constant time comparison

build.gradle

@@ -59,7 +59,7 @@ dependencies {
   testCompile(files("../jcardsim/jcardsim-3.0.5-SNAPSHOT.jar"))
   testCompile('org.web3j:core:2.3.1')
   testCompile('org.bitcoinj:bitcoinj-core:0.14.5')
-  testCompile('com.github.status-im.status-keycard-java:desktop:64aece4')
+  testCompile('com.github.status-im.status-keycard-java:desktop:15a61e1')
   testCompile('org.bouncycastle:bcprov-jdk15on:1.65')
   testCompile("org.junit.jupiter:junit-jupiter-api:5.1.1")
   testRuntime("org.junit.jupiter:junit-jupiter-engine:5.1.1")

buildSrc/build.gradle

@@ -4,5 +4,5 @@ repositories {
 }
 
 dependencies {
-  compile 'com.github.status-im.status-keycard-java:desktop:64aece4'
+  compile 'com.github.status-im.status-keycard-java:desktop:15a61e1'
 }

src/main/java/im/status/keycard/Crypto.java

@@ -89,11 +89,7 @@ public class Crypto {
 
     addm256(output, outOff, data, dataOff, SECP256k1.SECP256K1_R, (short) 0, output, outOff);
 
-    if (isZero256(output, outOff)) {
-      return false;
-    }
-
-    return true;
+    return !isZero256(output, outOff);
   }
 
   /**
@@ -202,17 +198,22 @@ public class Crypto {
    * @return the comparison result
    */
   private short ucmp256(byte[] a, short aOff, byte[] b, short bOff) {
-    short ai, bi;
+    short gt = 0;
+    short eq = 1;
+    
     for (short i = 0 ; i < 32; i++) {
-      ai = (short)(a[(short)(aOff + i)] & 0x00ff);
-      bi = (short)(b[(short)(bOff + i)] & 0x00ff);
+      short l = (short)(a[(short)(aOff + i)] & 0x00ff);
+      short r = (short)(b[(short)(bOff + i)] & 0x00ff);
+      short d = (short)(r - l);
+      short l_xor_r = (short)(l ^ r);
+      short l_xor_d = (short)(l ^ d);
+      short d_xored = (short)(d ^ (short)(l_xor_r & l_xor_d));
 
-      if (ai != bi) {
-        return (short)(ai - bi);
-      }
+      gt |= (d_xored >>> 15) & eq;
+      eq &= ((short)(l_xor_r - 1) >>> 15);
     }
 
-    return 0;
+    return (short) ((gt + gt + eq) - 1);
   }
 
   /**
@@ -223,16 +224,13 @@ public class Crypto {
    * @return true if a is 0, false otherwise
    */
   private boolean isZero256(byte[] a, short aOff) {
-    boolean isZero = true;
+    byte acc = 0;
 
-    for (short i = 0; i < (byte) 32; i++) {
-      if (a[(short)(aOff + i)] != 0) {
-        isZero = false;
-        break;
-      }
+    for (short i = 0; i < 32; i++) {
+      acc |= a[(short)(aOff + i)];
     }
 
-    return isZero;
+    return acc == 0;
   }
 
   /**