diff --git a/bignum.c b/bignum.c
index 67abe5d834903dac2c66c47713a8988eb2f4457e..d923d702d739bac04047eaed4d70df4694d1b32c 100644
--- a/bignum.c
+++ b/bignum.c
@@ -34,29 +34,54 @@
#include <assert.h>
#include <string.h>
+/* Two's complement negation means that -x = ~x + 1, ~x = -(x+1),
+ * and we use that x = ~~x = ~(-x-1).
+ *
+ * Examples:
+ *
+ * x ~x = -x+1 ~~x = x
+ * -1 0 ff
+ * -2 1 fe
+ * -7f 7e 81
+ * -80 7f 80
+ * -81 80 ff7f
+ */
+
+/* Including extra sign bit, if needed. Also one byte for zero. */
unsigned
-nettle_mpz_sizeinbase_256(const mpz_t x)
+nettle_mpz_sizeinbase_256_s(const mpz_t x)
{
- return (mpz_sizeinbase(x, 2) + 7) / 8;
+ if (mpz_sgn(x) >= 0)
+ return 1 + mpz_sizeinbase(x, 2) / 8;
+ else
+ {
+ /* We'll output ~~x, so we need as many bits as for ~x */
+ unsigned size;
+ mpz_t c;
+
+ mpz_init(c);
+ mpz_com(c, x); /* Same as c = - x - 1 = |x| + 1 */
+ size = 1 + mpz_sizeinbase(c,2) / 8;
+ mpz_clear(c);
+
+ return size;
+ }
}
-void
-nettle_mpz_get_str_256(unsigned length, uint8_t *s, const mpz_t x)
+unsigned
+nettle_mpz_sizeinbase_256_u(const mpz_t x)
+{
+ return (mpz_sizeinbase(x,2) + 7) / 8;
+}
+
+static void
+nettle_mpz_to_octets(unsigned length, uint8_t *s,
+ const mpz_t x, uint8_t sign)
{
uint8_t *dst = s + length - 1;
unsigned size = mpz_size(x);
unsigned i;
-
- if (!length)
- {
- /* x must be zero */
- assert(!mpz_sgn(x));
- return;
- }
- assert(mpz_sgn(x) >= 0);
- assert(nettle_mpz_sizeinbase_256(x) <= length);
-
for (i = 0; i<size; i++)
{
mp_limb_t limb = mpz_getlimbn(x, i);
@@ -64,28 +89,58 @@ nettle_mpz_get_str_256(unsigned length, uint8_t *s, const mpz_t x)
for (j = 0; length && j < sizeof(mp_limb_t); j++)
{
- *dst-- = limb & 0xff;
+ *dst-- = sign ^ (limb & 0xff);
limb >>= 8;
length--;
}
}
if (length)
- memset(s, 0, length);
+ memset(s, sign, length);
}
void
-nettle_mpz_set_str_256(mpz_t x,
- unsigned length, const uint8_t *s)
+nettle_mpz_get_str_256(unsigned length, uint8_t *s, const mpz_t x)
+{
+ if (!length)
+ {
+ /* x must be zero */
+ assert(!mpz_sgn(x));
+ return;
+ }
+
+ if (mpz_sgn(x) >= 0)
+ {
+ assert(nettle_mpz_sizeinbase_256_u(x) <= length);
+ nettle_mpz_to_octets(length, s, x, 0);
+ }
+ else
+ {
+ mpz_t c;
+ mpz_init(c);
+ mpz_com(c, x);
+
+ assert(nettle_mpz_sizeinbase_256_u(c) <= length);
+ nettle_mpz_to_octets(length, s, c, 0xff);
+
+ mpz_clear(c);
+ }
+}
+
+/* Converting from strings */
+static void
+nettle_mpz_from_octets(mpz_t x,
+ unsigned length, const uint8_t *s,
+ uint8_t sign)
{
- size_t i;
+ unsigned i;
mpz_t digit;
mpz_init(digit);
- mpz_set_ui(x, 0);
+
for (i = 0; i < length; i++)
{
- mpz_set_ui(digit, s[i]);
+ mpz_set_ui(digit, sign ^ s[i]);
mpz_mul_2exp(digit, digit, (length - i - 1) * 8);
mpz_ior(x, x, digit);
}
@@ -93,11 +148,45 @@ nettle_mpz_set_str_256(mpz_t x,
}
void
-nettle_mpz_init_set_str_256(mpz_t x,
- unsigned length, const uint8_t *s)
+nettle_mpz_set_str_256_u(mpz_t x,
+ unsigned length, const uint8_t *s)
+{
+ mpz_set_ui(x, 0);
+ nettle_mpz_from_octets(x, length, s, 0);
+}
+
+void
+nettle_mpz_init_set_str_256_u(mpz_t x,
+ unsigned length, const uint8_t *s)
+{
+ mpz_init_set_ui(x, 0);
+ nettle_mpz_from_octets(x, length, s, 0);
+}
+
+void
+nettle_mpz_set_str_256_s(mpz_t x,
+ unsigned length, const uint8_t *s)
+{
+ mpz_set_ui(x, 0);
+
+ if (!length)
+ return;
+
+ if (s[0] & 0x80)
+ {
+ nettle_mpz_from_octets(x, length, s, 0xff);
+ mpz_com(x, x);
+ }
+ else
+ nettle_mpz_from_octets(x, length, s, 0);
+}
+
+void
+nettle_mpz_init_set_str_256_s(mpz_t x,
+ unsigned length, const uint8_t *s)
{
mpz_init(x);
- nettle_mpz_set_str_256(x, length, s);
+ nettle_mpz_set_str_256_s(x, length, s);
}
#endif /* HAVE_LIBGMP */
diff --git a/bignum.h b/bignum.h
index f4c9ed07847ecc6848bcadc11fe63a24097a114e..3ba8ea5deb5f5af83d62f475de71d0582480d474 100644
--- a/bignum.h
+++ b/bignum.h
@@ -31,21 +31,39 @@
#include <gmp.h>
#include <inttypes.h>
+/* Size needed for signed encoding, including extra sign byte if
+ * necessary. */
unsigned
-nettle_mpz_sizeinbase_256(const mpz_t x);
+nettle_mpz_sizeinbase_256_s(const mpz_t x);
-/* Writes an unsigned integer as length octets, using big endian byte
- * order. */
+/* Size needed for unsigned encoding */
+unsigned
+nettle_mpz_sizeinbase_256_u(const mpz_t x);
+
+/* Writes an integer as length octets, using big endian byte order,
+ * and two's complement for negative numbers. */
+/* FIXME: Change order of arguments, putting the mpz_t first? */
void
nettle_mpz_get_str_256(unsigned length, uint8_t *s, const mpz_t x);
+/* Reads a big endian, two's complement, integer. */
+void
+nettle_mpz_set_str_256_s(mpz_t x,
+ unsigned length, const uint8_t *s);
+
+void
+nettle_mpz_init_set_str_256_s(mpz_t x,
+ unsigned length, const uint8_t *s);
+
+/* Similar, but for unsigned format. These function don't interpret
+ * the most significant bit as the sign. */
void
-nettle_mpz_set_str_256(mpz_t x,
- unsigned length, const uint8_t *s);
+nettle_mpz_set_str_256_u(mpz_t x,
+ unsigned length, const uint8_t *s);
void
-nettle_mpz_init_set_str_256(mpz_t x,
- unsigned length, const uint8_t *s);
+nettle_mpz_init_set_str_256_u(mpz_t x,
+ unsigned length, const uint8_t *s);
/* Returns a uniformly distributed random number 0 <= x < 2^n */
void