s2n-tls is a C library, and is built using Make. To clone the latest copy of s2n-tls from git use:
git clone https://github.com/aws/s2n-tls.git
cd s2n-tlsTo build s2n-tls with an existing libcrypto installation, store its root folder in the
LIBCRYPTO_ROOT environment variable.
# /usr/local/ssl/lib should contain libcrypto.a
LIBCRYPTO_ROOT=/usr/local/ssl makeThroughout this document, there are instructions for setting a LIBCRYPTO_ROOT environment variable, or setting install prefixes to s2n/lib-crypto-root. If you
are using CMake that step is unnecessary. Just follow the instructions here to use any build of libcrypto.
(Required): You need at least CMake version 3.0 to fully benefit from Modern CMake. See this for more information.
(Optional): Set the CMake variable CMAKE_PREFIX_PATH to the location libcrypto is installed to. If you do not,
the default installation on your machine will be used.
(Optional): Set the CMake variable BUILD_SHARED_LIBS=ON to build shared libraries. The default is static.
We recommend an out-of-source build. Suppose you have a directory s2n which contains the s2n-tls source code. At the same level
we can create a directory called s2n-build
For example, we can build and install shared libs using ninja as our build system, and the system libcrypto implementation.
mkdir s2n-build
cd s2n-build
cmake ../s2n-tls -DBUILD_SHARED_LIBS=ON -GNinja
ninja
ninja test
sudo ninja installFor another example, we can prepare an Xcode project using static libs using a libcrypto implementation in the directory $HOME/s2n-user/builds/libcrypto-impl.
mkdir s2n-build
cd s2n-build
cmake ../s2n-tls -DCMAKE_INSTALL_PREFIX=$HOME/s2n-user/builds/libcrypto-impl -G "Xcode"
# now open the project in Xcode and build from there, or use the Xcode CLIOr, for unix style vanilla builds:
mkdir s2n-build
cd s2n-build
cmake ../s2n-build
make
make test
sudo make installs2n-tls ships with modern CMake finder scripts if CMake is used for the build. To take advantage of this from your CMake script, all you need to do to compile and link against s2n-tls in your project is:
find_package(s2n)
....
target_link_libraries(yourExecutableOrLibrary AWS::s2n)And when invoking CMake for your project, do one of two things:
- Set the
CMAKE_INSTALL_PREFIXvariable with the path to your s2n-tls build. - If you have globally installed s2n-tls, do nothing, it will automatically be found.
We keep the build artifacts in the *-build directory:
cd libcrypto-buildOpenssl 3.0.5
curl -L -o openssl.tar.gz https://github.com/openssl/openssl/archive/refs/tags/openssl-3.0.5.tar.gz
tar -xzvf openssl-3.0.5.tar.gz
cd `tar ztf openssl-3.0.5.tar.gz | head -n1 | cut -f1 -d/`OpenSSL-1.1.1
curl -LO https://github.com/openssl/openssl/archive/refs/tags/OpenSSL_1_1_1.tar.gz
tar -xzvf OpenSSL_1_1_1.tar.gz
cd `tar ztf OpenSSL_1_1_1.tar.gz | head -n1 | cut -f1 -d/`OpenSSL-1.0.2
curl -LO https://github.com/openssl/openssl/archive/refs/tags/OpenSSL_1_0_2.tar.gz
tar -xzvf OpenSSL_1_0_2.tar.gz
cd `tar ztf OpenSSL_1_0_2.tar.gz | head -n1 | cut -f1 -d/`The following config command disables numerous Openssl features and algorithms which are not used by s2n-tls. A minimal feature-set can help prevent exposure to security vulnerabilities.
OpenSSL-1.1.1 and OpenSSL-3.0.5
./config -fPIC no-shared \
no-md2 no-rc5 no-rfc3779 no-sctp no-ssl-trace no-zlib \
no-hw no-mdc2 no-seed no-idea enable-ec_nistp_64_gcc_128 no-camellia\
no-bf no-ripemd no-dsa no-ssl2 no-ssl3 no-capieng \
-DSSL_FORBID_ENULL -DOPENSSL_NO_DTLS1 -DOPENSSL_NO_HEARTBEATS \
--prefix=`pwd`/../../libcrypto-root/
make
make installOpenSSL-1.0.2. Mac Users should replace "./config" with "./Configure darwin64-x86_64-cc".
./config -fPIC no-shared \
no-libunbound no-gmp no-jpake no-krb5 no-store \
no-md2 no-rc5 no-rfc3779 no-sctp no-ssl-trace no-zlib \
no-hw no-mdc2 no-seed no-idea enable-ec_nistp_64_gcc_128 no-camellia\
no-bf no-ripemd no-dsa no-ssl2 no-ssl3 no-capieng \
-DSSL_FORBID_ENULL -DOPENSSL_NO_DTLS1 -DOPENSSL_NO_HEARTBEATS \
--prefix=`pwd`/../../libcrypto-root/
make depend
make
make installOpenSSL-1.1.1 32-bit
setarch i386 ./config -fPIC no-shared \
-m32 no-md2 no-rc5 no-rfc3779 no-sctp no-ssl-trace no-zlib \
no-hw no-mdc2 no-seed no-idea no-camellia\
no-bf no-ripemd no-dsa no-ssl2 no-ssl3 no-capieng \
-DSSL_FORBID_ENULL -DOPENSSL_NO_DTLS1 -DOPENSSL_NO_HEARTBEATS \
--prefix=`pwd`/../../libcrypto-root/cd ../../ # root of project
makeTo build s2n-tls with LibreSSL, do the following:
# We keep the build artifacts in the *-build directory
cd libcrypto-build
# Download the latest version of LibreSSL
curl -O http://ftp.openbsd.org/pub/OpenBSD/LibreSSL/libressl-x.y.z.tar.gz
tar -xzvf libressl-x.y.z.tar.gz
# Build LibreSSL's libcrypto
cd libressl-x.y.z
./configure --prefix=`pwd`/../../libcrypto-root/
make CFLAGS=-fPIC install
# Build s2n-tls
cd ../../
makeonce built, static and dynamic libraries for s2n-tls will be available in the lib/ directory.
To build s2n-tls with BoringSSL, you must check out a copy of the BoringSSL directly via git. This procedure has been tested with fb68d6c901b98ffe15b8890d00bc819bf44c5f01 of BoringSSL.
# We keep the build artifacts in the *-build directory
cd libcrypto-build
# Clone BoringSSL
git clone https://boringssl.googlesource.com/boringssl
# Build BoringSSL
cd boringssl
mkdir build
cd build
cmake -DCMAKE_C_FLAGS="-fPIC" ../
make
# Copy the built library and includes
mkdir ../../../libcrypto-root/lib/
cp crypto/libcrypto.a ../../../libcrypto-root/lib/
cp -r ../include/ ../../../libcrypto-root/include
# Build s2n-tls
cd ../../../
makeonce built, static and dynamic libraries for s2n-tls will be available in the lib/ directory.
Internally s2n-tls uses mlock() to prevent memory from being swapped to disk. The s2n-tls build tests may fail in some environments where the default limit on locked memory is too low. To check this limit, run:
ulimit -lto raise the limit, consult the documentation for your platform.
To disable s2n-tls's mlock behavior, run your application with the S2N_DONT_MLOCK environment variable set.
s2n-tls also reads this for unit tests. Try S2N_DONT_MLOCK=1 make if you're having mlock failures during unit tests.
The API exposed by s2n-tls is the set of functions and declarations that are in the s2n.h header file. Any functions and declarations that are in the s2n.h file are intended to be stable (API and ABI) within major version numbers of s2n-tls releases. Other functions and structures used in s2n-tls internally can not be considered stable and their parameters, names, and sizes may change.
The VERSIONING.rst document contains more details about s2n's approach to versions and API changes.
s2n-tls uses Doxygen to document its public API. The latest s2n-tls documentation can be found on GitHub pages.
Documentation for older versions or branches of s2n-tls can be generated locally. To generate the documentation, install doxygen and run doxygen docs/doxygen/Doxyfile. The doxygen documentation can now be found at docs/doxygen/output/html/index.html.
Doxygen installation instructions are available at the Doxygen webpage.
The doxygen documentation should be used in conjunction with this guide.
Currently TLS 1.2 is our default version, but we recommend TLS 1.3 where possible. To use TLS 1.3 you need a security policy that supports TLS 1.3. See the Security Policies section for more information.
Note: s2n-tls does not support SSL2.0 for sending and receiving encrypted data, but does accept SSL2.0 hello messages.
s2n-tls functions that return 'int' return 0 to indicate success and -1 to indicate
failure. s2n-tls functions that return pointer types return NULL in the case of
failure. When an s2n-tls function returns a failure, s2n_errno will be set to a value
corresponding to the error. This error value can be translated into a string
explaining the error in English by calling s2n_strerror(s2n_errno, "EN").
A string containing human readable error name, can be generated with s2n_strerror_name.
A string containing internal debug information, including filename and line number, can be generated with s2n_strerror_debug.
This string is useful to include when reporting issues to the s2n-tls development team.
Example:
if (s2n_config_set_cipher_preferences(config, prefs) < 0) {
printf("Setting cipher prefs failed! %s : %s", s2n_strerror(s2n_errno, "EN"), s2n_strerror_debug(s2n_errno, "EN"));
return -1;
}
NOTE: To avoid possible confusion, s2n_errno should be cleared after processing an error: s2n_errno = S2N_ERR_T_OK
When using s2n-tls outside of C, the address of the thread-local s2n_errno may be obtained by calling the s2n_errno_location function.
This will ensure that the same TLS mechanisms are used with which s2n-tls was compiled.
s2n-tls organizes errors into different "types" to allow applications to handle error values without catching all possibilities.
Applications using non-blocking I/O should check the error type to determine if the I/O operation failed because it would block or for some other error. To retrieve the type for a given error use s2n_error_get_type().
Applications should perform any error handling logic using these high level types:
Here's an example that handles errors based on type:
#define SUCCESS 0
#define FAILURE 1
#define RETRY 2
s2n_errno = S2N_ERR_T_OK;
if (s2n_negotiate(conn, &blocked) < 0) {
switch(s2n_error_get_type(s2n_errno)) {
case S2N_ERR_T_BLOCKED:
/* Blocked, come back later */
return RETRY;
case S2N_ERR_T_CLOSED:
return SUCCESS;
case S2N_ERR_T_IO:
handle_io_err(errno);
return FAILURE;
case S2N_ERR_T_PROTO:
handle_proto_err();
return FAILURE;
case S2N_ERR_T_ALERT:
log_alert(s2n_connection_get_alert(conn));
return FAILURE;
/* Everything else */
default:
log_other_error();
return FAILURE;
}
}Blinding is a mitigation against timing side-channels which in some cases can leak information about encrypted data. By default s2n-tls will cause a thread to sleep between 10 and 30 seconds whenever tampering is detected.
Setting the S2N_SELF_SERVICE_BLINDING option with s2n_connection_set_blinding() turns off this behavior. This is useful for applications that are handling many connections in a single thread. In that case, if s2n_recv() or s2n_negotiate() return an error, self-service applications must call s2n_connection_get_delay() and pause activity on the connection for the specified number of nanoseconds before calling close() or shutdown(). s2n_shutdown() will fail if called before the blinding delay elapses.
s2n-tls has an mechanism to capture stacktraces when errors occur.
This mechanism is off by default, but can be enabled in code by calling s2n_stack_traces_enabled_set().
It can be enabled globally by setting the environment variable S2N_PRINT_STACKTRACE=1.
Call s2n_print_stacktrace() to print your stacktrace.
Note: Enabling stacktraces can significantly slow down unit tests, causing failures on tests (such as s2n_cbc_verify) that measure the timing of events.
The s2n-tls library must be initialized with s2n_init() before calling most library functions. s2n_init() MUST NOT be called more than once, even when an application uses multiple threads or processes. s2n attempts to clean up its thread-local memory at thread-exit and all other memory at process-exit. However, this may not work if you are using a thread library other than pthreads. In that case you should call s2n_cleanup() from every thread or process created after s2n_init().
Initialization can be modified by calling s2n_crypto_disable_init() or s2n_disable_atexit() before s2n_init().
An application can override s2n-tls’s internal memory management by calling s2n_mem_set_callbacks before calling s2n_init.
If you are trying to use FIPS mode, you must enable FIPS in your libcrypto library (probably by calling FIPS_mode_set(1)) before calling s2n_init().
Users will need to create a s2n_connection struct to store all of the state necessary for a TLS connection. Call s2n_connection_new() to create a new server or client connection. Call s2n_connection_free() to free the memory allocated for this struct when no longer needed.
The connection struct is roughly 4KB with some variation depending on how it is configured. Maintainers of the s2n-tls library carefully consider increases to the size of the connection struct as they are aware some users are memory-constrained.
A connection struct has memory allocated specifically for the TLS handshake. Memory-constrained users can free that memory by calling s2n_connection_free_handshake() after the handshake is successfully negotiated. Note that the handshake memory can be reused for another connection if s2n_connection_wipe() is called, so freeing it may result in more memory allocations later. Additionally some functions that print information about the handshake may not produce meaningful results after the handshake memory is freed.
The input and output buffers consume the most memory on the connection after the handshake. It may not be necessary to keep these buffers allocated when a connection is in a keep-alive or idle state. Call s2n_connection_release_buffers() to wipe and free the in and out buffers associated with a connection to reduce memory overhead of long-lived connections.
Connection objects can be re-used across many connections to reduce memory allocation. Calling s2n_connection_wipe() will wipe an individual connection's state and allow the connection object to be re-used for a new TLS connection.
s2n-tls provides many methods to retrieve details about the handshake and connection, such as the parameters negotiated with the peer. For a full list, see our doxygen guide.
s2n-tls provides multiple different methods to get the TLS protocol version of the connection. They should be called after the handshake has completed.
s2n_connection_get_actual_protocol_version(): The actual TLS protocol version negotiated during the handshake. This is the primary value referred to as "protocol_version", and the most commonly used.s2n_connection_get_server_protocol_version(): The highest TLS protocol version the server supports.s2n_connection_get_client_protocol_version(): The highest TLS protocol version the client advertised.
s2n_config objects are used to change the default settings of a s2n-tls connection. Use s2n_config_new() to create a new config object. To associate a config with a connection call s2n_connection_set_config(). A config should not be altered once it is associated with a connection as this will produce undefined behavior. It is not necessary to create a config object per connection; one config object should be used for many connections. Call s2n_config_free() to free the object when no longer needed. Only free the config object when all connections using it have been freed.
Most commonly, a s2n_config object is used to set the certificate key pair for authentication and change the default security policy. See the sections for certificates and security policies for more information on those settings.
Some s2n_config settings can be overridden on a specific connection if desired. For example, s2n_config_append_protocol_preference() appends a list of ALPN protocols to a s2n_config. Calling the s2n_connection_append_protocol_preference() API will override the list of ALPN protocols for an individual connection. Not all config APIs have a corresponding connection API so if there is one missing contact us with an explanation on why it is required for your use-case.
s2n-tls uses pre-made security policies to help avoid common misconfiguration mistakes for TLS.
s2n_config_set_cipher_preferences() sets a security policy, which includes the cipher/kem/signature/ecc preferences and protocol version.
The following chart maps the security policy version to protocol version and ciphersuites supported.
| version | SSLv3 | TLS1.0 | TLS1.1 | TLS1.2 | TLS1.3 | AES-CBC | ChaCha20-Poly1305 | ECDSA | AES-GCM | 3DES | RC4 | DHE | ECDHE | ChaCha20-Boosted |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| "default" | X | X | X | X | X | X | X | |||||||
| "20190214" | X | X | X | X | X | X | X | X | X | |||||
| "20170718" | X | X | X | X | X | X | ||||||||
| "20170405" | X | X | X | X | X | X | X | |||||||
| "20170328" | X | X | X | X | X | X | X | X | ||||||
| "20170210" | X | X | X | X | X | X | X | |||||||
| "20160824" | X | X | X | X | X | X | ||||||||
| "20160804" | X | X | X | X | X | X | X | |||||||
| "20160411" | X | X | X | X | X | X | X | |||||||
| "20150306" | X | X | X | X | X | X | X | |||||||
| "20150214" | X | X | X | X | X | X | X | |||||||
| "20150202" | X | X | X | X | X | X | ||||||||
| "20141001" | X | X | X | X | X | X | X | |||||||
| "20140601" | X | X | X | X | X | X | X | X | ||||||
| "20190120" | X | X | X | X | X | X | X | |||||||
| "20190121" | X | X | X | X | X | X | X | |||||||
| "20190122" | X | X | X | X | X | X | X | X | X | |||||
| "default_tls13" | X | X | X | X | X | X | X | X | X | |||||
| "20190801" | X | X | X | X | X | X | X | X | ||||||
| "20190802" | X | X | X | X | X | X | X | X | ||||||
| "20200207" | X | X | X | X | X | X | X | |||||||
| "20230317" | X | X | X | X | X | X | ||||||||
| "rfc9151" | X | X | X | X | X | X | ||||||||
| "CloudFront-TLS-1-2-2021" | X | X | X | X | X | X | ||||||||
| "CloudFront-TLS-1-2-2021-ChaCha20-Boosted" | X | X | X | X | X | X | X |
The "default" and "default_tls13" versions are special in that they will be updated with future s2n-tls changes and ciphersuites and protocol versions may be added and removed, or their internal order of preference might change. Numbered versions are fixed and will never change. In general, customers prefer to use numbered versions for production use cases to prevent impact from library updates.
"20230317" offers more limited but more secure options than "default". It only supports TLS1.2 and TLS1.3 and is FIPS compliant. Choose this policy if you don't need or want to support less secure legacy options like TLS1.1 or SHA1.
"20160411" follows the same general preference order as "default". The main difference is it has a CBC cipher suite at the top. This is to accommodate certain Java clients that have poor GCM implementations. Users of s2n-tls who have found GCM to be hurting performance for their clients should consider this version.
"20170405" is a FIPS compliant cipher suite preference list based on approved algorithms in the FIPS 140-2 Annex A. Similarly to "20160411", this preference list has CBC cipher suites at the top to accommodate certain Java clients. Users of s2n-tls who plan to enable FIPS mode should consider this version.
"rfc9151" is derived from Commercial National Security Algorithm (CNSA) Suite Profile for TLS and DTLS 1.2 and 1.3. This policy restricts the algorithms allowed for signatures on certificates in the certificate chain to RSA or ECDSA with sha384, which may require you to update your certificates.
s2n-tls does not expose an API to control the order of preference for each ciphersuite or protocol version. s2n-tls follows the following order:
NOTE: All ChaCha20-Poly1305 cipher suites will not be available if s2n-tls is not built with an Openssl 1.1.1 libcrypto. The underlying encrypt/decrypt functions are not available in older versions.
- Always prefer the highest protocol version supported
- Always use forward secrecy where possible. Prefer ECDHE over DHE.
- Prefer encryption ciphers in the following order: AES128, AES256, ChaCha20, 3DES, RC4.
- Prefer record authentication modes in the following order: GCM, Poly1305, SHA256, SHA1, MD5.
s2n-tls usually prefers AES over ChaCha20. However, some clients-- particularly mobile or IOT devices-- do not support AES hardware acceleration, making AES less efficient and performant than ChaCha20. In this case, clients will indicate their preference for ChaCha20 by listing it first during cipher suite negotiation. Usually s2n-tls servers ignore client preferences, but s2n-tls offers "ChaCha20 boosted" security policies that will choose ChaCha20 over AES if the client indicates a preference for ChaCha20.
| version | RSA PKCS1 | ECDSA | SHA-1 Legacy | RSA PSS |
|---|---|---|---|---|
| "default" | X | X | X | |
| "20190214" | X | X | X | |
| "20170718" | X | X | X | |
| "20170405" | X | X | X | |
| "20170328" | X | X | X | |
| "20170210" | X | X | X | |
| "20160824" | X | X | X | |
| "20160804" | X | X | X | |
| "20160411" | X | X | X | |
| "20150306" | X | X | X | |
| "20150214" | X | X | X | |
| "20150202" | X | X | X | |
| "20141001" | X | X | X | |
| "20140601" | X | X | X | |
| "20190120" | X | X | X | |
| "20190121" | X | X | X | |
| "20190122" | X | X | X | |
| "default_tls13" | X | X | X | X |
| "20190801" | X | X | X | X |
| "20190802" | X | X | X | X |
| "20200207" | X | X | X | X |
| "20230317" | X | X | X | |
| "rfc9151" | X | X | X |
Note that the default_tls13 security policy will never support legacy SHA-1 algorithms in TLS1.3, but will support legacy SHA-1 algorithms in CertificateVerify messages if TLS1.2 has been negotiated.
| version | secp256r1 | secp384r1 | x25519 |
|---|---|---|---|
| "default" | X | X | |
| "20190214" | X | X | |
| "20170718" | X | X | |
| "20170405" | X | X | |
| "20170328" | X | X | |
| "20170210" | X | X | |
| "20160824" | X | X | |
| "20160804" | X | X | |
| "20160411" | X | X | |
| "20150306" | X | X | |
| "20150214" | X | X | |
| "20150202" | X | X | |
| "20141001" | X | X | |
| "20140601" | X | X | |
| "20190120" | X | X | |
| "20190121" | X | X | |
| "20190122" | X | X | |
| "default_tls13" | X | X | X |
| "20190801" | X | X | X |
| "20190802" | X | X | |
| "20200207" | X | X | X |
| "rfc9151" | X |
TLS uses certificates to authenticate the server (and optionally the client). The handshake will fail if the client cannot verify the server’s certificate.
Authentication is usually the most expensive part of the handshake. To avoid the cost, consider using session resumption or pre-shared keys.
To validate the peer’s certificate, the local “trust store” must contain a certificate that can authenticate the peer’s certificate.
By default, s2n-tls will be initialized with the common trust store locations for the host operating system. To completely override those locations, call s2n_config_wipe_trust_store(). To add certificates to the trust store, call s2n_config_set_verification_ca_location() or s2n_config_add_pem_to_trust_store().
A server must have a certificate and private key pair to prove its identity. s2n-tls supports RSA, RSA-PSS, and ECDSA certificates, and allows one of each type to be added to a config.
Create a new certificate and key pair by calling s2n_cert_chain_and_key_new(), then load the pem-encoded data with s2n_cert_chain_and_key_load_pem_bytes(). Call s2n_config_add_cert_chain_and_key_to_store() to add the certificate and key pair to the config. When a certificate and key pair is no longer needed, it must be cleaned up with s2n_cert_chain_and_key_free().
A client can add restrictions on the certificate’s hostname by setting a custom s2n_verify_host_fn with s2n_config_set_verify_host_callback() or s2n_connection_set_verify_host_callback(). The default behavior is to require that the hostname match the server name set with s2n_set_server_name().
Client authentication is not enabled by default. However, the server can require that the client also provide a certificate, if the server needs to authenticate clients before accepting connections.
Client authentication can be configured by calling s2n_config_set_client_auth_type() or s2n_connection_set_client_auth_type() for both the client and server. Additionally, the client will need to load a certificate and key pair as described for the server in Server Authentication and the server will need to configure its trust store as described in Configuring the Trust Store.
When using client authentication, the server MUST implement the s2n_verify_host_fn, because the default behavior will likely reject all client certificates.
When using client authentication with TLS1.3, s2n_negotiate will report a successful
handshake to clients before the server validates the client certificate. If the server then
rejects the client certificate, the client may later receive an alert while calling s2n_recv,
potentially after already having sent application data with s2n_send. This is a quirk of the
TLS1.3 protocol message ordering: the server does not send any more handshake messages
after the client sends the client certificate (see the TLS1.3 state machine).
There is no security risk, since the client has already authenticated the server,
but it could make handshake failures and authentication errors more difficult to handle.
Applications may want to know which certificate was used by a server for authentication during a connection, since servers can set multiple certificates. s2n_connection_get_selected_cert() will return the local certificate chain object used to authenticate. s2n_connection_get_peer_cert_chain() will provide the peer's certificate chain, if they sent one. Use s2n_cert_chain_get_length() and s2n_cert_chain_get_cert() to parse the certificate chain object and get a single certificate from the chain. Use s2n_cert_get_der() to get the DER encoded certificate if desired.
Additionally s2n-tls has functions for parsing certificate extensions on a certificate. Use s2n_cert_get_x509_extension_value_length() and s2n_cert_get_x509_extension_value() to obtain a specific DER encoded certificate extension from a certificate. s2n_cert_get_utf8_string_from_extension_data_length() and s2n_cert_get_utf8_string_from_extension_data() can be used to obtain a specific UTF8 string representation of a certificate extension instead. These functions will work for both RFC-defined certificate extensions and custom certificate extensions.
Certificate revocation is how CAs inform validators that an active certificate should not be trusted. This commonly occurs when a private key has been leaked and the identity of the certificate's owner can no longer be trusted.
s2n-tls supports two methods of certificate revocation: OCSP stapling and CRLs. A fundamental difference between the two is that with OCSP stapling, the peer offering the certificate validates the revocation status of its own certificate. This peer can choose not to send a certificate status response, and applications will have to decide whether or not to fail certificate validation in this case. With CRLs, the application checks the revocation status of the certificate itself, without relying on the peer. However, CRLs must be retrieved and stored by the application, which requires more network and memory utilization than OCSP stapling.
Users who want certificate revocation should look closely at their use-case and decide which method is more appropriate. We suggest using OCSP stapling if you're sure your peer supports OCSP stapling. CRLs should be used if this assumption can't be made. However, s2n-tls does not enable applications to fetch CRLs for received certificates in real-time. This method should only be used if you're able to obtain CRLs in advance for all certificates you expect to encounter.
Online Certificate Status Protocol (OCSP) is a protocol to establish whether or not a certificate has been revoked. The requester (usually a client), asks the responder (usually a server), to ‘staple’ the certificate status information along with the certificate itself. The certificate status sent back will be either expired, current, or unknown, which the requester can use to determine whether or not to accept the certificate.
OCSP stapling can be applied to both client and server certificates when using TLS1.3, but only to server certificates when using TLS1.2.
To use OCSP stapling, the requester must call s2n_config_set_status_request_type() with S2N_STATUS_REQUEST_OCSP. The responder will need to call s2n_cert_chain_and_key_set_ocsp_data() to set the raw bytes of the OCSP stapling data.
The OCSP stapling information will be automatically validated if the underlying libcrypto supports OCSP validation. s2n_config_set_check_stapled_ocsp_response() can be called with "0" to turn this off. Call s2n_connection_get_ocsp_response() to retrieve the received OCSP stapling information for manual verification.
Note: the CRL validation feature in s2n-tls is currently considered unstable, meaning the CRL APIs are subject to change in a future release. To access the CRL APIs, include
api/unstable/crl.h.
Certificate Revocation Lists (CRLs) are lists of issued, unexpired certificates that have been revoked by the CA. CAs publish updated versions of these lists periodically. A validator wishing to verify a certificate obtains a CRL from the CA, validates the CRL, and checks to ensure the certificate is not contained in the list, and therefore has not been revoked by the CA.
The s2n CRL lookup callback must be implemented and set via s2n_config_set_crl_lookup_cb() to enable CRL validation in s2n-tls. This callback will be triggered once for each certificate in the certificate chain.
The CRLs for all certificates received in the handshake must be obtained in advance of the CRL lookup callback, outside of s2n-tls. It is not possible in s2n-tls to obtain CRLs in real-time. Applications should load these CRLs into memory, by creating s2n_crls via s2n_crl_new(), and adding the obtained CRL data via s2n_crl_load_pem(). The s2n_crl should be freed via s2n_crl_free() when no longer needed.
The application must implement a way to look up the correct CRL for a given certificate. This can be done by comparing the hash of the received certificate's issuer with the hash of the CRL's issuer. The certificate's issuer hash is retrieved via s2n_crl_lookup_get_cert_issuer_hash(), and the CRL's issuer hash is retrieved via s2n_crl_get_issuer_hash(). Once a CRL is found with a matching issuer hash, call s2n_crl_lookup_set() to provide s2n-tls with this CRL.
Call s2n_crl_lookup_ignore() to ignore a received certificate if its CRL can't be found. This will cause the certificate validation logic to fail with a S2N_ERR_CRL_LOOKUP_FAILED error if the certificate is needed in the chain of trust. The certificate validation logic will not fail if the ignored certificate ends up not being included in the chain of trust.
By default, the CRL validation logic will not fail on CRLs that are not yet active, or are expired. Timestamp validation can optionally be performed in the CRL lookup callback by calling s2n_crl_validate_active() and s2n_crl_validate_not_expired().
Certificate transparency is a framework to store public logs of CA-issued certificates. If requested, certificate owners can send a signed certificate timestamp (SCT) to prove that their certificate exists in these logs. The requester can choose whether or not to accept a certificate based on this information.
Certificate transparency information can be applied to both client and server certificates when using TLS1.3, but only to server certificates when using TLS1.2.
To use certificate transparency, the requester (usually the client) must call s2n_config_set_ct_support_level() with S2N_CT_SUPPORT_REQUEST. The responder (usually the server) must call s2n_cert_chain_and_key_set_sct_list() to set the raw bytes of the transparency information.
Call s2n_connection_get_sct_list() to retrieve the received certificate transparency information. The format of this data is the SignedCertificateTimestampList structure defined in section 3.3 of RFC 6962.
TLS handshake sessions are CPU-heavy due to the calculations involved in authenticating a certificate. These calculations can be skipped after the first connection by turning on session resumption. This mechanism stores state from the previous session and uses it to establish the next session, allowing the handshake to skip the costly authentication step while keeping the same cryptographic guarantees. The authentication step can be skipped because both the server and client will use their possession of the key from the previous session to prove who they are. We usually refer to the stored session state as a "session ticket". Note that this session ticket is encrypted by the server, so a server will have to set up an external key in order to do session resumption.
The key that encrypts and decrypts the session state is not related to the keys negotiated as part of the TLS handshake and has to be set by the server by calling s2n_config_add_ticket_crypto_key(). See RFC5077 for guidelines on securely generating keys.
Each key has two different expiration dates. The first expiration date signifies the time that the key can be used for both encryption and decryption. The second expiration date signifies the time that the key can be used only for decryption. This mechanism is to ensure that a session ticket can be successfully decrypted if it was encrypted by a key that was about to expire. The full lifetime of the key is therefore the encrypt-decrypt lifetime plus the decrypt-only lifetime. To alter the default key lifetime call s2n_config_set_ticket_encrypt_decrypt_key_lifetime() and s2n_config_set_ticket_decrypt_key_lifetime().
The server will stop issuing session resumption tickets if a user doesn't set up a new key before the previous key passes through its encrypt-decrypt lifetime. Therefore it is recommended to add a new key when half of the previous key's encrypt-decrypt lifetime has passed.
In stateless session resumption the server sends a session ticket to a client after a successful handshake, and the client can send that ticket back to the server during a new connection to skip the authentication step. This mechanism allows servers to avoid storing individual state for each client, and for that reason is the preferred method for resuming a session.
Servers should call s2n_config_set_session_tickets_onoff() to enable stateless session resumption. Additionally the server needs to set up an encryption key using s2n_config_add_ticket_crypto_key().
Clients should call s2n_config_set_session_tickets_onoff() to enable stateless session resumption and set a session ticket callback function using s2n_config_set_session_ticket_cb(), which will allow clients to receive a session ticket when it arrives. Then s2n_connection_set_session() should be called with that saved ticket when attempting to resume a new connection.
In stateful session resumption, also known as session caching, the server caches the session state per client and resumes a session based on the client's session ID. Note that session caching has not been implemented for > TLS1.2. If stateful session resumption is turned on and a TLS1.3 handshake is negotiated, the caching mechanism will not store that session and resumption will not be available the next time the client connects.
Servers should set the three caching callback functions: s2n_config_set_cache_store_callback(), s2n_config_set_cache_retrieve_callback(), and s2n_config_set_cache_delete_callback() and then call s2n_config_set_session_cache_onoff() to enable stateful session resumption. Session caching will not be turned on unless all three session cache callbacks are set prior to calling s2n_config_set_session_cache_onoff(). Additionally, the server needs to set up an encryption key using s2n_config_add_ticket_crypto_key().
Clients should call s2n_connection_get_session() to retrieve some serialized state about the session. Then s2n_connection_set_session() should be called with that saved state when attempting to resume a new connection.
In TLS1.2, session ticket messages are sent during the handshake and are automatically received as part of calling s2n_negotiate(). They will be available as soon as negotiation is complete.
In TLS1.3, session ticket messages are sent after the handshake as "post-handshake" messages, and may not be received as part of calling s2n_negotiate(). A s2n-tls server will send tickets immediately after the handshake, so clients can receive them by calling s2n_recv() immediately after the handshake completes. However, other server implementations may send their session tickets later, at any time during the connection.
Additionally, in TLS1.3, multiple session tickets may be issued for the same connection. Servers can call s2n_config_set_initial_ticket_count() to set the number of tickets they want to send and s2n_connection_add_new_tickets_to_send() to increase the number of tickets to send during a connection.
In TLS1.2, the secret stored inside the ticket is the original session's master secret. Because of this, TLS1.2 session tickets are not forward secret, meaning that compromising the resumed session's secret exposes the original session's encrypted data.
In contrast, in TLS1.3 the secret stored inside the ticket is derived from the original session's master secret. The derivation uses a cryptographic operation that can't be reversed by an attacker to retrieve the original master secret. Therefore, TLS1.3 session tickets are forward secret, meaning compromising the resumed session's secret will not expose the original session's encrypted data.
In TLS1.2, a full handshake can issue a session ticket encrypted with a specific session ticket encryption key. Connections that resume using that session ticket will not issue new session tickets. Therefore, the lifetime of the original "keying material"-- meaning the lifetime of any secret derived from the original full handshake-- is limited by the lifetime of the session ticket encryption key. Applications can set the session ticket encryption key lifetime with s2n_config_set_ticket_encrypt_decrypt_key_lifetime().
In TLS1.3, connections that resume using a session ticket CAN issue new session tickets. This is because TLS1.3 tickets are intended to be single-use, and each ticket contains a different secret: see Session Resumption Forward Secrecy. These new session tickets may be encrypted with newer session ticket encryption keys, allowing the original "keying material" to outlive the original session ticket encryption key. However, TLS1.3 enforces a specific separate "keying material" lifetime, which servers can configure with s2n_connection_set_server_keying_material_lifetime(). This effectively places a limit on how long sessions can be resumed before a new full handshake is required.
s2n-tls stores the received Client Hello and makes it available to the application. Call s2n_connection_get_client_hello() to get a pointer to the s2n_client_hello struct storing the Client Hello message. A NULL value will be returned if the connection has not yet received the Client Hello. The earliest point in the handshake when this struct is available is during the Client Hello Callback. The stored Client Hello message will not be available after calling s2n_connection_free_handshake().
Call s2n_client_hello_get_raw_message() to retrieve the complete Client Hello message with the random bytes on it zeroed out.
Call s2n_client_hello_get_cipher_suites() to retrieve the list of cipher suites sent by the client.
Call s2n_client_hello_get_session_id() to retrieve the session ID sent by the client in the ClientHello message. Note that this value may not be the session ID eventually associated with this particular connection since the session ID can change when the server sends the Server Hello. The official session ID can be retrieved with s2n_connection_get_session_id()after the handshake completes.
Call s2n_client_hello_get_extensions() to retrieve the entire list of extensions sent in the Client Hello. Calling s2n_client_hello_get_extension_by_id() allows you to interrogate the s2n_client_hello struct for a specific extension.
s2n-tls will not negotiate SSLv2, but will accept SSLv2 ClientHellos advertising a higher protocol version like SSLv3 or TLS1.0. This was a backwards compatibility strategy used by some old clients when connecting to a server that might only support SSLv2.
You can determine whether an SSLv2 ClientHello was received by checking the value
of s2n_connection_get_client_hello_version(). If an SSLv2 ClientHello was
received, then s2n_connection_get_client_protocol_version() will still report
the real protocol version requested by the client.
SSLv2 ClientHellos are formatted differently than ClientHellos in later versions.
s2n_client_hello_get_raw_message() and s2n_client_hello_get_cipher_suites()
will produce differently formatted data. See the documentation for those methods
for details about proper SSLv2 ClientHello parsing.
Users can access the Client Hello during the handshake by setting the callback s2n_config_set_client_hello_cb(). A possible use-case for this is to modify the s2n_connection based on information in the Client Hello. This should be done carefully, as modifying the connection in response to untrusted input can be dangerous. In particular, switching from an s2n_config that supports TLS1.3 to one that does not opens the server up to a possible version downgrade attack.
s2n_connection_server_name_extension_used() MUST be invoked before exiting the callback if any of the connection properties were changed on the basis of the Server Name extension. If desired, the callback can return a negative value to make s2n-tls terminate the handshake early with a fatal handshake failure alert.
The callback can be invoked in two modes: S2N_CLIENT_HELLO_CB_BLOCKING or S2N_CLIENT_HELLO_CB_NONBLOCKING. Use s2n_config_set_client_hello_cb_mode() to set the desired mode.
The default mode, "blocking mode", will wait for the Client Hello callback to succeed and then continue the handshake. Use this mode for light-weight callbacks that won't slow down the handshake or block the main thread, like logging or simple configuration changes.
In contrast, "non-blocking mode" will wait for the ClientHello callback to succeed and then pause the handshake, immediately returning from s2n_negotiate with an error indicating that the handshake is blocked on application input. This allows the application to do expensive or time-consuming work like network calls outside of the callback without blocking the main thread. Only when the application calls s2n_client_hello_cb_done() will the handshake be able to resume.
When sending data, s2n-tls uses a default maximum record size which experimentation has suggested provides a reasonable balance of performance and throughput.
s2n_connection_prefer_throughput can be called to increase the record size, which minimizes overhead. It also increases s2n-tls's memory usage.
s2n_connection_prefer_low_latency can be called to decrease the record size, which allows the receiver to decrypt the data faster. It also decreases s2n-tls's memory usage.
These options only affect the size of the records that s2n-tls sends, not the behavior of the peer.
The maximum number of bytes that can be sent in a TLS record is called the "maximum fragment length", and is set to 2^14 bytes by default. Regardless of the maximum record size that s2n-tls uses when sending, it may receive records containing up to 2^14 bytes of plaintext.
A client can request a lower maximum fragment length by calling s2n_config_send_max_fragment_length, reducing the size of TLS records sent and providing benefits similar to s2n_connection_prefer_low_latency. However, many TLS servers either ignore these requests or handle them incorrectly, so a client should never assume that a lower maximum fragment length will be honored. If a server accepts the requested maximum fragment length, the client will respect that maximum when sending.
By default, an s2n-tls server will ignore a client's requested maximum fragment length. If s2n_config_accept_max_fragment_length is called, the server will respect the client's requested maximum fragment length when sending, but will not reject client records with a larger fragment size.
If a maximum fragment length is negotiated during the connection, it will override the behavior configured by s2n_connection_prefer_throughput and s2n_connection_prefer_low_latency.
Sending smaller records at the beginning of a connection can decrease first byte latency, particularly if TCP slow start is used.
s2n_connection_set_dynamic_record_threshold can be called to initially send smaller records. The connection will send the first resize_threshold bytes in records small enough to fit in a single standard 1500 byte ethernet frame. Whenever timeout_threshold seconds pass without sending data, the connection will revert to this behavior and send small records again.
Dynamic record sizing doesn't completely override s2n_connection_prefer_throughput, s2n_connection_prefer_low_latency, or the negotiated maximum fragment length. Once resize_threshold is hit, records return to the maximum size configured for the connection. And if the maximum fragment length negotiated with the peer is lower than what dynamic record sizing would normally produce, the lower value will be used.
int s2n_connection_set_fd(struct s2n_connection *conn,
int readfd);
int s2n_connection_set_read_fd(struct s2n_connection *conn,
int readfd);
int s2n_connection_set_write_fd(struct s2n_connection *conn,
int writefd);s2n_connection_set_fd sets the file-descriptor for an s2n-tls connection. This file-descriptor should be active and connected. s2n-tls also supports setting the read and write file-descriptors to different values (for pipes or other unusual types of I/O).
Important Note: If the read end of the pipe is closed unexpectedly, writing to the pipe will raise a SIGPIPE signal. s2n-tls does NOT handle SIGPIPE. A SIGPIPE signal will cause the process to terminate unless it is handled or ignored by the application.
By default, s2n-tls automatically uses the configured private key to synchronously perform the signature and decryption operations required for a tls handshake. However, this default behavior may not work for some situations.
For example:
- An application may want to perform the CPU-expensive signature and decryption operations asynchronously to avoid blocking the main event loop. See Asynchronous private key operations
- An application may not have direct access to the private key, such as when using PKCS#11. See Offloading private key operations
To handle these use cases, s2n-tls provides a callback to allow users to control how these operations are performed. The callback is set via s2n_config_set_async_pkey_callback and is triggered every time s2n_negotiate performs an action involving the private key. The callback is passed op, an opaque object representing the private key operation. To avoid memory leaks, op must always eventually be freed by calling s2n_async_pkey_op_free.
The private key operation can be performed by calling s2n_async_pkey_op_perform (or s2n_async_pkey_op_set_output: see Offloading private key operations). The required private key can be retrieved using the s2n_connection_get_selected_cert and s2n_cert_chain_and_key_get_private_key calls. The operation can then be finalized with s2n_async_pkey_op_apply to continue the handshake.
When s2n-tls is used in non-blocking mode, private key operations can be completed asynchronously. This model can be useful to move execution of CPU-heavy private key operations out of the main event loop, preventing s2n_negotiate from blocking the loop for a few milliseconds each time the private key operation needs to be performed.
To handle private key operations asynchronously, return from the callback without calling s2n_async_pkey_op_perform or s2n_async_pkey_op_apply. Usually the user would do this by spawning a separate thread to perform op and immediately returning S2N_SUCCESS from the callback without waiting for that separate thread to complete. In response, s2n_negotiate will return S2N_FAILURE with an error of type S2N_ERR_T_BLOCKED and s2n_blocked_status set to S2N_BLOCKED_ON_APPLICATION_INPUT. All subsequent calls to s2n_negotiate will produce the same result until s2n_async_pkey_op_apply is called to finalize the op.
Note: It is not safe to call multiple functions on the same conn or op objects from 2 different threads at the same time. Doing so will produce undefined behavior. However it is safe to have a call to a function involving only conn at the same time as a call to a function involving only op, as those objects are not coupled with each other. It is also safe to free conn or op at any moment with respective function calls, with the exception that conn cannot be freed inside the s2n_async_pkey_fn callback.
Despite the "async" in the function names, private key operations can also be completed synchronously using the callback. To complete an operation synchronously, simply call s2n_async_pkey_op_perform and s2n_async_pkey_op_apply inside the callback. If the callback succeeds, the handshake will continue uninterrupted. If the callback fails, s2n_negotiate will fail with an error of type S2N_ERR_T_INTERNAL.
The s2n_async_pkey_op_perform call used to perform a private key operation requires direct access to the private key. In some cases, like when using PKCS#11, users may not have access to the private key. In these cases, we can substitute s2n_async_pkey_op_set_output for s2n_async_pkey_op_perform to tell s2n-tls the result of the operation rather than having s2n-tls perform the operation itself.
s2n-tls provides a number of calls to gather the information necessary for an outside module or library to perform the operation. The application can query the type of private key operation by calling s2n_async_pkey_op_get_op_type. In order to perform an operation, the application must ask s2n-tls to copy the operation's input into an application supplied buffer. The appropriate buffer size can be determined by calling s2n_async_pkey_op_get_input_size. Once a buffer of the proper size is allocated, the application can request the input data by calling s2n_async_pkey_op_get_input. After the operation is completed, the finished output can be copied back to S2N by calling s2n_async_pkey_op_set_output. Once the output is set, the private key operation can be completed by calling s2n_async_pkey_op_apply as usual.
Offloading can be performed either synchronously or asynchronously. If the offloaded operation fails synchronously, simply return S2N_FAILURE from the callback. If the offloaded operation fails asynchronously, s2n-tls does not provide a way to communicate that result. Instead, simply shutdown and cleanup the connection as you would for any other error.
s2n-tls supports pre-shared keys (PSKs) as of TLS1.3. PSKs allow users to establish secrets outside of the handshake, skipping certificate exchange and authentication.
Using pre-shared keys can avoid the need for public key operations. This is useful in performance-constrained environments with limited CPU power. PSKs may also be more convenient from a key management point of view: If the system already has a mechanism for sharing secrets, that mechanism can be reused for TLS PSKs.
A PSK must not be shared between more than one server and one client. An entity that acts as both a server and a client should not use the same PSK for both roles. For more information see: Selfie: reflections on TLS 1.3 with PSK.
Both clients and servers will need to create and append a PSK to a connection in order to negotiate that PSK. Call s2n_external_psk_new() to allocate memory for a PSK object. Call s2n_psk_set_identity() to set a unique identifier for the PSK. Note that this identity will be transmitted over the network unencrypted, so do not include any confidential information in it. Call s2n_psk_set_secret() to set the secret value for a given PSK. Deriving a shared secret from a password or other low-entropy source is not secure and is subject to dictionary attacks. See this RFC for more guidelines on creating a secure PSK secret. Call s2n_psk_set_hmac() to change the hmac algorithm (defaults to S2N_PSK_HMAC_SHA256) for a given PSK. Note that the hmac algorithm may influence server cipher-suite selection.
Call s2n_connection_append_psk() to append the newly created PSK to the connection. Both the server and client should call this API to add PSKs to their connection. PSKs that are appended first will be preferred over PSKs appended last unless custom PSK selection logic is implemented. Use s2n_psk_free() to free the memory allocated for a PSK once you have associated it with a connection.
External PSKs and Session Resumption cannot both be used in TLS13. Therefore, users must pick which mode they are using by calling s2n_config_set_psk_mode() prior to the handshake. Additionally, s2n_connection_set_psk_mode() overrides the PSK mode set on the config for a particular connection.
By default, a server chooses the first identity in its PSK list that also appears in the client's PSK list. The s2n_psk_selection_callback is available if you would like to implement your own PSK selection logic. Currently, this callback is not asynchronous. Call s2n_config_set_psk_selection_callback() to associate your s2n_psk_selection_callback with a config.
The s2n_psk_selection_callback will provide the list of PSK identities sent by the client in the psk_list input parameter. You will need to create an offered PSK object by calling s2n_offered_psk_new() and pass this object as a parameter in s2n_offered_psk_list_next() in order to populate the offered PSK object. Call s2n_offered_psk_list_has_next() prior to calling s2n_offered_psk_list_next() to determine if there exists another PSK in the psk_list. Call s2n_offered_psk_get_identity() to get the identity of a particular s2n_offered_psk.
Call s2n_offered_psk_list_choose_psk() to choose a particular s2n_offered_psk to be used for the connection. Note that the server must have already configured the corresponding PSK on the connection using s2n_connection_append_psk(). To disable PSKs for the connection and perform a full handshake instead, set the PSK identity to NULL. Call s2n_offered_psk_free() once you have chosen a particular PSK to free the memory allocated.
If desired, s2n_offered_psk_list_reread() returns the offered PSK list to its original read state. After s2n_offered_psk_list_reread() is called, the next call to s2n_offered_psk_list_next() will return the first PSK in the offered PSK list.
Use s2n_connection_get_negotiated_psk_identity() to retrieve the PSK identity selected by the server for the connection.
In the following example, s2n_psk_selection_callback chooses the first client offered PSK identity present in an external store.
int s2n_psk_selection_callback(struct s2n_connection *conn, void *context,
struct s2n_offered_psk_list *psk_list)
{
struct s2n_offered_psk *offered_psk = s2n_offered_psk_new();
while (s2n_offered_psk_list_has_next(psk_list)) {
uint8_t *client_psk_id = NULL;
uint16_t client_psk_id_len = 0;
s2n_offered_psk_list_next(psk_list, offered_psk);
s2n_offered_psk_get_identity(offered_psk, &client_psk_id, &client_psk_id_len);
struct s2n_psk *psk = user_lookup_identity_db(client_psk_id, client_psk_id_len);
if (psk) {
s2n_connection_append_psk(conn, psk);
s2n_offered_psk_list_choose_psk(psk_list, offered_psk);
break;
}
}
s2n_offered_psk_free(&offered_psk);
return S2N_SUCCESS;
}s2n-tls supports both blocking and non-blocking I/O. To use s2n-tls in non-blocking mode, set the underlying file descriptors as non-blocking (i.e. with fcntl). In blocking mode, each s2n-tls I/O function will not return until it is complete. In non-blocking mode an s2n-tls I/O function may return while there is still I/O pending. In this case the value of the blocked parameter will be set to either S2N_BLOCKED_ON_READ or S2N_BLOCKED_ON_WRITE, depending on the direction in which s2n-tls is blocked.
s2n-tls I/O functions should be called repeatedly until the blocked parameter is S2N_NOT_BLOCKED.
If the read end of the pipe is closed unexpectedly, writing to the pipe will raise a SIGPIPE signal. s2n-tls does NOT handle SIGPIPE. A SIGPIPE signal will cause the process to terminate unless it is handled or ignored by the application.
int s2n_negotiate(struct s2n_connection *conn, s2n_blocked_status *blocked);s2n_negotiate performs the initial "handshake" phase of a TLS connection and must be called before any s2n_recv or s2n_send calls.
ssize_t s2n_send(struct s2n_connection *conn
void *buf,
ssize_t size,
s2n_blocked_status *blocked);s2n_send writes and encrypts size of buf data to the associated connection. s2n_send will return the number of bytes written, and may indicate a partial write. Partial writes are possible not just for non-blocking I/O, but also for connections aborted while active. NOTE: Unlike OpenSSL, repeated calls to s2n_send should not duplicate the original parameters, but should update buf and size per the indication of size written. For example;
s2n_blocked_status blocked;
int written = 0;
char data[10]; /* Some data we want to write */
do {
int w = s2n_send(conn, data + written, 10 - written, &blocked);
if (w < 0) {
/* Some kind of error */
break;
}
written += w;
} while (blocked != S2N_NOT_BLOCKED);ssize_t s2n_sendv_with_offset(struct s2n_connection *conn
const struct iovec *bufs,
ssize_t count,
ssize_t offs,
s2n_blocked_status *blocked);s2n_sendv_with_offset works in the same way as s2n_send except that it accepts vectorized buffers. s2n_sendv_with_offset will return the number of bytes written, and may indicate a partial write. Partial writes are possible not just for non-blocking I/O, but also for connections aborted while active. NOTE: Unlike OpenSSL, repeated calls to s2n_sendv_with_offset should not duplicate the original parameters, but should update bufs and count per the indication of size written. For example;
s2n_blocked_status blocked;
int written = 0;
char data[10]; /* Some data we want to write */
struct iovec iov[1];
iov[0].iov_base = data;
iov[0].iov_len = 10;
do {
int w = s2n_sendv_with_offset(conn, iov, 1, written, &blocked);
if (w < 0) {
/* Some kind of error */
break;
}
written += w;
} while (blocked != S2N_NOT_BLOCKED);ssize_t s2n_sendv(struct s2n_connection *conn
const struct iovec *bufs,
ssize_t count,
s2n_blocked_status *blocked);s2n_sendv works in the same way as s2n_sendv_with_offset except that the latter's offs parameter is implicitly assumed to be 0. Therefore in the partial write case, the caller would have to make sure that bufs and count fields are modified in a way that takes the partial writes into account.
ssize_t s2n_recv(struct s2n_connection *conn,
void *buf,
ssize_t size,
s2n_blocked_status *blocked);s2n_recv decrypts and reads *size to buf data from the associated connection. s2n_recv will return the number of bytes read and also return "0" on connection shutdown by the peer.
NOTE: By default, s2n_recv will return after reading a single TLS record. To change this behavior such that it will read up to size, the config for the connection can be updated by calling s2n_config_set_recv_multi_record.
NOTE: Unlike OpenSSL, repeated calls to s2n_recv should not duplicate the original parameters, but should update buf and size per the indication of size read. For example;
s2n_blocked_status blocked;
int bytes_read = 0;
char data[10];
do {
int r = s2n_recv(conn, data + bytes_read, 10 - bytes_read, &blocked);
if (r < 0) {
/* Some kind of error */
break;
}
bytes_read += r;
} while (blocked != S2N_NOT_BLOCKED);uint32_t s2n_peek(struct s2n_connection *conn);s2n_peek allows users of s2n-tls to peek inside the data buffer of an s2n-tls connection to see if there more data to be read without actually reading it. This is useful when using select() on the underlying s2n-tls file descriptor with a message based application layer protocol. As a single call to s2n_recv may read all data off the underlying file descriptor, select() will be unable to tell you there if there is more application data ready for processing already loaded into the s2n-tls buffer. s2n_peek can then be used to determine if s2n_recv needs to be called before more data comes in on the raw fd.
int s2n_connection_set_recv_cb(struct s2n_connection *conn, s2n_connection_recv recv);
int s2n_connection_set_send_cb(struct s2n_connection *conn, s2n_connection_send send);
int s2n_connection_set_recv_ctx(struct s2n_connection *conn, void *ctx);
int s2n_connection_set_send_ctx(struct s2n_connection *conn, void *ctx);s2n-tls also provides an I/O abstraction layer in the event the application would like to keep control over I/O operations. s2n_connection_set_recv_cb and s2n_connection_set_send_cb may be used to send or receive data with callbacks defined by the user. These may be blocking or nonblocking.
typedef int s2n_connection_send(void *io_context, const uint8_t *buf, uint32_t len);
typedef int s2n_connection_recv(void *io_context, uint8_t *buf, uint32_t len);These callbacks take as input a context containing anything needed in the function (for example, a file descriptor), the buffer holding data to be sent or received, and the length of the buffer. The io_context passed to the callbacks may be set separately using s2n_connection_set_recv_ctx and s2n_connection_set_send_ctx.
The callback may send or receive less than the requested length. The function should return the number of bytes sent/received, or set errno and return an error code < 0.
int s2n_shutdown(struct s2n_connection *conn,
s2n_blocked_status *blocked);s2n_shutdown attempts a closure at the TLS layer. It does not close the underlying transport. The call may block in either direction. Unlike other TLS implementations, s2n_shutdown attempts a graceful shutdown by default. It will not return with success unless a close_notify alert is successfully sent and received. As a result, s2n_shutdown may fail when interacting with a non-conformant TLS implementation. Once s2n_shutdown is complete:
- The s2n_connection handle cannot be used for reading for writing.
- The underlying transport can be closed. Most likely via
close(). - The s2n_connection handle can be freed via s2n_connection_free or reused via s2n_connection_wipe
TLS1.3 introduced the ability for clients to send data before completing the handshake when using external pre-shared keys or session resumption.
WARNING: Early data does not have the same security properties as regular data sent after a successful handshake.
- It is not forward secret. If the PSK or session resumption secret is compromised, then the early data is also compromised.
- It is susceptible to replay attacks unless proper precautions are taken. Early data can be captured and successfully resent by an attacker. See https://tools.ietf.org/rfc/rfc8446#appendix-E.5 for more details, and "Adding anti-replay protection" for how to implement counter measures.
Do not enable early data for your application unless you have understood and mitigated the risks.
To use early data with session tickets, early data must be enabled on a server by setting the maximum early data allowed to a non-zero value with s2n_config_set_server_max_early_data_size or s2n_connection_set_server_max_early_data_size. The server then begins issuing tickets that support early data, and clients can use early data when they use those tickets.
To use early data with pre-shared keys, individual pre-shared keys must support early data. In addition to configuring the maximum early data allowed, each pre-shared key needs an associated cipher suite and if applicable, application protocol. The server only accepts early data if the pre-shared key's associated cipher suite and application protocol match the cipher suite and the application protocol negotiated during the handshake.
The maximum early data allowed and cipher suite can be set with s2n_psk_configure_early_data. If the connection will negotiate an application protocol then the expected application protocol can be set with s2n_psk_set_application_protocol.
To send early data, your application should call s2n_send_early_data before it calls s2n_negotiate.
s2n_connection_get_remaining_early_data_size can be called to check how much more early data the client is allowed to send. If s2n_send_early_data exceeds the allowed maximum, s2n-tls returns a usage error.
Like other IO functions, s2n_send_early_data can potentially fail repeatedly with a blocking error before it eventually succeeds: see I/O Functions for more information. An application can stop calling s2n_send_early_data at any time, even if the function has not returned success yet. If s2n_send_early_data does return success, the connection is ready to complete the handshake and begin sending normal data. However, s2n_send_early_data can continue to be called to send more early data if desired.
Once a client finishes sending early data, you should call s2n_negotiate to complete the handshake just as you would for a handshake that did not include early data.
For example:
uint8_t early_data[] = "early data to send";
ssize_t total_data_sent = 0, len = sizeof(early_data);
while (total_data_sent < len) {
ssize_t data_sent = 0;
int r = s2n_send_early_data(client_conn, early_data + total_data_sent,
len - total_data_sent, &data_sent, &blocked);
total_data_sent += data_sent;
if (r == S2N_SUCCESS) {
break;
} else if (s2n_error_get_type(s2n_errno) != S2N_ERR_T_BLOCKED) {
exit(1);
}
}
while (s2n_negotiate(client_conn, &blocked) != S2N_SUCCESS) {
if (s2n_error_get_type(s2n_errno) != S2N_ERR_T_BLOCKED) {
exit(1);
}
}
To receive early data, your application should call s2n_recv_early_data before it calls s2n_negotiate.
Like other S2N IO functions, s2n_recv_early_data can potentially fail repeatedly with a blocking error before it eventually succeeds: see I/O Functions for more information. Once s2n_recv_early_data has been called, it must be called until it returns success. If an application stops calling s2n_recv_early_data early, some early data may be left unread and cause later calls to s2n_negotiate to return fatal errors. Calling s2n_recv_early_data again after it returns success is possible but has no effect on the connection.
Once a server has read all early data, you should call s2n_negotiate to complete the handshake just as you would for a handshake that did not include early data.
For example:
uint8_t early_data[MAX_EARLY_DATA] = { 0 };
ssize_t total_data_recv = 0, data_recv = 0;
while (s2n_recv_early_data(conn, early_data + total_data_recv, MAX_EARLY_DATA - total_data_recv,
&data_recv, &blocked) != S2N_SUCCESS) {
total_data_recv += data_recv;
if (s2n_error_get_type(s2n_errno) != S2N_ERR_T_BLOCKED) {
exit(1);
}
}
while (s2n_negotiate(conn, &blocked) != S2N_SUCCESS) {
if (s2n_error_get_type(s2n_errno) != S2N_ERR_T_BLOCKED) {
exit(1);
}
}
s2n-tls does not include anti-replay protection automatically. Effective anti-replay protection for a multi-server application requires an external state shared by all servers. Without shared state, an attacker can capture early data originally sent to server A and successfully replay it against server B.
The TLS1.3 specification suggests two possible anti-replay solutions that a user can implement:
- Single-Use Tickets (https://tools.ietf.org/rfc/rfc8446#section-8.1): Valid tickets are stored in a shared database and deleted after use. s2n_connection_get_negotiated_psk_identity_length and s2n_connection_get_negotiated_psk_identity can be used to get the ticket identifer, or "pre-shared key identity", associated with offered early data.
- Client Hello Recording (https://tools.ietf.org/rfc/rfc8446#section-8.2): Instead of recording outstanding valid tickets, unique values from recent ClientHellos can be stored. The client hello message can be retrieved with s2n_connection_get_client_hello and the pre-shared key identity can be retrieved with s2n_connection_get_negotiated_psk_identity_length and s2n_connection_get_negotiated_psk_identity, but s2n-tls does not currently provide methods to retrieve the validated binders or the ClientHello.random.
The s2n_early_data_cb can be used to hook an anti-replay solution into s2n-tls. The callback can be configured by using s2n_config_set_early_data_cb. Using the s2n_offered_early_data pointer offered by the callback, s2n_offered_early_data_reject or s2n_offered_early_data_accept can accept or reject the client request to use early data.
An example implementation:
int s2n_early_data_cb_impl(struct s2n_connection *conn, struct s2n_offered_early_data *early_data)
{
uint16_t identity_size = 0;
s2n_connection_get_negotiated_psk_identity_length(conn, &identity_size);
uint8_t *identity = malloc(identity_size);
s2n_connection_get_negotiated_psk_identity(conn, identity, identity_size);
if (user_verify_single_use_ticket(identity)) {
s2n_offered_early_data_accept(early_data);
} else {
s2n_offered_early_data_reject(early_data);
}
free(identity);
return S2N_SUCCESS;
}
The callback can also be implemented asynchronously by returning S2N_SUCCESS without either accepting or rejecting the early data. The handshake will then fail with an S2N_ERR_T_BLOCKED error type and s2n_blocked_status set to S2N_BLOCKED_ON_APPLICATION_INPUT until s2n_offered_early_data_reject or s2n_offered_early_data_accept is called asynchronously.
An example asynchronous implementation:
void *user_accept_or_reject_early_data(void *arg)
{
struct s2n_offered_early_data *early_data = (struct s2n_offered_early_data *) arg;
if (user_slowly_verify_early_data(early_data)) {
s2n_offered_early_data_accept(early_data);
} else {
s2n_offered_early_data_reject(early_data);
}
return NULL;
}
int s2n_early_data_cb_async_impl(struct s2n_connection *conn, struct s2n_offered_early_data *early_data)
{
pthread_t thread_id;
pthread_create(&thread_id, NULL, user_accept_or_reject_early_data, (void *) early_data);
return S2N_SUCCESS;
}
s2n_offered_early_data_get_context_length and s2n_offered_early_data_get_context can be called to examine the optional user context associated with the early data. Unlike most s2n-tls callbacks, the context is not configured when the callback is set. Instead, the context is associated with the specific pre-shared key or session ticket used for early data. The context can be set for external pre-shared keys by calling s2n_psk_set_early_data_context. For session tickets, s2n_connection_set_server_early_data_context can be used to set the context the server includes on its new session tickets. Because the server needs to serialize the context when creating a new session ticket, the context is a byte buffer instead of the usual void pointer.
To understand the API it may be easiest to see examples in action. s2n-tls's bin/ directory includes an example client (s2nc) and server (s2nd).