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## Exercises7.8Sage Exercises

###### 1.

Construct a keypair for Alice using the first two primes greater than $10^{12}\text{.}$ For your choice of $E\text{,}$ use a single prime number and use the smallest possible choice.

Output the values of $n\text{,}$ $E\text{,}$ and $D$ for Alice. Then use Sage commands to verify that Alice's encryption and decryption keys are multiplicative inverses.

###### 2.

Construct a keypair for Bob using the first two primes greater than $2\cdot 10^{12}\text{.}$ For your choice of $E\text{,}$ use a single prime number and use the smallest possible choice. Output the values of $n\text{,}$ $E\text{,}$ and $D$ for Alice.

Encode the word Math using ASCII values in the same manner as described in this section (keep the capitalization as shown). Create a signed message of this word for communication from Alice to Bob. Output the three integers: the message, the signed message and the signed, encrypted message.

###### 3.

Demonstrate how Bob converts the message received from Alice back into the word Math. Output the value of the intermediate computations and the final human-readable message.

###### 4.

Create a new signed message from Alice to Bob. Simulate the message being tampered with by adding $1$ to the integer Bob receives, before he decrypts it. What result does Bob get for the letters of the message when he decrypts and unsigns the tampered message?

###### 5.Classroom Exercise.

Organize a class into several small groups. Have each group construct key pairs with some minimum size (digits in $n$). Each group should keep their private key to themselves, but make their public key available to everybody in the room. It could be written on the board (error-prone) or maybe pasted in a public site like pastebin.com. Then each group can send a signed message to another group, where the groups could be arranged logically in a circular fashion for this purpose. Of course, messages should be posted publicly as well. Expect a success rate somewhere between 50% and 100%.

If you do not do this in class, grab a study buddy and send each other messages in the same manner. Expect a success rate of 0%, 50% or 100%.