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fix: spelling
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Andre Henriques 2023-12-30 16:32:06 +00:00
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@ -53,17 +53,17 @@
\subsection*{1.1}
The file ag01598\_6644818\_1\_1.spdl contains the base model of $\text{protocol}\Pi$.
I choose the names of the roles based on their functions since it would make the file more readable so R is Phone, S is Application, N is Network.
I choose the names of the roles based on their functions since it would make the file more readable, so R is Phone, S is Application, N is Network.
As the diagram shows the first message is sent from the phone do the network to request the generation of a new session key.
As the diagram shows, the first message is sent from the phone to the network to request the generation of a new session key.
The keys where modeled using a custom usertype called ``SessionKey'' and the time to live has modeled using a custom usertype called ``Timestamp''
The keys were modelled using a custom usertype called ``SessionKey'' and the time to live has modelled using a custom usertype called ``Timestamp''
The Network then aswers to the Phone and the Application the keys and the time to live and the hashed value of that using a hash function named ``Mac''.
The network then answers to the Phone and the Application the keys and the time to live and the hashed value of that using a hash function named ``Mac''.
The Phone and the Application verify the Mac and then the phone sends a nonce to the phone and the phone answers back with a new nonce and the original nonce.
The Mac was modeled as a hashing function then encryption this was done this way because scyther does not have a way of creating a mac function with keys so the hashing is done first followed by the encrypted so that an attacker can not modify it.
The Mac was modelled as a hashing function, then encryption. This was done this way because Scyther does not have a way of creating a mac function with keys, so the hashing is done first followed by the encrypted so that an attacker cannot modify it.
\subsection*{1.2}
The file ag01598\_6644818\_1\_2.spdl contains the base model of $\text{protocol}\Pi$ and the claims.
@ -82,70 +82,70 @@
The protocol as it stands does not guarantee secrecy and agreement.
\subsection*{1.3}
The file ag01598\_6644818\_1\_3.spdl contains the fixed version of $\text{protocol}\Pi$
The file ag01598\_6644818\_1\_3.spdl contains the fixed version of $\text{protocol}\Pi$.
The first change was to require the refresh keys request was to require the application to send a nounce, this nonce is then sent back to application to verify that the key was generated, was requested to the application and not by the attacker.
The first change was to require the refresh keys request was to require the application to send a nonce. This nonce is then sent back to the application to verify that the key was generated, was requested by the application and not by the attacker.
The second change was to make the network send the identity of the other party to party that is reciving the message. i.e. Sending the identity of the Phone to the Application encrypted with the key Network,Application. This is done to guarantee that the Party reciving the communication is using a key that was intended for this communication.
The second change was to make the network send the identity of the other party to the party that is receiving the message. i.e. Sending the identity of the Phone to the Application encrypted with the key Network, Application. This is done to guarantee that the Party receiving the communication is using a key that was intended for this communication.
\subsection*{1.4}
The original $\text{protocol}\Pi$ is not an apropiate solution of the the third party problem as it can not guarantee the secresy of the session key, and since an attacker can obtain the session key, $\text{protocol}\Pi$ is not an appropiate solution to the present problem.As scyther showed the are attacks in the Dolev-Yao model. There are also some attacks in outside of Dolev-Yao model.
The original $\text{protocol}\Pi$ is not an appropriate solution to the third-party problem as it cannot guarantee the secrecy of the session key, and since an attacker can obtain the session key, $\text{protocol}\Pi$ is not an appropriate solution to the present problem. As Scyther, showed that there are attacks on the Dolev-Yao model. There are also some attacks outside the Dolev-Yao model.
For example:
With the assumptions that:
With the assumptions, that:
\begin{itemize}
\item{Dolev-Yao attacker.}
\item{Strong cryptographic primitives}
\item{The time to live is implemented as a counter not an endate timetamp.}
\item{One of the previous keys where the encrypted version of the key and timestamp were recorded and leaked.}
\item{The time to live is implemented as a counter, and not as an end date timestamp.}
\item{One of the previous keys, where the encrypted version of the key and timestamp were recorded and leaked.}
\end{itemize}
In this senario an attacker can record the messages where the key and the time to live were encrypted. Then when the key gets leaked, the attacker can perform a replay attack.
In this scenario, an attacker can record the messages where the key and the time to live were encrypted. When the key gets leaked, the attacker can perform a replay attack.
The attacker resends the previously recorded keys to the application server and the phone, because the time to live is based on a counter and not on a timestamp the phone and the server accept the ``new'' key, and since the attacker already knows this key the protocol failed at guarantee, the secrecy of the session key.
The attacker resends the previously recorded keys to the application server and the phone because the time to live is based on a counter and not on a timestamp, the phone, and the server accept the ``new'' key. And since the attacker already knows this key, the protocol failed to guarantee, the secrecy of the session key.
% Other possible attacks are possible, for example a senario where the time to live is large, and the cryptographic primitives are weak for that time frame, i.e. RSA with 100 decimal digits and time to live of 2 days, an attacter with enought computer power would be abble to obtain the key
But it can be improved, as we saw in the answers for the question 1.3 by modifing the protocol slightly we can achive secrecy of the session key in the dolev-yao model. Although it does not resolve issues related with the time to live if the protocol was implemented with counters for time to live instead of timestamp.
But it can be improved, as we saw in the answers for the question 1.3 by modifying the protocol slightly we can achieve secrecy of the session key in the Dolev-Yao model. Although it does not resolve issues related with the time to live if the protocol was implemented with counters for time to live instead of timestamp.
\subsection*{1.5}
The file ag01598\_6644818\_1\_5.spdl contains the a more comunication efficient version of the $\text{protocol}\Pi$
The file ag01598\_6644818\_1\_5.spdl contains a more communication efficient version of the $\text{protocol}\Pi$.
This version trades off computational power for communication efficiency. This version creates bigger encrypted messages and as trade off reduces the number of messages that are sent.
This version trades off computational power for communication efficiency. This version creates bigger encrypted messages, and as a trade-off reduces the number of messages that are sent.
The message that is removed is the message where the network sends the key to the phone. This data in this message is still send but it's send when the application sends the m message.
The message that is removed is the message where the network sends the key to the phone. This data in this message is still sent, but it's sent when the application sends the m message.
And the data is sent to the Application inside the encrypted packed that is already sent to the Application when the phone recives the keys from the network.
And the data is sent to the Application inside the encrypted packed that is already sent to the Application when the phone receives the keys from the network.
It sends the session key that was send by the network, to the phone, with the message m.
It sends the session key that was sent by the network, to the phone, with the message m.
\section*{2}
\subsection*{2.1}
Using a system like gpg you can generate keys by running the command \begin{verbatim}gpg --gen-key\end{verbatim}.
Using a system like GPG, you can generate keys by running the command \begin{verbatim}gpg --gen-key\end{verbatim}.
The public key are then traded.
The public keys are then traded.
\subsection*{2.2}
There are multiple ways of securily exaging the keys.
For example if meeting up in person was a possiblity the keys cloud be put onto usb drives and the drives exchanged in person. And this would guarantee 100\% authentication, but meeting in person could not be feasable.
There are multiple ways of securely exchanging the keys.
For example, if meeting in person was a possibility, the keys cloud be put onto USB drives and the drives exchanged in person. And this would guarantee 100\% authentication, but meeting in person could not be feasible.
If meeting in person is not feasable an altertive method would be sending the public key via email, and then calling the collegue on the phone. Both parties would then hash key using for example \begin{verbatim}sha512sum key\end{verbatim} and both parties would readout their public keys hash to each other. Since the parties know eachother, and would recoginze the voice this would be a feazable way to exchange the public keys. With this they could verify that the password came from the correct person.
If meeting in person is not feasible, an alterative method would be sending the public key via email, and then calling the colleague on the phone. Both parties would then hash key using for example \begin{verbatim}sha512sum key\end{verbatim} and both parties would read out their public keys hash to each other. Since the parties know each other, and would recognize the voice, this would be a feasible way to exchange the public keys. With this, they could verify that the password came from the correct person.
\subsection*{2.3}
Assuming that the peers were able to exchange public keys.
One of the Peer A would generate a random 256 bit key.
One of the Peer A would generate a random 256-bit key.
Peer A would then encrypt the key using Peer B's public key, and sign it using it's own private key.
Peer A would then encrypt the key using Peer B's public key, and sign it using its own private key.
Peer A would then send the signed and encrypted semetric key to Peer B in an insecure channel.
Peer A would then send the signed and encrypted symmetric key to Peer B in an insecure channel.
Peer B upon reciving the encrypted and signed semetric key, would verify the signature, and decrypt the message and obtain the key.
Peer B upon receiving the encrypted and signed symmetric key, would verify the signature, and decrypt the message and obtain the key.
After this both A and B can comunicate using the semetric key.
After this, both A and B can communicate using the symmetric key.
Example:
@ -181,23 +181,23 @@
\subsection*{2.4}
A secure communication channel for the purpoposes of this answer is a communication channel that can maitain the secrecy, and integtaty.
A secure communication channel for the purposes of this answer is a communication channel that can maintain the secrecy, and integrity.
A communication channel can be achived by trading 2 semetric keys.
A communication channel can be achieved by trading 2 symmetric keys.
One of the keys would be used to encrypt the messages and the other would be used to MAC the messages this would gurantee that the messages are secret because of the encryption and that they have integraty because of the MAC.
One of the keys would be used to encrypt the messages and the other would be used to MAC the messages. This would guarantee that the messages are secret because of the encryption and that they have integrity because of the MAC.
The 2 diferent keys guarantee that the MAC that is generated is significanfly diferent than the encrypted message.
The 2 different keys guarantee that the MAC that is generated is significantly different from the encrypted message.
In this system messages should also contain the timestamp of when the messages was send this is usefull to maintain freshness.
In this system, messages should also contain the timestamp of when the messages were sent. This is useful to maintain freshness.
\subsection*{2.5}
The system would work under a computational system as the cryptographic primitives that were selected are computationaly hard to break.
The system would work under a computational system, as the cryptographic primitives that were selected are computationally challenging to break.
The system would also work with a man in the midle attacker as the attacker can not change the messages witout one of the people feeling suspicios and calling for a restart of the system.
The system would also work with a man in the middle attacker, as the attacker cannot change the messages without one of the people feeling suspicious and calling for a restart of the system.
This system would not work if the method for exachaging keys was via phone, and the attacker had a method of replicating the voice of one of the participants of the system i.e. using an AI.
This system would not work if the method for exchanging keys was via phone, and the attacker had a method of replicating the voice of one of the participants of the system, i.e. using an AI.
%The system can maintain integratity, in the public key exachange phase with the phone call and the hash, in the semetric key exahange phase with the signatures, and in the message exchange phase with the MACs.
@ -207,16 +207,10 @@
\subsection*{2.6}
An altertive solution to the problem is if both parties have a third party that they both know and trust. This system would have less comunication and computaional overhead as the public key exchange could have been skiped and the the semetric key could have been exchanged from the begging.
This solution would have a less level of security as the system relies on the trust of a third party, this includes more levels of failures as the third party could get currupted makeing the system less secure.
An alterative solution to the problem is if both parties have a third party that they both know and trust. This system would have less communication and computational overhead, as the public key exchange could have been skipped and the symmetric key could have been exchanged from the begging.
This solution would have a less level of security as the system relies on the trust of a third party, this includes more levels of failures as the third party could get corrupted, making the system less secure.
\end{document}