Hash Functions & Stream Ciphers: CTR/OFB Compatibility

TL;DR

No, hash functions are generally not suitable for direct use in Counter (CTR) or Output Feedback (OFB) modes. They lack the necessary properties – specifically, a predictable and controllable pseudorandom output sequence – required by these stream cipher modes.

Why Hash Functions Don’t Work with CTR/OFB

CTR and OFB rely on repeatedly encrypting an incrementing counter (CTR) or the previous ciphertext block (OFB) using a symmetric key. This generates a keystream that is then XORed with the plaintext to produce the ciphertext. Hash functions don’t behave like this.

Step-by-Step Explanation

1. Hash Function Properties: Hash functions are designed to be one-way (difficult to reverse) and collision-resistant (difficult to find different inputs that produce the same output). They take a variable-length input and produce a fixed-size hash value. Crucially, they don’t have a ‘key’ in the same way encryption algorithms do, nor do they provide a predictable sequence of outputs based on an incrementing counter or previous state.
   • Example: SHA-256 takes any data and produces a 256-bit hash. You can’t easily ask it for the *next* hash in a series without providing new input.
2. CTR Mode Requirements: CTR mode needs to encrypt incrementing counter values with the key. This creates a pseudorandom keystream.

   # Example (Conceptual) - NOT actual code
   keystream = encrypt(key, counter_0)
   keystream += encrypt(key, counter_1)
   keystream += encrypt(key, counter_2)
   ...

3. OFB Mode Requirements: OFB mode feeds the previous ciphertext block back into the encryption function to generate a keystream.

   # Example (Conceptual) - NOT actual code
   keystream = encrypt(key, IV)
   keystream += encrypt(key, keystream_0)
   keystream += encrypt(key, keystream_1)
   ...

4. Hash Function Limitations: Hash functions can’t do either of these things reliably.
   • No Key-Based Sequence: You can’t generate a predictable sequence from a hash function using just a key. Every input is treated independently.
   • Fixed Output Size: Hash functions produce fixed-size outputs, which might not be suitable for the block size required by CTR or OFB.

What Happens if You Try?

If you attempt to use a hash function in CTR or OFB mode, you’ll likely get:

• Poor Statistical Properties: The keystream won’t be sufficiently random.
• Security Weaknesses: The encryption will be easily broken. Attackers could potentially recover the plaintext without knowing the key.

Alternatives

Use a proper block cipher (like AES) or stream cipher (like ChaCha20) designed for CTR or OFB mode. These algorithms have the necessary properties to ensure secure encryption.

Summary

Hash functions are one-way functions, while CTR and OFB modes require a reversible, key-based pseudorandom sequence generator. They serve different purposes in cyber security and shouldn’t be used interchangeably.