Add SSL_SESSION_get_master_key.

Opaquifying SSL_SESSION is less important than the other structs, but this will
cause less turbulence in wpa_supplicant if we add this API too. Semantics and
name taken from OpenSSL 1.1.0 to match.

BUG=6

Change-Id: Ic39f58d74640fa19a60aafb434dd2c4cb43cdea9
Reviewed-on: https://boringssl-review.googlesource.com/7725
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/ssl.h

diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 26e504d..7cb12f1 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -1496,6 +1496,12 @@
  * TODO(davidben): This should return a const X509 *. */
 OPENSSL_EXPORT X509 *SSL_SESSION_get0_peer(const SSL_SESSION *session);
 
+/* SSL_SESSION_get_master_key writes up to |max_out| bytes of |session|'s master
+ * secret to |out| and returns the number of bytes written. If |max_out| is
+ * zero, it returns the size of the master secret. */
+OPENSSL_EXPORT size_t SSL_SESSION_get_master_key(const SSL_SESSION *session,
+                                                 uint8_t *out, size_t max_out);
+
 /* SSL_SESSION_set_time sets |session|'s creation time to |time| and returns
  * |time|. This function may be useful in writing tests but otherwise should not
  * be used. */

ssl/ssl_session.c

diff --git a/ssl/ssl_session.c b/ssl/ssl_session.c
index 5a6a10d..12d065e 100644
--- a/ssl/ssl_session.c
+++ b/ssl/ssl_session.c
@@ -231,6 +231,19 @@
   return session->peer;
 }
 
+size_t SSL_SESSION_get_master_key(const SSL_SESSION *session, uint8_t *out,
+                                  size_t max_out) {
+  /* TODO(davidben): Fix master_key_length's type and remove these casts. */
+  if (max_out == 0) {
+    return (size_t)session->master_key_length;
+  }
+  if (max_out > (size_t)session->master_key_length) {
+    max_out = (size_t)session->master_key_length;
+  }
+  memcpy(out, session->master_key, max_out);
+  return max_out;
+}
+
 long SSL_SESSION_set_time(SSL_SESSION *session, long time) {
   if (session == NULL) {
     return 0;