Ubuntu Manpage: SSL_export_keying_material, SSL_export_keying_material_early

Provided by: libssl-doc_3.0.8-1ubuntu1_all

NAME

       SSL_export_keying_material, SSL_export_keying_material_early - obtain keying material for
       application use

SYNOPSIS

        #include <openssl/ssl.h>

        int SSL_export_keying_material(SSL *s, unsigned char *out, size_t olen,
                                       const char *label, size_t llen,
                                       const unsigned char *context,
                                       size_t contextlen, int use_context);

        int SSL_export_keying_material_early(SSL *s, unsigned char *out, size_t olen,
                                             const char *label, size_t llen,
                                             const unsigned char *context,
                                             size_t contextlen);

DESCRIPTION

During the creation of a TLS or DTLS connection shared keying material is established
between the two endpoints. The functions SSL_export_keying_material() and
SSL_export_keying_material_early() enable an application to use some of this keying
material for its own purposes in accordance with RFC5705 (for TLSv1.2 and below) or
RFC8446 (for TLSv1.3).

SSL_export_keying_material() derives keying material using the exporter_master_secret
established in the handshake.

SSL_export_keying_material_early() is only usable with TLSv1.3, and derives keying
material using the early_exporter_master_secret (as defined in the TLS 1.3 RFC). For the
client, the early_exporter_master_secret is only available when the client attempts to
send 0-RTT data. For the server, it is only available when the server accepts 0-RTT data.

An application may need to securely establish the context within which this keying
material will be used. For example this may include identifiers for the application
session, application algorithms or parameters, or the lifetime of the context. The context
value is left to the application but must be the same on both sides of the communication.

For a given SSL connection s, olen bytes of data will be written to out. The application
specific context should be supplied in the location pointed to by context and should be
contextlen bytes long. Provision of a context is optional. If the context should be
omitted entirely then use_context should be set to 0. Otherwise it should be any other
value. If use_context is 0 then the values of context and contextlen are ignored. Note
that in TLSv1.2 and below a zero length context is treated differently from no context at
all, and will result in different keying material being returned. In TLSv1.3 a zero
length context is that same as no context at all and will result in the same keying
material being returned.

An application specific label should be provided in the location pointed to by label and
should be llen bytes long. Typically this will be a value from the IANA Exporter Label
Registry
(<https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#exporter-labels>).
Alternatively labels beginning with "EXPERIMENTAL" are permitted by the standard to be
used without registration. TLSv1.3 imposes a maximum label length of 249 bytes.

Note that this function is only defined for TLSv1.0 and above, and DTLSv1.0 and above.
Attempting to use it in SSLv3 will result in an error.

RETURN VALUES

       SSL_export_keying_material() returns 0 or -1 on failure or 1 on success.

       SSL_export_keying_material_early() returns 0 on failure or 1 on success.

HISTORY

       The SSL_export_keying_material_early() function was added in OpenSSL 1.1.1.

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