无同行评审下自制加密算法及哈希函数的安全性优化问询

Great question—let’s be clear upfront: rolling your own crypto is almost always a terrible idea. The odds of missing a subtle vulnerability that breaks everything are astronomically high. But if you’re stuck with no other option (small team, no access to peer-reviewed standards), here’s how to minimize risk as much as possible.

Part 1: Hardening a Custom Encryption Algorithm #

• Anchor to proven design principles: Never ignore Claude Shannon’s core rules of confusion and diffusion. Confusion means making the relationship between your key and ciphertext as complex as possible (so attackers can’t reverse-engineer the key from ciphertext). Diffusion ensures that changing a single bit of plaintext alters as many bits of ciphertext as possible (prevents statistical attacks). Build your algorithm around these—don’t invent random transformations just to “look secure.”
• Leverage existing secure primitives (don’t reinvent the wheel): Instead of designing a cipher from scratch, build on structures that have stood the test of time. For example:
  • Use a Feistel network (used in DES, Blowfish) for block ciphers—its security properties are well-understood, so you only need to focus on designing a secure round function.
  • Borrow ideas from AES for substitution-permutation networks (SPNs): use non-linear S-boxes (precompute them using known secure methods, not random ones) and linear permutations to spread bits around.
• Lock down against side-channel attacks: Most custom ciphers fail not because of mathematical flaws, but because of side channels like timing, power consumption, or electromagnetic leaks. To fix this:
  • Write code using constant-time operations—avoid conditional branches that depend on key or plaintext bits (e.g., replace if (key_bit) { ... } with bitwise operations).
  • Mask sensitive data (like keys) with random values during computations to hide power consumption patterns.
• Test rigorously for known attacks: You can’t do peer review, but you can simulate common attacks:
  • Run differential cryptanalysis: Generate pairs of plaintexts with small differences, compute their ciphertext differences, and check if any difference pattern appears more often than random (a sign of a weakness).
  • Run linear cryptanalysis: Look for linear relationships between plaintext bits, ciphertext bits, and key bits—if a relationship has a correlation significantly higher than 0.5, your cipher is vulnerable.
  • Use open-source crypto testing frameworks (like Crypto++ or libsodium’s test suites) to automate these checks.
• Limit use to low-risk scenarios: Never use your custom cipher for high-stakes data (financial records, medical data, user passwords). Restrict it to trivial, internal use cases—like encrypting team meeting notes or temporary files. And rotate keys frequently (every few weeks) to limit damage if a flaw is found.
• Keep it simple: Overcomplicating your cipher (adding extra rounds, weird transformations) doesn’t make it safer—it hides vulnerabilities that would be obvious in a simpler design. If you can’t explain every part of your cipher to a fellow developer, it’s probably too complex.

Part 2: Reducing Collision/Preimage Risks in Custom Hash Functions #

Hash functions have three core security goals: preimage resistance (can’t find a message that produces a given hash), second preimage resistance (can’t find a second message with the same hash as a given one), and collision resistance (can’t find any two messages with the same hash). Here’s how to protect against failures in these areas:

• Use a validated iterative structure: Don’t design a hash function from scratch—build on structures like:
  • Merkle-Damgård: Used in SHA-1, SHA-2. It chains a compression function over blocks of input. As long as your compression function is collision-resistant, the whole hash is too (with proper padding).
  • Sponge construction: Used in SHA-3. It’s more flexible and resistant to certain attacks than Merkle-Damgård. Learn how it works and adapt it instead of inventing your own structure.
• Design a secure compression function: The compression function is the heart of your hash. A safe approach is to build it using a secure block cipher (even if you’re using a standard one like AES here—no need to roll your own block cipher for this). The Davies-Meyer construction is a proven way to turn a block cipher into a compression function.
• Use a sufficiently long output:
  • For collision resistance: Your hash output needs to be at least 128 bits. The birthday paradox means attackers can find a collision in ~2^(n/2) operations—so 128 bits requires 2^64 operations, which is computationally infeasible for most attackers.
  • For preimage resistance: Aim for at least 160 bits. Preimage attacks require ~2^n operations, so 160 bits means 2^160 steps—way beyond current computing power.
• Test for collisions and preimages:
  • Run a birthday attack simulation: Generate millions of random inputs, compute their hashes, and check for duplicates. If you find a collision before generating ~2^(n/2) hashes, your function is broken.
  • Test preimage resistance: Pick a random hash value and try to find any input that produces it. If you succeed in less than ~2^n attempts, your function has a flaw.
• Avoid obvious pitfalls:
  • Don’t use linear transformations exclusively—non-linearity is critical to breaking statistical patterns.
  • Ensure every input bit affects every output bit (full diffusion). If changing one input bit only changes a few output bits, attackers can exploit that.
  • Never reuse internal state between hash computations without resetting it fully.

Final Critical Note #

Even if you follow all these steps, your custom crypto will never be as secure as peer-reviewed standards like AES, ChaCha20, SHA-256, or SHA-3. These algorithms have been picked apart by thousands of experts over decades—your small team can’t replicate that level of scrutiny. Use custom crypto only as a last resort.

内容的提问来源于stack exchange，提问作者DEP