[crypto] Add a placeholder for a proper random number generator

Currently it just calls random().

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/crypto/crandom.c

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2009 Joshua Oreman <oremanj@rwcr.net>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+/** @file
+ *
+ * Cryptographically strong random number generator
+ *
+ * Currently the cryptographic part is not implemented, and this just
+ * uses random().
+ */
+
+#include <gpxe/crypto.h>
+#include <stdlib.h>
+
+/**
+ * Get cryptographically strong random bytes
+ *
+ * @v buf	Buffer in which to store random bytes
+ * @v len	Number of random bytes to generate
+ *
+ * @b WARNING: This function is currently underimplemented, and does
+ * not give numbers any stronger than random()!
+ */
+void get_random_bytes ( void *buf, size_t len )
+{
+	u8 *bufp = buf;
+
+	/*
+	 * Somewhat arbitrarily, choose the 0x00FF0000-masked byte
+	 * returned by random() as having good entropy. PRNGs often
+	 * don't provide good entropy in lower bits, and the top byte
+	 * might show a pattern because of sign issues.
+	 */
+
+	while ( len-- ) {
+		*bufp++ = ( random() >> 16 ) & 0xFF;
+	}
+}

src/include/gpxe/crypto.h

@@ -151,4 +151,6 @@ extern struct digest_algorithm digest_null;
 extern struct cipher_algorithm cipher_null;
 extern struct pubkey_algorithm pubkey_null;
 
+void get_random_bytes ( void *buf, size_t len );
+
 #endif /* _GPXE_CRYPTO_H */