X-Git-Url: https://www.bearssl.org/gitweb//home/git/?p=BearSSL;a=blobdiff_plain;f=src%2Fec%2Fec_p256_m15.c;h=06eee868d53f40ee482281157fb1c7bfcecb5b9b;hp=0d40aef5ba8b416c8f643fb1b8e4dce59e277ce2;hb=bd3036844bd20b2b8d7bce7fee5ad010ce401915;hpb=89ea3b1876d6a17a754c1f80c74f5076eccda866
diff --git a/src/ec/ec_p256_m15.c b/src/ec/ec_p256_m15.c
index 0d40aef..06eee86 100644
--- 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;
@@ -1172,18 +1174,20 @@ 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;