bignum: introduce read/write_le

CONTRIBUTORS

@@ -4,6 +4,7 @@ Jochen Hoenicke <hoenicke@gmail.com>
 Dustin Laurence <dustin@laurences.net>
 Ondrej Mikle <ondrej.mikle@nic.cz>
 Roman Zeyde <roman.zeyde@gmail.com>
+Alex Beregszaszi <alex@rtfs.hu>
 netanelkl <netanel.keidar@gmail.com>
 Jan Pochyla <jpochyla@gmail.com>
 Ondrej Mikle <ondrej.mikle@gmail.com>

bignum.c

@@ -2,6 +2,7 @@
  * Copyright (c) 2013-2014 Tomas Dzetkulic
  * Copyright (c) 2013-2014 Pavol Rusnak
  * Copyright (c)      2015 Jochen Hoenicke
+ * Copyright (c)      2016 Alex Beregszaszi
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the "Software"),
@@ -69,6 +70,22 @@ inline void write_be(uint8_t *data, uint32_t x)
 	data[3] = x;
 }
 
+inline uint32_t read_le(const uint8_t *data)
+{
+	return (((uint32_t)data[3]) << 24) |
+	       (((uint32_t)data[2]) << 16) |
+	       (((uint32_t)data[1]) << 8)  |
+	       (((uint32_t)data[0]));
+}
+
+inline void write_le(uint8_t *data, uint32_t x)
+{
+	data[3] = x >> 24;
+	data[2] = x >> 16;
+	data[1] = x >> 8;
+	data[0] = x;
+}
+
 // convert a raw bigendian 256 bit value into a normalized bignum.
 // out_number is partly reduced (since it fits in 256 bit).
 void bn_read_be(const uint8_t *in_number, bignum256 *out_number)
@@ -104,6 +121,41 @@ void bn_write_be(const bignum256 *in_number, uint8_t *out_number)
 	}
 }
 
+// convert a raw little endian 256 bit value into a normalized bignum.
+// out_number is partly reduced (since it fits in 256 bit).
+void bn_read_le(const uint8_t *in_number, bignum256 *out_number)
+{
+	int i;
+	uint32_t temp = 0;
+	for (i = 0; i < 8; i++) {
+		// invariant: temp = (in_number % 2^(32i)) >> 30i
+		// get next limb = (in_number % 2^(32(i+1))) >> 32i
+		uint32_t limb = read_le(in_number + i * 4);
+		// temp = (in_number % 2^(32(i+1))) << 30i
+		temp |= limb << (2*i);
+		// store 30 bits into val[i]
+		out_number->val[i]= temp & 0x3FFFFFFF;
+		// prepare temp for next round
+		temp = limb >> (30 - 2*i);
+	}
+	out_number->val[8] = temp;
+}
+
+// convert a normalized bignum to a raw little endian 256 bit number.
+// in_number must be fully reduced.
+void bn_write_le(const bignum256 *in_number, uint8_t *out_number)
+{
+	int i;
+	uint32_t temp = in_number->val[8] << 16;
+	for (i = 0; i < 8; i++) {
+		// invariant: temp = (in_number >> 30*(8-i)) << (16 + 2i)
+		uint32_t limb = in_number->val[7 - i];
+		temp |= limb >> (14 - 2*i);
+		write_le(out_number + (7 - i) * 4, temp);
+		temp = limb << (18 + 2*i);
+	}
+}
+
 // sets a bignum to zero.
 void bn_zero(bignum256 *a)
 {

bignum.h

@@ -39,10 +39,20 @@ uint32_t read_be(const uint8_t *data);
 // write 4 big endian bytes
 void write_be(uint8_t *data, uint32_t x);
 
+// read 4 little endian bytes into uint32
+uint32_t read_le(const uint8_t *data);
+
+// write 4 little endian bytes
+void write_le(uint8_t *data, uint32_t x);
+
 void bn_read_be(const uint8_t *in_number, bignum256 *out_number);
 
 void bn_write_be(const bignum256 *in_number, uint8_t *out_number);
 
+void bn_read_le(const uint8_t *in_number, bignum256 *out_number);
+
+void bn_write_le(const bignum256 *in_number, uint8_t *out_number);
+
 void bn_zero(bignum256 *a);
 
 int bn_is_zero(const bignum256 *a);

tests.c

@@ -123,6 +123,36 @@ START_TEST(test_bignum_equal)
 }
 END_TEST
 
+START_TEST(test_bignum_read_le)
+{
+	bignum256 a;
+	bignum256 b;
+
+	bn_read_be(fromhex("c55ece858b0ddd5263f96810fe14437cd3b5e1fbd7c6a2ec1e031f05e86d8bd5"), &a);
+	bn_read_le(fromhex("d58b6de8051f031eeca2c6d7fbe1b5d37c4314fe1068f96352dd0d8b85ce5ec5"), &b);
+
+	ck_assert_int_eq(bn_is_equal(&a, &b), 1);
+}
+END_TEST
+
+START_TEST(test_bignum_write_le)
+{
+	bignum256 a;
+	bignum256 b;
+	uint8_t tmp[32];
+
+	bn_read_be(fromhex("c55ece858b0ddd5263f96810fe14437cd3b5e1fbd7c6a2ec1e031f05e86d8bd5"), &a);
+	bn_write_le(&a, tmp);
+
+	bn_read_le(tmp, &b);
+	ck_assert_int_eq(bn_is_equal(&a, &b), 1);
+
+	bn_read_be(fromhex("d58b6de8051f031eeca2c6d7fbe1b5d37c4314fe1068f96352dd0d8b85ce5ec5"), &a);
+	bn_read_be(tmp, &b);
+	ck_assert_int_eq(bn_is_equal(&a, &b), 1);
+}
+END_TEST
+
 // from https://github.com/bitcoin/bitcoin/blob/master/src/test/data/base58_keys_valid.json
 START_TEST(test_base58)
 {
@@ -2371,6 +2401,8 @@ Suite *test_suite(void)
 	tcase_add_test(tc, test_bignum_read_be);
 	tcase_add_test(tc, test_bignum_write_be);
 	tcase_add_test(tc, test_bignum_equal);
+	tcase_add_test(tc, test_bignum_read_le);
+	tcase_add_test(tc, test_bignum_write_le);
 	suite_add_tcase(s, tc);
 
 	tc = tcase_create("base58");