What symmetric cypher to use for encrypting messages?

Question

I haven't a clue about encryption at all. But I need it. How?

Say you have a system of nodes communicating with each other on a network via asynchronous messages. The nodes do not maintain session information about other nodes (this is a design restriction).

Say you want to make sure only your nodes can read the messages being sent. I believe encryption is the sollution to that.

Since the nodes are not maintaining a session and communication must work in a stateless, connectionless fashion, I am guessing that asymmetric encryption is ruled out.

So here is what I would like to do:

• messages are sent as UDP datagrams
• each message contains a timestamp to make messages differ (counter replay attacks)
• each message is encrypted with a shared secret symmetric key and sent over the network
• other end can decrypt with shared secret symmetric key

Keys can obviously be compromised by compromising any single node. At the same time, in this scenario, access to any single compromised node reveals all interesting information anyway, so the key is not the weakest link.

What cypher should I use for this encryption? What key length?

I would prefer to use something supported by ezPyCrypto.

Assuming, as most point out, I go with AES. What modes should I be using?

I couldn't figure out how to do it with ezPyCrypto, PyCrypto seems to be hung on a moderator swap and googles keyczar does not explain how to set this up - I fear if I don't just get it, then I run a risk of introducing insecurity. So barebones would be better. This guy claims to have a nice module for AES in python, but he also asserts that this is his first python project - Allthough he is probably loads smarter than I, maybe he got tripped up?

EDIT: I moved the search for the python implementation to another question to stop clobber...

Answer 1

I haven't a clue about encryption at all. But I need it. How?

DANGER! If you don't know much about cryptography, don't try to implement it yourself. Cryptography is hard to get right. There are many, many different ways to break the security of a cryptographic system beyond actually cracking the key (which is usually very hard).

If you just slap a cipher on your streaming data, without careful key management and other understanding of the subtleties of cryptographic systems, you will likely open yourself up to all kinds of vulnerabilities. For example, the scheme you describe will be vulnerable to man-in-the-middle attacks without some specific plan for key distribution among the nodes, and may be vulnerable to chosen-plaintext and/or known-plaintext attacks depending on how your distributed system communicates with the outside world, and the exact choice of cipher and mode of operation.

So... you will have to read up on crypto in general before you can use it securely.