From 85bb52acad3f5b7c4ff059ca12e675a2ac1a0385 Mon Sep 17 00:00:00 2001
From: robm <unknown>
Date: Wed, 16 Nov 2016 15:12:50 +0000
Subject: [PATCH] 8168728: DSA signing improvments Reviewed-by: xuelei

---
 .../classes/sun/security/provider/DSA.java    | 40 ++++++++++++++++++-
 1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/src/share/classes/sun/security/provider/DSA.java b/src/share/classes/sun/security/provider/DSA.java
index 071b5b0c0..75a36623d 100644
--- a/src/share/classes/sun/security/provider/DSA.java
+++ b/src/share/classes/sun/security/provider/DSA.java
@@ -366,13 +366,49 @@ abstract class DSA extends SignatureSpi {
         return t5.mod(q);
     }
 
-    // NOTE: This following impl is defined in FIPS 186-4 AppendixB.2.1.
     protected BigInteger generateK(BigInteger q) {
+        // Implementation defined in FIPS 186-4 AppendixB.2.1.
         SecureRandom random = getSigningRandom();
         byte[] kValue = new byte[(q.bitLength() + 7)/8 + 8];
 
         random.nextBytes(kValue);
-        return new BigInteger(1, kValue).mod(q.subtract(BigInteger.ONE)).add(BigInteger.ONE);
+        BigInteger k = new BigInteger(1, kValue).mod(
+                q.subtract(BigInteger.ONE)).add(BigInteger.ONE);
+
+        // Using an equivalent exponent of fixed length (same as q or 1 bit
+        // less than q) to keep the kG timing relatively constant.
+        //
+        // Note that this is an extra step on top of the approach defined in
+        // FIPS 186-4 AppendixB.2.1 so as to make a fixed length K.
+        k = k.add(q).divide(BigInteger.valueOf(2));
+
+        // An alternative implementation based on FIPS 186-4 AppendixB2.2
+        // with fixed-length K.
+        //
+        // Please keep it here as we may need to switch to it in the future.
+        //
+        // SecureRandom random = getSigningRandom();
+        // byte[] kValue = new byte[(q.bitLength() + 7)/8];
+        // BigInteger d = q.subtract(BigInteger.TWO);
+        // BigInteger k;
+        // do {
+        //     random.nextBytes(kValue);
+        //     BigInteger c = new BigInteger(1, kValue);
+        //     if (c.compareTo(d) <= 0) {
+        //         k = c.add(BigInteger.ONE);
+        //         // Using an equivalent exponent of fixed length to keep
+        //         // the g^k timing relatively constant.
+        //         //
+        //         // Note that this is an extra step on top of the approach
+        //         // defined in FIPS 186-4 AppendixB.2.2 so as to make a
+        //         // fixed length K.
+        //         if (k.bitLength() >= q.bitLength()) {
+        //             break;
+        //         }
+        //     }
+        // } while (true);
+
+        return k;
     }
 
     // Use the application-specified SecureRandom Object if provided.
-- 
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