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diff --git a/lib/af_alg.h b/lib/af_alg.h
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+/* af_alg.h - Compute message digests from file streams and buffers.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+
+   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 3, or (at your option) 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, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Matteo Croce <mcroce@redhat.com>, 2018.
+   Documentation by Bruno Haible <bruno@clisp.org>, 2018.  */
+
+/* Declare specific functions for computing message digests
+   using the Linux kernel crypto API, if available.  This kernel API gives
+   access to specialized crypto instructions (that would also be available
+   in user space) or to crypto devices (not directly available in user space).
+
+   For a more complete set of facilities that use the Linux kernel crypto API,
+   look at libkcapi.  */
+
+#ifndef AF_ALG_H
+# define AF_ALG_H 1
+
+# include <stdio.h>
+# include <errno.h>
+
+# ifdef __cplusplus
+extern "C" {
+# endif
+
+# if USE_LINUX_CRYPTO_API
+
+/* Compute a message digest of a memory region.
+
+   The memory region starts at BUFFER and is LEN bytes long.
+
+   ALG is the message digest algorithm; see the file /proc/crypto.
+
+   RESBLOCK points to a block of HASHLEN bytes, for the result.
+   HASHLEN must be the length of the message digest, in bytes, in particular:
+
+      alg    | hashlen
+      -------+--------
+      md5    | 16
+      sha1   | 20
+      sha224 | 28
+      sha256 | 32
+      sha384 | 48
+      sha512 | 64
+
+   If successful, fill RESBLOCK and return 0.
+   Upon failure, return a negated error number.  */
+int
+afalg_buffer (const char *buffer, size_t len, const char *alg,
+              void *resblock, ssize_t hashlen);
+
+/* Compute a message digest of data read from STREAM.
+
+   STREAM is an open file stream.  The last operation on STREAM should
+   not be 'ungetc', and if STREAM is also open for writing it should
+   have been fflushed since its last write.  Read from the current
+   position to the end of STREAM.  Handle regular files efficently.
+
+   ALG is the message digest algorithm; see the file /proc/crypto.
+
+   RESBLOCK points to a block of HASHLEN bytes, for the result.
+   HASHLEN must be the length of the message digest, in bytes, in particular:
+
+      alg    | hashlen
+      -------+--------
+      md5    | 16
+      sha1   | 20
+      sha224 | 28
+      sha256 | 32
+      sha384 | 48
+      sha512 | 64
+
+   If successful, fill RESBLOCK and return 0.
+   Upon failure, return a negated error number.
+   Unless returning 0 or -EIO, restore STREAM's file position so that
+   the caller can fall back on some other method.  */
+int
+afalg_stream (FILE *stream, const char *alg,
+              void *resblock, ssize_t hashlen);
+
+# else
+
+static inline int
+afalg_buffer (const char *buffer, size_t len, const char *alg,
+              void *resblock, ssize_t hashlen)
+{
+  return -EAFNOSUPPORT;
+}
+
+static inline int
+afalg_stream (FILE *stream, const char *alg,
+              void *resblock, ssize_t hashlen)
+{
+  return -EAFNOSUPPORT;
+}
+
+# endif
+
+# ifdef __cplusplus
+}
+# endif
+
+#endif /* AF_ALG_H */