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group: Save a normalize_to_zero in gej_add_ge

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The code currently switches to the alternative formula for lambda only if (R,M)
= (0,0) but the alternative formula works whenever M = 0: Specifically, M = 0
implies y1 = -y2. If x1 = x2, then a = -b this is the r = infinity case that we
handle separately. If x1 != x2, then the denominator in the alternative formula
is non-zero, so this formula is well-defined.

One needs to carefully check that the infinity assignment is still correct
because now the definition of m_alt at this point in the code has changed. But
this is true:

Case y1 = -y2:
  Then degenerate = true and infinity = ((x1 - x2)Z == 0) & ~a->infinity .
  a->infinity is handled separately.
  And if ~a->infinity, then Z = Z1 != 0,
  so infinity = (x1 - x2 == 0) = (a == -b) by case condition.

Case y1 != -y2:
  Then degenerate = false and infinity = ((y1 + y2)Z == 0) & ~a->infinity .
  a->infinity is handled separately.
  And if ~a->infinity, then Z = Z1 != 0,
  so infinity = (y1 + y2 == 0) = false by case condition.

Co-Authored-By: Pieter Wuille <pieter@wuille.net>
This commit is contained in:
Tim Ruffing
2022-02-04 15:11:38 +01:00
parent 1253a27756
commit ac71020ebe
2 changed files with 12 additions and 18 deletions
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7
src/group_impl.h
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@@ -558,10 +558,9 @@ static void secp256k1_gej_add_ge(secp256k1_gej *r, const secp256k1_gej *a, const
secp256k1_fe_negate(&m_alt, &u2, 1); /* Malt = -X2*Z1^2 */
secp256k1_fe_mul(&tt, &u1, &m_alt); /* tt = -U1*U2 (2) */
secp256k1_fe_add(&rr, &tt); /* rr = R = T^2-U1*U2 (3) */
/** If lambda = R/M = 0/0 we have a problem (except in the "trivial"
/** If lambda = R/M = R/0 we have a problem (except in the "trivial"
* case that Z = z1z2 = 0, and this is special-cased later on). */
degenerate = secp256k1_fe_normalizes_to_zero(&m) &
secp256k1_fe_normalizes_to_zero(&rr);
degenerate = secp256k1_fe_normalizes_to_zero(&m);
/* This only occurs when y1 == -y2 and x1^3 == x2^3, but x1 != x2.
* This means either x1 == beta*x2 or beta*x1 == x2, where beta is
* a nontrivial cube root of one. In either case, an alternate
@@ -573,7 +572,7 @@ static void secp256k1_gej_add_ge(secp256k1_gej *r, const secp256k1_gej *a, const
secp256k1_fe_cmov(&rr_alt, &rr, !degenerate);
secp256k1_fe_cmov(&m_alt, &m, !degenerate);
/* Now Ralt / Malt = lambda and is guaranteed not to be 0/0.
/* Now Ralt / Malt = lambda and is guaranteed not to be Ralt / 0.
* From here on out Ralt and Malt represent the numerator
* and denominator of lambda; R and M represent the explicit
* expressions x1^2 + x2^2 + x1x2 and y1 + y2. */