#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));
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.
Return values from the server callback are interpreted as follows:
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.