1 files changed, 117 insertions, 0 deletions
diff --git a/bcprov/src/main/java/org/bouncycastle/crypto/generators/Poly1305KeyGenerator.java b/bcprov/src/main/java/org/bouncycastle/crypto/generators/Poly1305KeyGenerator.java
new file mode 100644
index 0000000..59eee39
--- /dev/null
+++ b/bcprov/src/main/java/org/bouncycastle/crypto/generators/Poly1305KeyGenerator.java
@@ -0,0 +1,117 @@
+package org.bouncycastle.crypto.generators;
+
+import org.bouncycastle.crypto.CipherKeyGenerator;
+import org.bouncycastle.crypto.KeyGenerationParameters;
+import org.bouncycastle.crypto.macs.Poly1305;
+
+/**
+ * Generates keys for the Poly1305 MAC.
+ * <p>
+ * Poly1305 keys are 256 bit keys consisting of a 128 bit secret key used for the underlying block
+ * cipher followed by a 128 bit {@code r} value used for the polynomial portion of the Mac. <br>
+ * The {@code r} value has a specific format with some bits required to be cleared, resulting in an
+ * effective 106 bit key. <br>
+ * A separately generated 256 bit key can be modified to fit the Poly1305 key format by using the
+ * {@link #clamp(byte[])} method to clear the required bits.
+ *
+ * @see Poly1305
+ */
+public class Poly1305KeyGenerator
+    extends CipherKeyGenerator
+{
+    private static final byte R_MASK_LOW_2 = (byte)0xFC;
+    private static final byte R_MASK_HIGH_4 = (byte)0x0F;
+
+    /**
+     * Initialises the key generator.<br>
+     * Poly1305 keys are always 256 bits, so the key length in the provided parameters is ignored.
+     */
+    public void init(KeyGenerationParameters param)
+    {
+        // Poly1305 keys are always 256 bits
+        super.init(new KeyGenerationParameters(param.getRandom(), 256));
+    }
+
+    /**
+     * Generates a 256 bit key in the format required for Poly1305 - e.g.
+     * <code>k[0] ... k[15], r[0] ... r[15]</code> with the required bits in <code>r</code> cleared
+     * as per {@link #clamp(byte[])}.
+     */
+    public byte[] generateKey()
+    {
+        final byte[] key = super.generateKey();
+        clamp(key);
+        return key;
+    }
+
+    /**
+     * Modifies an existing 32 byte key value to comply with the requirements of the Poly1305 key by
+     * clearing required bits in the <code>r</code> (second 16 bytes) portion of the key.<br>
+     * Specifically:
+     * <ul>
+     * <li>r[3], r[7], r[11], r[15] have top four bits clear (i.e., are {0, 1, . . . , 15})</li>
+     * <li>r[4], r[8], r[12] have bottom two bits clear (i.e., are in {0, 4, 8, . . . , 252})</li>
+     * </ul>
+     *
+     * @param key a 32 byte key value <code>k[0] ... k[15], r[0] ... r[15]</code>
+     */
+    public static void clamp(byte[] key)
+    {
+        /*
+         * Key is k[0] ... k[15], r[0] ... r[15] as per poly1305_aes_clamp in ref impl.
+         */
+        if (key.length != 32)
+        {
+            throw new IllegalArgumentException("Poly1305 key must be 256 bits.");
+        }
+
+        /*
+         * r[3], r[7], r[11], r[15] have top four bits clear (i.e., are {0, 1, . . . , 15})
+         */
+        key[19] &= R_MASK_HIGH_4;
+        key[23] &= R_MASK_HIGH_4;
+        key[27] &= R_MASK_HIGH_4;
+        key[31] &= R_MASK_HIGH_4;
+
+        /*
+         * r[4], r[8], r[12] have bottom two bits clear (i.e., are in {0, 4, 8, . . . , 252}).
+         */
+        key[20] &= R_MASK_LOW_2;
+        key[24] &= R_MASK_LOW_2;
+        key[28] &= R_MASK_LOW_2;
+    }
+
+    /**
+     * Checks a 32 byte key for compliance with the Poly1305 key requirements, e.g.
+     * <code>k[0] ... k[15], r[0] ... r[15]</code> with the required bits in <code>r</code> cleared
+     * as per {@link #clamp(byte[])}.
+     *
+     * @throws IllegalArgumentException if the key is of the wrong length, or has invalid bits set
+     *             in the <code>r</code> portion of the key.
+     */
+    public static void checkKey(byte[] key)
+    {
+        if (key.length != 32)
+        {
+            throw new IllegalArgumentException("Poly1305 key must be 256 bits.");
+        }
+
+        checkMask(key[19], R_MASK_HIGH_4);
+        checkMask(key[23], R_MASK_HIGH_4);
+        checkMask(key[27], R_MASK_HIGH_4);
+        checkMask(key[31], R_MASK_HIGH_4);
+
+        checkMask(key[20], R_MASK_LOW_2);
+        checkMask(key[24], R_MASK_LOW_2);
+        checkMask(key[28], R_MASK_LOW_2);
+    }
+
+    private static void checkMask(byte b, byte mask)
+    {
+        if ((b & (~mask)) != 0)
+        {
+            throw new IllegalArgumentException("Invalid format for r portion of Poly1305 key.");
+        }
+    }
+
+}
+\ No newline at end of file