Re: [cryptography] Security Discussion: Password Based Key Derivation for Elliptic curve Diffie–Hellman key agreement
Thanks for all the comments so far! Is there a reason you did not consider using OTR? Or another of the many secure chat protocols? We did not want to use OTR, because we do not want to have forward secrecy and message deniability. Our idea is to built an encryption scheme that is completely transparent to the user, it should not appear different to him if he is chatting over an encrypted Facebook chat or not. This way we hope to make encryption easier, less of hassle and more mainstream. If we had session keys that expire after the conversation is over, the user wouldn't be able to read the messages later on (or on a different device) or send offline messages (all things possible with original Facebook Messenger). What safeguards do you have against a MITM attack? We were thinking to query the public key server over HTTPS and validate the certificate (either through a CA or hard coded in the plugin). Also, wouldn't you have to compromise the public key server (to deliver wrong pub keys to both parties) and the communication channel to Facebook (to intercept the message) at the same time? Therefore, we thought that only Facebook itself would have a realistic opportunity for MITM attacks (meaning the user would have to trust us, that we don't cooperate with them). We also thought about building a decentralized Web-of-Trust, but found it hard to establish a second secure channel (assuming that users don't necessarily engage in real life) without impacting usability. ___ cryptography mailing list cryptography@randombit.net http://lists.randombit.net/mailman/listinfo/cryptography
Re: [cryptography] Security Discussion: Password Based Key Derivation for Elliptic curve Diffie–Hellman key agreement
The app/plugin will be open source, so you can see what we are doing. Messages will only be sent to the Facebook XMPP server. On Dec 18, 2013, at 4:24 PM, Jason Goldberg jgoldb...@oneid.com wrote: A MITM attack is more than just trusting your SSL cert or Facebook. How do we know *you* aren’t secretly intercepting our messages? Does your platform assume we have to trust *you*? On Dec 18, 2013, at 3:36 AM, SafeChat.IM i...@safechat.im wrote: Thanks for all the comments so far! Is there a reason you did not consider using OTR? Or another of the many secure chat protocols? We did not want to use OTR, because we do not want to have forward secrecy and message deniability. Our idea is to built an encryption scheme that is completely transparent to the user, it should not appear different to him if he is chatting over an encrypted Facebook chat or not. This way we hope to make encryption easier, less of hassle and more mainstream. If we had session keys that expire after the conversation is over, the user wouldn't be able to read the messages later on (or on a different device) or send offline messages (all things possible with original Facebook Messenger). What safeguards do you have against a MITM attack? We were thinking to query the public key server over HTTPS and validate the certificate (either through a CA or hard coded in the plugin). Also, wouldn't you have to compromise the public key server (to deliver wrong pub keys to both parties) and the communication channel to Facebook (to intercept the message) at the same time? Therefore, we thought that only Facebook itself would have a realistic opportunity for MITM attacks (meaning the user would have to trust us, that we don't cooperate with them). We also thought about building a decentralized Web-of-Trust, but found it hard to establish a second secure channel (assuming that users don't necessarily engage in real life) without impacting usability. ___ cryptography mailing list cryptography@randombit.net http://lists.randombit.net/mailman/listinfo/cryptography ___ cryptography mailing list cryptography@randombit.net http://lists.randombit.net/mailman/listinfo/cryptography
[cryptography] Security Discussion: Password Based Key Derivation for Elliptic curve Diffie–Hellman key agreement
Dear mailing list, A friend and me are working on a plugin that enables encryption on top of Facebook messaging. The idea is to encrypt messages before they leave the chat client, sending only the cipher to Facebook and decrypt the message on the receiver client, before it is displayed. The plugin automatically realizes which users have it installed and only encrypts these chats. Since the reliability of the cryptographic system is a crucial part of the design, I would to discuss the protocol here: First, we use PBKDF2 to derive a 256 bit data block from a passphrase the user chooses and a salt (the username). We advise the user to use a long and hard-to-guess passphrase. We use Parvez Anandam’s JavaScript implementation [1]. This data block serves as the private key for a secp256r1 elliptic curve. We cannot use a random private key, as we have to be able to generate the same private key on different devices of the user. Given this private key, and another user’s public key (exchange through a public key server), we calculate the shared key as defined in the Elliptic curve Diffie–Hellman (ECDH) key agreement protocol: Given Alice’s private key ‘a’ and the elliptic curve ‘G’ (defined by the secp256r1 parameters), Alice’s public key ‘A’ is defined as: A = a*G (Analogously for Bob: B = b*G) If Alice has her private key ‘a’ and Bob’s public key B, she can calculate the shared key S S = a*B = a*b*G Bob has his private key ‘b’ and Alice’s public key ‘A’ to derive the same secret: S’ = b*A = b*a*G = a*b*G = S Tom Wu’s library [2] is used to implement all ECDH related stuff. The shared secret together with a random salt is used as a starting block to generate a 256bit AES key, which eventually encrypts the message. The cipher and the random salt are sent to the other person, so that he can reproduce the symmetric key. We use the Gibberish library for that purpose [3]. Our process is also depicted here: http://goo.gl/ghzWSl Do you see a problem with that approach? I am looking forward to comments and concerns. Thanks! Felix [1] http://anandam.com/pbkdf2/ [2] http://www-cs-students.stanford.edu/~tjw/jsbn/ [3] https://github.com/mdp/gibberish-aes___ cryptography mailing list cryptography@randombit.net http://lists.randombit.net/mailman/listinfo/cryptography
