tls: fix (what looks like) a rare corner case bug in P256

function                                             old     new   delta
static.sp_256_mont_sub_10                             30      46     +16

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/networking/tls_sp_c32.c b/networking/tls_sp_c32.c
index 7c6229ffd..99f9c6839 100644
--- a/networking/tls_sp_c32.c
+++ b/networking/tls_sp_c32.c
@@ -96,11 +96,13 @@ static void sp_256_to_bin_10(sp_digit* r, uint8_t* a)
         a[j] = 0;
         for (i = 0; i < 10 && j >= 0; i++) {
                 b = 0;
-                a[j--] |= r[i] << s; b += 8 - s;
+                a[j--] |= r[i] << s;
+                b += 8 - s;
                 if (j < 0)
                         break;
                 while (b < 26) {
-                        a[j--] = r[i] >> b; b += 8;
+                        a[j--] = r[i] >> b;
+                        b += 8;
                         if (j < 0)
                                 break;
                 }
@@ -297,6 +299,7 @@ static void sp_256_mont_sub_10(sp_digit* r, const sp_digit* a, const sp_digit* b
         if (r[9] >> 22)
                 sp_256_add_10(r, r, m);
         sp_256_norm_10(r);
+        r[9] &= 0x03fffff; /* truncate to 22 bits */
 }
 
 /* Double a Montgomery form number (r = a + a % m) */
@@ -864,8 +867,8 @@ static void sp_ecc_secret_gen_256(const sp_digit priv[10], const uint8_t *pub2x3
         dump_hex("        %s\n", pub2x32 + 32, 32);
 
         sp_256_point_from_bin2x32(point, pub2x32);
-        dump_hex("point->x %s\n", point->x, sizeof(point->x));
-        dump_hex("point->y %s\n", point->y, sizeof(point->y));
+        dump_512("point->x %s\n", point->x);
+        dump_512("point->y %s\n", point->y);
 
         sp_256_ecc_mulmod_10(point, point, priv);