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<title>s6-networking: the sbearssl library interface</title>
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<p>
<a href="../">s6-networking</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
</p>
<h1> The <tt>sbearssl</tt> library interface </h1>
<h2> General information </h2>
<p>
<tt>libsbearssl</tt> is a support library for the
<a href="../s6-tlsc.html">s6-tlsc</a> and
<a href="../s6-tlsd.html">s6-tlsd</a> executables when they're built
against the <a href="https://bearssl.org/">BearSSL</a>
backend. Among other things, it offers interfaces to read private
keys and certificates from a Unix filesystem, which BearSSL does
not provide on its own.
</p>
<h2> Compiling </h2>
<ul>
<li> Make sure the s6-networking headers, as well as the skalibs headers,
and the <tt>bearssl.h</tt> header, are visible in your header search path. </li>
<li> Use <tt>#include <s6-networking/sbearssl.h></tt> </li>
</ul>
<h2> Linking </h2>
<ul>
<li> Make sure the s6-networking libraries, as well as the skalibs
libraries, and the BearSSL libraries, are visible in your library search path. </li>
<li> Link against <tt>-lsbearssl</tt>, <tt>-lskarnet</tt>, <tt>-lbearssl</tt>,
<tt>`cat $sysdeps/socket.lib`</tt>, <tt>`cat $sysdeps/spawn.lib`</tt>, and
<tt>`cat $sysdeps/sysclock.lib`</tt>, where <tt>$sysdeps</tt> is your skalibs
sysdeps directory. </li>
</ul>
<h2> Programming </h2>
<h3> General concepts </h3>
<p>
<a href="https://bearssl.org/">BearSSL</a> provides engines
to decode PEM objects and X.509 certificates, and to run a
TLS/SSL connection. However, it does not store such objects:
it never allocates memory, and does not interact with the
filesystem. <tt>sbearssl</tt> provides functions to
address this.
</p>
<p>
When reading an object into memory, <tt>sbearssl</tt> stores all
the bytes of the object in a
<a href="//skarnet.org/software/skalibs/libstddjb/stralloc.html">stralloc</a>,
and the <tt>sbearssl_*</tt> structures contain indices of bytes in that
stralloc. That allows the structures to remain valid even when the stralloc
contents get reallocated and move to some other place in the heap. After
you have finished adding data to the stralloc and are sure its contents
will not move again, you can use the
<tt>sbearssl_*_to</tt> functions to convert <tt>sbearssl_*</tt> structures
to the corresponding <tt>br_*</tt> structures (native BearSSL), which
contain pointers to memory.
</p>
<h3> Private keys (typically for servers) </h3>
<p>
BearSSL handles two types of private keys: RSA keys and
EC keys (i.e. points on an elliptic curve). <tt>sbearssl</tt>
adds some generic functions to handle keys no matter their
type.
</p>
<h4> <code> int sbearssl_rsa_skey_from (sbearssl_rsa_skey *l, br_rsa_private_key const *k, stralloc *sa) </code> </h4>
<p>
Converts the RSA private key from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_rsa_skey_to (sbearssl_rsa_skey const *l, br_rsa_private_key *k, char *s) </code> </h4>
<p>
Converts the RSA private key from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h4> <code> int sbearssl_ec_skey_from (sbearssl_ec_skey *l, br_ec_private_key const *k, stralloc *sa) </code> </h4>
<p>
Converts the EC private key from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_ec_skey_to (sbearssl_ec_skey const *l, br_ec_private_key *k, char *s) </code> </h4>
<p>
Converts the EC private key from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h4> <code> int sbearssl_skey_from (sbearssl_skey *l, br_skey const *k, stralloc *sa) </code> </h4>
<p>
Converts the private key from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_skey_to (sbearssl_skey const *l, br_skey *k, char *s) </code> </h4>
<p>
Converts the private key from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h4> <code> int sbearssl_skey_readfile (char const *fn, sbearssl_skey *key, stralloc *sa) </code> </h4>
<p>
Reads a private key from the file named <em>fn</em> and stores it
in <tt>sbearssl</tt> format into the structure in *<em>key</em>,
the bytes of the key being added to the stralloc in *<em>sa</em>.
</p>
<p>
The private key in <em>fn</em> can be either DER-encoded (binary format)
or PEM-encoded (text format).
</p>
<p>
The function returns 0 on success. It returns a negative value in
case of a system error, in which case <tt>errno</tt> identifies the
error. It returns a positive value in case of an error returned by
a BearSSL decoder, in which case an appropriate message can be
obtained with the <tt>sbearssl_error_str()</tt> function.
</p>
<h3> Public keys </h3>
<p>
BearSSL handles two types of public keys: RSA keys and
EC keys (i.e. points on an elliptic curve). <tt>sbearssl</tt>
adds some generic functions to handle keys no matter their
type.
</p>
<p>
You normally should not handle public keys directly;
you should handle x509 certificate chains instead.
</p>
<h4> <code> int sbearssl_rsa_pkey_from (sbearssl_rsa_pkey *l, br_rsa_public_key const *k, stralloc *sa) </code> </h4>
<p>
Converts the RSA public key from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_rsa_pkey_to (sbearssl_rsa_pkey const *l, br_rsa_public_key *k, char *s) </code> </h4>
<p>
Converts the RSA public key from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h4> <code> int sbearssl_ec_pkey_from (sbearssl_ec_skey *l, br_ec_public_key const *k, stralloc *sa) </code> </h4>
<p>
Converts the EC public key from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_ec_pkey_to (sbearssl_ec_pkey const *l, br_ec_public_key *k, char *s) </code> </h4>
<p>
Converts the EC public key from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h4> <code> int sbearssl_pkey_from (sbearssl_pkey *l, br_x509_pkey const *k, stralloc *sa) </code> </h4>
<p>
Converts the public key from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_pkey_to (sbearssl_pkey const *l, br_x509_pkey *k, char *s) </code> </h4>
<p>
Converts the public key from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h3> Generic PEM objects </h3>
<p>
You normally should not have to call these functions
directly. Instead, you should use the higher-level functions for
private keys, X509 certificate chains and trust anchors, which
will perform the PEM decoding for you.
</p>
<h4> <code> int sbearssl_pem_decode_from_buffer (buffer *b, genalloc *list, stralloc *sa) </code> </h4>
<p>
Decodes a PEM object, reading from the
<a href="//skarnet.org/software/skalibs/libstddjb/buffer.html">buffer</a>
in *<em>b</em>. The decoded bytes are appended to *<em>sa</em>.
<em>list</em> points to a
<a href="//skarnet.org/software/skalibs/libstddjb/genalloc.html">genalloc</a>
containing objects of type <tt>sbearssl_pemobject</tt>.
One <tt>sbearssl_pemobject</tt> is appended to the genalloc per PEM entity
decoded from the byte stream read from the buffer.
</p>
<p>
The function returns 0 on success. It returns a negative value in
case of a system error, in which case <tt>errno</tt> identifies the
error. It returns a positive value in case of an error returned by
a BearSSL decoder, in which case an appropriate message can be
obtained with the <tt>sbearssl_error_str()</tt> function.
</p>
<h4> <code> int sbearssl_pem_decode_from_string (char const *s, size_t len, genalloc *list, stralloc *sa) </code> </h4>
<p>
Decodes a PEM object from the <em>len</em> bytes pointed to by <em>s</em>.
The decoded bytes are appended to *<em>sa</em>.
<em>list</em> points to a
<a href="//skarnet.org/software/skalibs/libstddjb/genalloc.html">genalloc</a>
containing objects of type <tt>sbearssl_pemobject</tt>.
One <tt>sbearssl_pemobject</tt> is appended to the genalloc per PEM entity
found in the bytes in <em>s</em>.
</p>
<p>
The function returns 0 on success. It returns a negative value in
case of a system error, in which case <tt>errno</tt> identifies the
error. It returns a positive value in case of an error returned by
a BearSSL decoder, in which case an appropriate message can be
obtained with the <tt>sbearssl_error_str()</tt> function.
</p>
<h3> X.509 certificates (typically for servers) </h3>
<h4> <code> int sbearssl_cert_from (sbearssl_cert *l, br_x509_certificate const *k, stralloc *sa) </code> </h4>
<p>
Converts a certificate from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_cert_to (sbearssl_cert const *l, br_x509_certificate *k, char *s) </code> </h4>
<p>
Converts a certificate from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h4> <code> int sbearssl_cert_readfile (char const *fn, genalloc *list, stralloc *sa) </code> </h4>
<p>
Reads one or more certificates from the file named <em>fn</em> and appends
them to the <a href="//skarnet.org/software/skalibs/libstddjb/genalloc.html">genalloc</a>
in *<em>list</em>, which is a dynamically growing list of
<tt>sbearssl_cert</tt> structures. The bytes of the
(maybe PEM-decoded, but still DER-encoded) certificate are
appended to the stralloc in *<em>sa</em>.
</p>
<p>
The <em>fn</em> file can be either DER-encoded (binary format)
or PEM-encoded (text format). If it is DER-encoded, it must
contain exactly one X.509 certificate. If it is PEM-encoded,
it may contain a chain of certificates as long as the PEM
file fits within the size limits.
</p>
<p>
<em>fn</em> must not be bigger than SBEARSSL_MAXCERTFILESIZE,
which is 8 kB. This function is meant to read individual
certificates, not files containing large certificate chains or
sets of trust anchors. To do that, use
<tt>sbearssl_cert_readbigpem()</tt> instead.
</p>
<p>
The function returns 0 on success. It returns a negative value in
case of a system error, in which case <tt>errno</tt> identifies the
error. It returns a positive value in case of an error returned by
a BearSSL decoder, in which case an appropriate message can be
obtained with the <tt>sbearssl_error_str()</tt> function.
</p>
<h4> <code> int sbearssl_cert_readbigpem (char const *fn, genalloc *, stralloc *sa) </code> </h4>
<p>
Reads one or more PEM-encoded certificates from the file named
<em>fn</em> and appends them to the
<a href="//skarnet.org/software/skalibs/libstddjb/genalloc.html">genalloc</a>
in *<em>list</em>, which is a dynamically growing list of
<tt>sbearssl_cert</tt> structures. The bytes of the PEM-decoded (but
still DER-encoded) certificates are appended to the stralloc
in *<em>sa</em>.
</p>
<p>
The function will refuse to read a file that is not valid PEM.
Inside the file, It will ignore PEM objects that are
not X.509 certificates.
</p>
<p>
The function returns 0 on success. It returns a negative value in
case of a system error, in which case <tt>errno</tt> identifies the
error. It returns a positive value in case of an error returned by
a BearSSL decoder, in which case an appropriate message can be
obtained with the <tt>sbearssl_error_str()</tt> function.
</p>
<h3> Trust anchors (typically for clients) </h3>
<p>
BearSSL clients do not use X.509-encoded certificates,
they use sets of <em>trust anchors</em>, i.e. structures
decoded from certificates representing (intermediate or)
root CAs.
</p>
<h4> <code> int sbearssl_ta_from (sbearssl_ta *l, br_x509_trust_anchor const *k, stralloc *sa) </code> </h4>
<p>
Converts a trust anchor from BearSSL format (reading from a structure pointed to by <em>k</em>)
to <tt>sbearssl</tt> format (writing to a structure pointed to by <em>l</em>).
The data from *<em>k</em>'s contents are copied into the stralloc in *<em>sa</em>.
The function returns 1 on success and 0 (and sets errno) on failure.
</p>
<h4> <code> void sbearssl_ta_to (sbearssl_ta const *l, br_x509_trust_anchor *k, char *s) </code> </h4>
<p>
Converts a trust anchor from <tt>sbearssl</tt> format (reading from a structure pointed to by <em>l</em>)
to BearSSL format (writing to a structure pointed to by <em>k</em>).
The indices in <em>l</em> must refer to data stored in the string <em>s</em>.
</p>
<h4> <code> int sbearssl_ta_readfile (char const *fn, genalloc *list, stralloc *sa) </code> </h4>
<p>
Reads a set of trust anchors from a PEM file named <em>fn</em>
which must contain a list of (intermediate or) root CA certificates.
The trust anchors are appended to the
<a href="//skarnet.org/software/skalibs/libstddjb/genalloc.html">genalloc</a>
in *<em>list</em>, which is a dynamically growing list of
<tt>sbearssl_ta</tt> structures. The contents of the trust anchors
are appended to *<em>sa</em>, which is a
<a href="//skarnet.org/software/skalibs/libstddjb/stralloc.html">stralloc</a>
used for storage.
</p>
<p>
The function returns 0 on success. It returns a negative value in
case of a system error, in which case <tt>errno</tt> identifies the
error. It returns a positive value in case of an error returned by
a BearSSL decoder, in which case an appropriate message can be
obtained with the <tt>sbearssl_error_str()</tt> function.
</p>
<h4> <code> int sbearssl_ta_readdir (char const *dir, genalloc *list, stralloc *sa) </code> </h4>
<p>
Reads a set of trust anchors from a directory named <em>dir</em>,
which must contain a list of (intermediate or) root CA certificates
stored as individual DER- or PEM-encoded files.
The trust anchors are appended to the
<a href="//skarnet.org/software/skalibs/libstddjb/genalloc.html">genalloc</a>
in *<em>list</em>, which is a dynamically growing list of
<tt>sbearssl_ta</tt> structures. The contents of the trust anchors
are appended to *<em>sa</em>, which is a
<a href="//skarnet.org/software/skalibs/libstddjb/stralloc.html">stralloc</a>
used for storage.
</p>
<p>
The function ignores files that do not contain valid DER
or PEM objects containing X.509 certificates representing
certification authorities.
</p>
<p>
The function returns 0 on success. It returns a negative value in
case of a system error, in which case <tt>errno</tt> identifies the
error. It returns a positive value in case of an error returned by
a BearSSL decoder, in which case an appropriate message can be
obtained with the <tt>sbearssl_error_str()</tt> function.
</p>
<h3> Miscellaneous utilities </h3>
<p>
You probably shouldn't need to call any of these functions
directly, except for the first one.
</p>
<h4> <code> char const *sbearssl_error_str (int err) </code> </h4>
<p>
Returns a fixed string containing an error message corresponding
to the <em>err</em> code, which must be non-negative. The return
value from a few <tt>sbearssl</tt> functions, if positive, can be
interpreted via this function.
</p>
<h4> <code> int sbearssl_isder (unsigned char const *s, size_t len) </code> </h4>
<p>
Tests whether the array of <em>len</em> bytes pointed to by <em>s</em>
looks like a DER-encoded object. Returns 1 if it does and 0 otherwise.
</p>
<h4> <code> int sbearssl_x509_minimal_set_tai (br_x509_minimal_context *ctx, tai_t t) </code> </h4>
<p>
Sets the validation time for the X.509 context in *<em>ctx</em> to
the absolute time contained in *<em>t</em>, which is a
<a href="//skarnet.org/software/skalibs/libstddjb/tai.html">tai_t</a>.
Returns 1 if it succeeds, or 0 if it fails - probably
because *<em>t</em> does not represent a valid time.
</p>
<h4> <code> int sbearssl_x509_minimal_set_tain (br_x509_minimal_context *ctx, tain_t a) </code> </h4>
<p>
Same as the above function, except the time is given as a
<a href="//skarnet.org/software/skalibs/libstddjb/tai.html">tain_t</a>,
i.e. a <tt>tai_t</tt> plus nanoseconds (which are simply ignored).
</p>
<h3> Running the TLS/SSL engine (both clients and servers) </h3>
<h4> <code> int sbearssl_run (br_ssl_engine_context *ctx, int *fds, pid_t pid, unsigned int verbosity, uint32_t options, tain_t const *tto) </code> </h4>
<p>
This function runs a full-duplex TLS/SSL engine, reading/writing
clear text from/to two file descriptors, and writing/reading
ciphertext to/from two other file descriptors, until the
connection is closed both ways (either with a SSL close, or
with EOF) or a given subprocess dies.
</p>
<ul>
<li> <em>ctx</em> is a pointer to a fully initialized context,
connected to <em>fds</em>[2] and <em>fds</em>[3]. The
TLS/SSL handshake does not have to be completed. </li>
<li> <em>fds</em> is an array of 5 file descriptors, in this
order: fd reading clear text, fd writing clear text, fd reading
ciphertext, fd writing ciphertext, selfpipe. </li>
<li> <em>pid</em> is the pid of the application subprocess.
When a SIGCHLD is detected on the selfpipe, the newly deceased
process is reaped, and if it was <em>pid</em>, then the function
returns as soon as it doesn't have anything left to write to
the network peer. </li>
<li> <em>verbosity</em> defines the engine's verbosity: the
higher the more verbose. This parameter is currently ignored. </li>
<li> <em>options</em> is a bitfield.
<ul>
<li> bit 0 tells the engine how to behave when
the local application closes the connection (i.e. when the engine
reads EOF on <em>fds</em>[0]). If the bit is clear, then the
engine will perform as SSL close: it will send a SSL close_notify,
and stop processing incoming records, waiting for a peer
acknowledgement of the close_notify. If the bit is set, then the
engine will not send a close_notify but simply transmit EOF to
the peer, while continuing to process incoming records until it
gets EOF back. close_notify is secure when handling protocols that
are not auto-terminated (such as HTTP 0.9), but it does not permit
separate closing of both ways. EOF allows full-duplex until the
very end, but is insecure if the application protocol does not
know in advance how many bytes it should get. Modern application
protocols should all work with EOF. </li>
</ul> </li>
<li> <em>tto</em> is a pointer to a
<a href="//skarnet.org/software/skalibs/libstddjb/tai.html">tain_t</a>
containing a relative time (i.e. a timeout) If *<em>tto</em> time elapses
with no application data being exchanged, the engine will forcibly close the
connection (with the method defined by <tt><em>options</em> & 1</tt>).
You can use <tt>&tain_infinite_relative</tt> as a value for <em>tto</em>
if you don't want the engine to ever timeout. </li>
</ul>
<p>
<tt>sbearssl_run</tt> will make the process die with an appropriate error
message if it encounters an unrecoverable error. If there were no problems and the
SSL/TLS connection closed cleanly, it returns -1. If the application
subprocess dies early, <tt>sbearssl_run</tt> returns the <em>wstat</em>
for that subprocess, i.e. the integer containing the information about
its exit code or crash signal. No matter how <tt>sbearssl_run</tt> returns,
the first four descriptors in <em>fds</em> are closed, but the
selfpipe is untouched and the caller should free <em>ctx</em> itself.
</p>
<h4> <code> int sbearssl_s6tlsc (char const *const *argv, char const *const *envp, tain_t const *tto, uint32_t preoptions, uint32_t options, uid_t uid, gid_t gid, unsigned int verbosity, char const *servername, int *sfd) </code> </h4>
<p>
This function implements <a href="../s6-tlsc.html">s6-tlsc</a> on top of BearSSL.
It has no other practical purpose; you're better off directly invoking
<a href="../s6-tlsc.html">s6-tlsc</a>.
</p>
<h4> <code> int sbearssl_s6tlsd (char const *const *argv, char const *const *envp, tain_t const *tto, uint32_t preoptions, uint32_t options, uid_t uid, gid_t gid, unsigned int verbosity) </code> </h4>
<p>
This function implements <a href="../s6-tlsd.html">s6-tlsd</a> on top of BearSSL.
It has no other practical purpose; you're better off directly invoking
<a href="../s6-tlsd.html">s6-tlsd</a>.
</p>
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