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obtain keying material for application use

#include <openssl/ssl.h>
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);

During the creation of a TLS or DTLS connection, shared keying material is established between the two endpoints. The function SSL_export_keying_material() enables an application to use some of this keying material for its own purposes in accordance with RFC 5705.
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.
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.
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.
Alternatively, labels beginning with "EXPERIMENTAL" are permitted by the standard to be used without registration.

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

SSL_export_keying_material() first appeared in OpenSSL 1.0.1 and has been available since OpenBSD 5.3.
March 23, 2018 OpenBSD-current