Instructions on how to submit Lab 3: Please download all the required files from the lab3-timing github repo and lab3-ssh github repo.

Upload (timing/ to the lab3-timing gradescope assignment.

Upload (ssh/, questions.txt) to the lab3-ssh gradescope assignment.

Running the Lab on Windows make check and make venv do not natively work on Windows.

If you are using a windows machine, please see the Windows Instructions.

Gradescope autograder: Your code will be graded with the Gradescope autograder with a total timeout of 40 minutes.

There is a STRICT 6.0GB memory limit on Gradescope. Reasonable solutions to this lab should not come close to approaching this memory limit.

Problem 1: Performance

In this problem, you will get some hands-on experience with the performance characteristics of different cryptographic algorithms.

For this lab, you will need access to a machine with OpenSSL installed. The Athena machines will work well for this if you don’t have a convenient local environment to use. To test whether you are on a machine with OpenSSL, run the shell command openssl version. You should see some output like this:

$ openssl version
OpenSSL 3.0.7 1 Nov 2022 (Library: OpenSSL 3.0.7 1 Nov 2022)

Now, run openssl help. It should display some output like this:

$ openssl help

Standard commands
asn1parse         ca                ciphers           cmp
cms               crl               crl2pkcs7         dgst
dhparam           dsa               dsaparam          ec
ecparam           enc               engine            errstr
fipsinstall       gendsa            genpkey           genrsa
help              info              kdf               list
mac               nseq              ocsp              passwd
pkcs12            pkcs7             pkcs8             pkey
pkeyparam         pkeyutl           prime             rand
rehash            req               rsa               rsautl
s_client          s_server          s_time            sess_id
smime             speed             spkac             srp
storeutl          ts                verify            version

Message Digest commands (see the `dgst' command for more details)
blake2b512        blake2s256        md4               md5
mdc2              rmd160            sha1              sha224
sha256            sha3-224          sha3-256          sha3-384
sha3-512          sha384            sha512            sha512-224
sha512-256        shake128          shake256          sm3

Cipher commands (see the `enc' command for more details)
aes-128-cbc       aes-128-ecb       aes-192-cbc       aes-192-ecb
aes-256-cbc       aes-256-ecb       aria-128-cbc      aria-128-cfb
aria-128-cfb1     aria-128-cfb8     aria-128-ctr      aria-128-ecb
aria-128-ofb      aria-192-cbc      aria-192-cfb      aria-192-cfb1
aria-192-cfb8     aria-192-ctr      aria-192-ecb      aria-192-ofb
aria-256-cbc      aria-256-cfb      aria-256-cfb1     aria-256-cfb8
aria-256-ctr      aria-256-ecb      aria-256-ofb      base64
bf                bf-cbc            bf-cfb            bf-ecb
bf-ofb            camellia-128-cbc  camellia-128-ecb  camellia-192-cbc
camellia-192-ecb  camellia-256-cbc  camellia-256-ecb  cast
cast-cbc          cast5-cbc         cast5-cfb         cast5-ecb
cast5-ofb         des               des-cbc           des-cfb
des-ecb           des-ede           des-ede-cbc       des-ede-cfb
des-ede-ofb       des-ede3          des-ede3-cbc      des-ede3-cfb
des-ede3-ofb      des-ofb           des3              desx
idea              idea-cbc          idea-cfb          idea-ecb
idea-ofb          rc2               rc2-40-cbc        rc2-64-cbc
rc2-cbc           rc2-cfb           rc2-ecb           rc2-ofb
rc4               rc4-40            seed              seed-cbc
seed-cfb          seed-ecb          seed-ofb          sm4-cbc
sm4-cfb           sm4-ctr           sm4-ecb           sm4-ofb

You can see benchmark each of the message-digest and cipher algorithms listed here by running openssl speed <ALG_NAME>. For example openssl speed rc4 will give you performance numbers for the RC4 block cipher.

A few additional algorithms not listed above are:

  • rsa1024, rsa2048, rsa4096 – RSA keypair generation and signing

  • ecdh – Elliptic-curve Diffie-Hellman key exchange

  • dsa – The digital signature algorithm, working over the integers modulo a prime p

  • ecsda – Elliptic-curve digital signature algorithm

In addition, the following command runs AES encryption using hardware acceleration (if your machine supports it), with a 256-bit key in Galois counter mode:

openssl speed -evp aes-256-gcm

The command openssl genrsa 4096 will generate a 4096-bit keypair.

The command openssl speed -help will give you more options that you can pass to the speed command.


  1. You are designing a file-storage application that requires computing a MAC over large files. You have the option between using HMAC-SHA256 and AES-128-GMAC. Both of these MACs give 128-bit security. Which has better performance for encrypting >1MB files? (You may have to do a little bit of research on the design of both of these primitives to come up with a good answer.)

  2. Your boss tells you that to protect against quantum computers, your company will have to switch from using AES-128 encryption to AES-256 encryption. Roughly how much longer will it take to encrypt a 100MB file after increasing the key size? Explain why in at most three sentences.

  3. MIT has asked you to redesign the software on the MIT certificate authority (CA). They are deciding between using RSA, DSA, and ECDSA signatures.

    1. What is the minimum keylength you must use for each of these three signature algorithms to achieve 128-bit security under the best-known attacks today?

      Answer the following sub-questions assuming that you use key sizes for each algorithm that achieve 128-bit security.

    2. Which of these three algorithms is fastest/slowest for signing?

    3. Which of these three algorithms is fastest/slowest for signature verification?

    4. Which of these three algorithms is fastest/slowest for keypair generation? (You should be able to infer the answer to this question from the output of the openssl commands given above.)

  4. MIT’s Touchstone authentication service allows users to authenticated using a username and password. Which password-hashing function would you use for storing hashed passwords on the server? Explain why in at most three sentences.

Problem 2: Timing side-channel attack

In this problem you will mount your own attack to extract a secret password from a server using an insecure authentication scheme.

The code for this assignment is in timing.


Bob runs a payments service that, after Bob authenticates by sending a password to the server, runs the send_money routine to process a payment. (In this toy example, send_money is a no-op.)

In a secure implementation, Bob’s server would use a robust off-the-shelf authenticated transport protocol (SSH, TLS 1.3 with pre-shared keys, etc.). But since Bob has not taken 6.1600 yet, he cooked up his own ad-hoc authentication protocol.

Bob’s server accepts requests from the network, where each request contains a password. Bob’s server checks the request’s password against the true password and calls the send_money function only if the passwords match.

More specifically

On initialization, a BadServer instance generates a secret password using fresh randomness and saves it as a hexadecimal string i.e., one with characters from 0 to 9 or a to f. Note two hexadecimal characters represent one byte of data.

The BadServer allows any user to submit a VerifyRequest with some password. The server responds with a VerifyResponse, which contains a single boolean value. This value is True if the password in the request matches the server’s secret. Otherwise, the value is False.

Implementation errors make it possible for you, the attacker, to violate this property. In particular, software side channels (specifically, timing side channels) foil Bob’s attempt to achieve this property.

Your job

Your job is to implement steal_password in timing/ to steal the secret password from the server.

Timing side channels have nondeterministic behavior. Thus, while we have provided an autograder to help you develop your solution locally, you will need it to submit your solution to our autograder on Gradescope to receive an accurate evaluation.

In particular,

  • The autograder will test whether you can extract passwords of different lengths. The length in bytes is the l parameter to steal_password.
  • Every test will wait 20 minutes for the attacker to extract the secret password. Your attack must complete by this time (or the autograder will reject it).
  • Your attack must not crash or fail (or the autograder will reject it).
  • To compute your final grade, the autograder will only be run a limited number of times per student. Make sure that your attack succeeds with a comfortably-high probability.

Finally, you must not access private variables of the BadServer instance.

Problem 3: SSH Security

In this problem, you will mount two different attacks on a real SSH implementation. The SSH client and server are built with the very slick paramiko library.

The starter code for this assignment is in ssh.

You will have to implement two functions in ssh/ You should not change any of the other file – we will grade your solution against fresh copies of these files.

Getting started

Run make venv to set up your development environment.

Running the server

Run venv/bin/python to start the ssh server.

If everything works, you should see the following message:

Listening for connection ...

To run the grader, open a new shell (separate from the shell that is running the server) and run:


It might be helpful for you to enable paramiko’s debug-level log messages. The following lines of code will enable logging:

import logging


Part (a): Compress then encrypt

The SSH protocol supports compression: the client first compresses the data it wants to send to the server, and then the client encrypts it. In this problem, you will see how an attacker can abuse compress-then-encrypt to decrypt encrypted traffic. This attack works against real SSH implementations.

The code in grade_decrypt of ssh/ chooses the names of three capital cities at random and puts them into a JSON object like this:

"city0": "Victoria, Seychelles",
"city1": "Seoul, South Korea",
"city2": "Paris, France",

The grader stores this object in a string called secret, and then sends it via SSH to the SSH server.

As the attacker in this problem, you are able to somehow convince the SSH client to send a string evil of your choosing alongside the string secret. So the string that the SSH client sends to the server is actually evil+secret. (This scenario can occur in a number of contexts. On the web, for example, a malicious website can trick a client into sending HTTPS requests that combine attacker-controlled fields with client secrets.)

Your task: You must implement the function attack_decrypt in ssh/ Your code may call client_fn many times as:

(bytes_out, bytes_in) = client_fn("evil string")

This instructs the SSH client to send the string "evil string" + secret to the SSH server over the encrypted channel. The function returns the number of bytes sent to the server (bytes_out) and bytes received from the server (bytes_in) during the client-server interaction.

Your job is to recover the string secret exactly.

NOTE: Your attack does not need to succeed with probability 1. It is good enough if your attack works with probability 10% or so – as long as you can convince the grader to accept your solution.

Part (b): Tampering with packets

A standard SSH implementation applies a MAC to each SSH-protocol packet. In this part, you will attack a broken implementation of SSH that does not use MACs at all. (More precisely, our implementation uses a MAC that always outputs a 160-bit string of zeros. The code for this no-op MAC is in ssh/

The code in grade_tamper of ssh/ starts an SSH client, connects to the SSH server, and sends the shell command ls ./file/* to the server.

Your task: You will play the role of an in-network attacker that tampers with the SSH packets as they flow from the client to the server. Your task is to cause the server to receive the shell command rm -r /, instead of the command that the client intended to send..

In particular, you must implement the function AttackTamper.handle_data in ssh/ The grader code will invoke your handle_data function each time the SSH client sends data to the SSH server. The function takes as input the raw bytes of the data that the SSH client sent and it outputs whatever bytes the attacker wants to relay to the SSH server. If you want to save state across invocations of handle_data, you can store it as members of AttackTamper. (That is, handle_data can set self.blah = 7 to store the value of blah.)

If the SSH server receives the special string rm -r /, it will reply to the client with a special message that the grader will use to determine that your attack has worked.

You may find it helpful to read a bit about the SSH protocol in RFC 4253 and RFC 4254, though you should not need to go very deep into either to complete the problem.

Part (c): Extra credit

Repeat the attack of Part (b), except that now you must mount the attack against an SSH client that uses packet compression.