4 ms·

You're right about the theory, but you're also wrong about how to implement - which is the point of telling people not to roll their own crypto. What about res

by IncRnd 9y ago

You're right about the theory, but you're also wrong about how to implement - which is the point of telling people not to roll their own crypto. What about reseeding or prediction resistance? How do you handle biased entropy input from that hairdryer someone is blowing on your chips? Oops, the quantum random number generator card doesn't work anymore after adding that extra GPGPU to the system.

ramphastidae 9y ago
> How do you handle biased entropy input from that hairdryer someone is blowing on your chips? Sounds fascinating. Can you please tell us more?
- the_stc 9y ago
  If you had a RNG that sampled some hardware noise but then you altered the environment by introducing extreme temperatures, maybe the RNG would end up with biased output. A simple example: you can use a camera as a good source of randomness, even taking a picture of a wall indoors. But if an attacker was able to flood that room with light and overwhelm your sensor, he could predict the image would be 100% white pixels or near to it and guess the random output. But the comment is exaggerating. Encryption algorithms do not handle the case of your entropy source being compromised. Good RNG algorithms like Yarrow try to mitigate this somewhat.
  - JohnStrange 9y ago
    Well, to be fair, a good implementation should throw some simple statistical sanity checks at RNG input during self-test and throw an error when it doesn't pass. Passing such tests doesn't indicate that the RNG is cryptographically secure, but failing them indicates that it is insecure.
  - pkolaczk 9y ago
    Even if you need a real entropy-based RNG, e.g. for key generation, you need to gather entropy once at the beginning to seed the RNG. Then running RNG does not need any more entropy, so your hairdryer trick won't work. A common myth I hear is that "entropy" gets "depleted" during RNG operation, but in fact it does not. You only need enough bits of entropy once at the beginning, equal to the internal RNG state length.
    - the_stc 9y ago
      Strong RNG designs re-seed to deal with compromise or poor seeding. That way if an attacker has temporary read access they do not permanently compromise the system going forward. Or if on boot you are in a compromised or low entropy environment the system will improve over time. Perhaps the utility of this is arguable. If an attacker is able to see your private RNG state then it is most likely game over. But it is a cheap operation with little downside and again, designs like Yarrow put considerable time into thinking it through.
syrrim 9y ago
You wouldn't want true random numbers anyways. If you're encrypting it, then you'll want to decrypt it later. That means you'll need the random data you used again. If you had a secure channel over which to send or store the random data, then you could just use that channel to send or store the plaintext itself (note that the random data will be the same length as the plaintext). Such a system could only be useful if the cryptographic key was shorter than the plaintext. So it would have to be a deterministic RNG, and you would only need the seed and the ciphertext in order to decrypt. Also, if someone has acccess to the hardware, then you're screwed. No one would bother wasting time with a hairdryer. If the plaintext was on your harddrive, they would just grab that and run.
- IncRnd 9y ago
  I think you missed the point of the comment, which wasn't about physical possession of a computer. You also make assumptions, such as chips being inside the computer, when they can in fact be in an outside environment-based RNG. The hairdryer example is the canonical and prototypical weakness in that particular type of "rng". I'll try to be explicit about that in the future.
  - pkolaczk 9y ago
    I didn't write anything about environment-based RNG. RNG must be deterministic and seeded by preshared encryption key. One point of using a cryptographically secure RNG is that it should not be possible to learn the seed by observing the output of the RNG without bruteforcing the key.

by IncRnd 9y ago

ramphastidae 9y ago

> How do you handle biased entropy input from that hairdryer someone is blowing on your chips? Sounds fascinating. Can you please tell us more?

the_stc 9y ago
If you had a RNG that sampled some hardware noise but then you altered the environment by introducing extreme temperatures, maybe the RNG would end up with biased output. A simple example: you can use a camera as a good source of randomness, even taking a picture of a wall indoors. But if an attacker was able to flood that room with light and overwhelm your sensor, he could predict the image would be 100% white pixels or near to it and guess the random output. But the comment is exaggerating. Encryption algorithms do not handle the case of your entropy source being compromised. Good RNG algorithms like Yarrow try to mitigate this somewhat.
- JohnStrange 9y ago
  Well, to be fair, a good implementation should throw some simple statistical sanity checks at RNG input during self-test and throw an error when it doesn't pass. Passing such tests doesn't indicate that the RNG is cryptographically secure, but failing them indicates that it is insecure.
- pkolaczk 9y ago
  Even if you need a real entropy-based RNG, e.g. for key generation, you need to gather entropy once at the beginning to seed the RNG. Then running RNG does not need any more entropy, so your hairdryer trick won't work. A common myth I hear is that "entropy" gets "depleted" during RNG operation, but in fact it does not. You only need enough bits of entropy once at the beginning, equal to the internal RNG state length.
  - the_stc 9y ago
    Strong RNG designs re-seed to deal with compromise or poor seeding. That way if an attacker has temporary read access they do not permanently compromise the system going forward. Or if on boot you are in a compromised or low entropy environment the system will improve over time. Perhaps the utility of this is arguable. If an attacker is able to see your private RNG state then it is most likely game over. But it is a cheap operation with little downside and again, designs like Yarrow put considerable time into thinking it through.

syrrim 9y ago

You wouldn't want true random numbers anyways. If you're encrypting it, then you'll want to decrypt it later. That means you'll need the random data you used again. If you had a secure channel over which to send or store the random data, then you could just use that channel to send or store the plaintext itself (note that the random data will be the same length as the plaintext). Such a system could only be useful if the cryptographic key was shorter than the plaintext. So it would have to be a deterministic RNG, and you would only need the seed and the ciphertext in order to decrypt. Also, if someone has acccess to the hardware, then you're screwed. No one would bother wasting time with a hairdryer. If the plaintext was on your harddrive, they would just grab that and run.

IncRnd 9y ago
I think you missed the point of the comment, which wasn't about physical possession of a computer. You also make assumptions, such as chips being inside the computer, when they can in fact be in an outside environment-based RNG. The hairdryer example is the canonical and prototypical weakness in that particular type of "rng". I'll try to be explicit about that in the future.
- pkolaczk 9y ago
  I didn't write anything about environment-based RNG. RNG must be deterministic and seeded by preshared encryption key. One point of using a cryptographically secure RNG is that it should not be possible to learn the seed by observing the output of the RNG without bruteforcing the key.