netdev - Re: [PATCH 3/5] xfrm: Traffic Flow Confidentiality for IPv4 ESP

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Message-ID: <20101203073403.GA2292@gondor.apana.org.au>
Date:	Fri, 3 Dec 2010 15:34:03 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Martin Willi <martin@...ongswan.org>
Cc:	linux-crypto@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 3/5] xfrm: Traffic Flow Confidentiality for IPv4 ESP

On Tue, Nov 30, 2010 at 03:49:13PM +0000, Martin Willi wrote:
>
> +	if (skb->len >= tfcpadto) {
> +		clen = ALIGN(skb->len + 2, blksize);
> +	} else if (x->tfc.flags & XFRM_TFC_ESPV3 &&
> +		   x->props.mode == XFRM_MODE_TUNNEL) {
> +		/* ESPv3 TFC padding, append bytes to payload */
> +		tfclen = tfcpadto - skb->len;
> +		clen = ALIGN(skb->len + 2 + tfclen, blksize);
> +	} else {
> +		/* ESPv2 TFC padding. If we exceed the 255 byte maximum, use
> +		 * random padding to hide payload length as good as possible. */
> +		clen = ALIGN(skb->len + 2 + tfcpadto - skb->len, blksize);
> +		if (clen - skb->len - 2 > 255) {
> +			clen = ALIGN(skb->len + (u8)random32() + 2, blksize);
> +			if (clen - skb->len - 2 > 255)
> +				clen -= blksize;
> +		}

What is the basis of this random length padding?

Also, what happens when padto exceeds the MTU? Doesn't this
effectively disable PMTU-discovery?

I know that your last patch allows the padto to be set by PMTU.
But why would we ever want to use a padto that isn't clamped by
PMTU?

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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