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Encrypting data is the only sure way to secure data transmitted over a network. Encrypt data with a key, and the receiver or receivers can later decrypt it. To fully protect data transmitted over a network, you should not only encrypt it but also check that it has not been modified, as well as confirm that it was actually created by the claimed author. An encrypted message could still be intercepted and modified, and then reencrypted. Integrity checks such as modification digests make sure that the data was not altered. Though encryption and integrity checks protect the data, they do not authenticate it. You also need to know that the person who claimed to send a message actually is the one who sent it, rather than an imposter. To authenticate a message, the author can sign it using a digital signature. This signature can also be encrypted, allowing the receiver to validate it. Digital signatures ensure that the message you receive is authentic.
Encryption uses a key to encrypt data in such a way that a corresponding key can decrypt it. In the past, older forms of encryption used the same key to both encrypt and decrypt a message. This, however, involved providing the receiver with the key, opening up the possibility that anyone who obtained the key could decrypt the data. Public-key encryption uses two keys to encrypt and decrypt a message, a private key and a public key. The private key you always keep and use to decrypt messages you have received. The public key you make available to those you send messages to. They then use your public key to encrypt any message they want to send to you. The private key decrypts messages, and the public key encrypts them. Each user has a set of private and public keys. Reciprocally, if you want to send messages to another user, you would first obtain the user's public key and use it to encrypt the message you want to send to the user. The user then decrypts the messages with their own private key. In other words, your public key is used by others to encrypt the messages you receive, and you use other users' public keys to encrypt messages you send to them. Each user on your Linux system can have their own public and private keys. They will use the gpg program to generate them and keep their private key in their own directory.
A digital signature is used to both authenticate a message and provide an integrity check. Authentication guarantees that the message has not been modified—that it is the original message sent by you—and the integrity check verifies that it has not been changed. Though usually combined with encrypted messages to provide a greater level of security, digital signatures can also be used for messages that can be sent in the clear. For example, you would want to know if a public notice of upgrades of a Red Hat release was actually sent by Red Hat, and not by someone trying to spread confusion. Such a message still needs to be authenticated, checked to see if it was actually sent by the sender or, if sent by the original sender, was not somehow changed en route. Verification like this protects against modification or substitution of the message by someone pretending to be the sender.
Digitally signing a message involves generating a checksum value from the contents of the message using an encryption algorithm such as the MD5 modification digest algorithm. This is a unique value that accurately represents the size and contents of your message. Any changes to the message of any kind would generate a different value. Such a value provides a way to check the integrity of the data. The MD5 value is then itself encrypted with your private key. When the user receives your message, they decrypt your digital signature with your public key. The user then generates an MD5 value of the message received and compares it with the MD5 value you sent. If they are the same, the message is authenticated—it is the original message sent by you, not a false one sent by a user pretending to be you. The user can use GnuPG (described in the next section) to decrypt and check digital signatures.
Normally, digital signatures are combined with encryption to provide a more secure level of transmission. The message would be encrypted with the recipient's public key, and the digital signature encrypted with your private key. The user would decrypt both the message (with their own private key) and then the signature (with your public key). They would then compare the signature with one the user generates from the message to authenticate it. When GnuPG decodes a message, it will also decode and check a digital signature automatically. Figure 16-1 shows the process for encrypting and digitally signing a message.
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