Which SSL protocol takes an application message to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies a MAC, encrypts, adds a header, and transmits the resulting unit in a TCP segment?
A.
SSL Alert Protocol
B.
SSL Change CipherSpec Protocol
C.
SSL Record Protocol
D.
SSL Handshake Protocol
Explanation:
SSL ArchitectureSSL is designed to make use of TCP to provide a reliable end-to-end secure service. SSL is not a single protocol but rather two layers of protocols.The SSL Record Protocol provides basic security services to various higher-layer protocols. In particular, the HTTP, which provides the transfer service for Web client/server interaction, can operate on top of SSL. Three higher-layer protocols are defined as part of SSL: the Handshake Protocol , the Change CipherSpec Protocol , and the Alert Protocol. These SSL-specific protocols are used in the management of SSL exchanges.SSL Record ProtocolThe SSL Record Protocol provides two services for SSL connections: confidentiality, by encrypting application data; and message integrity, by using a message authentication code (MAC). The Record Protocol is a base protocol that can be utilized by some of the upper-layer protocols of SSL. One of these is the handshake protocol which, as described later, is used to exchange the encryption and authentication keys. It is vital that this key exchange be invisible to anyone who may be watching this session.Figure 1 indicates the overall operation of the SSL Record Protocol. The Record Protocol takes an application message to be transmitted, fragments the data into manageable blocks, optionally compresses the data, applies a MAC, encrypts, adds a header, and transmits the resulting unit in a TCP segment. Received data is decrypted, verified, decompressed, and reassembled and then delivered to the calling application, such as the browser.
The first step is fragmentation. Each upper-layer message is fragmented into blocks of 2 14 bytes (16, 384 bytes) or less. Next, compression is optionally applied. In SLLv3 (as well as the current version of TLS), no compression algorithm is specified, so the default compression algorithm is null. However, specific implementations may include a compression algorithm.The next step in processing is to compute a message authentication code over the compressed data. For this purpose, a shared secret key is used. In essence, the hash code (for example, MD5) is calculated over a combination of the message, a secret key, and some padding. The receiver performs the same calculation and compares the incoming MAC value with the value it computes. If the two values match, the receiver is assured that the message has not been altered in transit. An attacker would not be able to alter both the message and the MAC, because the attacker does not know the secret key needed to generate the MAC.Next, the compressed message plus the MAC are encrypted using symmetric encryption. A variety of encryption algorithms may be used, including the Data Encryption Standard (DES) and triple DES. The final step of SSL Record Protocol processing is to prepend a header, consisting of the following fields:
Content Type (8 bits): The higher-layer protocol used to process the enclosed fragment. Major Version (8 bits): Indicates major version of SSL in use. For SSLv3, the value is 3. Minor Version (8 bits): Indicates minor version in use. For SSLv3, the value is 0. Compressed Length (16 bits): The length in bytes of the plain-text fragment (or compressed fragment if compression is used).
The content types that have been defined are change_cipher_spec, alert, handshake, and application_data. The first three are the SSL-specific protocols, mentioned previously. The application-data type refers to the payload from any application that would normally use TCP but is now using SSL, which in turn uses TCP. In particular, the HTTP protocol that is used for Web transactions falls into the application-data category. A message from HTTP is passed down to SSL, which then wraps this message into an SSL record.
