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Improve documenation of internal functions

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Matthias Schiffer 2015-10-03 18:41:12 +02:00
parent 55178f5f41
commit 320daa4838
1 changed files with 34 additions and 10 deletions
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44
src/ec25519.c
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@ -37,6 +37,10 @@
* double operations. * double operations.
* *
* Doxygen comments for public APIs can be found in the public header file. * Doxygen comments for public APIs can be found in the public header file.
*
* Invariant that must be held by all public API: the components of an
* \ref ecc_25519_work_t are always in the range \f$ [0, 2p) \f$.
* Integers in this range will be called \em squeezed in the following.
*/ */
#include <libuecc/ecc.h> #include <libuecc/ecc.h>
@ -74,7 +78,11 @@ static void add(unsigned int out[32], const unsigned int a[32], const unsigned i
u += a[31] + b[31]; out[31] = u; u += a[31] + b[31]; out[31] = u;
} }
/** Subtracts two unpacked integers (modulo p) */ /**
* Subtracts two unpacked integers (modulo p)
*
* b must be \em squeezed.
*/
static void sub(unsigned int out[32], const unsigned int a[32], const unsigned int b[32]) { static void sub(unsigned int out[32], const unsigned int a[32], const unsigned int b[32]) {
unsigned int j; unsigned int j;
unsigned int u; unsigned int u;
@ -88,7 +96,11 @@ static void sub(unsigned int out[32], const unsigned int a[32], const unsigned i
out[31] = u; out[31] = u;
} }
/** Performs carry and reduce on an unpacked integer */ /**
* Performs carry and reduce on an unpacked integer
*
* The result is not always fully reduced, but it will be significantly smaller than \f$ 2p \f$.
*/
static void squeeze(unsigned int a[32]) { static void squeeze(unsigned int a[32]) {
unsigned int j; unsigned int j;
unsigned int u; unsigned int u;
@ -103,7 +115,7 @@ static void squeeze(unsigned int a[32]) {
/** /**
* Ensures that the output of a previous \ref squeeze is fully reduced * Ensures that the output of a previous \ref squeeze is fully reduced
* *
* After a \ref freeze, only the lower byte of each integer part holds a meaningful value * After a \ref freeze, only the lower byte of each integer part holds a meaningful value.
*/ */
static void freeze(unsigned int a[32]) { static void freeze(unsigned int a[32]) {
static const unsigned int minusp[32] = { static const unsigned int minusp[32] = {
@ -123,7 +135,11 @@ static void freeze(unsigned int a[32]) {
for (j = 0; j < 32; j++) a[j] ^= negative & (aorig[j] ^ a[j]); for (j = 0; j < 32; j++) a[j] ^= negative & (aorig[j] ^ a[j]);
} }
/** Multiplies two unpacked integers (modulo p) */ /**
* Multiplies two unpacked integers (modulo p)
*
* The result will be \em squeezed.
*/
static void mult(unsigned int out[32], const unsigned int a[32], const unsigned int b[32]) { static void mult(unsigned int out[32], const unsigned int a[32], const unsigned int b[32]) {
unsigned int i; unsigned int i;
unsigned int j; unsigned int j;
@ -138,7 +154,11 @@ static void mult(unsigned int out[32], const unsigned int a[32], const unsigned
squeeze(out); squeeze(out);
} }
/** Multiplies an unpacked integer with a small integer (modulo p) */ /**
* Multiplies an unpacked integer with a small integer (modulo p)
*
* The result will be \em squeezed.
*/
static void mult_int(unsigned int out[32], unsigned int n, const unsigned int a[32]) { static void mult_int(unsigned int out[32], unsigned int n, const unsigned int a[32]) {
unsigned int j; unsigned int j;
unsigned int u; unsigned int u;
@ -151,7 +171,11 @@ static void mult_int(unsigned int out[32], unsigned int n, const unsigned int a[
u += out[j]; out[j] = u; u += out[j]; out[j] = u;
} }
/** Squares an unpacked integer */ /**
* Squares an unpacked integer
*
* The result will be sqeezed.
*/
static void square(unsigned int out[32], const unsigned int a[32]) { static void square(unsigned int out[32], const unsigned int a[32]) {
unsigned int i; unsigned int i;
unsigned int j; unsigned int j;
@ -185,9 +209,9 @@ static int check_equal(const unsigned int x[32], const unsigned int y[32]) {
} }
/** /**
* Checks if an unpacked integer equals zero * Checks if an unpacked integer equals zero (modulo p)
* *
* The intergers must be \ref squeeze "squeezed" before. * The interger must be squeezed before.
*/ */
static int check_zero(const unsigned int x[32]) { static int check_zero(const unsigned int x[32]) {
static const unsigned int p[32] = { static const unsigned int p[32] = {
@ -473,7 +497,7 @@ int ecc_25519_load_packed(ecc_25519_work_t *out, const ecc_int256_t *in) {
if (!square_root(Y, Y2)) if (!square_root(Y, Y2))
return 0; return 0;
/* No squeeze is necessary for subtractions from zero */ /* No squeeze is necessary after subtractions from zero if the subtrahend is squeezed */
sub(Yt, zero, Y); sub(Yt, zero, Y);
select(out->Y, Y, Yt, (in->p[31] >> 7) ^ (Y[0] & 1)); select(out->Y, Y, Yt, (in->p[31] >> 7) ^ (Y[0] & 1));
@ -507,7 +531,7 @@ void ecc_25519_negate(ecc_25519_work_t *out, const ecc_25519_work_t *in) {
out->Z[i] = in->Z[i]; out->Z[i] = in->Z[i];
} }
/* No squeeze is necessary for subtractions from zero */ /* No squeeze is necessary after subtractions from zero if the subtrahend is squeezed */
sub(out->X, zero, in->X); sub(out->X, zero, in->X);
sub(out->T, zero, in->T); sub(out->T, zero, in->T);
} }