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Cryptography

This material was developed with funding from the
National Science Foundation under Grant # DUE 1601612

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Symmetric

Algorithm
a mathematical procedure performed on data

Key
specifies the transformation of plaintext into ciphertext

Take a Quiz

Asymmetric

Digital
Signature

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Click on each box for more information.

The use of cryptography to protect secrets by using codes and ciphers began thousands of years ago. Modern cryptography uses algorithms that use a key to encrypt and decrypt information.

As a child, you may have opened your cereal box to find a secret decoder ring as the prize inside the box. You could now send secret messages to your best friend that only he would be able to read.

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If you want to send your friend a secret message, you would write out the message and use your secret decoder ring to change the characters of the original message to create ciphertext. If someone else found the ciphertext, he would not know what the message meant.

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Original Plaintext

Decryption

Ciphertext

Symmetric Key

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The secret decoder ring is an example of using a symmetric-key algorithm—the same cryptographic key both encrypts the original plaintext into ciphertext and decrypts the ciphertext back into plaintext.

Encryption

Asymmetric encryption uses two different, mathematically related keys to encrypt and decrypt messages (a key pair). A user has a public key which is available to anyone who wants to communicate securely. The user's private key remains protected on the device. The key pair is based on prime numbers having many digits. Since there is an infinite number of prime numbers available, the key pair remains secure.

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Main Menu

Public Key

Private Key

Key Pair

Symmetric-key algorithms require that both the sender and the recipient of the message have the same secret key. Alice needs the same secret decoder ring as Bob to decode the message. How is Bob going to get Alice the same decoder ring? Distributing the key in a secure way is one of the challenges of symmetric encryption. For example, what happens to Bob's message if he loses or misplaces the ring?

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Once Alice has a secret decoder ring, she also needs to know how Bob created the ciphertext. Bob’s ring used a very simple methodology—each alphabetic character is replaced by a numerical value. The key is that A=19, B=20, C=21; the value 1 corresponds to the letter “I”.

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Alice’s
Private Key

If Bob wants to send Alice a message that only she could read, Bob would use Alice’s Public Key to encrypt the message. Because Alice’s key pair is mathematically related, whatever is encrypted with Alice’s Public Key can only be decrypted by Alice’s Private Key. The use of encryption maintains the confidentiality of the original message.

Confidentiality
information is not made available or disclosed to unauthorized individuals

Alice’s
Public Key

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If Carol intercepts the message, she will not be able to decrypt and read it even though she also has access to Alice’s Public Key. Asymmetric encryption algorithms are secure because there is no way to calculate the second key if a user has access to one of the keys. Therefore, Carol will not be able to calculate Alice’s Private Key even though she has access to Alice’s Public Key.

Since Alice is the only one who has access to the corresponding Private Key, she is the only one who can decrypt the message. Asymmetric cryptography does not have the key distribution or key management problem associated with symmetric encryption.

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Say Bob needs an app for mobile messaging. He finds one that allows messages and free calls with other users that have the app. This app uses end-to-end encryption which is an implementation of asymmetric encryption.

Bob’s
Public Key

Carol's
Private Key

When Bob installs the app on his smartphone, his public key gets registered with the app’s server and his private key remains protected on his smartphone. His friend, Carol, also has the app installed on her smartphone, and her Public Key is also stored on the app’s server. When using this app, Bob and Carol can maintain the confidentiality of their messages.

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Carol’s
Public Key

Bob's
Private Key

Nonrepudiation
a sender in a transaction cannot deny having sent the communication

Another practical implementation of cryptography is the digital signature. A digital signature provides proof of authentication and nonrepudiation. Manually signing a document and sending it to the needed location can be time-consuming. There may be a delay to complete the manual signature process that may negatively impact the transaction. Digitally signing the documents can avoid a delay.

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Digital Signature

Authentication
the act of verifying a claim of identity

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A digital signature uses asymmetric-key encryption. Bob and Carol need to sign a contract. Bob and Carol both have a public-private key pair. To create a digital signature, Bob uses his private key to sign the document and transfers it to Carol.

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Carol uses Bob’s public key (which she has access to since it is publicly available) to decrypt the message. Carol is now sure that the message came from Bob since he is the only one who has access to his Private Key. This message is not confidential, though, since anyone that has access to Bob’s Public Key can decrypt it.
Additionally, Bob cannot deny that he sent the message which is called non-repudiation.

If Bob wanted to send Carol a confidential message, what key would he use to encrypt the message?
Click on one of the keys to choose the correct key.

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That’s correct!
All users’ Public Keys are stored on a third-party server maintained by the app. Since you want only Carol to read the message, you would need to use her Public Key to encrypt the message. She uses her Private Key (stored on her smartphone) to decrypt the message.

The act of verifying a claim of identity

Confidentiality

Algorithm

Note for Jasmine When the all correct keys are dropped please show
“final feedback”
Could they also hear the sound “confirmation2.mp3"

A sender in a transaction cannot deny having sent the communication

Great Job!

Information is not made available or disclosed to unauthorized individuals

A mathematical procedure performed on data

Authentication

Element
with Audio
HTML

Nonrepudiation

Note for Jasmine When the correct term is dropped please show
“checkmark” over the drop box
Could they also hear the sound “confirmation2.mp3"

Drag each term to its appropriate description.

Alice's
Private Key

Great Job!
All users’ Public Keys are stored on a third-party server. Since only Bob should read the message, Alice uses Bob’s Public Key to encrypt the message. Bob uses his Private Key to decrypt the message.

Note for Jasmine Answer:Drag Bob’s Public Key over encryptionDrag Bob’s Private Key over decryption

Drag and drop the correct key here

Alice and Bob are interested in maintaining confidentiality. What keys should be used?
Drag and drop the correct keys to the corresponding spaces above.

Note for Jasmine When the correct key is dropped please show
“checkmark” over the key for a couple seconds
Could they also hear the sound “confirmation2.mp3"

Great Job!
In this case, Carol and Alice are not looking to keep their communication confidential. Anyone having access to Carol’s Public Key will be able to see the message.

Carol needs to provide assurance to Alice that she sent the communication that Alice received. What keys should be used?
Drag and drop the correct keys to the corresponding spaces above.

Note for Jasmine When the correct key is dropped please show
“checkmark” over the key for a couple seconds
Could they also hear the sound “confirmation2.mp3"

Note for Jasmine Answer:Drag Carol’s Private Key over encryptionDrag Carol’s Public Key over decryption

Symmetric encryption ensures nonrepudiation but not confidentiality

Someone can calculate the private key by accessing the public key

Securely distributing the key

Storage of Public Key

What is the primary issue when using symmetric encryption?
Click to choose the correct answer.

That’s correct!
Since the same key is used to encrypt and decrypt the original plaintext, the challenge for Bob is making sure that Alice has the same key he used.