/**
* Creates a new {@code HKDF} instance.
*
* @param algorithm the MAC algorithm to use.
* @param info optional context and application specific information, may be {@code null} or
* empty.
* @param dkLen the desired length for derived keys, in bytes.
* @throws NullPointerException if {@code algorithm} is {@code null}.
* @throws IllegalArgumentException if {@code dkLen} is negative, or if the MAC algorithm is
* unknown, or if {@code dkLen} is greater than 255 * MAC algorithm's output length.
*/
HKDF(Algorithm<MAC> algorithm, byte[] info, int dkLen) {
Parameters.checkCondition(dkLen > 0);
MAC mac = Factory.newMAC(algorithm, new byte[0]);
Parameters.checkCondition(dkLen <= 255 * mac.length());
this.algorithm = algorithm;
this.dkLen = dkLen;
this.info = info == null ? new byte[0] : XArrays.copyOf(info);
}
private byte[] expand(byte[] key) {
MAC mac = Factory.newMAC(algorithm, key);
ByteBuffer t = new ByteBuffer(dkLen + mac.length());
int n = (int) Math.ceil((double) dkLen / mac.length());
byte[] u = new byte[0];
for (int i = 1; i <= n; i++) {
u = mac.update(u).update(info).digest((byte) i);
t.append(u);
}
return t.toByteArray(0, dkLen);
}
private byte[] extract(byte[] key, byte[] salt) {
return Factory.newMAC(algorithm, salt).digest(key);
}