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seen Sep 5 '13 at 12:20

Apr
13
comment Signature schemes for underpowered devices (8bit microcontroller)
@fgrieu: The main problem is not the cryptographic stack itself, but the other functions the device must support in order to be compliant to this communication bus I am developing. The RAM isn't the only problem. FLASH space itself is a problem when using this setup. For now, I'm putting the smaller devices aside, for I am not quite ready to reimplement several blocks in assembly. Remember that I have a communications protocol and other tasks as well running simultaneously and there is a lot of code.
Apr
13
revised Signature schemes for underpowered devices (8bit microcontroller)
added 517 characters in body
Apr
10
comment Signature schemes for underpowered devices (8bit microcontroller)
@fgrieu: I have one final comment: if I were to inject a public, long-term key when the slave is new I might as well have injected a secret key for a symmetric system. So I don't know if staying asymmetric is worth it
Apr
10
comment Signature schemes for underpowered devices (8bit microcontroller)
@fgrieu: This is getting very interesting and I thank you for your answers. I have begun to see the point in the long-term key, but then there is another issue to address: if the slave is not required to have any previous knowledge of the bus's master it is connecting to when the system is started up, then security is impossible, because an attacker could do the same thing described in your update 5b. To me, this means that the master's public key must be pre-programmed on the device, defeating the purpose of an asymmetric system.
Apr
9
comment Signature schemes for underpowered devices (8bit microcontroller)
Another question came to mind just now. Why should the tendency to exaggerate the key size so readily accepted? In an application like this, its not likely an attacker will have access to a supercomputer to factor the key. Thanks again.
Apr
9
comment Signature schemes for underpowered devices (8bit microcontroller)
Well, your answer is very complete. Thanks for the ideas. I still fail to see why the "chain of certificates" wouldn't do, as this isn't a really serious application. One of my main concerns is the signature size, which must reside in RAM and so will eat a good chunk of it. Also, the signed messages will get really big.
Apr
9
awarded  Scholar
Apr
9
accepted Signature schemes for underpowered devices (8bit microcontroller)
Apr
9
comment Signature schemes for underpowered devices (8bit microcontroller)
Secondly, I can't see the attacker blocking the line in this fashion because intercepting the key doesn't serve him any purpose, as this is only a public key. Also, the slaves are connected in parallel (RS485), so it is impossible to block some part of the bus from listening. An option is that the slave uses a timer to refuse commands if its key has expired and the master can then signal an intrusion alert.
Apr
9
comment Signature schemes for underpowered devices (8bit microcontroller)
Well, first, the attacker couldn't replay a block and get away with it; the internal structure of this block would be changing and the slave ignores a command if its block is the same as the last one.
Apr
9
awarded  Editor
Apr
9
revised Signature schemes for underpowered devices (8bit microcontroller)
added 894 characters in body
Apr
9
comment Signature schemes for underpowered devices (8bit microcontroller)
You were right. With the tools you mentioned I was able to factor a ~160bit integer in a matter of seconds, so this is out of the question. However, in my tests, a ~320bit integer took some 20 minutes of crunching and didn't get factored, so I started thinking on the lines of key renewal again. I could, say, produce a new key every 10 minutes and broadcast it to the slaves. I get the feeling that this is kind of an awkward move, but really this is an exceptional situation where I could indeed issue new keys. The constraints are really stringent, anything more than 320bits will be difficult.
Apr
9
comment Signature schemes for underpowered devices (8bit microcontroller)
This isn't the case, as I've pointed out, the slaves do not en/decrypt or sign anything; they just verify the masters' signature. The trouble is that this setup won't work, as the master isn't required to have any knowledge about a slave that is connected to the system at some point, so this isn't the way for me, but thanks
Apr
9
awarded  Student
Apr
9
asked Signature schemes for underpowered devices (8bit microcontroller)