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Re: Crypto Question
From: Adam Newhard <atnewhard () microstrain com>
Date: Fri, 07 Nov 2003 13:20:49 -0500

Unfortunately, no. IMHO, it should be "the encrypted file/email is only as strong as the encryption algorithm, the generated key(s), your passphrase, the person you share the data with, and your ability to hold onto your keys."

1. Encryption algorithm - This is pretty obvious, otherwise, people wouldn't be coming up w/new and "better" ways to encrypt things and RSA wouldn't be trying to generate random numbers using lava lamps (if it wasn't RSA, someone correct me). 2. your generated keys - This isn't just oh my key is 128 bits and yours is 64 so mine is better. It's definitely not better if your friend's key is truly random and you're one of the unlucky ones to generate a key of all 1's, all 0's, or alternating 1's and 0's (DES weak keys). Not like I'd know what your key was on your encrypted data, but if I wanted to brute force the two of you, I'd aim for the algorithm's weak keys first and keep my hopes up. I mean, yeah, if you don't have a weak key it'll take me on average 2^64 tries to get your key and 2^32 tries to get your friend's key. That's just 1 example. 3. Your passphrase - This is only worthwhile if someone has your keys...the real question is, why'd you let them get those in the first place??? (See number 5) Of course, if I'm going to dictionary attack you, the most common thing is to just use normal words. A password that's 7 characters versus 6 is going to take me 26^7 - 26^6 tries more. If you include numbers in that password all of the sudden it's 36^7 - 36^6. If you include all other ascii, etc...you get the idea. So yeah, your passphrase is definitely important, but again, why do they have your keys anyway? 4. The person you share your data with - Ok, so lets say you and your friend are sharing a key to encrypt data between each other...you mentioned email so you're obviously doing that or at least using asymmetric encryption (public/private keys). If you're a genious and your friend is a moron, who do you think I'm going to attack first to read conversations...obviously, not you. I'd say this is one of the biggest holes people have to deal with. If I'm encrypting stuff for me, myself, and I then I'm feeling pretty good...but if I'm encrypting for me, myself, and the other guy, I'm a little weary of what I disclose in writing. The more people you share encrypted data with, the less of a reason there is to encrypt it. If this wasn't true, we could encrypt everything we do and have one worldwide key...complete inefficiency (although, yeah, that is taking it a little too far). 5. Your ability to hold onto your keys - Lets say you're using symmetric encryption. If you don't have a passphrase on it, I'm already in. If you do have a passphrase, hey, at least I have your keys and I can dictionary you...and if worse comes to worse...social engineering is always an adventure (this goes back to number 3 and number 4). If you do assymetric encryption, your key is either posted online or I could probably find our your email and ask you to send me over your public key. There's half the deal. To get your private key, I'd have to go to your machine or wherever you keep it. Granted, if I had stuff to encrypt that was incredibly sensative and I had to access it frequently or at my convenience, I'd probably put the private key on a usb pen drive and put it in my pants...think about it...are you that desperate to go in my pants to get the private key? Maybe...but most people aren't. If it was the same data that I hardly accessed, maybe burning it to a dvd/cd or even putting it on a pen drive in a safe place like a safe deposit box at a bank would be in order.

I know you wanted a short answer, but I think that's about as short as it gets. If I missed anything or made any errors, lemme know. I can't correct myself unless someone corrects me.

Microstrain, Inc.

McGill, Lachlan wrote:

Am I right in assuming that an encrypted file/email is only as secure as the passphrase used for the private key? i.e. If i use 
the passphrase 'password' then does it become irrelevant what key size I use to encrypt the data?

If someone can please briefly explain this to me I would be much appreciative.


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