Function s2n_tls_sys::s2n_connection_get_master_secret

pub unsafe extern "C" fn s2n_connection_get_master_secret(
    conn: *const s2n_connection,
    secret_bytes: *mut u8,
    max_size: usize
) -> c_int

Provides access to the TLS master secret.

This is a dangerous method and should not be used unless absolutely necessary. Mishandling the master secret can compromise both the current connection and any past or future connections that use the same master secret due to session resumption.

This method is only supported for older TLS versions, and will report an S2N_ERR_INVALID_STATE usage error if called for a TLS1.3 connection. TLS1.3 includes a new key schedule that derives independent secrets from the master secret for specific purposes, such as separate traffic, session ticket, and exporter secrets. Using the master secret directly circumvents that security feature, reducing the security of the protocol.

If you need cryptographic material tied to the current TLS session, consider s2n_connection_tls_exporter instead. Although s2n_connection_tls_exporter currently only supports TLS1.3, there is also an RFC that describes exporters for older TLS versions: https://datatracker.ietf.org/doc/html/rfc5705 Using the master secret as-is or defining your own exporter is dangerous.

@param conn A pointer to the connection. @param secret_bytes Memory to copy the master secret into. The secret is always 48 bytes long. @param max_size The size of the memory available at secret_bytes. Must be at least 48 bytes. @returns S2N_SUCCESS on success, S2N_FAILURE otherwise. secret_bytes will be set on success.