pub const Tls_1_2 = extern struct {
    client_write_MAC_key: [mac_key_length]u8,
    server_write_MAC_key: [mac_key_length]u8,
    client_write_key: [enc_key_length]u8,
    server_write_key: [enc_key_length]u8,
    client_write_IV: [fixed_iv_length]u8,
    server_write_IV: [fixed_iv_length]u8,
    // non-standard entropy
    client_salt: [record_iv_length]u8,
}