Fixed some typographic errors in comments.

[BearSSL] / src / ec / ec_p256_m15.c

diff --git a/src/ec/ec_p256_m15.c b/src/ec/ec_p256_m15.c
index 99c7224..6ce57e0 100644 (file)
--- a/src/ec/ec_p256_m15.c
+++ b/src/ec/ec_p256_m15.c
@@ -1101,18 +1101,20 @@ mul_f256(uint32_t *d, const uint32_t *a, const uint32_t *b)
        }
 
        /*
-        * Propagate carries. Since the operation above really is a
-        * truncature, followed by the addition of nonnegative values,
-        * the result will be positive. Moreover, the carry cannot
-        * exceed 5 bits (we performed 20 additions with values smaller
-        * than 256 bits).
+        * Propagate carries. This is a signed propagation, and the
+        * result may be negative. The loop above may enlarge values,
+        * but not two much: worst case is the chain involving t[i - 3],
+        * in which a value may be added to itself up to 7 times. Since
+        * starting values are 13-bit each, all words fit on 20 bits
+        * (21 to account for the sign bit).
         */
        cc = norm13(t, t, 20);
 
        /*
         * Perform modular reduction again for the bits beyond 256 (the carry
-        * and the bits 256..259). This time, we can simply inject full
-        * word values.
+        * and the bits 256..259). Since the largest shift below is by 10
+        * bits, and the values fit on 21 bits, values fit in 32-bit words,
+        * thereby allowing injecting full word values.
         */
        cc = (cc << 4) | (t[19] >> 9);
        t[19] &= 0x01FF;
@@ -1120,6 +1122,22 @@ mul_f256(uint32_t *d, const uint32_t *a, const uint32_t *b)
        t[14] -= cc << 10;
        t[7] -= cc << 5;
        t[0] += cc;
+
+       /*
+        * If the carry is negative, then after carry propagation, we may
+        * end up with a value which is negative, and we don't want that.
+        * Thus, in that case, we add the modulus. Note that the subtraction
+        * result, when the carry is negative, is always smaller than the
+        * modulus, so the extra addition will not make the value exceed
+        * twice the modulus.
+        */
+       cc >>= 31;
+       t[0] -= cc;
+       t[7] += cc << 5;
+       t[14] += cc << 10;
+       t[17] -= cc << 3;
+       t[19] += cc << 9;
+
        norm13(d, t, 20);
 }
 
@@ -1172,18 +1190,20 @@ square_f256(uint32_t *d, const uint32_t *a)
        }
 
        /*
-        * Propagate carries. Since the operation above really is a
-        * truncature, followed by the addition of nonnegative values,
-        * the result will be positive. Moreover, the carry cannot
-        * exceed 5 bits (we performed 20 additions with values smaller
-        * than 256 bits).
+        * Propagate carries. This is a signed propagation, and the
+        * result may be negative. The loop above may enlarge values,
+        * but not two much: worst case is the chain involving t[i - 3],
+        * in which a value may be added to itself up to 7 times. Since
+        * starting values are 13-bit each, all words fit on 20 bits
+        * (21 to account for the sign bit).
         */
        cc = norm13(t, t, 20);
 
        /*
         * Perform modular reduction again for the bits beyond 256 (the carry
-        * and the bits 256..259). This time, we can simply inject full
-        * word values.
+        * and the bits 256..259). Since the largest shift below is by 10
+        * bits, and the values fit on 21 bits, values fit in 32-bit words,
+        * thereby allowing injecting full word values.
         */
        cc = (cc << 4) | (t[19] >> 9);
        t[19] &= 0x01FF;
@@ -1191,6 +1211,22 @@ square_f256(uint32_t *d, const uint32_t *a)
        t[14] -= cc << 10;
        t[7] -= cc << 5;
        t[0] += cc;
+
+       /*
+        * If the carry is negative, then after carry propagation, we may
+        * end up with a value which is negative, and we don't want that.
+        * Thus, in that case, we add the modulus. Note that the subtraction
+        * result, when the carry is negative, is always smaller than the
+        * modulus, so the extra addition will not make the value exceed
+        * twice the modulus.
+        */
+       cc >>= 31;
+       t[0] -= cc;
+       t[7] += cc << 5;
+       t[14] += cc << 10;
+       t[17] -= cc << 3;
+       t[19] += cc << 9;
+
        norm13(d, t, 20);
 }
 
@@ -1987,6 +2023,14 @@ api_order(int curve, size_t *len)
        return P256_N;
 }
 
+static size_t
+api_xoff(int curve, size_t *len)
+{
+       (void)curve;
+       *len = 32;
+       return 1;
+}
+
 static uint32_t
 api_mul(unsigned char *G, size_t Glen,
        const unsigned char *x, size_t xlen, int curve)
@@ -2079,6 +2123,7 @@ const br_ec_impl br_ec_p256_m15 = {
        (uint32_t)0x00800000,
        &api_generator,
        &api_order,
+       &api_xoff,
        &api_mul,
        &api_mulgen,
        &api_muladd