java/src/main/java/com/google/crypto/tink/subtle/prf/StreamingPrf.java

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//
//      http://www.apache.org/licenses/LICENSE-2.0
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////////////////////////////////////////////////////////////////////////////////
package com.google.crypto.tink.subtle.prf;

import java.io.InputStream;

/**
 * Streaming API Interface for Pseudo Random Function (Prf)
 *
 * <h3>Security guarantees</h3>
 *
 * <p>Pseudorandom functions provide a mapping from an input to an output string which is
 * indistinguishable from a pure random function.
 *
 * <p>In Tink, the pseudorandom interface produces a stream of pseudorandom bytes, from which the
 * user can read. The resulting stream may be of (virtually) infinite length, producing bytes as
 * long as the user reads from it, or it may produce a finite length stream of a certain length.
 * More formally, ignoring timing, every implementation is indistinguishable in the input/output
 * behavior from the following ideal implementation, for some length {@code LENGTH}.
 * <pre>{@code
 *   public class IdealPRF {
 *     private Map<byte[], byte[]> cache = new ArrayMap<>();
 *     private static final int LENGTH = ...; // Any value; conceptually, Infinite would also be ok.
 *
 *     public InputStream apply(final byte[] input) {
 *       if (!cache.containsKey(input)) {
 *         cache.put(input, Random.getBytes(LENGTH)));
 *       }
 *       return new ByteArrayInputStream(cache.get(input));
 *     }
 *   }
 * }</pre>
 */
public interface StreamingPrf {
  /**
   * Returns an {@link InputStream} which is indistinguishable from a stream returning random bytes
   * in the above sense.
   */
  InputStream computePrf(final byte[] input);
}