5 ms·

> Unless you’re using a true random number generator that works on a mechanical/electrical process… …you're not using a one-time pad. OTP requires the pad to b

> Unless you’re using a true random number generator that works on a mechanical/electrical process… …you're not using a one-time pad. OTP requires the pad to be truly random: at least one bit of unique, never-used-elsewhere entropy for every bit in the message. Merely XORing some plaintext with a pseudo-random stream based on a smaller seed, which as you say is the basis for various other encryption algorithms, is not a one-time pad. The real problem with OTP is key distribution: You need to share pads with everyone you might want to communicate with, one pad per sender/receiver pair, and those pads need to be at least as large as all the message you'll eventually want to exchange. There is no OTP equivalent to public-key cryptography where you only need one private/public keypair per recipient.

generalizations 5y ago
Which is why it seems like there should at least be an RFC for OTP communication - if there's a small group that wants to communicate securely together, it may be worth their time and effort to share that pad securely. Right now, there's no official standard for that.
- rtkwe 5y ago
  It's not a complex system, you share the noise and XOR your plaintext with it before sending.
rtkwe 5y ago
That's what I was saying a true OTP is a cumbersome version of stream cyphers that trades some improvement in security for a huge increase in fragility and difficulty of use and distribution.
- generalizations 5y ago
  > With a single key much smaller than a megabyte Unless that was a preshared key, you're not talking about a one time pad. One time pads are inherently symmetric. If you're comparing symmetric to assymmetric crypto systems, you might as well just say that 'if my grandmother had wheels, she'd be a bicycle'.
  - boloust 5y ago
    I don't think "symmetric cryptography" means what you think it means. Stream ciphers and OTPs are both symmetric. The "symmetric" part is the fact that the same key is used for encryption and decryption, not that the plaintext and key are the same length.
- nybble41 5y ago
  The OTP algorithm can be seen as a form of stream cipher (where the key is the stream), but what you actually said was "a lot of encryption algorithms are various ways of creating a one-time pad". This is false since a one-time pad must be truly random and the stream produced by the other algorithms is only pseudo-random; you can't "create" a one-time pad using a pseudo-random number generator. This is a qualitative difference in security because a pseudo-random stream is subject to brute-force searches to locate the seed which decodes the ciphertext to a meaningful message (the number of possible seeds is much smaller than the number of possible messages, so you can generally determine when you've found the right key) whereas with a properly implemented OTP for any message of size equal to or smaller than the ciphertext there exists a key which will decode the ciphertext into that message (possibly with some padding), so brute-force search is impossible. I wouldn't call OTP "fragile" or "hard to use", but you're correct about the key distribution difficulties.

generalizations 5y ago

Which is why it seems like there should at least be an RFC for OTP communication - if there's a small group that wants to communicate securely together, it may be worth their time and effort to share that pad securely. Right now, there's no official standard for that.

rtkwe 5y ago
It's not a complex system, you share the noise and XOR your plaintext with it before sending.

rtkwe 5y ago

That's what I was saying a true OTP is a cumbersome version of stream cyphers that trades some improvement in security for a huge increase in fragility and difficulty of use and distribution.

generalizations 5y ago
> With a single key much smaller than a megabyte Unless that was a preshared key, you're not talking about a one time pad. One time pads are inherently symmetric. If you're comparing symmetric to assymmetric crypto systems, you might as well just say that 'if my grandmother had wheels, she'd be a bicycle'.
- boloust 5y ago
  I don't think "symmetric cryptography" means what you think it means. Stream ciphers and OTPs are both symmetric. The "symmetric" part is the fact that the same key is used for encryption and decryption, not that the plaintext and key are the same length.
nybble41 5y ago
The OTP algorithm can be seen as a form of stream cipher (where the key is the stream), but what you actually said was "a lot of encryption algorithms are various ways of creating a one-time pad". This is false since a one-time pad must be truly random and the stream produced by the other algorithms is only pseudo-random; you can't "create" a one-time pad using a pseudo-random number generator. This is a qualitative difference in security because a pseudo-random stream is subject to brute-force searches to locate the seed which decodes the ciphertext to a meaningful message (the number of possible seeds is much smaller than the number of possible messages, so you can generally determine when you've found the right key) whereas with a properly implemented OTP for any message of size equal to or smaller than the ciphertext there exists a key which will decode the ciphertext into that message (possibly with some padding), so brute-force search is impossible. I wouldn't call OTP "fragile" or "hard to use", but you're correct about the key distribution difficulties.