Implemented eddsa compression and decompression.

1 files changed, 62 insertions, 0 deletions

diff --git a/eddsa-compress.c b/eddsa-compress.c
new file mode 100644
index 00000000..40959586
--- /dev/null
+++ b/eddsa-compress.c
@@ -0,0 +1,62 @@
+/* eddsa-compress.c
+
+   Copyright (C) 2014 Niels Möller
+
+   This file is part of GNU Nettle.
+
+   GNU Nettle is free software: you can redistribute it and/or
+   modify it under the terms of either:
+
+     * the GNU Lesser General Public License as published by the Free
+       Software Foundation; either version 3 of the License, or (at your
+       option) any later version.
+
+   or
+
+     * the GNU General Public License as published by the Free
+       Software Foundation; either version 2 of the License, or (at your
+       option) any later version.
+
+   or both in parallel, as here.
+
+   GNU Nettle 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 copies of the GNU General Public License and
+   the GNU Lesser General Public License along with this program.  If
+   not, see http://www.gnu.org/licenses/.
+*/
+
+#if HAVE_CONFIG_H
+# include "config.h"
+#endif
+
+#include "eddsa.h"
+
+#include "ecc-internal.h"
+#include "gmp-glue.h"
+
+mp_size_t
+_eddsa_compress_itch (const struct ecc_curve *ecc)
+{
+  return 2*ecc->p.size + ecc->h_to_a_itch;
+}
+
+void
+_eddsa_compress (const struct ecc_curve *ecc, uint8_t *r, mp_limb_t *p,
+		 mp_limb_t *scratch)
+{
+#define xp scratch
+#define yp (scratch + ecc->p.size)
+#define scratch_out (scratch + 2*ecc->p.size)
+
+  ecc->h_to_a (ecc, 0, xp, p, scratch_out);
+  /* Encoding is the y coordinate and an appended "sign" bit, which is
+     the low bit of x. Bit order is not specified explicitly, but for
+     little-endian encoding, it makes most sense to append the bit
+     after the most significant bit of y. */
+  mpn_get_base256_le (r, 1 + ecc->p.bit_size / 8, yp, ecc->p.size);
+  r[ecc->p.bit_size / 8] += (xp[0] & 1) << (ecc->p.bit_size & 7);
+}