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[release-branch.go1.11-security] crypto/elliptic: reduce subtraction …

…term to prevent long busy loop

If beta8 is unusually large, the addition loop might take a very long
time to bring x3-beta8 back positive.

This would lead to a DoS vulnerability in the implementation of the
P-521 and P-384 elliptic curves that may let an attacker craft inputs
to ScalarMult that consume excessive amounts of CPU.

This fixes CVE-2019-6486.

Change-Id: Ia969e8b5bf5ac4071a00722de9d5e4d856d8071a
Reviewed-on: https://team-review.git.corp.google.com/c/399777
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: Julie Qiu <julieqiu@google.com>
(cherry picked from commit 746d6abe2dfb9ce7609f8e1e1a8dcb7e221f423e)
Reviewed-on: https://team-review.git.corp.google.com/c/401142
Reviewed-by: Filippo Valsorda <valsorda@google.com>
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FiloSottile authored and Julie Qiu committed Jan 22, 2019
1 parent 4601a4c commit 42b42f71cf8f5956c09e66230293dfb5db652360
Showing with 2 additions and 1 deletion.
  1. +2 −1 src/crypto/elliptic/elliptic.go
@@ -210,8 +210,9 @@ func (curve *CurveParams) doubleJacobian(x, y, z *big.Int) (*big.Int, *big.Int,

x3 := new(big.Int).Mul(alpha, alpha)
beta8 := new(big.Int).Lsh(beta, 3)
beta8.Mod(beta8, curve.P)
x3.Sub(x3, beta8)
for x3.Sign() == -1 {
if x3.Sign() == -1 {
x3.Add(x3, curve.P)
}
x3.Mod(x3, curve.P)

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