1 files changed, 93 insertions, 0 deletions
diff --git a/src/main/java/org/whispersystems/libaxolotl/kdf/HKDF.java b/src/main/java/org/whispersystems/libaxolotl/kdf/HKDF.java
new file mode 100644
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+++ b/src/main/java/org/whispersystems/libaxolotl/kdf/HKDF.java
@@ -0,0 +1,93 @@
+/**
+ * Copyright (C) 2013 Open Whisper Systems
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.whispersystems.libaxolotl.kdf;
+
+import java.io.ByteArrayOutputStream;
+import java.security.InvalidKeyException;
+import java.security.NoSuchAlgorithmException;
+
+import javax.crypto.Mac;
+import javax.crypto.spec.SecretKeySpec;
+
+public abstract class HKDF {
+
+  private static final int HASH_OUTPUT_SIZE  = 32;
+
+  public static HKDF createFor(int messageVersion) {
+    switch (messageVersion) {
+      case 2:  return new HKDFv2();
+      case 3:  return new HKDFv3();
+      default: throw new AssertionError("Unknown version: " + messageVersion);
+    }
+  }
+
+  public byte[] deriveSecrets(byte[] inputKeyMaterial, byte[] info, int outputLength) {
+    byte[] salt = new byte[HASH_OUTPUT_SIZE];
+    return deriveSecrets(inputKeyMaterial, salt, info, outputLength);
+  }
+
+  public byte[] deriveSecrets(byte[] inputKeyMaterial, byte[] salt, byte[] info, int outputLength) {
+    byte[] prk = extract(salt, inputKeyMaterial);
+    return expand(prk, info, outputLength);
+  }
+
+  private byte[] extract(byte[] salt, byte[] inputKeyMaterial) {
+    try {
+      Mac mac = Mac.getInstance("HmacSHA256");
+      mac.init(new SecretKeySpec(salt, "HmacSHA256"));
+      return mac.doFinal(inputKeyMaterial);
+    } catch (NoSuchAlgorithmException | InvalidKeyException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+  private byte[] expand(byte[] prk, byte[] info, int outputSize) {
+    try {
+      int                   iterations     = (int) Math.ceil((double) outputSize / (double) HASH_OUTPUT_SIZE);
+      byte[]                mixin          = new byte[0];
+      ByteArrayOutputStream results        = new ByteArrayOutputStream();
+      int                   remainingBytes = outputSize;
+
+      for (int i= getIterationStartOffset();i<iterations + getIterationStartOffset();i++) {
+        Mac mac = Mac.getInstance("HmacSHA256");
+        mac.init(new SecretKeySpec(prk, "HmacSHA256"));
+
+        mac.update(mixin);
+        if (info != null) {
+          mac.update(info);
+        }
+        mac.update((byte)i);
+
+        byte[] stepResult = mac.doFinal();
+        int    stepSize   = Math.min(remainingBytes, stepResult.length);
+
+        results.write(stepResult, 0, stepSize);
+
+        mixin          = stepResult;
+        remainingBytes -= stepSize;
+      }
+
+      return results.toByteArray();
+    } catch (NoSuchAlgorithmException | InvalidKeyException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+  protected abstract int getIterationStartOffset();
+
+}