# Porting from OpenSSL to BoringSSL

BoringSSL is an OpenSSL derivative and is mostly source-compatible, for the
subset of OpenSSL retained. Libraries ideally need little to no changes for
BoringSSL support, provided they do not use removed APIs. In general, see if the
library compiles and, on failure, consult the documentation in the header files
and see if problematic features can be removed.

In some cases, BoringSSL-specific code may be necessary. In that case, the
`OPENSSL_IS_BORINGSSL` preprocessor macro may be used in `#ifdef`s. This macro
should also be used in lieu of the presence of any particular function to detect
OpenSSL vs BoringSSL in configure scripts, etc., where those are necessary.
Before using the preprocessor, however, contact the BoringSSL maintainers about
the missing APIs. If not an intentionally removed feature, BoringSSL will
typically add compatibility functions for convenience.

For convenience, BoringSSL defines upstream's `OPENSSL_NO_*` feature macros
corresponding to removed features. These may also be used to disable code which
uses a removed feature.

Note: BoringSSL does *not* have a stable API or ABI. It must be updated with its
consumers. It is not suitable for, say, a system library in a traditional Linux
distribution. For instance, Chromium statically links the specific revision of
BoringSSL it was built against. Likewise, Android's system-internal copy of
BoringSSL is not exposed by the NDK and must not be used by third-party
applications.


## Major API changes

### Integer types

Some APIs have been converted to use `size_t` for consistency and to avoid
integer overflows at the API boundary. (Existing logic uses a mismash of `int`,
`long`, and `unsigned`.)  For the most part, implicit casts mean that existing
code continues to compile. In some cases, this may require BoringSSL-specific
code, particularly to avoid compiler warnings.

Most notably, the `STACK_OF(T)` types have all been converted to use `size_t`
instead of `int` for indices and lengths.

### Reference counts

Some external consumers increment reference counts directly by calling
`CRYPTO_add` with the corresponding `CRYPTO_LOCK_*` value.

These APIs no longer exist in BoringSSL. Instead, code which increments
reference counts should call the corresponding `FOO_up_ref` function, such as
`EVP_PKEY_up_ref`. Note that not all of these APIs are present in OpenSSL and
may require `#ifdef`s.

### Error codes

OpenSSL's errors are extremely specific, leaking internals of the library,
including even a function code for the function which emitted the error! As some
logic in BoringSSL has been rewritten, code which conditions on the error may
break (grep for `ERR_GET_REASON` and `ERR_GET_FUNC`). This danger also exists
when upgrading OpenSSL versions.

Where possible, avoid conditioning on the exact error reason. Otherwise, a
BoringSSL `#ifdef` may be necessary. Exactly how best to resolve this issue is
still being determined. It's possible some new APIs will be added in the future.

Function codes have been completely removed. Remove code which conditions on
these as it will break with the slightest change in the library, OpenSSL or
BoringSSL.

### `*_ctrl` functions

Some OpenSSL APIs are implemented with `ioctl`-style functions such as
`SSL_ctrl` and `EVP_PKEY_CTX_ctrl`, combined with convenience macros, such as

    # define SSL_CTX_set_mode(ctx,op) \
            SSL_CTX_ctrl((ctx),SSL_CTRL_MODE,(op),NULL)

In BoringSSL, these macros have been replaced with proper functions. The
underlying `_ctrl` functions have been removed.

For convenience, `SSL_CTRL_*` values are retained as macros to `doesnt_exist` so
existing code which uses them (or the wrapper macros) in `#ifdef` expressions
will continue to function. However, the macros themselves will not work.

Switch any `*_ctrl` callers to the macro/function versions. This works in both
OpenSSL and BoringSSL. Note that BoringSSL's function versions will be
type-checked and may require more care with types. See the end of this
document for a table of functions to use.

### HMAC `EVP_PKEY`s

`EVP_PKEY_HMAC` is removed. Use the `HMAC_*` functions in `hmac.h` instead. This
is compatible with OpenSSL.

### DSA `EVP_PKEY`s

`EVP_PKEY_DSA` is deprecated. It is currently still possible to parse DER into a
DSA `EVP_PKEY`, but signing or verifying with those objects will not work.

### DES

The `DES_cblock` type has been switched from an array to a struct to avoid the
pitfalls around array types in C. Where features which require DES cannot be
disabled, BoringSSL-specific codepaths may be necessary.

### TLS renegotiation

OpenSSL enables TLS renegotiation by default and accepts renegotiation requests
from the peer transparently. Renegotiation is an extremely problematic protocol
feature, so BoringSSL rejects peer renegotiations by default.

To enable renegotiation, call `SSL_set_renegotiate_mode` and set it to
`ssl_renegotiate_once` or `ssl_renegotiate_freely`. Renegotiation is only
supported as a client in SSL3/TLS and the HelloRequest must be received at a
quiet point in the application protocol. This is sufficient to support the
common use of requesting a new client certificate between an HTTP request and
response in (unpipelined) HTTP/1.1.

Things which do not work:

* There is no support for renegotiation as a server. (Attempts by clients will
  result in a fatal alert so that ClientHello messages cannot be used to flood
  a server and escape higher-level limits.)

* There is no support for renegotiation in DTLS.

* There is no support for initiating renegotiation; `SSL_renegotiate` always
  fails and `SSL_set_state` does nothing.

* Interleaving application data with the new handshake is forbidden.

* If a HelloRequest is received while `SSL_write` has unsent application data,
  the renegotiation is rejected.

* Renegotiation does not participate in session resumption. The client will
  not offer a session on renegotiation or resume any session established by a
  renegotiation handshake.

* The server may not change its certificate in the renegotiation. This mitigates
  the [triple handshake attack](https://mitls.org/pages/attacks/3SHAKE). Any new
  stapled OCSP response and SCT list will be ignored. As no authentication state
  may change, BoringSSL will not re-verify the certificate on a renegotiation.
  Callbacks such as `SSL_CTX_set_custom_verify` will only run on the initial
  handshake.

### Lowercase hexadecimal

BoringSSL's `BN_bn2hex` function uses lowercase hexadecimal digits instead of
uppercase. Some code may require changes to avoid being sensitive to this
difference.

### Legacy ASN.1 functions

OpenSSL's ASN.1 stack uses `d2i` functions for parsing. They have the form:

    RSA *d2i_RSAPrivateKey(RSA **out, const uint8_t **inp, long len);

In addition to returning the result, OpenSSL places it in `*out` if `out` is
not `NULL`. On input, if `*out` is not `NULL`, OpenSSL will usually (but not
always) reuse that object rather than allocating a new one. In BoringSSL, these
functions are compatibility wrappers over a newer ASN.1 stack. Even if `*out`
is not `NULL`, these wrappers will always allocate a new object and free the
previous one.

Ensure that callers do not rely on this object reuse behavior. It is
recommended to avoid the `out` parameter completely and always pass in `NULL`.
Note that less error-prone APIs are available for BoringSSL-specific code (see
below).

### Memory allocation

OpenSSL provides wrappers `OPENSSL_malloc` and `OPENSSL_free` over the standard
`malloc` and `free`. Memory allocated by OpenSSL should be released with
`OPENSSL_free`, not the standard `free`. However, by default, they are
implemented directly using `malloc` and `free`, so code which mixes them up
usually works.

In BoringSSL, these functions maintain additional book-keeping to zero memory
on `OPENSSL_free`, so any mixups must be fixed.

## Optional BoringSSL-specific simplifications

BoringSSL makes some changes to OpenSSL which simplify the API but remain
compatible with OpenSSL consumers. In general, consult the BoringSSL
documentation for any functions in new BoringSSL-only code.

### Return values

Most OpenSSL APIs return 1 on success and either 0 or -1 on failure. BoringSSL
has narrowed most of these to 1 on success and 0 on failure. BoringSSL-specific
code may take advantage of the less error-prone APIs and use `!` to check for
errors.

### Initialization

OpenSSL has a number of different initialization functions for setting up error
strings and loading algorithms, etc. All of these functions still exist in
BoringSSL for convenience, but they do nothing and are not necessary.

The one exception is `CRYPTO_library_init`. In `BORINGSSL_NO_STATIC_INITIALIZER`
builds, it must be called to query CPU capabitilies before the rest of the
library. In the default configuration, this is done with a static initializer
and is also unnecessary.

### Threading

OpenSSL provides a number of APIs to configure threading callbacks and set up
locks. Without initializing these, the library is not thread-safe. Configuring
these does nothing in BoringSSL. Instead, BoringSSL calls pthreads and the
corresponding Windows APIs internally and is always thread-safe where the API
guarantees it.

### ASN.1

BoringSSL is in the process of deprecating OpenSSL's `d2i` and `i2d` in favor of
new functions using the much less error-prone `CBS` and `CBB` types.
BoringSSL-only code should use those functions where available.


## Replacements for `CTRL` values

When porting code which uses `SSL_CTX_ctrl` or `SSL_ctrl`, use the replacement
functions below. If a function has both `SSL_CTX` and `SSL` variants, only the
`SSL_CTX` version is listed.

Note some values correspond to multiple functions depending on the `larg`
parameter.

`CTRL` value | Replacement function(s)
-------------|-------------------------
`DTLS_CTRL_GET_TIMEOUT` | `DTLSv1_get_timeout`
`DTLS_CTRL_HANDLE_TIMEOUT` | `DTLSv1_handle_timeout`
`SSL_CTRL_CHAIN` | `SSL_CTX_set0_chain` or `SSL_CTX_set1_chain`
`SSL_CTRL_CHAIN_CERT` | `SSL_add0_chain_cert` or `SSL_add1_chain_cert`
`SSL_CTRL_CLEAR_EXTRA_CHAIN_CERTS` | `SSL_CTX_clear_extra_chain_certs`
`SSL_CTRL_CLEAR_MODE` | `SSL_CTX_clear_mode`
`SSL_CTRL_CLEAR_OPTIONS` | `SSL_CTX_clear_options`
`SSL_CTRL_EXTRA_CHAIN_CERT` | `SSL_CTX_add_extra_chain_cert`
`SSL_CTRL_GET_CHAIN_CERTS` | `SSL_CTX_get0_chain_certs`
`SSL_CTRL_GET_CLIENT_CERT_TYPES` | `SSL_get0_certificate_types`
`SSL_CTRL_GET_EXTRA_CHAIN_CERTS` | `SSL_CTX_get_extra_chain_certs` or `SSL_CTX_get_extra_chain_certs_only`
`SSL_CTRL_GET_MAX_CERT_LIST` | `SSL_CTX_get_max_cert_list`
`SSL_CTRL_GET_NUM_RENEGOTIATIONS` | `SSL_num_renegotiations`
`SSL_CTRL_GET_READ_AHEAD` | `SSL_CTX_get_read_ahead`
`SSL_CTRL_GET_RI_SUPPORT` | `SSL_get_secure_renegotiation_support`
`SSL_CTRL_GET_SESSION_REUSED` | `SSL_session_reused`
`SSL_CTRL_GET_SESS_CACHE_MODE` | `SSL_CTX_get_session_cache_mode`
`SSL_CTRL_GET_SESS_CACHE_SIZE` | `SSL_CTX_sess_get_cache_size`
`SSL_CTRL_GET_TLSEXT_TICKET_KEYS` | `SSL_CTX_get_tlsext_ticket_keys`
`SSL_CTRL_GET_TOTAL_RENEGOTIATIONS` | `SSL_total_renegotiations`
`SSL_CTRL_MODE` | `SSL_CTX_get_mode` or `SSL_CTX_set_mode`
`SSL_CTRL_NEED_TMP_RSA` | `SSL_CTX_need_tmp_RSA` is equivalent, but [*do not use this function*](https://freakattack.com/). (It is a no-op in BoringSSL.)
`SSL_CTRL_OPTIONS` | `SSL_CTX_get_options` or `SSL_CTX_set_options`
`SSL_CTRL_SESS_NUMBER` | `SSL_CTX_sess_number`
`SSL_CTRL_SET_CURVES` | `SSL_CTX_set1_curves`
`SSL_CTRL_SET_ECDH_AUTO` | `SSL_CTX_set_ecdh_auto`
`SSL_CTRL_SET_MAX_CERT_LIST` | `SSL_CTX_set_max_cert_list`
`SSL_CTRL_SET_MAX_SEND_FRAGMENT` | `SSL_CTX_set_max_send_fragment`
`SSL_CTRL_SET_MSG_CALLBACK` | `SSL_set_msg_callback`
`SSL_CTRL_SET_MSG_CALLBACK_ARG` | `SSL_set_msg_callback_arg`
`SSL_CTRL_SET_MTU` | `SSL_set_mtu`
`SSL_CTRL_SET_READ_AHEAD` | `SSL_CTX_set_read_ahead`
`SSL_CTRL_SET_SESS_CACHE_MODE` | `SSL_CTX_set_session_cache_mode`
`SSL_CTRL_SET_SESS_CACHE_SIZE` | `SSL_CTX_sess_set_cache_size`
`SSL_CTRL_SET_TLSEXT_HOSTNAME` | `SSL_set_tlsext_host_name`
`SSL_CTRL_SET_TLSEXT_SERVERNAME_ARG` | `SSL_CTX_set_tlsext_servername_arg`
`SSL_CTRL_SET_TLSEXT_SERVERNAME_CB` | `SSL_CTX_set_tlsext_servername_callback`
`SSL_CTRL_SET_TLSEXT_TICKET_KEYS` | `SSL_CTX_set_tlsext_ticket_keys`
`SSL_CTRL_SET_TLSEXT_TICKET_KEY_CB` | `SSL_CTX_set_tlsext_ticket_key_cb`
`SSL_CTRL_SET_TMP_DH` | `SSL_CTX_set_tmp_dh`
`SSL_CTRL_SET_TMP_DH_CB` | `SSL_CTX_set_tmp_dh_callback`
`SSL_CTRL_SET_TMP_ECDH` | `SSL_CTX_set_tmp_ecdh`
`SSL_CTRL_SET_TMP_ECDH_CB` | `SSL_CTX_set_tmp_ecdh_callback`
`SSL_CTRL_SET_TMP_RSA` | `SSL_CTX_set_tmp_rsa` is equivalent, but [*do not use this function*](https://freakattack.com/). (It is a no-op in BoringSSL.)
`SSL_CTRL_SET_TMP_RSA_CB` | `SSL_CTX_set_tmp_rsa_callback` is equivalent, but [*do not use this function*](https://freakattack.com/). (It is a no-op in BoringSSL.)

## Significant API additions

In some places, BoringSSL has added significant APIs. Use of these APIs goes beyound “porting” and means giving up on OpenSSL compatibility.

One example of this has already been mentioned: the [CBS and CBB](https://commondatastorage.googleapis.com/chromium-boringssl-docs/bytestring.h.html) functions should be used whenever parsing or serialising data.

### CRYPTO\_BUFFER

With the standard OpenSSL APIs, when making many TLS connections, the certificate data for each connection is retained in memory in an expensive `X509` structure. Additionally, common certificates often appear in the chains for multiple connections and are needlessly duplicated in memory.

A [`CRYPTO_BUFFER`](https://commondatastorage.googleapis.com/chromium-boringssl-docs/pool.h.html) is just an opaque byte string. A `CRYPTO_BUFFER_POOL` is an intern table for these buffers, i.e. it ensures that only a single copy of any given byte string is kept for each pool.

The function `TLS_with_buffers_method` returns an `SSL_METHOD` that avoids creating `X509` objects for certificates. Additionally, `SSL_CTX_set0_buffer_pool` can be used to install a pool on an `SSL_CTX` so that certificates can be deduplicated across connections and across `SSL_CTX`s.

When using these functions, the application also needs to ensure that it doesn't call other functions that deal with `X509` or `X509_NAME` objects. For example, `SSL_get_peer_certificate` or `SSL_get_peer_cert_chain`. Doing so will trigger an assert in debug mode and will result in NULLs in release mode. Instead, call the buffer-based alternatives such as `SSL_get0_peer_certificates`. (See [ssl.h](https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html) for functions taking or returning `CRYPTO_BUFFER`.) The buffer-based alternative functions will work even when not using `TLS_with_buffers_method`, thus application code can transition gradually.

In order to use buffers, the application code also needs to implement its own certificate verification using `SSL_[CTX_]set_custom_verify`. Otherwise all connections will fail with a verification error. Auto-chaining is also disabled when using buffers.

Once those changes have been completed, the whole of the OpenSSL X.509 and ASN.1 code should be eliminated by the linker if BoringSSL is linked statically.

### Asynchronous and opaque private keys

OpenSSL offers the ENGINE API for implementing opaque private keys (i.e. private keys where software only has oracle access because the secrets are held in special hardware or on another machine). While the ENGINE API has been mostly removed from BoringSSL, it is still possible to support opaque keys in this way. However, when using such keys with TLS and BoringSSL, you should strongly prefer using `SSL_PRIVATE_KEY_METHOD` via `SSL[_CTX]_set_private_key_method`. This allows a handshake to be suspended while the private operation is in progress. It also supports more forms of opaque key as it exposes higher-level information about the operation to be performed.