6 ms·

One time pad. without the original key, the message could litterally be anything. There's no way to analyze the text at all

andai 9y ago
This. Provided, however, that the key is literally random (not pseudorandom), and never reused.
- Faaak 9y ago
  Exactly. And provided that the length of the one time pad is EQUAL TO or BIGGER than the plaintext. But this is not very practical
  - anikain 9y ago
    The question was not about usability. It was about secure communication. OTP does provide perfect secrecy
  - Twotriads 9y ago
    As with other forms of encryption, there's still the issue of key management / delivery over a secure channel. If you can't guarantee the pad cannot be intercepted, this method is also insecure...
  - e12e 9y ago
    I don't know. You can easily get sdhc cards that are 128GB, ditto for usb sticks. You'd need a high-bandwidth source of random data - or possibly proxy that with high-quality seed for a stream/bloc cipher (conceivably, it's easier to guarantee a one-time, controlled secure generation of a 256 bit symmetric key that's thrown away after generating your stream, than handling of session keys and complicated calculations on a message by message basis) - but I'm not aware of any secure system designed to work with such a one-time pad. You'd need a way to keep track of sections of the pad [probably framing the message, maybe a start keyword could work - it'd give away the section of the pad, but as that section is never re-used... or, if you can assume few messages will be lost, just keep a local counter, and do a sliding search] - and there are other issues. Split the pad into sender/receiver sections (A only encrypt and send with a-apd, B only encrypts with b-pad - to avoid two messages encrypted with the same section of pad, crossing each other). Still, if Eve gets a hold of a plaintext message from Bob, and can intercept the ciphertext before it gets to Alice - Eve gains access to a section of the otp by xoring - and can send anything to Bob (up to the same message length). I'd be interesting to hear if there are modern cryptosystems /research based around one time pads?
    - elcapitan 9y ago
      > I'd be interesting to hear if there are modern cryptosystems /research based around one time pads? There's this implementation from 2015: https://techcrunch.com/2015/03/24/one-time-pads-ride-again/ https://techcrunch.com/2015/03/24/one-time-pads-ride-again/
libeclipse 9y ago
Yep, this. If the key is pseudorandom instead of purely random, then you have yourself a stream cipher and not the OTP.
deleted 9y ago
[deleted]
eb0la 9y ago
I bet you cannot beat OTP written/printed/punched on toilet paper: Key hard to discover plus medium is easy to carry, easy to destroy, and degrades with time (bad archival medium).
y7 9y ago
One time pads don't give you message integrity, so it's arguably less secure than most standard cryptography in use today.
- lb1lf 9y ago
  -Then again, nothing to prevent you from digitally signing the message encoded in OTP. So, you verify the integrity by a mechanism separate from the encoding. (This may or may not be sufficient, depending on the needs of the OP.) Also, of course - the odds of someone tampering with a message encoded using OTP and having the tamper still appear a valid message once decoded could probably best be described as astronomical.
  - aninhumer 9y ago
    Not really astronomical. If you're just XORing your message with a pad, they can trivially flip bits by position. There are a lot of things an attacker can do with that, especially if they have some idea what you're sending.
eeZah7Ux 9y ago
Not again! Your OTP is extremely sensitive to the quality of randomness and requires a lot of it - which makes things very difficult. It provides no authenticity and integrity or at least proof of tampering. It does not protect from message reordering and capture+retransmission. It obviously leaks metadata in a real-world usage: sender, receiver, msg length, time of message.
- falcolas 9y ago
  All of these problems are present with any pure encryption method. That's why authentication hashes, message ids, high quality randomness sources, and so forth exist. OTP can use these just as well as any other encryption method.
  - irundebian 9y ago
    What do you mean by "pure encryption method"? No one (no smart people) uses AES purely, but of course in some mode of operation such as GCM which provides integrity.
  - deleted 9y ago
    [deleted]
  - eeZah7Ux 9y ago
    The question was about secure communication, not just "pure" encryption.
jug 9y ago
The problem may become delivering the key securely though. It probably has to be agreed upon in advance by the people being physically in the same location. But then each key still only usable for a single message. I think number stations communicating with spies use one time pads?

andai 9y ago

This. Provided, however, that the key is literally random (not pseudorandom), and never reused.

Faaak 9y ago
Exactly. And provided that the length of the one time pad is EQUAL TO or BIGGER than the plaintext. But this is not very practical
- anikain 9y ago
  The question was not about usability. It was about secure communication. OTP does provide perfect secrecy
- Twotriads 9y ago
  As with other forms of encryption, there's still the issue of key management / delivery over a secure channel. If you can't guarantee the pad cannot be intercepted, this method is also insecure...
- e12e 9y ago
  I don't know. You can easily get sdhc cards that are 128GB, ditto for usb sticks. You'd need a high-bandwidth source of random data - or possibly proxy that with high-quality seed for a stream/bloc cipher (conceivably, it's easier to guarantee a one-time, controlled secure generation of a 256 bit symmetric key that's thrown away after generating your stream, than handling of session keys and complicated calculations on a message by message basis) - but I'm not aware of any secure system designed to work with such a one-time pad. You'd need a way to keep track of sections of the pad [probably framing the message, maybe a start keyword could work - it'd give away the section of the pad, but as that section is never re-used... or, if you can assume few messages will be lost, just keep a local counter, and do a sliding search] - and there are other issues. Split the pad into sender/receiver sections (A only encrypt and send with a-apd, B only encrypts with b-pad - to avoid two messages encrypted with the same section of pad, crossing each other). Still, if Eve gets a hold of a plaintext message from Bob, and can intercept the ciphertext before it gets to Alice - Eve gains access to a section of the otp by xoring - and can send anything to Bob (up to the same message length). I'd be interesting to hear if there are modern cryptosystems /research based around one time pads?
  - elcapitan 9y ago
    > I'd be interesting to hear if there are modern cryptosystems /research based around one time pads? There's this implementation from 2015: https://techcrunch.com/2015/03/24/one-time-pads-ride-again/ https://techcrunch.com/2015/03/24/one-time-pads-ride-again/

libeclipse 9y ago

Yep, this. If the key is pseudorandom instead of purely random, then you have yourself a stream cipher and not the OTP.

deleted 9y ago

[deleted]

eb0la 9y ago

I bet you cannot beat OTP written/printed/punched on toilet paper: Key hard to discover plus medium is easy to carry, easy to destroy, and degrades with time (bad archival medium).

y7 9y ago

One time pads don't give you message integrity, so it's arguably less secure than most standard cryptography in use today.

lb1lf 9y ago
-Then again, nothing to prevent you from digitally signing the message encoded in OTP. So, you verify the integrity by a mechanism separate from the encoding. (This may or may not be sufficient, depending on the needs of the OP.) Also, of course - the odds of someone tampering with a message encoded using OTP and having the tamper still appear a valid message once decoded could probably best be described as astronomical.
- aninhumer 9y ago
  Not really astronomical. If you're just XORing your message with a pad, they can trivially flip bits by position. There are a lot of things an attacker can do with that, especially if they have some idea what you're sending.

eeZah7Ux 9y ago

Not again! Your OTP is extremely sensitive to the quality of randomness and requires a lot of it - which makes things very difficult. It provides no authenticity and integrity or at least proof of tampering. It does not protect from message reordering and capture+retransmission. It obviously leaks metadata in a real-world usage: sender, receiver, msg length, time of message.

falcolas 9y ago
All of these problems are present with any pure encryption method. That's why authentication hashes, message ids, high quality randomness sources, and so forth exist. OTP can use these just as well as any other encryption method.
- irundebian 9y ago
  What do you mean by "pure encryption method"? No one (no smart people) uses AES purely, but of course in some mode of operation such as GCM which provides integrity.
- deleted 9y ago
  [deleted]
- eeZah7Ux 9y ago
  The question was about secure communication, not just "pure" encryption.

jug 9y ago

The problem may become delivering the key securely though. It probably has to be agreed upon in advance by the people being physically in the same location. But then each key still only usable for a single message. I think number stations communicating with spies use one time pads?