linux 20distribution. For instance, Chromium statically links the specific revision of 21BoringSSL it was built against. Likewise, Android's system-internal copy of 22BoringSSL is not exposed by the NDK and must not be used by third-party 23applications. 24 25 26## Major API changes 27 28### Integer types 29 30Some APIs have been converted to use `size_t` for consistency and to avoid 31integer overflows at the API boundary. (Existing logic uses a mismash of `int`, 32`long`, and `unsigned`.) For the most part, implicit casts mean that existing 33code continues to compile. In some cases, this may require BoringSSL-specific 34code, particularly to avoid compiler warnings. 35 36Most notably, the `STACK_OF(T)` types have all been converted to use `size_t` 37instead of `int` for indices and lengths. 38 39### Reference counts 40 41Some external consumers increment reference counts directly by calling 42`CRYPTO_add` with the corresponding `CRYPTO_LOCK_*` value. 43 44These APIs no longer exist in BoringSSL. Instead, code which increments 45reference counts should call the corresponding `FOO_up_ref` function, such as 46`EVP_PKEY_up_ref`. Note that not all of these APIs are present in OpenSSL and 47may require `#ifdef`s. 48 49### Error codes 50 51OpenSSL's errors are extremely specific, leaking internals of the library, 52including even a function code for the function which emitted the error! As some 53logic in BoringSSL has been rewritten, code which conditions on the error may 54break (grep for `ERR_GET_REASON` and `ERR_GET_FUNC`). This danger also exists 55when upgrading OpenSSL versions. 56 57Where possible, avoid conditioning on the exact error reason. Otherwise, a 58BoringSSL `#ifdef` may be necessary. Exactly how best to resolve this issue is 59still being determined. It's possible some new APIs will be added in the future. 60 61Function codes have been completely removed. Remove code which conditions on 62these as it will break with the slightest change in the library, OpenSSL or 63BoringSSL. 64 65### `*_ctrl` functions 66 67Some OpenSSL APIs are implemented with `ioctl`-style functions such as 68`SSL_ctrl` and `EVP_PKEY_CTX_ctrl`, combined with convenience macros, such as 69 70 # define SSL_CTX_set_mode(ctx,op) \ 71 SSL_CTX_ctrl((ctx),SSL_CTRL_MODE,(op),NULL) 72 73In BoringSSL, these macros have been replaced with proper functions. The 74underlying `_ctrl` functions have been removed. 75 76For convenience, `SSL_CTRL_*` values are retained as macros to `doesnt_exist` so 77existing code which uses them (or the wrapper macros) in `#ifdef` expressions 78will continue to function. However, the macros themselves will not work. 79 80Switch any `*_ctrl` callers to the macro/function versions. This works in both 81OpenSSL and BoringSSL. Note that BoringSSL's function versions will be 82type-checked and may require more care with types. 83 84### HMAC `EVP_PKEY`s 85 86`EVP_PKEY_HMAC` is removed. Use the `HMAC_*` functions in `hmac.h` instead. This 87is compatible with OpenSSL. 88 89### DSA `EVP_PKEY`s 90 91`EVP_PKEY_DSA` is deprecated. It is currently still possible to parse DER into a 92DSA `EVP_PKEY`, but signing or verifying with those objects will not work. 93 94### DES 95 96The `DES_cblock` type has been switched from an array to a struct to avoid the 97pitfalls around array types in C. Where features which require DES cannot be 98disabled, BoringSSL-specific codepaths may be necessary. 99 100### TLS renegotiation 101 102OpenSSL enables TLS renegotiation by default and accepts renegotiation requests 103from the peer transparently. Renegotiation is an extremely problematic protocol 104feature, so BoringSSL rejects peer renegotiations by default. 105 106To enable renegotiation, call `SSL_set_renegotiate_mode` and set it to 107`ssl_renegotiate_once` or `ssl_renegotiate_freely`. Renegotiation is only 108supported as a client in SSL3/TLS and the HelloRequest must be received at a 109quiet point in the application protocol. This is sufficient to support the 110common use of requesting a new client certificate between an HTTP request and 111response in (unpipelined) HTTP/1.1. 112 113Things which do not work: 114 115* There is no support for renegotiation as a server. 116 117* There is no support for renegotiation in DTLS. 118 119* There is no support for initiating renegotiation; `SSL_renegotiate` always 120 fails and `SSL_set_state` does nothing. 121 122* Interleaving application data with the new handshake is forbidden. 123 124* If a HelloRequest is received while `SSL_write` has unsent application data, 125 the renegotiation is rejected. 126 127### Lowercase hexadecimal 128 129BoringSSL's `BN_bn2hex` function uses lowercase hexadecimal digits instead of 130uppercase. Some code may require changes to avoid being sensitive to this 131difference. 132 133 134## Optional BoringSSL-specific simplifications 135 136BoringSSL makes some changes to OpenSSL which simplify the API but remain 137compatible with OpenSSL consumers. In general, consult the BoringSSL 138documentation for any functions in new BoringSSL-only code. 139 140### Return values 141 142Most OpenSSL APIs return 1 on success and either 0 or -1 on failure. BoringSSL 143has narrowed most of these to 1 on success and 0 on failure. BoringSSL-specific 144code may take advantage of the less error-prone APIs and use `!` to check for 145errors. 146 147### Initialization 148 149OpenSSL has a number of different initialization functions for setting up error 150strings and loading algorithms, etc. All of these functions still exist in 151BoringSSL for convenience, but they do nothing and are not necessary. 152 153The one exception is `CRYPTO_library_init`. In `BORINGSSL_NO_STATIC_INITIALIZER` 154builds, it must be called to query CPU capabitilies before the rest of the 155library. In the default configuration, this is done with a static initializer 156and is also unnecessary. 157 158### Threading 159 160OpenSSL provides a number of APIs to configure threading callbacks and set up 161locks. Without initializing these, the library is not thread-safe. Configuring 162these does nothing in BoringSSL. Instead, BoringSSL calls pthreads and the 163corresponding Windows APIs internally and is always thread-safe where the API 164guarantees it. 165