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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.");
}
}
}
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