CCM (CTR Mode With CBC-MAC)

CCM (Counter with CBC-MAC) is a mode of operation for symmetric key block ciphers that provides both confidentiality and authenticity. It was designed to be used in low-power devices such as wireless sensor networks, where energy consumption and processing power are limited. CCM is a widely used mode of operation, standardized by the National Institute of Standards and Technology (NIST) in the Special Publication 800-38C.

In CCM mode, the plaintext is divided into blocks of fixed size and encrypted using a symmetric key block cipher. The ciphertext is then divided into two parts: the first part is used for authentication, and the second part is used for confidentiality.

The authentication part of the ciphertext is generated using CBC-MAC (Cipher Block Chaining Message Authentication Code) mode, which is a message authentication code based on the CBC mode of operation. CBC-MAC takes the plaintext as input, pads it to a multiple of the block size, and applies the block cipher to each block in turn, using the previous block's ciphertext as the initialization vector for the next block. The resulting ciphertext is the authentication tag, which is appended to the end of the ciphertext.

The confidentiality part of the ciphertext is generated using CTR (Counter) mode, which is a stream cipher mode of operation. CTR mode generates a stream of key bits by encrypting a counter value concatenated with a nonce using the block cipher. The counter is incremented for each block of plaintext to be encrypted. The resulting stream of key bits is XORed with the plaintext to produce the ciphertext.

CCM mode combines these two modes of operation in a unique way. It uses the counter from the CTR mode to generate the nonce for the CBC-MAC mode. This allows the nonce to be generated dynamically without requiring an additional field in the message header. The nonce is generated by concatenating the following values:

1. Flags field: A bit field that specifies the lengths of the associated data (A), nonce (N), and plaintext (P).
2. Nonce field: A unique value that is used to generate the CTR mode keystream and the CBC-MAC nonce.
3. Counter field: A value that is incremented for each block of plaintext.

The length of the nonce field depends on the length of the block cipher used. For example, if AES is used as the block cipher, the nonce field length can be 7 bytes, 8 bytes, or 13 bytes. The length of the counter field is determined by the length of the plaintext.

CCM mode provides both confidentiality and authenticity with a single pass over the data. The authentication tag is generated using the CBC-MAC mode and appended to the end of the ciphertext. The receiver can then verify the authenticity of the message by recomputing the CBC-MAC and comparing it to the received authentication tag. If the tags match, the message is authentic. If the tags do not match, the message has been tampered with.

CCM mode also provides a mechanism for including associated data (A) in the authentication tag. Associated data is data that is not encrypted but is included in the authentication tag to ensure its integrity. This can include message headers, timestamps, or other metadata that is not secret but needs to be protected from modification.

The use of CTR mode in CCM mode provides several advantages. CTR mode allows for parallel encryption and decryption of the ciphertext, making it more efficient than CBC mode. It also provides better resistance to message replay attacks than CBC mode, since the counter value is unique for each block of plaintext. Additionally, CTR mode allows for random access to the ciphertext, which is useful in some applications.

However, there are also some disadvantages to CCM mode. One potential weakness is that the same key is used for both encryption and authentication. This means that if an attacker can obtain the key, they can both decrypt the ciphertext and generate authentic-looking messages. Therefore, it is important to ensure that the key is kept secure and is not compromised.

Another potential weakness is that the same nonce should never be used with the same key. If a nonce is reused, an attacker could potentially recover the key or forge authentic-looking messages. Therefore, it is important to ensure that the nonce is unique for each message encrypted with the same key.

In summary, CCM (Counter with CBC-MAC) is a mode of operation for symmetric key block ciphers that provides both confidentiality and authenticity. It combines the CTR mode of operation for confidentiality and the CBC-MAC mode of operation for authentication. CCM is widely used in low-power devices such as wireless sensor networks and is standardized by NIST. However, care must be taken to ensure the key is kept secure and the nonce is unique for each message encrypted with the same key.