Security newbie here.

I've read in some books the 'goals of information security', which includes non-repudiation.

My understanding of non-repudiation is that if Alice sends a message to Bob, Bob is not only convinced that the message came from Alice but he can also prove to Carol that the message indeed came from Alice (assuming Carol doesn't trust Bob)

Recently, while watching a talk of Moxie Marlinspike, I learnt that non-repudiation is not necessarily a good thing (you might want to deny authorship of a message to the world), and hence he spent time to develop this new protocol (Axolotl) which has a thing called 'plausible deniablilty' which I assume is that if Alice sends a message to Bob, Bob can be certain that it came from Alice, but Bob cannot prove to Carol that it did actually come from Alice.

Now, to a beginner, those two ideas are kind of contradictory and hence the confusion and this question (and the following sub-questions).

• Have the goals of information security changed?
• Are there specific usecases where either of the two (non-repudiation and plausible deniability) is useful to have? (An example would help a lot)

Thanks!

I've read in some books the 'goals of information security', which includes non-repudiation.

My understanding of non-repudiation is that if Alice sends a message to Bob, Bob is not only convinced that the message came from Alice but he can also prove to Carol that the message indeed came from Alice (assuming Carol doesn't trust Bob)

Recently, while watching a talk of Moxie Marlinspike, I learned that non-repudiation is not necessarily a good thing (you might want to deny authorship of a message to the world), and hence he spent time to develop this new protocol (Axolotl) which has a thing called 'plausible deniability' which I assume is that if Alice sends a message to Bob, Bob can be certain that it came from Alice, but Bob cannot prove to Carol that it did actually come from Alice.

Now, to a beginner, those two ideas are kind of contradictory and hence the confusion and this question (and the following sub-questions).

• Have the goals of information security changed?
• Are there specific use cases where either of the two (non-repudiation and plausible deniability) is useful to have? (An example would help a lot)