程序代写代做代考 algorithm Lab title: Symmetric (secret key) and asymmetric (public key) encryption
Lab title: Symmetric (secret key) and asymmetric (public key) encryption
Learning outcomes: Learn how to perform symmetric and asymmetric encryption.
In this lab we will use OpenPGP which is a non-proprietary protocol for encryption using public key
cryptography. It is based on the original PGP (Pretty Good Privacy) software. Specifically we will use
GNU Privacy Guard (GnuPG) which is an open-source implementation of OpenPGP. We will use this
tool via the Linux terminal. A description of how to use this tool can be found at the corresponding
Linux man page:
http://linux.die.net/man/1/gpg2
Symmetric encryption
We will now encrypt and decrypt a file using symmetric key encryption. Create a file which you wish
to encrypt using symmetric encryption (for example secret.txt). With the -c option, gpg2 encrypts the
file using a symmetric key. The file secret.txt can be encrypted using the following command:
gpg2 -c secret.txt
This generates an encrypted file with extension gpg (secret.txt.gpg). Compare the sizes of the original
and encrypted files. With the -d option, gpg2 encrypts the file using a symmetric key. The file
secret.txt.gpg can be decrypted using the following command:
gpg2 -d secret.txt.gpg
Asymmetric encryption
We will now encrypt and decrypt a file using asymmetric key encryption.
1. Generating public and private keypair
Enter the following command into the terminal
gpg2 –full-gen-key
A list of options corresponding to different public-key cryptosystems will be provided. Hit enter to
select the default RSA algorithm. RSA is the most popular asymmetric cryptographic algorithm, and is
built on the difficulty of factoring extremely large composites (a composite number is a positive
integer greater than one which is not prime).
GnuPG next asks you to choose the length of your keys. Longer keys are more secure but take longer
to compute. For the purpose of this lab, press enter to choose the default length of 2014 bits.
Next you will be asked how long you want the key to be valid for. Make an appropriate choice.
The generator will next ask you to answer a series of questions.
Eventually a dialog box will appear asking for a passphrase. This is necessary to ensure that even if
someone does gain access to your secret key, they will not be able to use it.
http://linux.die.net/man/1/gpg2
Finally gpg2 will begin to generate a random key. This requires many random bits and therefore you
will be asked to perform random actions such as moving the mouse randomly. If you chose a large
key size earlier this might take a while. However this only occurs once per keypair that you generate.
Following key generation a block of text will be displayed. The public key ID is a string containing
eigth characters on the line beginning with pub.
To return a list of keypairs stored on your computer (your keyring) enter the following command:
gpg2 –list-keys
The keypair just created should be displayed.
2. Publish public key to keyserver
A keyserver is a repository of public keys. It allows other people to import your public key into their
local keyring given your ID. To publish your public key to a keyserver enter the following command:
gpg2 –send-keys PUB_KEY_ID
Where PUB_KEY_ID is the key ID. GnuPG2 will default to using the keys.gnupg.net public keyserver;
most public keyservers synchronize with one another, so you can safely leave this as the default. You
can confirm that your key has been sent by going to the site http://keys.gnupg.net/ and searching for
your name.
If one does not wish to use a keyserver they can export their public key using the command:
gpg2 –export PUB_KEY_ID > PUB_KEY_ID.pub
This will create a file with extension pub. This file can then be shared and imported by somebody
into their local keyring using the command:
gpg2 –import PUB_KEY_ID.pub
3. Encrypting a document
Create a file which you wish to encrypt using asymmetric encryption (for example secret.txt). You can
encrypt this file using the following command:
gpg2 –encrypt secret.txt
You will be asked to enter the ID for the recipient of this document. The ID in question must be into
your local keyring. For now use the ID you just created.
4. Decrypting a document
To decrypt a document use the following command:
gpg2 –decrypt secret.txt.gpg
http://keys.gnupg.net/
You will be asked for the private key corresponding to the public key used to perform encryption. If
the private key is already stored on your keyring you will be asked for the password to access it.
Communicate with one of your fellow students using asymmetric encryption. You will need to import
their public key into your local keyring from the keyserver. This can be achieved using the following
command (replace PUB_KEY_ID with the public ID of the person in question):
gpg2 –recv-keys PUB_KEY_ID
To return a list of keypairs stored on your computer (your keyring) enter the following command:
gpg2 –list-keys
The keypair just imported should be displayed.
