SSL_CTX_use_psk_identity_hint.3ssl man page
SSL_CTX_use_psk_identity_hint, SSL_use_psk_identity_hint, SSL_CTX_set_psk_server_callback, SSL_set_psk_server_callback — set PSK identity hint to use
#include <openssl/ssl.h> int SSL_CTX_use_psk_identity_hint(SSL_CTX *ctx, const char *hint); int SSL_use_psk_identity_hint(SSL *ssl, const char *hint); void SSL_CTX_set_psk_server_callback(SSL_CTX *ctx, unsigned int (*callback)(SSL *ssl, const char *identity, unsigned char *psk, int max_psk_len)); void SSL_set_psk_server_callback(SSL *ssl, unsigned int (*callback)(SSL *ssl, const char *identity, unsigned char *psk, int max_psk_len));
SSL_CTX_use_psk_identity_hint() sets the given NULL-terminated PSK identity hint hint to SSL context object ctx. SSL_use_psk_identity_hint() sets the given NULL-terminated PSK identity hint hint to SSL connection object ssl. If hint is NULL the current hint from ctx or ssl is deleted.
In the case where PSK identity hint is NULL, the server does not send the ServerKeyExchange message to the client.
A server application must provide a callback function which is called when the server receives the ClientKeyExchange message from the client. The purpose of the callback function is to validate the received PSK identity and to fetch the pre-shared key used during the connection setup phase. The callback is set using functions SSL_CTX_set_psk_server_callback() or SSL_set_psk_server_callback(). The callback function is given the connection in parameter ssl, NULL-terminated PSK identity sent by the client in parameter identity, and a buffer psk of length max_psk_len bytes where the pre-shared key is to be stored.
SSL_CTX_use_psk_identity_hint() and SSL_use_psk_identity_hint() return 1 on success, 0 otherwise.
Return values from the server callback are interpreted as follows:
PSK identity was not found. An "unknown_psk_identity" alert message will be sent and the connection setup fails.
PSK identity was found and the server callback has provided the PSK successfully in parameter psk. Return value is the length of psk in bytes. It is an error to return a value greater than max_psk_len.
If the PSK identity was not found but the callback instructs the protocol to continue anyway, the callback must provide some random data to psk and return the length of the random data, so the connection will fail with decryption_error before it will be finished completely.