I'm trying to get Android 2.1 phones to connect to our corporate VPN, but it would appear they only support DES rather than 3DES for encryption.
Is it safe to use DES, am I going to get hacked if I change the encryption to DES?
In standard terms (i.e. FIPS 46-3, which defined DES and 3DES from the US federal point of view [withdrawn in 2005...]), 3DES is a "kind of DES". So it is possible that by claiming "DES support" your VPN software actually implements 3DES. I am now assuming that you have checked that point and that the software in Android knows only of the original DES, not 3DES (which is kind of surprising for an OS as recent as Android 2.1, but who knows).
Cracking a DES key is feasible but not easy. This is a 255 average work factor. It has been done with specialized hardware (the EFF "Deep Crack" machine), then again with standard PC and other contributed machines (but a lot of them). An FPGA-based generic engine has been demonstrated to cost about 10000€ and to be able to crack a 56-bit key in less than a week (on average). So one has to assume that in a corporate setting (assuming that the "attackers" have huge incentive to discover trade secrets) a VPN protected by a 56-bit DES key can be cracked.
Still, using smartphones to access sensitive data over the VPN strikes me as a much weaker security point. Smartphones are used in unprotected environments (that's the point of having a mobile system...) and are possibly the most frequently stolen object these days. For a determinate attacker, using a FPGA machine is not very expensive -- but mugging the smartphone owner with a 5$ wrench even less so, and, lo! the attacker gets a free smartphone afterwards). If the VPN gives access to anything sensitive, then letting people use it from smartphones is a kind of offence.
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The reason DES has been deprecated for most uses is that it can be brute forced in a small amount of time (in 1999 a DES key was brute forced in 22 hours - so the expectation is that it would be much faster now)
It depends what type of connection you want though -
If you are just connecting to non-sensitive email, and any key used is temporary and short lived then it may still be appropriate to use DES.
If you are connecting to business sensitive or personal data, or use a long lived key then you may want to invest in other mitigating controls if you have to use DES.
What sort of context will this be used in?