apacheGH-524: Poly1305Mac: avoid needless byte copying

The update() implementation copied _all_ bytes (successively) first into
an internal 16-byte buffer and then processed that buffer. This is no
needed if the input is long.

Use the internal 16-byte buffer only for inputs shorter than 16 bytes,
or if there is a leftover of less than 16 bytes at the end of a long
input. In between process 16-byte chunks directly from the input byte
array.

For 32kB inputs this saves us some 2048 calls to System.arraycopy()
copying all those 32kB. The speedup is minimal but noticeable in
benchmarking.

Bug: apache#524

sshd-common/src/main/java/org/apache/sshd/common/mac/Poly1305Mac.java

@@ -95,20 +95,36 @@ public void init(byte[] key) throws Exception {
         k1 = unpackIntLE(key, 20);
         k2 = unpackIntLE(key, 24);
         k3 = unpackIntLE(key, 28);
+
+        currentBlockOffset = 0;
     }
 
     @Override
     public void update(byte[] in, int offset, int length) {
-        while (length > 0) {
-            if (currentBlockOffset == BLOCK_SIZE) {
-                processBlock();
-            }
-
+        if (currentBlockOffset > 0) {
+            // There is a partially filled block.
             int toCopy = Math.min(length, BLOCK_SIZE - currentBlockOffset);
             System.arraycopy(in, offset, currentBlock, currentBlockOffset, toCopy);
             offset += toCopy;
             length -= toCopy;
             currentBlockOffset += toCopy;
+            if (currentBlockOffset == BLOCK_SIZE) {
+                processBlock(currentBlock, 0, BLOCK_SIZE);
+                currentBlockOffset = 0;
+            }
+            if (length == 0) {
+                return;
+            }
+        }
+        while (length >= BLOCK_SIZE) {
+            processBlock(in, offset, BLOCK_SIZE);
+            offset += BLOCK_SIZE;
+            length -= BLOCK_SIZE;
+        }
+        if (length > 0) {
+            // Put remaining bytes into internal buffer (length < BLOCK_SIZE here).
+            System.arraycopy(in, offset, currentBlock, 0, length);
+            currentBlockOffset = length;
         }
     }
 
@@ -125,7 +141,14 @@ public void doFinal(byte[] out, int offset) throws Exception {
             throw new BufferOverflowException();
         }
         if (currentBlockOffset > 0) {
-            processBlock();
+            if (currentBlockOffset < BLOCK_SIZE) {
+                // padding
+                currentBlock[currentBlockOffset] = 1;
+                for (int i = currentBlockOffset + 1; i < BLOCK_SIZE; i++) {
+                    currentBlock[i] = 0;
+                }
+            }
+            processBlock(currentBlock, 0, currentBlockOffset);
         }
 
         h1 += h0 >>> 26;
@@ -179,27 +202,20 @@ public void doFinal(byte[] out, int offset) throws Exception {
         reset();
     }
 
-    private void processBlock() {
-        if (currentBlockOffset < BLOCK_SIZE) {
-            // padding
-            currentBlock[currentBlockOffset] = 1;
-            for (int i = currentBlockOffset + 1; i < BLOCK_SIZE; i++) {
-                currentBlock[i] = 0;
-            }
-        }
+    private void processBlock(byte[] block, int offset, int length) {
 
-        int t0 = unpackIntLE(currentBlock, 0);
-        int t1 = unpackIntLE(currentBlock, 4);
-        int t2 = unpackIntLE(currentBlock, 8);
-        int t3 = unpackIntLE(currentBlock, 12);
+        int t0 = unpackIntLE(block, offset);
+        int t1 = unpackIntLE(block, offset + 4);
+        int t2 = unpackIntLE(block, offset + 8);
+        int t3 = unpackIntLE(block, offset + 12);
 
         h0 += t0 & 0x3ffffff;
         h1 += (t0 >>> 26 | t1 << 6) & 0x3ffffff;
         h2 += (t1 >>> 20 | t2 << 12) & 0x3ffffff;
         h3 += (t2 >>> 14 | t3 << 18) & 0x3ffffff;
         h4 += t3 >>> 8;
 
-        if (currentBlockOffset == BLOCK_SIZE) {
+        if (length == BLOCK_SIZE) {
             h4 += 1 << 24;
         }
 
@@ -226,8 +242,6 @@ private void processBlock() {
         h0 += (int) (tp4 >>> 26) * 5;
         h1 += h0 >>> 26;
         h0 &= 0x3ffffff;
-
-        currentBlockOffset = 0;
     }
 
     private void reset() {