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Alice wants to share a message with Bob, but Alice and Bob can never be in the same place at the same time. We can assume they both know each others public keys (or agreed on a shared key, if that makes a difference).

Is it safe for Alice to broadcast the ciphertext over the TV/radio/public internet/etc, or should she send Bob the ciphertext over video chat/phone/email?

I understand that modern algorithms are resistant to ciphertext-only attacks, but is the layer of obscuring the email from the public practically beneficial?

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    See Numbers Station - it is normal practise to broadcast ciphertext. – RedGrittyBrick Mar 13 '15 at 10:06
  • Relying on people to not find it would be security through obscurity, which is a bad thing. – Jon Mar 13 '15 at 14:29
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    You have asked a security question without stating the attack you wish to protect against. Does sharing the ciphertext make you more or less vulnerable to having your house broken into? State the attack you are concerned about or the resource you are interested in protecting when you ask a security question. – Eric Lippert Mar 13 '15 at 20:51
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    @Chipperyman 'Security by obscurity' is not a bad thing. It is however, bad to rely on it. It actually has several potentially good uses. – user2338816 Mar 14 '15 at 6:45
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    For all we know, especially these days, sending the ciphertext via email is for all intents and purposes the same thing as broadcasting it to the world. – a CVn Mar 14 '15 at 20:09
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That is exactly what encryption is designed to safely enable. If Bob and Alice could safely share the message without allowing attackers and eavesdroppers access to it, they would not, in fact, need encryption at all. So, yes, it is safe to allow any and everyone access to the ciphertext. You do want to authenticate it so that it cannot be tampered with in transit, but done correctly, we don't believe there is any non-negligible risk of compromising the confidentiality of the message.

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    I don't agree. If Alice and Bob are residents of North Korea, then simply broadcasting the ciphertext may trigger a round of rubber hose cryptoanalysis, followed by a trip to the firing squad. Even in more tolerant environments, the attacker may be more interested in that Alice is communicating with Bob than the actual message. – emory Mar 13 '15 at 14:49
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    @emory On the other hand, when the cyphertext is posted in a public place where many people access it, there is no way to find out that Bob is the intended recipient. Qbja jvgu Ovt Oebgure! - who did I just sent this message to? – Philipp Mar 13 '15 at 14:51
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    @emory the OP's intent was to talk about the threat vectors to the message, not the messenger. – schroeder Mar 13 '15 at 15:38
  • @emory I think this question can be interpreted in multiple ways and broken ways. The message is threatened if people target it, but if the sender is sufficiently obscured and its broadcast and received by many that it secure in a way - note that is different from putting it on pastebin or some site where the adversary can monitor who goes to access it, therefore the list of possible recipients is minimal since people don't just visit random links in general. – Eric G Mar 13 '15 at 19:23
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    @emory On the other hand, the case where Bob is in North Korea and Alice isn't is exactly what numbers stations were designed for. In that case, broadcasting gives huge amounts of plausible deniability. – David Richerby Mar 14 '15 at 11:45
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Exposure of ciphertext does not inherently decrease is security of the algorithm.

However, if the ciphertext is more easily accessible it is more likely to be found by an adversary. If your adversary has means to steal your private/shared keys, rubber hose you, etc its more risky to increase exposure. You may always want to consider that in the future a weakness may be found in the algorithm allowing for cryptoanalysis, known plaintext, etc.

Lastly, you may also want to consider the value of the metadata (signals) vs the message. Your adversary may not care about the content of your message if they can correlate the timing, the people, etc with other actions and other signals. i.e., the risk may be knowledge that you communicated at all rather than what the content of the message contained.

Consider a layered approach of various means to limit access to increase confidentiality. Obscurity alone is not security, but may serve to make it more difficult to attack.

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    As an example, when you make information easily accessible it will be easily accessed: computerworld.com.au/article/204457/… Clarkson would not have easily been targeted had he not broadcast the info. In the case of crypto, it is always possible that in the future the crypto may be broken or that there was a failure at either end in securing the keys. – Eric G Mar 13 '15 at 15:43
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Yes, sharing the ciphertext is safe. Kerckhoffs' principle, which modern cryptosystems are designed around, states that the system should be secure if everything about it is known except the specific key used for encryption/decryption. Additionally, security through obscurity is rarely effective.

What you're (somewhat) getting at in your question is that of a secure channel. The whole point of crypto is to be able to transmit your enciphered message over an insecure channel and have confidentiality, authenticity, and non-repudiation.

If you are using an already-secure channel (e.g. PGP encrypted email), there's no need to use encryption on the message before that. But just broadcasting your ciphertext (e.g. on TV) is legit.

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Every time you browse GMail or your bank's website using HTTPS from your laptop in a coffee shop or airport, your are broadcasting all sorts of ciphertexts to everyone who cares to listen in.

It had better be safe publicly to share ciphertexts. Assuming a great many things about the strengths of the chosen ciphersuite, crypto implementations, and applications.

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