19 ms·

That is not rolling your own crypto. Rolling your own would be relying on your custom construction, not double encrypting and relying on the known construction

by dsacco 9y ago

That is not rolling your own crypto. Rolling your own would be relying on your custom construction, not double encrypting and relying on the known construction for safety. ...Furthermore I'm confused as to why someone would do that. If you already have a safe way to encrypt your plaintext and you're the only one with both keys, why would you bother double encrypting? You gain absolutely nothing because it's redundant, and you lose performance. What you're doing is tantamount to a weird sort of encoding that doesn't offer any benefit.

madez 9y ago
Doing your own encryption does clearly not exclude also using other encryption. Or is implementing AES already not anymore doing your own encryption, because AES wasn't invented by you? You _always_ use work of others. Doing your own crypto can be used as way to safeguard against unknown vulnerabilities in the tools you would use otherwise. If the recommended library is safe, then fine, you are good to go. How do we know it is safe? My idea above is a way to increase the safety margin in a way that is orthogonal to the increase of the key space of the recommended tool. That sounds like a very good thing to me.
- dsacco 9y ago
  I'm sorry, I don't understand what you're trying to say. Can you clarify? What I am trying to say is that you gain nothing by, say, implementing your own encryption algorithm, using it on a plaintext, then encrypting the resulting ciphertext with a known good implementation of a known good algorithm like AES. That is how I interpreted your previous comment. So - you don't gain anything from that, if that's what you meant. If you mean just implementing a known good algorithm on your own (again, like AES) - why? You are overwhelmingly more likely to shoot yourself in the foot, and there are plenty of open source implementations that you can inspect if you don't trust the code. Finally, if you distrust the design of a known good algorithm, you're out of luck, because (again) you're almost certainly not going to come up with something better. If you do, by all means use it, but it's simply not likely whatsoever. And again: you're not increasing the safety margin by double encrypting, unless you know both algorithms and implementations are solid. And even if they both are, the safety margin improvement will be negligible compared to the decrease in performance.
  - jmulho 9y ago
    > "If you mean just implementing a known good algorithm on your own (again, like AES) - why? You are overwhelmingly more likely to shoot yourself in the foot" I've never understood this argument. Encryption algorithms are deterministic. If you implement a known good algorithm on your own, and it gets the same outputs given the same inputs, then you are highly unlikely to shoot yourself in the foot. This is not what people are talking about when they warn not to roll your own crypto.
    - bascule 9y ago
      Just because an AES implementation matches the test vectors does not make it correct or safe. Here is an excellent visual depiction of several different AES implementations, many of which do not run in constant time: https://cr.yp.to/mac/variability1.html https://cr.yp.to/mac/variability1.html Implementing AES with variable time secret-dependent operations can leak the key. There are much bigger and more terrifying subtleties to implementing crypto correctly than simply matching the test vectors.
      - dsacco 9y ago
        To briefly expand on this (good) comment: In cryptography, the output of an algorithm is far less important than how that output was generated (with very precise specificity). Crypto code requires far more heavy lifting in the "how" of the output than other software does. This is not intuitive, because ordinarily there are several ways to do things and you can compare on output, and it's probably fine to develop something in a functional but unorthodox way (minus performance consequences, maybe). As the parent comment here mentions, there are extremely bad consequences to subtle mistakes in how the algorithm and its implementation arrives at the output, even if the output is correct. If Alice sends Bob an encrypted message and the custom crypto provides an incorrect output, Bob can't decrypt the message. But if Alice sends Bob an encrypted message with the correct output generated in an unsafe way, there are many ways by which the secret key could theoretically be recovered, which is far worse than Bob not being able to read the message in the first place.
      - 9y ago
- monsieurbanana 9y ago
  That's far from what I'd call "doing crypto as a side-hobby".
dankent 9y ago
If a vulnerability is found for the outer encryption method, additional encryption of the inner message by a different method may provide some defence. If that is the goal, though, it would be better to use two well studied encryption methods rather than something homemade.
- dsacco 9y ago
  Right, that observation is correct on the surface. But the reason why that's almost never done is because the goal of a cryptographic algorithm is to contribute enough safety margin on its own. Instead of encrypting twice, it's better to encrypt with a greater number of rounds, or to come up with a superior algorithm. In practice you sacrifice an unreasonable amount of performance double encrypting in a production environment for a threat model that is fantastically unrealistic.
  - dankent 9y ago
    You find it fantastically unrealistic that a vulnerability could be found for a well known and widely used encryption method?
    - dsacco 9y ago
      Depending on the algorithm: yes. I'm not saying all algorihms that are widely used and well known are extremely unlikely to be broken. I would happily bet $10,000 that AES will not be broken in the next ten years. I'd make the same bet for several hash functions. And as a corollary to this point: I think it's incredibly foolish to try and combat the threat of an encryption algorithm being broken by double encrypting a plaintext, including with that algorithm.
      - computerex 9y ago
        > I would happily bet $10,000 that AES will not be broken in the next ten years. What are you talking about: http://cr.yp.to/antiforgery/cachetiming-20050414.pdf http://cr.yp.to/antiforgery/cachetiming-20050414.pdf You'd lose the bet so hard. Don't take anything for granted. Even the most supposedly secure and widely used primitives should be scrutinized and are subjects to constant attacks.

by dsacco 9y ago

madez 9y ago

Doing your own encryption does clearly not exclude also using other encryption. Or is implementing AES already not anymore doing your own encryption, because AES wasn't invented by you? You _always_ use work of others. Doing your own crypto can be used as way to safeguard against unknown vulnerabilities in the tools you would use otherwise. If the recommended library is safe, then fine, you are good to go. How do we know it is safe? My idea above is a way to increase the safety margin in a way that is orthogonal to the increase of the key space of the recommended tool. That sounds like a very good thing to me.

dsacco 9y ago
I'm sorry, I don't understand what you're trying to say. Can you clarify? What I am trying to say is that you gain nothing by, say, implementing your own encryption algorithm, using it on a plaintext, then encrypting the resulting ciphertext with a known good implementation of a known good algorithm like AES. That is how I interpreted your previous comment. So - you don't gain anything from that, if that's what you meant. If you mean just implementing a known good algorithm on your own (again, like AES) - why? You are overwhelmingly more likely to shoot yourself in the foot, and there are plenty of open source implementations that you can inspect if you don't trust the code. Finally, if you distrust the design of a known good algorithm, you're out of luck, because (again) you're almost certainly not going to come up with something better. If you do, by all means use it, but it's simply not likely whatsoever. And again: you're not increasing the safety margin by double encrypting, unless you know both algorithms and implementations are solid. And even if they both are, the safety margin improvement will be negligible compared to the decrease in performance.
- jmulho 9y ago
  > "If you mean just implementing a known good algorithm on your own (again, like AES) - why? You are overwhelmingly more likely to shoot yourself in the foot" I've never understood this argument. Encryption algorithms are deterministic. If you implement a known good algorithm on your own, and it gets the same outputs given the same inputs, then you are highly unlikely to shoot yourself in the foot. This is not what people are talking about when they warn not to roll your own crypto.
  - bascule 9y ago
    Just because an AES implementation matches the test vectors does not make it correct or safe. Here is an excellent visual depiction of several different AES implementations, many of which do not run in constant time: https://cr.yp.to/mac/variability1.html https://cr.yp.to/mac/variability1.html Implementing AES with variable time secret-dependent operations can leak the key. There are much bigger and more terrifying subtleties to implementing crypto correctly than simply matching the test vectors.
    - dsacco 9y ago
      To briefly expand on this (good) comment: In cryptography, the output of an algorithm is far less important than how that output was generated (with very precise specificity). Crypto code requires far more heavy lifting in the "how" of the output than other software does. This is not intuitive, because ordinarily there are several ways to do things and you can compare on output, and it's probably fine to develop something in a functional but unorthodox way (minus performance consequences, maybe). As the parent comment here mentions, there are extremely bad consequences to subtle mistakes in how the algorithm and its implementation arrives at the output, even if the output is correct. If Alice sends Bob an encrypted message and the custom crypto provides an incorrect output, Bob can't decrypt the message. But if Alice sends Bob an encrypted message with the correct output generated in an unsafe way, there are many ways by which the secret key could theoretically be recovered, which is far worse than Bob not being able to read the message in the first place.
    - 9y ago
monsieurbanana 9y ago
That's far from what I'd call "doing crypto as a side-hobby".

dankent 9y ago

If a vulnerability is found for the outer encryption method, additional encryption of the inner message by a different method may provide some defence. If that is the goal, though, it would be better to use two well studied encryption methods rather than something homemade.

dsacco 9y ago
Right, that observation is correct on the surface. But the reason why that's almost never done is because the goal of a cryptographic algorithm is to contribute enough safety margin on its own. Instead of encrypting twice, it's better to encrypt with a greater number of rounds, or to come up with a superior algorithm. In practice you sacrifice an unreasonable amount of performance double encrypting in a production environment for a threat model that is fantastically unrealistic.
- dankent 9y ago
  You find it fantastically unrealistic that a vulnerability could be found for a well known and widely used encryption method?
  - dsacco 9y ago
    Depending on the algorithm: yes. I'm not saying all algorihms that are widely used and well known are extremely unlikely to be broken. I would happily bet $10,000 that AES will not be broken in the next ten years. I'd make the same bet for several hash functions. And as a corollary to this point: I think it's incredibly foolish to try and combat the threat of an encryption algorithm being broken by double encrypting a plaintext, including with that algorithm.
    - computerex 9y ago
      > I would happily bet $10,000 that AES will not be broken in the next ten years. What are you talking about: http://cr.yp.to/antiforgery/cachetiming-20050414.pdf http://cr.yp.to/antiforgery/cachetiming-20050414.pdf You'd lose the bet so hard. Don't take anything for granted. Even the most supposedly secure and widely used primitives should be scrutinized and are subjects to constant attacks.