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package org.spongycastle.crypto.generators;
import org.spongycastle.crypto.DataLengthException;
import org.spongycastle.crypto.DerivationFunction;
import org.spongycastle.crypto.DerivationParameters;
import org.spongycastle.crypto.Digest;
import org.spongycastle.crypto.macs.HMac;
import org.spongycastle.crypto.params.HKDFParameters;
import org.spongycastle.crypto.params.KeyParameter;
/**
* HMAC-based Extract-and-Expand Key Derivation Function (HKDF) implemented
* according to IETF RFC 5869, May 2010 as specified by H. Krawczyk, IBM
* Research & P. Eronen, Nokia. It uses a HMac internally to compute de OKM
* (output keying material) and is likely to have better security properties
* than KDF's based on just a hash function.
*/
public class HKDFBytesGenerator
implements DerivationFunction
{
private HMac hMacHash;
private int hashLen;
private byte[] info;
private byte[] currentT;
private int generatedBytes;
/**
* Creates a HKDFBytesGenerator based on the given hash function.
*
* @param hash the digest to be used as the source of generatedBytes bytes
*/
public HKDFBytesGenerator(Digest hash)
{
this.hMacHash = new HMac(hash);
this.hashLen = hash.getDigestSize();
}
public void init(DerivationParameters param)
{
if (!(param instanceof HKDFParameters))
{
throw new IllegalArgumentException(
"HKDF parameters required for HKDFBytesGenerator");
}
HKDFParameters params = (HKDFParameters)param;
if (params.skipExtract())
{
// use IKM directly as PRK
hMacHash.init(new KeyParameter(params.getIKM()));
}
else
{
hMacHash.init(extract(params.getSalt(), params.getIKM()));
}
info = params.getInfo();
generatedBytes = 0;
currentT = new byte[hashLen];
}
/**
* Performs the extract part of the key derivation function.
*
* @param salt the salt to use
* @param ikm the input keying material
* @return the PRK as KeyParameter
*/
private KeyParameter extract(byte[] salt, byte[] ikm)
{
hMacHash.init(new KeyParameter(ikm));
if (salt == null)
{
// TODO check if hashLen is indeed same as HMAC size
hMacHash.init(new KeyParameter(new byte[hashLen]));
}
else
{
hMacHash.init(new KeyParameter(salt));
}
hMacHash.update(ikm, 0, ikm.length);
byte[] prk = new byte[hashLen];
hMacHash.doFinal(prk, 0);
return new KeyParameter(prk);
}
/**
* Performs the expand part of the key derivation function, using currentT
* as input and output buffer.
*
* @throws DataLengthException if the total number of bytes generated is larger than the one
* specified by RFC 5869 (255 * HashLen)
*/
private void expandNext()
throws DataLengthException
{
int n = generatedBytes / hashLen + 1;
if (n >= 256)
{
throw new DataLengthException(
"HKDF cannot generate more than 255 blocks of HashLen size");
}
// special case for T(0): T(0) is empty, so no update
if (generatedBytes != 0)
{
hMacHash.update(currentT, 0, hashLen);
}
hMacHash.update(info, 0, info.length);
hMacHash.update((byte)n);
hMacHash.doFinal(currentT, 0);
}
public Digest getDigest()
{
return hMacHash.getUnderlyingDigest();
}
public int generateBytes(byte[] out, int outOff, int len)
throws DataLengthException, IllegalArgumentException
{
if (generatedBytes + len > 255 * hashLen)
{
throw new DataLengthException(
"HKDF may only be used for 255 * HashLen bytes of output");
}
if (generatedBytes % hashLen == 0)
{
expandNext();
}
// copy what is left in the currentT (1..hash
int toGenerate = len;
int posInT = generatedBytes % hashLen;
int leftInT = hashLen - generatedBytes % hashLen;
int toCopy = Math.min(leftInT, toGenerate);
System.arraycopy(currentT, posInT, out, outOff, toCopy);
generatedBytes += toCopy;
toGenerate -= toCopy;
outOff += toCopy;
while (toGenerate > 0)
{
expandNext();
toCopy = Math.min(hashLen, toGenerate);
System.arraycopy(currentT, 0, out, outOff, toCopy);
generatedBytes += toCopy;
toGenerate -= toCopy;
outOff += toCopy;
}
return len;
}
}
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