How do you compare and contrast block and stream encryption algorithms?

The most popular block encryption algorithm is Advanced Encryption Standard (AES), which has been a NIST standard since 2002.

Block encryption is well-suited for encrypting large amounts of data, and encrypt each block independently using the same key. There is several advantages: 1. Encrypting and decrypting individual blocks can be parallelised, allowing for efficient processing on modern computing systems with multiple cores or distributed systems. 2. Block encryption algorithms enable random access to the encrypted data. Since each block is encrypted independently, it is possible to access and decrypt specific blocks without decrypting the entire dataset. 3. Block encryption algorithms have been widely implemented and standardised, making them compatible across different systems and platforms.

I always focus on how the algorithms perform under different real-world scenarios. For example, in a recent project involving secure communications for a financial institution, we chose AES (a block encryption algorithm) for encrypting sensitive transactional data due to its strong security and ability to handle large volumes of data efficiently. However, for securing real-time communications, like voice calls between executives, we opted for ChaCha20 (a stream encryption algorithm) because of its speed and low latency. In practice, block ciphers like AES provide robust security but can introduce overhead due to padding and require careful implementation to avoid vulnerabilities like replay attacks.

In order to have comprehensive security, do not focus only on encryption algorithms, but think around encryption schemes. Block ciphers can work functionally equivalent to stream ciphers by using appropriate encryption modes and secure padding schemes. Similarly stream ciphers may be used as block ciphers as well. Block cipher modes like CBC, CTR ensure that every block is encrypted using a different (derived) key to prevent statistical analysis. Secure padding schemes prevent side channel attacks derived based length of plain text. Although in theory, stream ciphers can use variable length keys, in practice they usually have defined length and use key derivation functions to expand the key to size of input.

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During a project where we secured IoT devices in a constrained environment, we found that traditional block ciphers like AES required too much computational power and memory. Instead, we leveraged lightweight stream ciphers, which provided the necessary encryption without taxing the limited resources of the devices. Additionally, it's essential to consider future-proofing your encryption strategy. With advancements in quantum computing on the horizon, certain encryption algorithms may become obsolete. In my work, I’m increasingly focusing on exploring post-quantum cryptography, ensuring that the encryption methods we choose today will continue to protect data in the future.