What can the public half of a key pair do?

• Unlock anything locked by anyone
• Lock a message only the private half unlocks
• Replace the private half entirely with a copy

The private half of a key pair is never revealed to anyone.

A pair (3, 4) is scrambled by multiplying [[1, 2], [0, 1]] times the pair as a column. What is the first number of the new pair?

Maya scrambles a secret note by multiplying each pair of numbers by the matrix [[1, 2], [0, 1]]. To turn the scrambled note back into the real one, what do you multiply by?

• The same matrix [[1, 2], [0, 1]] again
• The undoing matrix [[1, -2], [0, 1]]
• Any matrix that has the same numbers in it
• Nothing can undo it

Why do computers swap a symmetric key instead of carrying the chat on public-key methods?

• Public-key methods are slower, so the fast cipher carries the talk
• Symmetric keys cannot be swapped at all
• Chats are too short for symmetric ciphers to handle

A note has the pair (3, 4). It is scrambled by multiplying the matrix [[1, 2], [0, 1]] times the pair as a column, which gives a new pair. What is the first number of the new pair?

In this matrix secret-message method, if you publish the scrambling matrix for everyone to see, people can work out the undoing matrix from it, so this method cannot safely have one public half.

You publish the toy scrambling matrix for all to see. Why is there no safe public half?

• Nobody can multiply matrices in public
• Anyone can compute the undoing matrix
• Published matrices stop working entirely for everyone

A new pair (5, 1) is scrambled by multiplying [[1, 2], [0, 1]] times the pair as a column. What is the first number of the result?

• 5
• 6
• 7

Two strangers exchange key messages while somebody records everything. What does the watcher end with?

• The agreed secret, read straight from the recordings themselves
• Recorded messages but nothing that reveals the secret
• Nothing at all, the line goes silent