added first commitment implementation

5 files changed, 213 insertions, 1 deletions

diff --git a/src/main/java/org/bouncycastle/crypto/Commitment.java b/src/main/java/org/bouncycastle/crypto/Commitment.java
new file mode 100644
index 00000000..f1dc05a3
--- /dev/null
+++ b/src/main/java/org/bouncycastle/crypto/Commitment.java
@@ -0,0 +1,42 @@
+package org.bouncycastle.crypto;
+
+/**
+ * General holding class for a commitment.
+ */
+public class Commitment
+{
+    private final byte[] secret;
+    private final byte[] commitment;
+
+    /**
+     * Base constructor.
+     *
+     * @param secret  an encoding of the secret required to reveal the commitment.
+     * @param commitment  an encoding of the sealed commitment.
+     */
+    public Commitment(byte[] secret, byte[] commitment)
+    {
+        this.secret = secret;
+        this.commitment = commitment;
+    }
+
+    /**
+     * The secret required to reveal the commitment.
+     *
+     * @return an encoding of the secret associated with the commitment.
+     */
+    public byte[] getSecret()
+    {
+        return secret;
+    }
+
+    /**
+     * The sealed commitment.
+     *
+     * @return an encoding of the sealed commitment.
+     */
+    public byte[] getCommitment()
+    {
+        return commitment;
+    }
+}
diff --git a/src/main/java/org/bouncycastle/crypto/Committer.java b/src/main/java/org/bouncycastle/crypto/Committer.java
new file mode 100644
index 00000000..5c93e5d1
--- /dev/null
+++ b/src/main/java/org/bouncycastle/crypto/Committer.java
@@ -0,0 +1,24 @@
+package org.bouncycastle.crypto;
+
+/**
+ * General interface fdr classes that produce and validate commitments.
+ */
+public interface Committer
+{
+    /**
+     * Generate a commitment for the passed in message.
+     *
+     * @param message the message to be committed to,
+     * @return a Commitment
+     */
+    Commitment commit(byte[] message);
+
+    /**
+     * Return true if the passed in commitment represents a commitment to the passed in maessage.
+     *
+     * @param commitment a commitment previously generated.
+     * @param message the message that was expected to have been committed to.
+     * @return true if commitment matches message, false otherwise.
+     */
+    boolean isRevealed(Commitment commitment, byte[] message);
+}
diff --git a/src/main/java/org/bouncycastle/crypto/commitments/HashCommitter.java b/src/main/java/org/bouncycastle/crypto/commitments/HashCommitter.java
new file mode 100644
index 00000000..1494c3ce
--- /dev/null
+++ b/src/main/java/org/bouncycastle/crypto/commitments/HashCommitter.java
@@ -0,0 +1,81 @@
+package org.bouncycastle.crypto.commitments;
+
+import java.security.SecureRandom;
+
+import org.bouncycastle.crypto.Commitment;
+import org.bouncycastle.crypto.Committer;
+import org.bouncycastle.crypto.DataLengthException;
+import org.bouncycastle.crypto.Digest;
+import org.bouncycastle.crypto.ExtendedDigest;
+import org.bouncycastle.util.Arrays;
+
+/**
+ * A basic hash-committer as described in "Making Mix Nets Robust for Electronic Voting by Randomized Partial Checking",
+ * by Jakobsson, Juels, and Rivest (11th Usenix Security Symposium, 2002).
+ */
+public class HashCommitter
+    implements Committer
+{
+    private final Digest digest;
+    private final int byteLength;
+    private final SecureRandom random;
+
+    /**
+     * Base Constructor. The maximum message length that can be committed to is half the length of the internal
+     * block size for the digest (ExtendedDigest.getBlockLength()).
+     *
+     * @param digest digest to use for creating commitments.
+     * @param random source of randomness for generating secrets.
+     */
+    public HashCommitter(ExtendedDigest digest, SecureRandom random)
+    {
+        this.digest = digest;
+        this.byteLength = digest.getByteLength();
+        this.random = random;
+    }
+
+    /**
+     * Generate a commitment for the passed in message.
+     *
+     * @param message the message to be committed to,
+     * @return a Commitment
+     */
+    public Commitment commit(byte[] message)
+    {
+        if (message.length > byteLength / 2)
+        {
+            throw new DataLengthException("Message to be committed to too large for digest.");
+        }
+
+        byte[] w = new byte[byteLength - message.length];
+
+        random.nextBytes(w);
+
+        return new Commitment(w, calculateCommitment(w, message));
+    }
+
+    /**
+     * Return true if the passed in commitment represents a commitment to the passed in maessage.
+     *
+     * @param commitment a commitment previously generated.
+     * @param message the message that was expected to have been committed to.
+     * @return true if commitment matches message, false otherwise.
+     */
+    public boolean isRevealed(Commitment commitment, byte[] message)
+    {
+        byte[] calcCommitment = calculateCommitment(commitment.getSecret(), message);
+
+        return Arrays.constantTimeAreEqual(commitment.getCommitment(), calcCommitment);
+    }
+
+    private byte[] calculateCommitment(byte[] w, byte[] message)
+    {
+        byte[] commitment = new byte[digest.getDigestSize()];
+
+        digest.update(w, 0, w.length);
+        digest.update(message, 0, message.length);
+        digest.doFinal(commitment, 0);
+
+        return commitment;
+    }
+}
diff --git a/src/test/java/org/bouncycastle/crypto/test/HashCommitmentTest.java b/src/test/java/org/bouncycastle/crypto/test/HashCommitmentTest.java
new file mode 100644
index 00000000..d4bd83de
--- /dev/null
+++ b/src/test/java/org/bouncycastle/crypto/test/HashCommitmentTest.java
@@ -0,0 +1,64 @@
+package org.bouncycastle.crypto.test;
+
+import java.security.SecureRandom;
+
+import org.bouncycastle.crypto.Commitment;
+import org.bouncycastle.crypto.Committer;
+import org.bouncycastle.crypto.DataLengthException;
+import org.bouncycastle.crypto.commitments.HashCommitter;
+import org.bouncycastle.crypto.digests.SHA1Digest;
+import org.bouncycastle.crypto.digests.SHA256Digest;
+import org.bouncycastle.util.encoders.Hex;
+import org.bouncycastle.util.test.SimpleTest;
+
+public class HashCommitmentTest
+    extends SimpleTest
+{
+    public String getName()
+    {
+        return "HashCommitmentTest";
+    }
+
+    public void performTest()
+        throws Exception
+    {
+        byte[] data = Hex.decode("4e6f77206973207468652074696d6520666f7220616c6c20");
+
+        Committer committer = new HashCommitter(new SHA256Digest(), new SecureRandom());
+
+        Commitment c = committer.commit(data);
+
+        committer = new HashCommitter(new SHA256Digest(), new SecureRandom());
+
+        if (!committer.isRevealed(c, data))
+        {
+            fail("commitment failed to validate");
+        }
+
+        committer = new HashCommitter(new SHA1Digest(), new SecureRandom());
+
+        if (committer.isRevealed(c, data))
+        {
+            fail("commitment validated!!");
+        }
+
+        // SHA1 has a block size of 512 bits, try a message that's too big
+
+        try
+        {
+            c = committer.commit(new byte[33]);
+        }
+        catch (DataLengthException e)
+        {
+            if (!e.getMessage().equals("Message to be committed to too large for digest."))
+            {
+                fail("exception thrown but wrong message");
+            }
+        }
+    }
+
+    public static void main(String[] args)
+    {
+        runTest(new HashCommitmentTest());
+    }
+}
diff --git a/src/test/java/org/bouncycastle/crypto/test/RegressionTest.java b/src/test/java/org/bouncycastle/crypto/test/RegressionTest.java
index 906eb910..7f3732d6 100644
--- a/src/test/java/org/bouncycastle/crypto/test/RegressionTest.java
+++ b/src/test/java/org/bouncycastle/crypto/test/RegressionTest.java
@@ -116,7 +116,8 @@ public class RegressionTest
         new OCBTest(),
         new NonMemoableDigestTest(),
         new RSAKeyEncapsulationTest(),
-        new ECIESKeyEncapsulationTest()
+        new ECIESKeyEncapsulationTest(),
+        new HashCommitmentTest()
     };
 
     public static void main(