Add new functions to extract {client,server}_random, master_key

Tor uses these values to implement a low-rent clone of RFC 5705 (which,
in our defense, we came up with before RFC 5705 existed).  But now that
ssl_st is opaque, we need another way to get at them.

Includes documentation, with suitable warnings about not actually
using these functions.

Signed-off-by: Nick Mathewson <nickm@torproject.org>
Signed-off-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>

1 files changed, 73 insertions, 0 deletions

diff --git a/doc/ssl/SSL_get_client_random.pod b/doc/ssl/SSL_get_client_random.pod
new file mode 100644
index 0000000000..e31d3f0a28
--- /dev/null
+++ b/doc/ssl/SSL_get_client_random.pod
@@ -0,0 +1,73 @@
+=pod
+
+=head1 NAME
+
+SSL_get_client_random, SSL_get_server_random, SSL_SESSION_get_master_key - retrieve internal TLS/SSL random values and master key
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int SSL_get_client_random(const SSL *ssl, unsigned char *out, int outlen);
+ int SSL_get_server_random(const SSL *ssl, unsigned char *out, int outlen);
+ int SSL_SESSION_get_master_key(const SSL_SESSION *session, unsigned char *out, int outlen);
+
+=head1 DESCRIPTION
+
+SSL_get_client_random() extracts the random value sent from the client
+to the server during the initial SSL/TLS handshake.  It copies this
+value into the buffer provided in B<out>, which must have at least
+B<outlen> bytes available. It returns the total number of bytes that were
+actually copied.
+If B<outlen> is less than 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.
+
+
+=head1 NOTES
+
+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 protoccol.  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.
+
+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.
+
+
+=head1 RETURN VALUES
+
+If B<outlen> is at least 0, these functions return the number of bytes
+actually copied, which will be less than or equal to B<outlen>.
+
+If B<outlen> is less than 0, these functions return the maximum number
+of bytes they would copy--that is, the length of the underlying field.
+
+=head1 SEE ALSO
+
+L<ssl(3)|ssl(3)>,
+L<RAND_bytes(3)|RAND_bytes(3)>,
+L<SSL_export_keying_material(3)|SSL_export_keying_material(3)>
+
+
+=cut