Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I've been given as an exercise to show an encryption scheme that is CCA-secure but is not a secure message transmission scheme.

What is a message transmission scheme, and how does it differ from a regular encryption scheme?

share|improve this question
up vote 2 down vote accepted

An "encryption scheme" defines the encryption/decryption of data.

A "message transmission scheme" is about securing transmission and defines both "privacy" and "authenticity" between a sender and a receiver.

Since you haven't asked about the definition of CCA-secure (encryption) schemes and since you've been given this as an exercise, I won't mention anything about CCA-secure schemes because that would quickly provide an answer which would almost solve your exercise for you (and you wouldn't learn from that). But, when you keep in mind what I wrote above, you should be able to quickly come up with an encryption scheme that is CCA-secure while not being a secure transmission scheme.


Your CCA-secure scheme will be missing something a "message transmission scheme" provides… look closely at what I wrote, ask yourself what a "CCA-secure scheme" defines at a minimum, and then start thinking. You should be able to wrap your brain around it easily if you look at what each scheme defines.

share|improve this answer
Isn't it true that any CCA-secure encryption scheme that doesn't provide authenticity embedded in it will not be a secure message transmission scheme? – Bush Dec 7 '13 at 16:55
@Bush Correct, CCA security does not imply authenticated encryption. Since you're on the correct path, I can provide an example: encrypting a single block with a block cipher is supposed to be CCA-secure (except for the fact that it isn't randomized — yet, you could add a nonce to a short message and then encrypt) but it doesn't offer any authentication. Note that this is a very simplified (meaning: not copy-and-paste ready) example. But I think you can take it from here and come up with a better example, can't you? ;) – e-sushi Dec 7 '13 at 17:26

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.