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{{InfoboxWarningbox|WARNING: This page under construction!}}
The following is an overview of the AES-CMM attack done by Eyal Ronenet al., detailed in his their draft/limited release paper [http://iotworm.eyalro.net/ IoT Goes Nuclear: Creating a ZigBee Chain Reaction(research paper website)], [http://eprint.iacr.org/2016/1047 IACR E-print submission]. If using this attack please '''do not cite this page''', instead cite the research paper only. The paper is currently a draft so there is no proceedings information etc as it has not yet been presented anywhere, but you can cite the E-Print version.
This page is presented as an example of using Python/ChipWhisperer to perform attacks against the AES-CCM cipher, without needing to do a more complex attack against AES-CTR mode.
[[File:cbc_mac_source.png]]
The "Calculated Tag" and "Expected Tag" are compared together, and only if they match is the decrypted data used. A change of any of the data blocks OR the header would change the calculated tag, resulting in an error.
Some nice features of AES-CCM:
* Can decrypt any data block, or decrypt blocks out of orderdue to AES-CTR usage.
* Authentication Tag provides authentication that data has not been modified in transit.
* Auth tag can include non-encrypted information, such as a header with address or length information.
* Auth tag can be shortened (i.e., not full 16-byte length) for use with protocols with very sensitive length limitations.
'''Cipher Block Chaining (CBC):''' The plaintext is XORed with the previous ciphertext before being encrypted. There is no ciphertext before the first plaintext, so a randomly chosen initialization vector (IV) is used instead:
* A nonce format is required for AES-CTR. This nonce can be based on information in the packet, such as source address, or be random.
* An IV is required for the AES-CCM block. This I.V. can be sent (possibly encrypted) to the AES-CCM block, or be part of secret information stored in the bootloader.
== Example Bootloader Details ==
To perform this attack, we will define a simple bootloader. We will be using this example since we cannot use AES-CCM "standalone" without having a little bit of structure (i.e., how we do the AES-ECB demos). This `bootloader' is really just a demo of something that downloads a block of encrypted data, since it doesn't perform any actual bootloading.
The example bootloader has a simplified AES-CCM implementation (NB: do NOT use this as a reference for a good implementation!). It accomplishes the basic goals only of having:
* Header data that is authenticated but not encrypted
* An encrypted MAC tag
* A bunch of encrypted firmware blocks
Each block sent to the bootloader is 19 bytes long. The first byte indicates the type - header, auth tag, or data. If a new 'header' message is received it will abort any ongoing processing of existing data and restart the bootloader process.
All messages share this feature:
* CRC-16: A 16-bit checksum using the CRC-CCITT polynomial (0x1021). The LSB of the CRC is sent first, followed by the MSB. The bootloader will reply over the serial port, describing whether or not this CRC check was valid.
'''Header Frame'''
* <code>0x01</code>: 1 byte of fixed header
* Header Info: 14 bytes of "header" data which could be version or other such stuff.
* Length: Number of encrypted data frames (NOT including the auth-tag frame) that will follow.
Note the 16 bytes of the header info + length are fed into the AES-CBC algorithm as part of the auth-tag generation. That is this data is authenticated but not encrypted.
<pre>
+------+------+------+------+ .... +------+------+------+------+
| 0x01 | Header Info (14 bytes)| Length | CRC-16 |
+------+------+------+------+ .... +------+------+------+------+
</pre>
'''Auth Tag Frame'''
* <code>0x02</code>: 1 byte of fixed header
* Auth-Tag: The expected output of the AES-CBC algorithm after processing the authenticated only data + decrypted data frames. This is then encrypted in AES-CTR mode with the CTR set to 0.
<pre>
|<----- Encrypted block (16 bytes) ------>|
| AES-CTR Encryption with CTR=0 |
| |
+------+------+------+------+ .... +------+------+------+------+
| 0x02 | Auth-Tag (encrypted MAC) | CRC-16 |
+------+------+------+------+ .... +------+------+------+------+
</pre>
'''Data block frame'''
* <code>0x03</code>: 1 byte of fixed header
* Encrypted Data: Data encrypted in AES-CTR mode, with the CTR starting at 1 and incrementing.
<pre>
|<----- Encrypted block (16 bytes) ------>|
| AES-CTR Encryption with CTR=1,2,3..N |
| |
+------+------+------+------+ .... +------+------+------+------+
| 0x03 | Data (16 Bytes) | CRC-16 |
+------+------+------+------+ .... +------+------+------+------+
</pre>
The bootloader responds to each command with a single byte indicating if the CRC-16 was OK or not:
<pre>
+------+
CRC-OK: | 0xA1 |
+------+
+------+
CRC Failed: | 0xA4 |
+------+
</pre>
Once ALL messages are received, the bootloader will respond with a signature OK or not message:
<pre>
+------+
Sig-OK: | 0xB1 |
+------+
+------+
Sig Failed: | 0xB4 |
+------+
</pre>
Note details of the AES-CTR nonce, AES-CBC I.V., and key are stored in the firmware itself. In this example they are not downloaded as part of the encrypted firmware file.
== Background on Attack ==
The following will use the notation from [http://iotworm.eyalro.net/ IoT Goes Nuclear: Creating a ZigBee Chain Reaction]. This notation will be adopted throughout this wiki page.
From the figure of AES-CCM, you can see that we don't directly control the input to any of the AES blocks. It could be possible to perform an AES-CTR attack (such as described by J. Jaffe in [https://www.iacr.org/archive/ches2007/47270001/47270001.pdf A First-Order DPA Attack Against AES in Counter Mode with Unknown Initial Counter]), but this will have issues if the bootloader limits the number of blocks we can send.
But, as it turns out we can still break AES-CBC with a little additional computational effort (but not requiring any additional traces).
=== Breaking AES-CBC Encryption with unknown I.V. ===
If you've performed a standard CPA attack, you'll realize the problem with attacking AES-CBC is we don't directly control the input, which we call <math>PT</math>. Instead it's XORd with some unknown bytes (the AES-CBC ciphertext output).
But if we are always attacking the same block (that is, we reset the AES state to initial by say resetting the device, and rerun the algorithm up to the first block), the unknown bytes are constant. As it turns out this is a pretty easy problem to solve. The first step is to perform a standard CPA attack. The only issue is we won't recover the actual encryption key used <math>k</math>, instead we recover <math>k \oplus CBC_{m-1} \oplus PT</math>, since we basically roll all the constant inputs into what we call a `modified key'. Note <math>CBC_{m-1}</math> is the output of the previous-block AES-CBC ciphertext.
In what might seem like magic, we can use this modified key to directly determine the second-round key (the true key). This was originally presented by J. Jaffe in [https://www.iacr.org/archive/ches2007/47270001/47270001.pdf A First-Order DPA Attack Against AES in Counter Mode with Unknown Initial Counter]. The reason this works is if you remember we recovered <math>k' = k \oplus CBC_{m-1}</math>. In the AES algorithm the first thing we do is the AddRoundKey, which is:
=== Building Example ===
There is an example of a simple bootloader which uses AES-CCM in the firmware directory. A hex-file is also present for the Atmel XMEGA device on the ChipWhisperer-Lite board.
=== Collecting Traces ===
Collecting traces requires the following steps:
# Program into the target the aes-ccm bootloader. This bootloader can be found in the git repo, which also includes a .hex file.# Set the target type to the special AES-CCM bootloader driver. This target module is detailed in the appendix on this page, you can copy that code into a new file in the `target' directory.# Run a capture with ~500 traces. Use 1x CLKGEN for the ADC speed and the full point-range to be able to capture both software AES rounds. See capture script example for details.
=== Step #1: AES-CBC MAC Block #1 ===
== Bootloader Interface Code ==
<syntaxhighlightlang="python">
#!/usr/bin/python
# -*- coding: utf-8 -*-
raise IOError("Failed to communicate, no response")
</syntaxhighlight>
[[Category:Examples]]
