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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for the Android platform. Android unfortunately ships with a stripped-down version of Bouncy Castle, which prevents easy upgrades - Spongy Castle overcomes this and provides a full, up-to-date version of the Bouncy Castle cryptographic libs.

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package org.spongycastle.crypto.tls;

/**
 * RFC 4347 4.1.2.5 Anti-replay
 * 
 * Support fast rejection of duplicate records by maintaining a sliding receive window
 * 
 */
class DTLSReplayWindow
{
    private static final long VALID_SEQ_MASK = 0x0000FFFFFFFFFFFFL;

    private static final long WINDOW_SIZE = 64L;

    private long latestConfirmedSeq = -1;
    private long bitmap = 0;

    /**
     * Check whether a received record with the given sequence number should be rejected as a duplicate.
     *
     * @param seq the 48-bit DTLSPlainText.sequence_number field of a received record.
     * @return true if the record should be discarded without further processing.
     */
    boolean shouldDiscard(long seq)
    {
        if ((seq & VALID_SEQ_MASK) != seq)
        {
            return true;
        }

        if (seq <= latestConfirmedSeq)
        {
            long diff = latestConfirmedSeq - seq;
            if (diff >= WINDOW_SIZE)
            {
                return true;
            }
            if ((bitmap & (1L << diff)) != 0)
            {
                return true;
            }
        }

        return false;
    }

    /**
     * Report that a received record with the given sequence number passed authentication checks.
     *
     * @param seq the 48-bit DTLSPlainText.sequence_number field of an authenticated record.
     */
    void reportAuthenticated(long seq)
    {
        if ((seq & VALID_SEQ_MASK) != seq)
        {
            throw new IllegalArgumentException("'seq' out of range");
        }

        if (seq <= latestConfirmedSeq)
        {
            long diff = latestConfirmedSeq - seq;
            if (diff < WINDOW_SIZE)
            {
                bitmap |= (1L << diff);
            }
        }
        else
        {
            long diff = seq - latestConfirmedSeq;
            if (diff >= WINDOW_SIZE)
            {
                bitmap = 1;
            }
            else
            {
                bitmap <<= (int)diff;        // for earlier JDKs
                bitmap |= 1;
            }
            latestConfirmedSeq = seq;
        }
    }

    /**
     * When a new epoch begins, sequence numbers begin again at 0
     */
    void reset()
    {
        latestConfirmedSeq = -1;
        bitmap = 0;
    }
}