Essay on Security

1.a. Suppose that Alice, for confidentiality, wants to send Bob a short message using a secret key algorithm (such as the Vigenère cipher). Before she can do this, she and Bob need to have established a secret key. Alice uses this key to encrypt the message, creating the ciphertext. She sends the ciphertext to Bob. Bob receives the ciphertext and decrypts it using the same key. Contrast this to the use of a public key encryption algorithm (such as RSA).
The encryption scheme described in the 1a assessment is called symmetric encryption: it is relatively fast, but it has several significant disadvantages. The major problem is that Alice and Bob have to exchange secret keys, and there’s ultimate need to secure this procedure. For any other recipient, secure key exchange must me maintained. Also, if one of the recipients discloses the secret key, Alice’s mailing privacy will be destroyed.
Asymmetric encryption (such as RSA, DSA etc.), also referred to as public key encryption, overcomes these disadvantages in the following way (Panko, 2009). Alice generates a pair of keys using special mathematical algorithm, one is secret and no one but her knows this key. The other key is public and should be sent to all recipients. To communicate with Bob, Alice needs his public key as well. Before sending the message, Alice uses Bob’s public key to encode the message. Bob uses his secret key to decode the e-mail (Stallings, 2010).
Using public key cryptography, it is much easier to exchange keys. The system also protects against interception of public key since it can neither be used for decrypting the message nor for finding out the secret key (Schneier, 2000). Often, this encryption method is combined with symmetric encryption: public key is encrypted using a symmetric scheme, and then asymmetric encryption is used for sending the message (Stallings, 2010).
In general, asymmetric encryption is safer to a certain extent, and makes it easier to exchange keys. However, symmetric encryption also has advantages: it is faster and loss of secret key will not lead to inability to read all messages (as it is with asymmetric scheme).
1.b. The fundamental operation of public key encryption and decryption was outlined by your answer for 1.a. without mentioning certificates. Very concisely, explain how certificates become involved. Don’t get into chains of certificates and the public-key infrastructure.
Public key cryptography might also be used for authentication purposes, via creating digital signatures. In the above-considered case, Alice needs to hash her message and encrypt it with her private key, thus creating a digital signature. Once Bob receives the message, he decrypts the message with Alice’s public key and computes hash of the message. If these two coincide, Bob can be sure that the message is coming from Alice and that the message was delivered successfully. Also, the pairs of keys may be used to encrypt the message. Thus, public key cryptography provides means for privacy protection, authentication and identity verification.
However, there is a major weak place in this scheme: keys exchange security, their possible loss and revocation (Stallings, 2010). Bob and Alice need to be sure that public keys really belong to the owner, and not to the attacker. In order to secure this exchange, a system of certificates is used (Panko, 2009).
Generally, the certificates usage scheme is the following: there is one entity that is mutually trusted (called God, for example) (Schneier, 2000). God signs with his digital signature public keys distributed by Alice and Bob. When Bob wants to check whether Alice’s public key truly belongs to her, he verifies God’s signature using God’s public key. Bob also might use the same check for other recipients. Thus, instead of securing multiple key distribution processes, it is possible to have only one trusted entity and use this entity’s signature (certificate) to verify other keys. In reality, the system of certificates is hierarchical and more complicated; however, the idea is the same.