Understanding MAC-then-Encrypt: rand-CTR vs. rand-OFB

Welcome to our exploration of MAC-then-Encrypt, a fundamental method for securing digital communications.The process begins with a plaintext message. First, we generate a Message Authentication Code, or MAC, from this plaintext.The MAC provides integrity protection. It creates a unique tag that can detect any tampering with the message and verify its authenticity.After generating the MAC, both the original message and the MAC tag are encrypted together.The encryption step provides confidentiality, ensuring that both the message and its MAC tag remain hidden from unauthorized parties.The order of operations is crucial in MAC-then-Encrypt. By generating the MAC first, we ensure that the authentication tag itself is protected by encryption.This combination provides strong security properties. The MAC cannot be modified without detection, and encryption prevents attackers from forging valid MACs.Now that we understand the basics of MAC-then-Encrypt, let's explore how it works with specific encryption modes.In Counter Mode encryption, we start with a random counter value that adds unpredictability to our encryption process.The counter value is encrypted using AES to generate the first keystream block.For subsequent blocks, we increment the counter value before encryption.Each encrypted counter value is XORed with a plaintext block to produce the corresponding ciphertext block.One major advantage of Counter Mode is that it supports parallel processing. Each block can be encrypted independently.Unlike some other modes, Counter Mode doesn't require padding of the plaintext, making it more efficient for various message lengths.The encrypted counter values form a keystream that can be generated independently of the plaintext.This random counter implementation provides both security and efficiency in our MAC-then-Encrypt scheme.Let's compare Output Feedback Mode with Counter Mode, examining their key differences and similarities.In OFB mode, each block's output feeds into the next encryption, creating a sequential process. CTR mode, however, operates independently for each block.Let's examine the key differences between these modes in terms of their operational characteristics.A major advantage of Counter Mode is its ability to process blocks in parallel, while OFB must operate sequentially. This makes CTR generally faster in practice.Both modes handle errors similarly - a bit error in the ciphertext only affects the corresponding bit in the plaintext, without propagating to other blocks.Both modes require random initialization values, but use them differently - OFB needs a random IV, while CTR uses a random counter start value.Let's examine the security properties shared by both modes.When choosing between these modes, consider your specific requirements.This concludes our exploration of encryption modes and MAC-then-Encrypt compositions.