Reference no: EM132854270

CCM4332 Network Security - Middlesex University London

Public-Key Encryption Laboratory

To explore this system in more depth, you will be exchanging encrypted messages with a partner. Choose your partner now.

1. Launch Microsoft Excel and open the spreadsheet

You may see a warning message informing you that the workbook contains macros. Since you will not need these macros to use the workbook (they are left over from an older and less efficient version of this lab), click on the button.

2. This spreadsheet makes use of some specialized functions that are not part of the standard function set in Microsoft Excel. However, they are included in an extra set of functions called the Analysis Toolpak. From the Tools menu, choose Add-Ins.... In the dialogue box, click on the check box to the left of the entry Analysis Toolpak. When a checkmark appears, click on OK.

3. If necessary, click on the tab for the Key Selection worksheet. Use a random process to choose two different prime numbers p and q between 137 and 311 (displayed in a list in cells g5:I15). Enter these primes in cells B6 and B7. Be sure that cells C6 and C7 both display the message "OK". The spreadsheet automatically computes the modulus (the product p*q) in cell B8 and the Euler totient (the product (p-1)*(q-1)) in cell B9. Note that the Euler totient would be difficult to determine from the modulus by itself; one needs to know the two primes. Write your two primes, your modulus, and your Euler totient below:

p:       q:

modulus:         Euler totient:

4. Choose a small number (no more that two digits) that has no factors (except 1) in common with the Euler totient. Enter this number as your public key and enter it in cell B15. If cell C15 displays the message Invalid Public Key, you need to select a different public key. When you have chosen a valid public key, the message OK will appear in cell C15. The spreadsheet will automatically compute your private key in cell B20. The private key is chosen so that (Public Key)*(Private Key) leaves a remainder of one when divided by the Euler totient. (This would not be possible if the private had a factor other than 1 in common with the Euler totient.) Write your public and private keys below:

5. Once both you and your partner have each created a modulus and pair of keys, you are ready to exchange encrypted messages. Give your modulus and public key to your partner. Do not give your partner your private key or Euler totient. In return, your partner will give you her/his public key and modulus.

6. Click on the tab for the Encoding worksheet. Enter your partner's modulus and public key in cells B6 and B7. Write these values below:

Partner's modulus:

Partner's public key:

7. Enter a message in cell B11. This message should consist of a string of fifteen or more CAPITAL LETTERS with no spaces or punctuation marks. The spreadsheet will encipher only the first fifteen letters of your message. Your message could be a short phrase or sentence, your mother's name or your pet iguana's name. For example, I used RUTHJULIABENHAM (my mother's name) and PLEASEHELPMENOW to test this spreadsheet. Note that a message to be enciphered is usually called plaintext. The enciphered form of the message is called the ciphertext.

8. The enciphered form of the message (the ciphertext) should appear in cell B13. (This may take a few seconds.) The spreadsheet determines the ciphertext as follows:

• Split the plaintext up into blocks of three letters (called trigraphs).
• Obtain a numeric representation for each letter based on its position in the alphabet (A→0, B→1, etc.).
• Compute a numeric code for each trigraph using the formula

(First Letter Code) * 262 + (Second Letter Code) * 26 + (Third Letter code).

For the mathematically inclined, this is interpreting each trigraph as a number in base twenty-six.

• Encipher each plaintext trigraph code by computing
(Plaintext trigraph code)Public Key, dividing the result by the Modulus and taking the remainder.
• Convert each enciphered trigraph code into a quadragraph - a block of four letters - as follows:

Divide the code by 263. The quotient is the code for the first letter of the quadragraph. The spreadsheet uses the remainder to get codes for the other three letters.
Divide the remainder from the first step by 262. The quotient is the code for the second letter. The spreadsheet uses the remainder to get the codes for the other two letters.

Divide the remainder from the second step by 26. The quotient is the code for the third letter and the remainder is the code for the fourth letter.

For the mathematically inclined, this quadragraph calculation determines the representation of the enciphered message as a four-digit number in base twenty-six (using the letters of the alphabet as our digits).

Some of the details of this calculation appear in cells A16:K38 of the Encoding worksheet. Enter the plaintext and ciphertext below. Show the steps of the conversion process in the table.

9. Give the ciphertext (but not the plaintext) to your partner. In return, your partner will give you a ciphertext message. Record the ciphertext message from your partner below. In the rest of this exercise, you will be deciphering this message.

Ciphertext from partner:

10. Click on the tab for the Decoding worksheet. Enter your modulus and your private key in cells B6 and B7 of this worksheet. Enter the ciphertext you received from your partner as the "Encrypted Message" in cell B13. The deciphering process is similar to the enciphering process:

• Split the ciphertext up into quadragraphs (instead of trigraphs).
• Obtain the numeric representation for each letter and compute a numeric code for each trigraph using the formula

(First Letter Code) * 263 + (Second Letter Code) * 262 + (Third Letter Code) * 26 + (Fourth Letter Code).

Encipher each ciphertext quadragraph code by computing (Ciphertext quadragraph code)Private Key,
dividing the result by the Modulus and taking the remainder

• Convert each deciphered quadragraph code into a trigraph.
Divide the code by 262. The quotient is the code for the first letter.
Divide the remainder from the first step by 26. The quotient will be the code for the second letter and the remainder the code for the third.

Note that deciphering uses the private key in place of the public key. Some of the details of this calculation appear in cells A19:D23 of the Decoding Worksheet. The deciphered message should appear in cell B13. Record the results of each deciphering step in the table below.

Now, write the deciphered message (plaintext) below.

Deciphered message:

11. Quit Excel. You do not need to save changes. Shut down the computer. When you're finished, hand this lab in to the instructor.

If you forgotten how to calculate RSA keys then, the following should offer you some help.

1. Choose two large prime numbers p, q. (e.g., 1024 bits each)

2. Compute n = pq, z = (p-1)(q-1)

3. Choose e (with e<n) that has no common factors with z. (e, z are "relatively prime").

4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ).

5. Public key is (n,e). Private key is (n,d).

Wireshark Lab: SSL

In this lab, we'll investigate the Secure Sockets Layer (SSL) protocol, focusing on the SSL records sent over a TCP connection. We'll do so by analyzing a trace of the SSL records sent between your host and an e-commerce server. We'll investigate the various SSL record types as well as the fields in the SSL messages.

1. Capturing packets in an SSL session

The first step is to capture the packets in an SSL session. To do this, you should go to your favorite e-commerce site and begin the process of purchasing an item (but terminating before making the actual purpose!). After capturing the packets with Wireshark, you should set the filter so that it displays only the Ethernet frames that contain SSL records sent from and received by your host. (An SSL record is the same thing as an SSL message.) You should obtain something like screenshot on the previous page.

If you have difficulty creating a trace, you should download and extract the ssl-ethereal- trace-1 packet trace.

2. A look at the captured trace

Your Wireshark GUI should be displaying only the Ethernet frames that have SSL records. It is important to keep in mind that an Ethernet frame may contain one or more SSL records. (This is very different from HTTP, for which each frame contains either one complete HTTP message or a portion of a HTTP message.) Also, an SSL record may not completely fit into an Ethernet frame, in which case multiple frames will be needed to carry the record.

1. For each of the first 8 Ethernet frames, specify the source of the frame (client or server), determine the number of SSL records that are included in the frame, and list the SSL record types that are included in the frame. Draw a timing diagram between client and server, with one arrow for each SSL record.

2. Each of the SSL records begins with the same three fields (with possibly different values). One of these fields is "content type" and has length of one byte. List all three fields and their lengths.

ClientHello Record:

3. Expand the ClientHello record. (If your trace contains multiple ClientHello records, expand the frame that contains the first one.) What is the value of the content type?

4. Does the ClientHello record contain a nonce (also known as a "challenge")? If so, what is the value of the challenge in hexadecimal notation?

5. Does the ClientHello record advertise the cyber suites it supports? If so, in the first listed suite, what are the public-key algorithm, the symmetric-key algorithm, and the hash algorithm?

ServerHello Record:

6. Locate the ServerHello SSL record. Does this record specify a chosen cipher suite? What are the algorithms in the chosen cipher suite?

7. Does this record include a nonce? If so, how long is it? What is the purpose of the client and server nonces in SSL?

8. Does this record include a session ID? What is the purpose of the session ID?

9. Does this record contain a certificate, or is the certificate included in a separate record. Does the certificate fit into a single Ethernet frame?

Client Key Exchange Record:

10. Locate the client key exchange record. Does this record contain a pre-master secret? What is this secret used for? Is the secret encrypted? If so, how? How long is the encrypted secret?

Change Cipher Spec Record (sent by client) and Encrypted Handshake Record:

11. What is the purpose of the Change Cipher Spec record? How many bytes is the record in your trace?

12. In the encrypted handshake record, what is being encrypted? How?

13. Does the server also send a change cipher record and an encrypted handshake record to the client? How are those records different from those sent by the client?

Application Data

14. How is the application data being encrypted? Do the records containing application data include a MAC? Does Wireshark distinguish between the encrypted application data and the MAC?

15. Comment on and explain anything else that you found interesting in the trace.

Attachment:- Public-Key Encryption.rar