Homework #2 (14p)

Deadline: extended to 31st of October (the solution has to be submitted before Monday)

Recommended reading

• Announcing the first SHA1 collision
• SHAttered
• HTTPS Certificate Revocation is broken, and it’s time for some new tools

Written tasks

PKC, PKI and HTTPS

1. Hashing
   1. What are the two main differences between encryption and hashing? The answer must be formatted as a list. This question can have multiple correct answers. (1p)
2. Read the following article: HTTPS Certificate Revocation is broken, and it’s time for some new tools. Answer the following questions:
   1. Why would a certificate owner want to revoke a valid certificate? Name at least two common reasons. (1p)
   2. Why are certificate revocation lists not guaranteed to work in practice? (1p)
   3. Online Certificate Status Protocol has also some issues, list them. (1p)

Smartcards, Mobile-ID and e-voting

1. In an opinion story Otto de Voogd wrote about the possibility of the state having access to the secret keys on the Estonian ID-card. As a response to the opinion Agu Kivimägi wrote how private keys are generated. Name two main reasons why the Estonian government can not access / know the secret key that is on your ID-card. The ID-card vulnerabilities that were described in 2017 are out of scope of this question. The answer must be formatted as a list. (1p)
2. Since the spring of 2017 it is possible to find collisions for SHA-1. Currently it is no longer possible to use SHA-1 for issuing legally binding digital signatures. However, does this finding affect the signatures that were given before 2017? Hint: check the security requirements for cryptographic hash functions. Give answers to the following questions.
   
   
   1. Is it now possible to forge digital signatures that were issued in 2010 with SHA-1? You will have to write the reasoning with your own words. (1p)
      
      
   2. What should one do now in order to prevent legal disputes over future forgeries of digital signatures that were previously given using SHA-1? E.g., assume that you signed a contract in 2010 by using SHA-1 and the same contract should be valid for the next 30 years. What would you have to do with the existing digitally signed contract to prevent legal disputes in the future regarding the authenticity of the contract in case someone will be able to forge the signature in the future? The signature has to be in a digital format due to the business model and it is not possible to get a written confirmation by a third party about the validity of the signature. You will have to provide reasoning! (1p)

Lab tasks (7p)

There are four lab tasks that have to be solved. The submission forms are available at the lab page.

1. Task is about image metadata.
2. Task is about TLS and certification information.
3. Task is about syncing files with Syncthing.
4. Task is about using Signal for end-to-end encrypted messaging. As an alternative, also a theory task is offered.

Submission form for the written tasks

The solution for the written tasks has to be submitted through this website. The solution can be submitted once you have logged in with the university credentials. We accept solutions only in .pdf format if it is not stated otherwise in the homework task.

We would like to get feedback about the difficulty of the homework and therefore we would kindly ask you to write in the comments box an estimate of how much time it took to solve the homework tasks.