In addition to Owen's salient points about compression working efficiently on repetitive strings in plaintext/binary data (e.g. whitespace in a Word document) and not on random data (e.g. encrypted data), some encryption algorithms can actually be weakened by compressing the resulting data, giving a cryptanalyzer clues to the inner workings of the algorithm.
The bottom line here is that SSL works on the socket/transport layer and not at the application layer. If you're generating a .gz file on-the-fly within Apache (mod_gzip, etc), the result will still be encrypted *after* compression. The output chain of Apache applies SSL as the last stage, so something like mod_gzip will operate *before* SSL. Most modern browsers produced in the last four or five years will decompress a .gz file (not .zip!) for the user - even on Windows (just tested IE6 on XP .. works fine). If you've ever experimented with VRML, one "best practices" is to send files as .wrl.gz and not straight .wrl. As for SSL packets being larger - they're not to any appreciable degree - for the exact reason Owen pointed out below. Even symmetric cipher algorithms don't produce appreciably larger amounts of data. For example, using Chained Block Cipher (CBC) mode will only increase the amount of data by 8 bytes from adding an Initialization Vector (IV) to the beginning of the ciphertext and padding the end of the ciphertext to get a complete final block (with an 8 byte block cipher like Blowfish, the largest amount of padding will only be 7 bytes). So, at most, you've added 15 bytes to even the largest amount of plaintext data using Blowfish in CBC mode. There are a few exotic exceptions here, like interleaved chaining block ciphers which will add an IV (of the same size as above) per parallel operation (so if you've got four parallel encryption operations using interleaved CBC, you're adding 24 bytes at the beginning of the ciphertext). However, these are exceptionally rare and typically limited to proprietary implementations/applications. Addressing one other misconception here.. a packet can contain up to 1500 bytes - including headers (assuming your network handles MTUs of 1500, some are less (like ATM @ 53 bytes [48 bytes of payload w/5 bytes of header), some are more (like Frame Relay @ up to 4500 bytes), but hey, not many desktops are connected with ATM or Frame, so we'll call the connection standard ethernet with a MTU of 1500. The way networks operate and packets are forwarded, smaller packets actually transmit *less* data for any given amount of time over larger packets. Switches and routers (OSI layer 2 and 3 devices) operate on packet forwarding rates, regardless of the amount of data in the packet. The more data in the packet, the more data you're going to get for X period of time - this is one factor that introduces latency into a network. Lots of small packets going through a network simply transmit less data than lots of large packets .. and since the only appreciable metric is the number of packets and the packet forwarding rate of the network device, the larger the packet, the happier the network and the more data getting to the end user. The *only* place this is going to make a difference is if you've got an -inline- intrusion detection/prevension system (IDS/IPS), in which case you've got what most network engineers would consider to be a design flaw anyway. In this case, each packet needs to be inspected and the more data there is, the more there is to be inspected. Most IDS sensors will simply discard packets being inspected rather than slow the network down (Snort does this when it's either misconfigured or overloaded). So.. go for it. Use mod_gzip (or similar) to generate .gz docs on the fly .. let Apache handle your SSL. If anything, your win comes from SSL having to encrypt *less* data. This won't speed up the handshake phase, but will speed up the rest of the transaction since there's simply less data to encrypt and transmit. How much speed improvement you get is completely dependent on how much compression you're getting. If you can take a 100K document and compress it to 25K, that's a 75% reduction in the amount of data SSL needs to encrypt and reduces the number of packets from about 66 to around 16 - again, not including the SSL handshake/setup and general TCP setup/teardown. If you're bogging down your server with all the SSL transactions, look at investing in a SSL accelerator. If your business model depends on both security *and* performance, then the cost (starting around 20K$USD) should be easily justified. But that's the subject of another mail and I've got some coffee getting cold over here. ;-) Hope this didn't glaze your eyes over. :-) Best~ -dsp -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Boyle Owen Sent: Tuesday, August 19, 2003 7:02 AM To: [EMAIL PROTECTED] Subject: RE: configuration question >-----Original Message----- >From: Arthur Chan [mailto:[EMAIL PROTECTED] > >Hi Boyle, >I've been debating with myself over whether to encrypt >everything, that's a >cogent argument you have offered. I have a few questions myself : >(1) assuming an openssl encrypted packet is bigger than a >plain text one, Why would you assume this? Essentially; encrypted_text = f(plain_text, key) where f() is a mathematical function. I guess the 2nd law of thermodynamics ("entropy increases") would tend to cause the output to increase but not necessarily by much. In the simple case of a substitutional cipher, the encrypted text would be precisely the same size as the plain text. >would mod_gzip shrink it significantly to warrant the effort? Zipping algorithms work by replacing repetitive sequences in the input with shorter instructions to regenerate them (e.g. 1000 blue pixels -> "1 blue pixel x 1000"). Compression works best with highly structured input data (bitmaps, WAV files, human language etc). With random data, it can't make much difference and will even cause the file to grow! (try repeatedly zipping a file to see this happening). >(2) and would that slow down the client browser display of content ? Unzipping requires the client to have winzip - not a default on a windows client! Probably this would slow the whole thing down. Remember that SSL is well-defined on the web and all recent browsers contain fast and effective SSL software - I would trust it to do its job and not try to re-invent the wheel. Rgds, Owen Boyle Disclaimer: Any disclaimer attached to this message may be ignored. >On the other hand, with these new 1GHz+ P4 desk- and lap-tops >around, maybe >not. > >----- Original Message ----- >From: "Boyle Owen" <[EMAIL PROTECTED]> >To: <[EMAIL PROTECTED]> >Sent: Tuesday, August 19, 2003 04:49 PM >Subject: RE: configuration question > > >>-----Original Message----- >>From: Henrik Bentel [mailto:[EMAIL PROTECTED] >> >>I have a web app which serves both static and non static content, both >>secure and unsecure(https and http). >>Now, all my ssl configuration is under my secure virtual host, >>such that it applies to everything. However, I have quite a bit static >>content(images, css, javascript.,...) which doesn't need to be very >secure. I >>somewhat only want to secure my dynamic content. > >To add to Cliff's comment about browsers complaining about the mix of >secure an insecure content there is a genuine security reason for *not* >doing what you propose. > >Put yourself in the position of a crook who has gained access to the >datastream flowing into your SSL server. As you are probably aware, all >encryption ciphers can be cracked by a brute force attack (making >repeated attempts at guesssing the key). Hopefully, the time-to-crack >will be "long", but you don't know how fast the crook's computer is. If >he works for the NSA, it might be very fast indeed. If you serve all >content via SSL, he has no idea which packets are important and which >are just images etc. so he has to crack everything. If you decide to >save a teeny bit of processing on the server by encrypting only the >important things, he then sees lots of "en clair" packets (containing >image data etc.) which he can safely ignore and only a few occasional >nuggets of encrypted data which he can be sure are worth cracking. Thus >he can focus his efforts on these. Therefore, you make life >easy for the >cracker by highlighting the packets that are worth cracking! In other >words, the best place to hide a leaf is in the forest. > >You shouldn't need to worry about the processing load of the SSL >encryption. If it is slowing your server, then, frankly, your server is >not powerful enough to serve the traffic you have - get more memory, >upgrade the chipset, do whatever is necessary to get up to speed. > >Rgds, >Owen Boyle >Disclaimer: Any disclaimer attached to this message may be ignored. > >>But, I don't want to generate absolute URLs on the fly to link to >>non-secure static content. What I want is to make request to >>certain urls >>"less secure" such that processing is faster. For example, I have a >>directory called art, which is just a defined alias for a >>directory. Is >>there a way to make ssl processing for this directory less >>restrictive than >>for the "generic requests" to the virtual host so that >>processing is faster? >> >>Home someone can help >> >>Henrik Bentel >> >>______________________________________________________________________ >>Apache Interface to OpenSSL (mod_ssl) www.modssl.org >>User Support Mailing List [EMAIL PROTECTED] >>Automated List Manager [EMAIL PROTECTED] >> >Diese E-mail ist eine private und persönliche Kommunikation. Sie hat >keinen Bezug zur Börsen- bzw. Geschäftstätigkeit der SWX Swiss >Exchange. >This e-mail is of a private and personal nature. It is not related to >the exchange or business activities of the SWX Swiss Exchange. 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