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fix(jsbn2): prevent modInverse hangs on zero and negative inputs #648

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Merged
kjur merged 2 commits into from
Feb 20, 2026
+19 −4
Merged

fix(jsbn2): prevent modInverse hangs on zero and negative inputs #648

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2af4cb4
fix(jsbn2): prevent modInverse hangs on zero and negative inputs
Kr0emer Feb 20, 2026
7a3ae70
Merge branch 'master' into fix/bug-004-modinverse-dos
kjur Feb 20, 2026
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10 changes: 6 additions & 4 deletions ext/jsbn2.js
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Original file line number Diff line number Diff line change
Expand Up @@ -508,15 +508,17 @@ function bnpModInt(n) {

// (public) 1/this % m (HAC 14.61)
function bnModInverse(m) {
if(m.signum() == 0) return BigInteger.ZERO;
var x = this.mod(m);
var ac = m.isEven();
if((this.isEven() && ac) || m.signum() == 0) return BigInteger.ZERO;
var u = m.clone(), v = this.clone();
if((x.isEven() && ac) || x.signum() == 0) return BigInteger.ZERO;
var u = m.clone(), v = x.clone();
var a = nbv(1), b = nbv(0), c = nbv(0), d = nbv(1);
while(u.signum() != 0) {
while(u.isEven()) {
u.rShiftTo(1,u);
if(ac) {
if(!a.isEven() || !b.isEven()) { a.addTo(this,a); b.subTo(m,b); }
if(!a.isEven() || !b.isEven()) { a.addTo(x,a); b.subTo(m,b); }
a.rShiftTo(1,a);
}
else if(!b.isEven()) b.subTo(m,b);
Expand All @@ -525,7 +527,7 @@ function bnModInverse(m) {
while(v.isEven()) {
v.rShiftTo(1,v);
if(ac) {
if(!c.isEven() || !d.isEven()) { c.addTo(this,c); d.subTo(m,d); }
if(!c.isEven() || !d.isEven()) { c.addTo(x,c); d.subTo(m,d); }
c.rShiftTo(1,c);
}
else if(!d.isEven()) d.subTo(m,d);
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13 changes: 13 additions & 0 deletions test/qunit-do-crypto.html
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Original file line number Diff line number Diff line change
Expand Up @@ -257,6 +257,19 @@
"reject mod(0)");
});

test("BigInteger.modInverse returns quickly for zero input", function() {
var biM = new BigInteger("97", 10);
var biR = new BigInteger("0", 10).modInverse(biM);
equal(biR.compareTo(BigInteger.ZERO), 0, "0 has no inverse");
});

test("BigInteger.modInverse normalizes negative input", function() {
var biM1 = new BigInteger("7", 10);
var biM2 = new BigInteger("97", 10);
equal(new BigInteger("-1", 10).modInverse(biM1).toString(10), "6", "-1 mod 7");
equal(new BigInteger("-13", 10).modInverse(biM2).toString(10), "82", "-13 mod 97");
});

test("MessageDigest test", function() {
expect(10);
var md1 = new KJUR.crypto.MessageDigest({"alg": "sha1", "prov": "cryptojs"});
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