Difference between revisions of "AES-CCM Attack"

AES-CCM Overview

AES-CCM provides both encryption and authentication using the AES block cipher. This is a widely used mode since it requires only a single cryptographic primitive. That primitive is used in two different modes: CBC and CTR mode. The difference is explained below:

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:

Counter (CTR): An incrementing counter is encrypted to produce a sequence of blocks, which are XORed with the plaintexts to produce the ciphertexts:

Background on Attack

The following uses the notation from IoT Goes Nuclear: Creating a ZigBee Chain Reaction.

Assume first the basic AES-ECB cipher is $CT = E_k(PT)$, where we are encrypting a block with secret key $k$.

AES-CCM combines AES-CTR mode and AES-CBC mode as mentioned. We could consider AES-CTR to be performing the following operation:

Performing Attack

Example Bootloader

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

• 0x01: 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.

+------+------+------+------+ .... +------+------+------+------+
| 0x01 |     Header Info (14 bytes)|    Length   |   CRC-16    |
+------+------+------+------+ .... +------+------+------+------+

Auth Tag Frame

• 0x02: 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.

       
       |<----- Encrypted block (16 bytes) ------>|
       |    AES-CTR Encryption with CTR=0        |
       |                                         |
+------+------+------+------+ .... +------+------+------+------+
| 0x02 |     Auth-Tag (encrypted MAC)            |   CRC-16    |
+------+------+------+------+ .... +------+------+------+------+

Data block frame

• 0x03: 1 byte of fixed header
• Encrypted Data: Data encrypted in AES-CTR mode, with the CTR starting at 1 and incrementing.

       |<----- Encrypted block (16 bytes) ------>|
       |   AES-CTR Encryption with CTR=1,2,3..N  |
       |                                         |
+------+------+------+------+ .... +------+------+------+------+
| 0x03 |            Data (16 Bytes)              |   CRC-16    |
+------+------+------+------+ .... +------+------+------+------+

The bootloader responds to each command with a single byte indicating if the CRC-16 was OK or not:

            +------+
CRC-OK:     | 0xA1 |
            +------+

            +------+
CRC Failed: | 0xA4 |
            +------+

Once ALL messages are received, the bootloader will respond with a signature OK or not message:

            +------+
Sig-OK:     | 0xB1 |
            +------+

            +------+
Sig Failed: | 0xB4 |
            +------+

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.