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TextSecure's Private Group Messaging

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Re: TextSecure's Private Group Messaging

#51

Post by kaeporan »

sillysaurus3 wrote: post #49
kaeporan wrote: post #7

Earlier quoted context omitted.

I'm quite certain that the current TextSecure chat allows my proposed scenario with Alice, Bob and Carol to go through without issue. This is the main problem here. So while transcript consistency is discussed in the blog post, it remains the case that Alice can send a different message to Bob and Carol without being detected.

I'm quite certain that the current TextSecure chat allows my proposed scenario with Alice, Bob and Carol to go through without issue. This is the main problem here. So while transcript consistency is discussed in the blog post, it remains the case that Alice can send a different message to Bob and Carol without being detected. Are you sure this is correct? From the blog post, it seems like this is impossible: https:/…

The attacker could send different Ks to each user so that K decrypts C to a different plaintext.

Re: TextSecure's Private Group Messaging

#52

Post by sillysaurus3 »

kaeporan wrote: post #51
sillysaurus3 wrote: post #49

Earlier quoted context omitted.

I'm quite certain that the current TextSecure chat allows my proposed scenario with Alice, Bob and Carol to go through without issue. This is the main problem here. So while transcript consistency is discussed in the blog post, it remains the case that Alice can send a different message to Bob and Carol without being detected. Are you sure this is correct? From the blog post, it seems like this is impossible: https:/…

The attacker could send different Ks to each user so that K decrypts C to a different plaintext.

If you try to decrypt C with any key other than the K that was used to encrypt it, you'll get gibberish (the decryption process will fail).

As far as I know, it isn't possible for an attacker to generate a message ciphertext C such that two different decryption keys would both decrypt to valid plaintexts. There can only be one valid decryption key; trying to decrypt C using any other key would yield random output, wouldn't it?

Re: TextSecure's Private Group Messaging

#53

Post by tptacek »

kaeporan wrote: post #51
sillysaurus3 wrote: post #49

Earlier quoted context omitted.

I'm quite certain that the current TextSecure chat allows my proposed scenario with Alice, Bob and Carol to go through without issue. This is the main problem here. So while transcript consistency is discussed in the blog post, it remains the case that Alice can send a different message to Bob and Carol without being detected. Are you sure this is correct? From the blog post, it seems like this is impossible: https:/…

The attacker could send different Ks to each user so that K decrypts C to a different plaintext.

Hence the continuous transcript consistency feature of the TextSecure protocol. Meanwhile: I'm curious about your answer to the question 'sdevlin posted downthread:

https://news.ycombinator.com/item?id=7701510

Does your transcript consistency rely on the security of your server?

Re: TextSecure's Private Group Messaging

#54

Post by phaer »

dublinben wrote: post #44
phaer wrote: post #18

Earlier quoted context omitted.

Afaik TextSecures server infrastructure consists mainly of Google Play Services which comes at no financial costs for them but with the downside of depending on Google to temporary store encrypted text.

I would rather pay a nominal fee to support their infrastructure than have to rely on Google Play Services for a supposedly "secure" messaging service.

I see you point but it should not weaken the security because encryption happens at the client, Google "only" gets metadata which at least authorities will get anyway.

Besides, TextSecure is free software so it might possible to run your own server at least in the future.

Re: TextSecure's Private Group Messaging

#55

Post by kaeporan »

tptacek wrote: post #53
kaeporan wrote: post #51

Earlier quoted context omitted.

The attacker could send different Ks to each user so that K decrypts C to a different plaintext.

Hence the continuous transcript consistency feature of the TextSecure protocol. Meanwhile: I'm curious about your answer to the question 'sdevlin posted downthread: https://news.ycombinator.com/item?id=7701510 Does your transcript consistency rely on the security of your server?

Sorry, Thomas, but after you repeatedly replied to my private requests for conflict resolution with threats and abusive remarks, I refuse to interact with you entirely, publicly or privately. Those curious as to why I'm saying this should read Ptacek's other comments on this thread for a primer.

Zed Shaw was right: http://zedshaw.com/essays/thomas_ptacek_should_apologize.htm...

Re: TextSecure's Private Group Messaging

#56

Post by sillysaurus3 »

kaeporan wrote: post #55
tptacek wrote: post #53

Earlier quoted context omitted.

Hence the continuous transcript consistency feature of the TextSecure protocol. Meanwhile: I'm curious about your answer to the question 'sdevlin posted downthread: https://news.ycombinator.com/item?id=7701510 Does your transcript consistency rely on the security of your server?

Sorry, Thomas, but after you repeatedly replied to my private requests for conflict resolution with threats and abusive remarks, I refuse to interact with you entirely, publicly or privately. Those curious as to why I'm saying this should read Ptacek's other comments on this thread for a primer. Zed Shaw was right: http://zedshaw.com/essays/thomas_ptacek_should_apologize.htm...

Instead of writing your comments for Thomas, why not write them for the HN community? We're all very interested in hearing your thoughts. Thomas poses some good questions; why not consider answering them for us?

Re: TextSecure's Private Group Messaging

#57

Post by [deleted] »

sillysaurus3 wrote: post #56
kaeporan wrote: post #55

Earlier quoted context omitted.

Sorry, Thomas, but after you repeatedly replied to my private requests for conflict resolution with threats and abusive remarks, I refuse to interact with you entirely, publicly or privately. Those curious as to why I'm saying this should read Ptacek's other comments on this thread for a primer. Zed Shaw was right: http://zedshaw.com/essays/thomas_ptacek_should_apologize.htm...

Instead of writing your comments for Thomas, why not write them for the HN community? We're all very interested in hearing your thoughts. Thomas poses some good questions; why not consider answering them for us?

[deleted]

Re: TextSecure's Private Group Messaging

#58

Post by higherpurpose »

moxie wrote: post #46
codelike wrote: post #11

I'm a big fan of TextSecure and recommended it to all my friends, both those in IT and 'normal' people. Usually, I managed to convince them that the open source nature of TextSecure and the crypto experts behind it (e.g. Moxie) make it more secure than Threema/... . However, the more sceptical ones among my friends always asked two questions, which I didn't have a good answer for: 1. What is TextSecure's business mod…

> 1. What is TextSecure's business model? Who pays for the server infrastructure? It's a good question. TextSecure is not a business, so we don't really have a business model in the traditional sense. Open Whisper Systems is a collective project made up of volunteers and a growing number of contributors, who are sometimes paid by donations ( https://whispersystems.org/blog/bithub/ ) and grants. Thus far, we've been a…

What's the relation between the server and the SMS verification when setting up the app? I haven't tried TextSecure in a while, but when I tried right after you announced TextSecure v2, I kept getting failure to send me the SMS, and I had to receive the robo-call. I had a friend from another country who experienced the same. Is that still a problem?

Re: TextSecure's Private Group Messaging

#59

Post by kaeporan »

sillysaurus3 wrote: post #52
kaeporan wrote: post #51

Earlier quoted context omitted.

The attacker could send different Ks to each user so that K decrypts C to a different plaintext.

If you try to decrypt C with any key other than the K that was used to encrypt it, you'll get gibberish (the decryption process will fail). As far as I know, it isn't possible for an attacker to generate a message ciphertext C such that two different decryption keys would both decrypt to valid plaintexts. There can only be one valid decryption key; trying to decrypt C using any other key would yield random output, wo…

I hope I'm not mistaken, but my understanding is that you can have the same message C that would decrypt with K1 to the plaintext "Hello world" but with K2 to another plaintext of your choice ("Jello Warld" or whatever.)

Re: TextSecure's Private Group Messaging

#60

Post by higherpurpose »

phaer wrote: post #54
dublinben wrote: post #44

Earlier quoted context omitted.

I would rather pay a nominal fee to support their infrastructure than have to rely on Google Play Services for a supposedly "secure" messaging service.

I see you point but it should not weaken the security because encryption happens at the client, Google "only" gets metadata which at least authorities will get anyway. Besides, TextSecure is free software so it might possible to run your own server at least in the future.

Google gets the metadata? That's news to me. I thought the metadata was encrypted by the TextSecure server?
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