otr_mackey - Process Off-the-Record Messaging transcripts



otr_parse, otr_sesskeys, otr_mackey, otr_readforge, otr_modify, otr_remac − Process Off-the-Record Messaging transcripts


our_privkey their_pubkey
aes_enc_key [newmsg]
mackey old_text new_text offset
mackey sender_instance receiver_instance flags snd_keyid rcv_keyid pubkey counter encdata revealed_mackeys


Off-the-Record (OTR) Messaging allows you to have private conversations over IM by providing:
- Encryption
- No one else can read your instant messages.
- Authentication
- You are assured the correspondent is who you think it is.
- Deniability
- The messages you send do not have digital signatures that are
checkable by a third party. Anyone can forge messages after a
conversation to make them look like they came from you. However,
a conversation, your correspondent is assured the messages
he sees are authentic and unmodified.
- Perfect forward secrecy
- If you lose control of your private keys, no previous conversation
is compromised.

The OTR Toolkit is useful for analyzing and/or forging OTR messages. Why do we offer this? Primarily, to make absolutely sure that transcripts of OTR conversations are really easy to forge after the fact. [Note that during an OTR conversation, messages can’t be forged without real-time access to the secret keys on the participants’ computers, and in that case, all security has already been lost.] Easily-forgeable transcripts help us provide the "Deniability" property: if someone claims you said something over OTR, they’ll have no proof, as anyone at all can modify a transcript to make it say whatever they like, and still have all the verification come out correctly.

Here are the six programs in the toolkit:

- otr_parse
- Parse OTR messages given on stdin, showing the values of all the
fields in OTR protocol messages.

- otr_sesskeys our_privkey their_pubkey
- Shows our public key, the session id, two AES and two MAC keys
derived from the given Diffie-Hellman keys (one private, one public).

- otr_mackey aes_enc_key
- Shows the MAC key derived from the given AES key.

- otr_readforge aes_enc_key [newmsg]
- Decrypts an OTR Data message using the given AES key, and displays
the message.
- If newmsg is given, replace the message with that one, encrypt
and MAC it properly, and output the resulting OTR Data Message.
This works even if the given key was not correct for the original
message, so as to enable complete forgeries.

- otr_modify mackey old_text new_text offset
- Even if you can’t read the data because you don’t know either
the AES key or the Diffie-Hellman private key, but you can make a
good guess that the substring "old_text" appears at the given
offset in the message, replace the old_text with the new_text
(which must be of the same length), recalculate the MAC with the
given mackey, and output the resulting Data message.
- Note that, even if you don’t know any text in an existing message,
you can still forge messages of your choice using the
otr_readforge command, above.

- otr_remac mackey sender_instance receiver_instance flags snd_keyid rcv_keyid pubkey counter encdata revealed_mackeys
- Make a new OTR protocol version 3 Data Message, with the given
pieces (note that the data part is already encrypted). MAC it
with the given mackey.


Off-the-Record Messaging, at https://www.cypherpunks.ca/otr/


otr_toolkit was written by the OTR Dev Team <[email protected]>.

Updated 2024-01-29 - jenkler.se | uex.se