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Check points for validity on load

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This commit is contained in:
Matthias Schiffer 2013-01-09 10:18:39 +01:00
parent 131597e511
commit a88af14481
2 changed files with 38 additions and 12 deletions
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4
include/libuecc/ecc.h
View file

@ -65,10 +65,10 @@ typedef struct _ecc_25519_work {
* @{ * @{
*/ */
void ecc_25519_load_xy(ecc_25519_work_t *out, const ecc_int256_t *x, const ecc_int256_t *y); int ecc_25519_load_xy(ecc_25519_work_t *out, const ecc_int256_t *x, const ecc_int256_t *y);
void ecc_25519_store_xy(ecc_int256_t *x, ecc_int256_t *y, const ecc_25519_work_t *in); void ecc_25519_store_xy(ecc_int256_t *x, ecc_int256_t *y, const ecc_25519_work_t *in);
void ecc_25519_load_packed(ecc_25519_work_t *out, const ecc_int256_t *in); int ecc_25519_load_packed(ecc_25519_work_t *out, const ecc_int256_t *in);
void ecc_25519_store_packed(ecc_int256_t *out, const ecc_25519_work_t *in); void ecc_25519_store_packed(ecc_int256_t *out, const ecc_25519_work_t *in);
int ecc_25519_is_identity(const ecc_25519_work_t *in); int ecc_25519_is_identity(const ecc_25519_work_t *in);

46
src/ec25519.c
View file

@ -40,6 +40,10 @@
#include <libuecc/ecc.h> #include <libuecc/ecc.h>
static const unsigned int zero[32] = {0};
static const unsigned int one[32] = {1};
/** Adds two unpacked integers (modulo p) */ /** Adds two unpacked integers (modulo p) */
static void add(unsigned int out[32], const unsigned int a[32], const unsigned int b[32]) { static void add(unsigned int out[32], const unsigned int a[32], const unsigned int b[32]) {
unsigned int j; unsigned int j;
@ -114,7 +118,7 @@ static void mult(unsigned int out[32], const unsigned int a[32], const unsigned
} }
/** Multiplies an unpacked integer with a small integer (modulo p) */ /** Multiplies an unpacked integer with a small integer (modulo p) */
static void mult_int(unsigned int out[32], const 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;
@ -165,7 +169,6 @@ static int check_equal(const unsigned int x[32], const unsigned int y[32]) {
* The intergers must be must be \ref squeeze "squeezed" before. * The intergers must be must be \ref squeeze "squeezed" before.
*/ */
static int check_zero(const unsigned int x[32]) { static int check_zero(const unsigned int x[32]) {
static const unsigned int zero[32] = {0};
static const unsigned int p[32] = { static const unsigned int p[32] = {
0xed, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xed, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
@ -214,9 +217,9 @@ static void select(unsigned int out[32], const unsigned int r[32], const unsigne
/** /**
* Computes the square root of an unpacked integer (in the prime field modulo p) * Computes the square root of an unpacked integer (in the prime field modulo p)
* *
* If the given integer has no square root, the result is undefined. * If the given integer has no square root, 0 is returned, 1 otherwise.
*/ */
static void square_root(unsigned int out[32], const unsigned int z[32]) { static int square_root(unsigned int out[32], const unsigned int z[32]) {
static const unsigned int minus1[32] = { static const unsigned int minus1[32] = {
0xec, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xec, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
@ -303,6 +306,10 @@ static void square_root(unsigned int out[32], const unsigned int z[32]) {
mult(z2_252_1_rho_s, z2_252_1, rho_s); mult(z2_252_1_rho_s, z2_252_1, rho_s);
select(out, z2_252_1, z2_252_1_rho_s, check_equal(t1, minus1)); select(out, z2_252_1, z2_252_1_rho_s, check_equal(t1, minus1));
/* Check the root */
square(t0, out);
return check_equal(t0, z);
} }
/** Computes the reciprocal of an unpacked integer (in the prime field modulo p) */ /** Computes the reciprocal of an unpacked integer (in the prime field modulo p) */
@ -373,8 +380,9 @@ static void recip(unsigned int out[32], const unsigned int z[32]) {
} }
/** Loads a point with given coordinates into its unpacked representation */ /** Loads a point with given coordinates into its unpacked representation */
void ecc_25519_load_xy(ecc_25519_work_t *out, const ecc_int256_t *x, const ecc_int256_t *y) { int ecc_25519_load_xy(ecc_25519_work_t *out, const ecc_int256_t *x, const ecc_int256_t *y) {
int i; int i;
unsigned int X2[32], Y2[32], aX2[32], dX2[32], dX2Y2[32], aX2_Y2[32], _1_dX2Y2[32], r[32];
for (i = 0; i < 32; i++) { for (i = 0; i < 32; i++) {
out->X[i] = x->p[i]; out->X[i] = x->p[i];
@ -382,7 +390,23 @@ void ecc_25519_load_xy(ecc_25519_work_t *out, const ecc_int256_t *x, const ecc_i
out->Z[i] = (i == 0); out->Z[i] = (i == 0);
} }
/* Check validity */
square(X2, out->X);
square(Y2, out->Y);
mult_int(aX2, 486664, X2);
mult_int(dX2, 486660, X2);
mult(dX2Y2, dX2, Y2);
add(aX2_Y2, aX2, Y2);
add(_1_dX2Y2, one, dX2Y2);
sub(r, aX2_Y2, _1_dX2Y2);
squeeze(r);
if (!check_zero(r))
return 0;
mult(out->T, out->X, out->Y); mult(out->T, out->X, out->Y);
return 1;
} }
/** /**
@ -414,10 +438,7 @@ void ecc_25519_store_xy(ecc_int256_t *x, ecc_int256_t *y, const ecc_25519_work_t
} }
/** Loads a packed point into its unpacked representation */ /** Loads a packed point into its unpacked representation */
void ecc_25519_load_packed(ecc_25519_work_t *out, const ecc_int256_t *in) { int ecc_25519_load_packed(ecc_25519_work_t *out, const ecc_int256_t *in) {
static const unsigned int zero[32] = {0};
static const unsigned int one[32] = {1};
int i; int i;
unsigned int X2[32] /* X^2 */, aX2[32] /* aX^2 */, dX2[32] /* dX^2 */, _1_aX2[32] /* 1-aX^2 */, _1_dX2[32] /* 1-aX^2 */; unsigned int X2[32] /* X^2 */, aX2[32] /* aX^2 */, dX2[32] /* dX^2 */, _1_aX2[32] /* 1-aX^2 */, _1_dX2[32] /* 1-aX^2 */;
unsigned int _1_1_dX2[32] /* 1/(1-aX^2) */, Y2[32] /* Y^2 */, Y[32], Yt[32]; unsigned int _1_1_dX2[32] /* 1/(1-aX^2) */, Y2[32] /* Y^2 */, Y[32], Yt[32];
@ -436,12 +457,17 @@ void ecc_25519_load_packed(ecc_25519_work_t *out, const ecc_int256_t *in) {
sub(_1_dX2, one, dX2); sub(_1_dX2, one, dX2);
recip(_1_1_dX2, _1_dX2); recip(_1_1_dX2, _1_dX2);
mult(Y2, _1_aX2, _1_1_dX2); mult(Y2, _1_aX2, _1_1_dX2);
square_root(Y, Y2);
if (!square_root(Y, Y2))
return 0;
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));
mult(out->T, out->X, out->Y); mult(out->T, out->X, out->Y);
return 1;
} }
/** Stores a point into its packed representation */ /** Stores a point into its packed representation */