[rng] Add ANS X9.82 Approved Hash_df derivation function

ANS X9.82 specifies several Approved derivation functions for use in
distributing entropy throughout a buffer.  One such derivation
function is Hash_df, which can be implemented using the existing iPXE
SHA-1 functionality.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/hash_df.h

@@ -0,0 +1,30 @@
+#ifndef _IPXE_HASH_DF_H
+#define _IPXE_HASH_DF_H
+
+/** @file
+ *
+ * Hash-based derivation function (Hash_df)
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdint.h>
+#include <ipxe/sha1.h>
+
+/** Use SHA-1 as the underlying hash algorithm
+ *
+ * Hash_df using SHA-1 is an Approved algorithm in ANS X9.82.
+ */
+#define hash_df_algorithm sha1_algorithm
+
+/** Underlying hash algorithm output length (in bytes) */
+#define HASH_DF_OUTLEN_BYTES SHA1_DIGEST_SIZE
+
+/** Underlying hash algorithm context size (in bytes) */
+#define HASH_DF_CTX_SIZE SHA1_CTX_SIZE
+
+extern void hash_df ( const void *input, size_t input_len, void *output,
+		      size_t output_len );
+
+#endif /* _IPXE_HASH_DF_H */