SSL_get_client_random.3ssl man page
SSL_get_client_random, SSL_get_server_random, SSL_SESSION_get_master_key — retrieve internal TLS/SSL random values and master key
#include <openssl/ssl.h> size_t SSL_get_client_random(const SSL *ssl, unsigned char *out, size_t outlen); size_t SSL_get_server_random(const SSL *ssl, unsigned char *out, size_t outlen); size_t SSL_SESSION_get_master_key(const SSL_SESSION *session, unsigned char *out, size_t outlen);
SSL_get_client_random() extracts the random value sent from the client to the server during the initial SSL/TLS handshake. It copies as many bytes as it can of this value into the buffer provided in out, which must have at least outlen bytes available. It returns the total number of bytes that were actually copied. If outlen is zero, SSL_get_client_random() copies nothing, and returns the total size of the client_random value.
SSL_get_server_random() behaves the same, but extracts the random value sent from the server to the client during the initial SSL/TLS handshake.
SSL_SESSION_get_master_key() behaves the same, but extracts the master secret used to guarantee the security of the SSL/TLS session. This one can be dangerous if misused; see Notes below.
You probably shouldn't use these functions.
These functions expose internal values from the TLS handshake, for use in low-level protocols. You probably should not use them, unless you are implementing something that needs access to the internal protocol details.
Despite the names of SSL_get_client_random() and SSL_get_server_random(), they ARE NOT random number generators. Instead, they return the mostly-random values that were already generated and used in the TLS protocol. Using them in place of RAND_bytes() would be grossly foolish.
The security of your TLS session depends on keeping the master key secret: do not expose it, or any information about it, to anybody. If you need to calculate another secret value that depends on the master secret, you should probably use SSL_export_keying_material() instead, and forget that you ever saw these functions.
In current versions of the TLS protocols, the length of client_random (and also server_random) is always SSL3_RANDOM_SIZE bytes. Support for other outlen arguments to the SSL_get_*_random() functions is provided in case of the unlikely event that a future version or variant of TLS uses some other length there.
Finally, though the "client_random" and "server_random" values are called "random", many TLS implementations will generate four bytes of those values based on their view of the current time.
If outlen is greater than 0, these functions return the number of bytes actually copied, which will be less than or equal to outlen.
If outlen is 0, these functions return the maximum number of bytes they would copy--that is, the length of the underlying field.
ssl(3), RAND_bytes(3), SSL_export_keying_material(3)
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