Why is the one-time pad impossible?

[Kroze]

Sorry for a dumb question. Is it true that the key space is limited?? If so, why would the one time pad be impossible to break?

 

[Phanuel]

Er, no, why would the space be that limited? Just make a bigger pad. http://en.wikipedia.org/wiki/One-time_pad

 

[Kroze]

Er, no, why would the space be that limited? Just make a bigger pad. http://en.wikipedia.org/wiki/One-time_pad

I read that already but only understand just a grasp of it. That's why I'm asking the question here.

Say the cipher text is "HELLO". isn't true that the keyspace is 26*26*26*26*26? Therefore with a fast enough computer, it doesn't take long to decode it at all.

 

[Phanuel]

Well, you'd kinda need to know the message to decipher it.

The essence of a one-time pad is that the actual cipher key, the random numbers added to your base message, is only held in two locations and thrown away after use.

You can only send a one time message once, and only to whomever has the other copy of the same pad.

I've probably not answered your question properly though.

 

[repoman0]

Sure you can decode "HELLO" but how many other five letter words are there? A lot. The message could be any of them for all you know.

Hopefully I still remember what a one-time pad is, it's been a while.

 

[Paperlantern]

It wouldnt be impossible to BREAK, eventually you can brute force any message, OTP or no OTP. Yes it would take longer and longer as the size of the message grows because then the keyspace grows. The rub on the OTP is not it's security, it's the impracticality. In order to use a OTP you need to have a secure channel to exchange the given key for the Pad... if you have a secure channel to exchange the key, why not just exchange the message?

 

[Kroze]

Been playing call of duty: black ops, got me curious. LOL

 

[Phanuel]

It wouldnt be impossible to BREAK, eventually you can brute force any message, OTP or no OTP. Yes it would take longer and longer as the size of the message grows because then the keyspace grows. The rub on the OTP is not it's security, it's the impracticality. In order to use a OTP you need to have a secure channel to exchange the given key for the Pad... if you have a secure channel to exchange the key, why not just exchange the message?

Er, the big challenge for OTPs is just getting a pad to both ends. Beyond that, the key is kinda optional. That's the benefit to OTPs, you can send the encoded message over non-secure channels and know that it'll take a long long time to crack that one message, if ever. And the next message you send will be using a completely different unrelated series of random ciphering that cannot be cracked using historical ciphers.

 

[unokitty]

Sorry for a dumb question. Is it true that the key space is limited?? If so, why would the one time pad be impossible to break?

In theory, the one time pad is the only cipher that can be mathematically proven to be impossible to break. See cryptosmith. Or see One Time Pad FAQ.

In practice, the one time pad has proven very difficult to implement properly. See Project Venona. Or Venona and the Cold War.

Uno