Reading Dirk Rijmenants's work on One Time Pad and he frequently mentions that it is suited to short plaintexts.

I presume this is just because of the practicality of encrypting and decrypting one time pad ciphers? If not why is it not mathematically appropriate for longer plaintexts 1000 characters plus?

Presumably if the one time pad was longer and totally plaintext this cipher works with plaintexts of lengths through to infinity.

Thanks for the help with this small point.


The one-time pad guarantees secrecy for messages of arbitrary length, assuming the key is uniformly random of (at least) the same length. There are no caveats to this information-theoretic theorem, it's just that securely exchanging big amounts of key material may be considered to be impractical in some scenarios.

(But frankly, nowadays you would just hand over an SD card or something and have enough key material for hundreds of gigabytes of messages. If you were to engage in that kind of activities rather than using a modern cipher, that is.)


Yes, OTPs work equally securely on any size message. The clearest explanation for Dirk's recommendation of short messages is probably this paragraph from §7.1:-

The one-time pad system should be used with nothing more than a pencil and paper, and for good reasons. There are some critical security issues to consider when a computer or other peripheral devices are used. Readable data can, and most often will reside unintentionally on computers, in their memory, in temporary or swap files on the hard disk, or in memory buffers of peripherals. No single network connected computer is secure and will always be vulnerable to malicious software, spy ware or intrusion by skilled hackers or professional organisations.

He's suggesting that OTP operations should ideally be performed by hand. Thus short messages that are easily manipulated with pen and paper. His opinion has merit.


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