I am trying to figure out a way to implement decent crypto on a micro-controller project. I have an ARMv4 based MCU that will control my garage door and receive commands over a WiFi module.
The MCU will run a TCP/IP server, that will listen for commands from Android clients that can connect from anywhere on the Internet, which is why I need to implement crypto.
I understand how to use AES with shared secret key to properly encrypt traffic, but I am finding it difficult to deal with Replay Attacks. All solutions I see so far have serious drawbacks.
There are two fundamental problems which prevent me from using well
established methods like session tokens, timestamps or nonces:
The MCU has no reliable source of entropy, so I can't generate
quality random numbers.
The attacker can reset the MCU by cutting power to the garage,
thus erasing any stored state at will, and resetting time counter to
zero (or just wait 49 days until it loops).
With these restrictions in mind, I can see only one approach that seems
ok to me (i.e. I don't see how to break it yet). Unfortunately, this
requires non-volatile storage, which means writing to external flash,
which introduces some serious complexity due to a variety of technical details.
I would love to get some feedback on the following solution. Even better, is there a solution I am missing that does not require non-volatile storage?
Please note that the primary purpose of this project is education. I realize that I could simplify this problem by setting up a secure relay inside my firewall, and let that handle Internet traffic, instead of exposing the MCU directly. But what would be the fun in that? ;)
= Proposed Solution =
A pair of shared AES keys will be used. One key to turn a Nonce into an IV for the CBC stage, and another for encrypting the messages themselves:
Shared message Key
Shared IV_Key
Here's a picture of what I am doing:
https://www.youtube.com/watch?v=JNsUrOVQKpE#t=10m11s
1) Android takes current time in milliseconds (Ti) (64-bit long) and
uses it as a nonce input into the CBC stage to encrypt the command:
a) IV(i) = AES_ECB(IV_Key, Ti)
b) Ci = AES_CBC(Key, IV(i), COMMAND)
2) Android utilizes /dev/random to generate the IV_Response that the
MCU will use to answer current request.
3) Android sends [<Ti, IV_Response, Ci>, <== HMAC(K)]
4) MCU receives and verifies integrity using HMAC, so attacker can't
modify plain text Ti.
5) MCU checks that Ti > T(i-1) stored in flash. This ensures that
recorded messages can't be replayed.
6) MCU calculates IV(i) = AES_ECB(IV_Key, Ti) and decrypts Ci.
7) MCU responds using AES_CBC(Key, IV_Response, RESPONSE)
8) MCU stores Ti in external flash memory.
Does this work? Is there a simpler approach?
EDIT: It was already shown in comments that this approach is vulnerable to a Delayed Playback Attack. If the attacker records and blocks messages from reaching the MCU, then the messages can be played back at any later time and still be considered valid, so this algorithm is no good.
As suggested by #owlstead, a challenge/response system is likely required. Unless I can find a way around that, I think I need to do the following:
Port or implement a decent PRGA. (Any recommendations?)
Pre-compute a lot of random seed values for the PRGA. A new seed will be used for every MCU restart. Assuming 128-bit seeds, 16K of storage buys be a 1000 unique seeds, so the values won't loop until the MCU successfully uses at least one PRGA output value and restarts a 1000 times. That doesn't seem too bad.
Use the output of PRGA to generate the challenges.
Does that sound about right?
Having an IV_KEY is unnecessary. IVs (and similar constructs, such as salts) do not need to be encrypted, and if you look at image you linked to in the youtube video you'll see their description of the payload includes the IV in plaintext. They are used so that the same plaintext does not encode to the same ciphertext under the same key every time, which presents information to an attacker. The protection against the IV being altered is the HMAC on the message, not the encryption. As such, you can remove that requirement. EDIT: This paragraph is incorrect, see discussion below. As noted though, your approach described above will work fine.
Your system does have a flaw though, namely the IV_Response. I assume, from that you include it in your system, that it serves some purpose. However, because it is not in any way encoded, it allows an attacker to respond affirmatively to a device's request without the MCU receiving it. Let's say that your device's were instructing an MCU that was running a small robot to throw a ball. Your commands might look like.
1) Move to loc (x,y).
2) Lower anchor to secure bot table.
3) Throw ball
Our attacker can allow messages 1 and 3 to pass as expected, and block 2 from getting to the MCU while still responding affirmatively, causing our bot to be damaged when it tosses the ball without being anchored. This does have an imperfect solution. Append the response (which should fit into a single block) to the command so that it is encrypted as well, and have the MCU respond with AES_ECB(Key, Response), which the device will verify. As such, the attacker will not be able to forge (feasibly) a valid response. Note that as you consider /dev/random untrustworthy this could provide an attacker with plaintext-ciphertext pairs, which can be used for linear cryptanalysis of the key provided an attacker has a large set of pairs to work with. As such, you'll need to change the key with some regularity.
Other than that, your approach looks good. Just remember it is crucial that you use the stored Ti to protect against the replay attack, and not the MCU's clock. Otherwise you run into synchronization problems.
Related
IF, someone was able to retrieve the intentionally lost information from the SHA function by finding a vulnerability in the loose compression of data & able to get the input of every SHA256 output & verify a block that does not belong in the main BTC network, can't it be used to create a fake dust transaction and manually verify that transaction? - Ultimately creating an unlimited BTC in seconds?
Be aware that for a given SHA256 output there may be several inputs that result to the given SHA256 output.
AFAIK Bitcoin strongly relies on not being able to reverse SHA256 function. So without talking about faking blocks or making dust transactions, if you are able to reverse the SHA256, then you can instantly mine Bitcoin blocks making you reward all the remaining bitcoins not yet mined. However if such an extreme miner appeared, he would be spotted by the whole network and so the value of bitcoins would be lost as the network isn't working as it was designed.
I am trying to create a special military RADIO transmitter.
Basically, the flow is:
A solider will receive a message to transmit (about 10 times a day). Each message is of length 1024 bits exactly.
He will insert this message into the radio and validate it is inserted correctly.
The RADIO will repetitively transmit this message.
This is very important that the transmitter will not be hacked, because its very important in times of emergencies.
So, the assistance I ask from you is, how to preform stage 2 without risking getting infected.
If I will transfer the data using a DOK, it may be hacked.
If I will make the user type in 1024 bits, it will be safe but exhausting.
Any Ideas? (unlimited budget)
(It’s important to say that the data being transmitted is not a secret)
Thanks for any help you can supply!
Danny
Edit:
Basically, I want to create the most secure way to transfer a fixed number of bits (in this case 1024), from one (may be infected computer) to the other (air gaped computer).
without having any risk of a virus being transferred as well.
I don't mind if an hacker will change the data that is transferred from the infected computer, I just want that the length of the data will be exactly 1024, and avoiding virus to be inserted to the other computer.
Punch card (https://en.wikipedia.org/wiki/Punched_card) sounds like a good option, but an old one.
Any alternatives?
The transmitter is in the field, and is one dead soldier away from falling into enemy hands at any time. The enemy will take it apart, dissect it, learn how it works and use the protocol to send fraudulent messages that may contain exploit code to you, with or without the original equipment. You simply cannot prevent a trasmitter or otherwise mocked up "enemy" version of a transmitter from potentially transmitting bad stuff, because those are outside of your control. This is the old security adage "Never trust the client" taken to its most extreme level. Even punch cards could be tampered with.
Focus on what you can control: The receiving (or host) computer (which, contrary to your description, is not airgapped as it is receiving outside communication in your model) will need to validate the messages that come in from the client source; this validation will need to check for malicious messages and handle them safely (don't do anything functional with them, just log it, alert somebody and move on with life).
Your protocol should only be treating inbound messages like text or identifiers for message types. Under no circumstances should you be trying to interpret them as machine language instructions, and any SQL queries or strings that this message is appended to should be properly sanitized. This will prevent the host from executing any nasties that do come in.
I'm developing own protocol for secure message exchanging.
Each message contains the following fields: HMAC, time, salt, and message itself. HMAC is computed over all other fields using known secret key.
Protocol should protect against reply attack. On large time interval "time" record protects against replay attack (both sides should have synchronized clocks). But for protection against replay attack on short time intervals (clocks are not too accurate) I'm planning replace "salt" field with counter increasing every time, when new message is send. Receiving party will throw away messages with counter value less or equal to the previous message counter.
What I'm doing wrong?
Initial counter value can be different (I can use party identifier as initial value), but it will be known to the attacker (party identifier transmitted in unencrypted form).
(https://security.stackexchange.com/questions/8246/what-is-a-good-enough-salt-for-a-saltedhash)
But attacker can precompute rainbow tables for counter+1, counter+2, counter+3... if I will not use really random salt?
I'm not certain of your design and requirements, so some of this may be off base; hopefully some of it is also useful.
First, I'm having a little trouble understanding the attack; I'm probably just missing something. Alice sends a message to Bob that includes a counter, a payload, and an HMAC of (counter||payload). Eve intercepts and replays the message. Bob has seen that one, so he throws it away. Eve tries to compute a new message with counter+1, but she is unable to compute the HMAC for this message (since the counter is different), so Bob throws it away. As long as there is a secret available, Eve should never be able to forge a message, and replaying a message does nothing.
So what is the "known secret key?" Is this key known to the attacker? (And if it is, then he can trivially forge messages, so the HMAC isn't helpful.) Since you note that you have DH, are you using that to negotiate a key?
Assuming I'm missing the attack, thinking through the rest of your question: If you have a shared secret, why not use that to encrypt the message, or at least the time+counter? By encrypting the time and counter together, a rainbow table should be impractical.
If there is some shared secret, but you don't have the processor available to encrypt, you could still do something like MD5(secret+counter) to prevent an attacker guessing ahead (you must already have MD5 available for your HMAC-MD5).
I have attacked this problem before with no shared secret and no DH. In that case, the embedded device needed a per-device public/private keypair (ideally installed during manufacturing, but it can be computed during first power-on and stored in nonvolatile memory; randomness is hard, one option is to let the server provide a random number service; if you have any piece of unique non-public information on the chip, like a serial number, that can be used to seed your key, too. Worst case, you can use your MAC plus the time plus as much entropy as you can scrounge from the network.)
With a public/private key in place, rather than using HMAC, the device just signs its messages, sending its public key to the server in its first message. The public key becomes the identifier of the device. The nice thing about this approach is that there is no negotiation phase. The device can just start talking, and if the server has never heard of this public key, it creates a new record.
There's a small denial-of-service problem here, because attackers could fill your database with junk. The best solution to that is to generate the keys during manufacturing, and immediately insert the public keys into your database. That's impractical for some contract manufacturers. So you can resort to including a shared secret that the device can use to authenticate itself to the server the first time. That's weak, but probably sufficient for the vast majority of cases.
I have created a small wireless network using a few PIC microcontrollers and nRF24L01 wireless RF modules. One of the PICs is PIC18F46K22 and it is used as the main controller which sends commands to all other PICs. All other (slave) microcontrollers are PIC16F1454, there are 5 of them so far. These slave controllers are attached to various devices (mostly lights). The main microcontroller is used to transmit commands to those devices, such as turn lights on or off. These devices also report the status of the attached devices back to the main controller witch then displays it on an LCD screen. This whole setup is working perfectly fine.
The problem is that anybody who has these cheap nRF24L01 modules could simply listen to the commands which are being sent by the main controller and then repeat them to control the devices.
Encrypting the commands wouldn’t be helpful as these are simple instructions and if encrypted they will always look the same, and one does not need to decrypt it to be able to retransmit the message.
So how would I implement a level of security in this system?
What you're trying to do is to prevent replay attacks. The general solution to this involves two things:
Include a timestamp and/or a running message number in all your messages. Reject messages that are too old or that arrive out of order.
Include a cryptographic message authentication code in each message. Reject any messages that don't have the correct MAC.
The MAC should be at least 64 bits long to prevent brute force forgery attempts. Yes, I know, that's a lot of bits for small messages, but try to resist the temptation to skimp on it. 48 bits might be tolerable, but 32 bits is definitely getting into risky territory, at least unless you implement some kind of rate limiting on incoming messages.
If you're also encrypting your messages, you may be able to save a few bytes by using an authenticated encryption mode such as SIV that combines the MAC with the initialization vector for the encryption. SIV is a pretty nice choice for encrypting small messages anyway, since it's designed to be quite "foolproof". If you don't need encryption, CMAC is a good choice for a MAC algorithm, and is also the MAC used internally by SIV.
Most MACs, including CMAC, are based on block ciphers such as AES, so you'll need to find an implementation of such a cipher for your microcontroller. A quick Google search turned up this question on electronics.SE about AES implementations for microcontrollers, as well as this blog post titled "Fast AES Implementation on PIC18F4550". There are also small block ciphers specifically designed for microcontrollers, but such ciphers tend to be less thoroughly analyzed than AES, and may harbor security weaknesses; if you can use AES, I would. Note that many MAC algorithms (as well as SIV mode) only use the block cipher in one direction; the decryption half of the block cipher is never used, and so need not be implemented.
The timestamp or message number should be long enough to keep it from wrapping around. However, there's a trick that can be used to avoid transmitting the entire number with each message: basically, you only send the lowest one or two bytes of the number, but you also include the higher bytes of the number in the MAC calculation (as associated data, if using SIV). When you receive a message, you reconstruct the higher bytes based on the transmitted value and the current time / last accepted message number and then verify the MAC to check that your reconstruction is correct and the message isn't stale.
If you do this, it's a good idea to have the devices regularly send synchronization messages that contain the full timestamp / message number. This allows them to recover e.g. from prolonged periods of message loss causing the truncated counter to wrap around. For schemes based on sequential message numbering, a typical synchronization message would include both the highest message number sent by the device so far as well as the lowest number they'll accept in return.
To guard against unexpected power loss, the message numbers should be regularly written to permanent storage, such as flash memory. Since you probably don't want to do this after every message, a common solution is to only save the number every, say, 1000 messages, and to add a safety margin of 1000 to the saved value (for the outgoing messages). You should also design your data storage patterns to avoid directly overwriting old data, both to minimize wear on the memory and to avoid data corruption if power is lost during a write. The details of this, however, are a bit outside the scope of this answer.
Ps. Of course, the MAC calculation should also always include the identities of the sender and the intended recipient, so that an attacker can't trick the devices by e.g. echoing a message back to its sender.
(I'm doing this on my network, just for science). I was using airodump-ng to capture handshake. After that, I was able to open file with captured information in WireShark and find part with 4 handshake messages of EAPOL protocol. I know about millions of years needed for brute-force and I know that I can use aircrack-ng for dictionary attack.
I would like to extract just password from those 4 messages. I assume it is transfered as some sort of salted hash value. What I don't know is, in which message password resides (wireless password, for connection) and how exactly is sent? For example SHA1 of "password"+"ssid"... I would like to be able to compute exact same hash in my program (of course, that would be possible only for my network because I know my password). I'm gonna need that also for some demonstration on university.
Thanks!
The 802.11i "4 way handshake" that you have captured is where both parties agree on shared Group (read: broadcast) and Pairwise (read: unicast) transient keys. I.e. the keys generated here only exist for the duration of the 802.11 Association, or until the next rekey is issued from the AP.
Before you can even begin to decrypt the 4 way handshake messages you need the pairwise master key (PMK), which is what gets derived from the user-entered passphrase using a key derivation function (PBKDF2), or is the result of a WPS exchange which is based on Diffie-Hellman.
The point here is the ASCII passphrase you are seeking to extract is not exchanged in any of the 4 messages, as it has already been shared amongst all parties involved in the transaction (client and AP in this case) and used to generate a 256 bit PMK. And unless you have this PMK, the contents of the 4 way handshake messages are as good as random data.
The best that you can do, if you already know the PMK, is extract the GTK and PTK from M2 and M3 of the 4 way handshake, and from those pull the temporal key which can be XORed with the payload in subsequent frames to get the plaintext data - which wireshark will also do for you if you enter the PMK or passphrase into the IEE802.11 settings and enable decryption.