How do you handle error propagation and ciphertext manipulation in CBC mode encryption?

There are many different symmetric encryption schemes called "modes of operation". All modes provide secrecy but only some provide message integrity (aka authenticity). CBC (Cipher-Block Chaining) while common, is an older mode that does not provide any integrity and so is vulnerable to propagation errors and malicious tampering. The best solution is to not use CBC at all and instead use a mode of operation that provides integrity as well (such as GCM mode). However, if CBC must be used (say to support a legacy system), error correcting codes like CRC will not provide sufficient integrity protection. They can help identify propagation errors but won't stop a malicious attacker. The only secure option is to use a MAC (HMAC, PMAC etc).

In Cipher Block Chaining (CBC) mode, error propagation occurs when an error in one ciphertext block affects subsequent plaintext blocks during decryption. In CBC mode, each plaintext block is XORed with the previous ciphertext block before being encrypted, and each ciphertext block depends on the previous one. This means that if a single bit error occurs in a ciphertext block, it will cause the corresponding plaintext block to be decrypted incorrectly. Additionally, the error will also affect the decryption of the next plaintext block, though in a more limited way. The impact of this error propagation is significant in applications where data integrity is crucial.

This article highlights the risks of relying on AI for answers because the recommendation here is completely wrong! Ciphertext manipulation in unauthenticated encryption modes (like CBC, CTR etc) is very easy. However, the only mitigation is to use a secure integrity check such as including a Message Authentication Code (MAC) with the message or to use an encryption mode that provides integrity/authenticity like GCM, GCM-SIV or CCM. NIST publishes guidelines on appropriate encryption modes of operation and their constraints in articles SP 800-38C and SP 800-38D. Correct message padding is important to security but it doesn't prevent ciphertext manipulation and the specific padding mechanism varies between modes.

Ciphertext manipulation refers to intentional alterations of the ciphertext to produce predictable changes in the decrypted plaintext. In CBC mode, an attacker who can modify the ciphertext can exploit the XOR operation between plaintext and ciphertext blocks to manipulate the decrypted plaintext. For instance, flipping a bit in a ciphertext block will flip the corresponding bit in the next plaintext block after decryption. This vulnerability makes CBC mode susceptible to certain types of attacks, such as bit-flipping attacks. To protect against ciphertext manipulation, it is crucial to use additional cryptographic techniques, such as message authentication codes (MACs) or authenticated encryption modes.

The GCM mode of operation (Galois Counter Mode) is almost always the best mode to use with AES. It provides message authenticity as well as secrecy, has no ciphertext expansion except for the storage of a 12-byte IV (initialization vector) and a tag (typically 8-bytes for an authenticity check), and benefits from the hardware acceleration built into almost all modern CPUs. Older modes like CBC should only be used if legacy system support is required and even then should be combined with a MAC (like HMAC) to provide integrity checks. Note that even GCM has gotchas. Notably, if an initiatization vector is repeated more than once for the same key the encryption is completely broken! An extension mode, GCM-SIV addresses this but is slower.

While CBC mode has been widely used, it has several limitations, particularly in terms of error propagation and susceptibility to ciphertext manipulation. Alternatives to CBC mode, such as Counter (CTR) mode and Galois/Counter Mode (GCM), offer different approaches to encryption that address some of these weaknesses. CTR mode, for example, converts a block cipher into a stream cipher by encrypting a counter value and XORing the result with the plaintext. This approach eliminates error propagation, as each block is encrypted independently, and a single bit error in the ciphertext affects only the corresponding bit in the plaintext.

CBC mode is not parallelizable which makes it slow in practice. Like I said, it really isn't a good choice in most cases! Modes that use a counter like CTR, GCM or CCM are generally parallelizable but of these, only GCM and CCM provide integrity. At CipherStash we exclusively use GCM or GCM-SIV for all of encryption because these modes are fast: parallelizable and CPU accelerated, and secure: provide both secrecy and authenticity. I would recommend against developers implementing their own encryption and instead find a reputable library that implements GCM or GCM-SIV. The former is faster but implementors *must* guarantee that every IV is unique.

Choosing the best encryption mode depends on the specific requirements and constraints of your application. If error resilience and data integrity are critical, modes like GCM or Encrypt-then-MAC provide robust protection against manipulation and error propagation. These modes offer authenticated encryption, ensuring that any alteration of the ciphertext is detected, and the integrity of the decrypted plaintext is maintained. Additionally, GCM is highly efficient, making it suitable for high-performance applications where both security and speed are essential.

This advice is generally correct, with some caveats. TLS and SSH both use symmetric encryption so it wouldn't necessarily provide any additional protection. However, I'll point out that CBC mode was removed from the suite of supported encryption modes in TLS 1.3 because of its shortcomings! So in that sense, an extra layer of encryption with TLS (so long as its v1.3) isn't a bad idea! I also strongly advise against implementing your own encryption and instead use a reputable library like OpenSSL, BoringSSL, Rust Crypto etc.

Implementing CBC mode encryption involves several key steps to ensure secure and effective encryption. First, you need to generate a secure random initialization vector (IV) for each encryption session. The IV should be the same size as the block size of the cipher and must be unique and unpredictable to ensure the security of the encryption process. The IV is typically transmitted along with the ciphertext, either prepended to the ciphertext or in a separate field, so it can be used for decryption. Next, the plaintext is divided into blocks of the cipher's block size. Each plaintext block is XORed with the previous ciphertext block (or the IV for the first block) before being encrypted with the block cipher..

When using CBC mode encryption, it is essential to consider the overall security architecture of the system. This includes using strong cryptographic primitives, secure key management practices, and ensuring that the IV is properly generated and handled. Secure key management involves generating, storing, and distributing cryptographic keys in a way that minimizes the risk of key compromise. Hardware security modules (HSMs) and secure key storage solutions can help protect cryptographic keys from unauthorized access. Another important consideration is the implementation environment.