Page 122 - DCAP309_INFORMATION_SECURITY_AND_PRIVACY

Information Security and Privacy

Notes Self Assessment

Fill in the blanks:
9. ....................... techniques are designed to provide the digital counterpart to handwritten
signatures and can be achieved using cryptography.

10. Certification is a means for a ....................... to bind the identity of a user to a public key.

8.5 Cryptographic Algorithms

8.5.1 Symmetric (or Shared) Key Cryptography

Symmetric-key cryptography is also known as shared-key, secret-key, single-key, one-key and
eventually private-key cryptography.
With symmetric key cryptography, a single key is used for both encryption and decryption. The
sender uses the key (or some set of rules) to encrypt the plaintext and sends the cipher text to the
receiver. The receiver applies the same key (or ruleset) to decrypt the message and recover the
plaintext. Because a single key is used for both functions, symmetric key cryptography is also
called symmetric encryption.
With this form of cryptography, it is obvious that the key must be known to both the sender and
the receiver; that, in fact, is the shared. The biggest difficulty with this approach, of course, is the
distribution of the key.
Symmetric key cryptography schemes are generally categorized as being either stream ciphers
or block ciphers. Stream ciphers operate on a single bit (byte or computer word) at a time and
implement some form of feedback mechanism so that the key is constantly changing.
A block cipher is so-called because the scheme encrypts one block of data at a time using the
same key on each block. In general, the same plaintext block will always encrypt to the same
ciphertext when using the same key in a block cipher whereas the same plaintext will encrypt to
different ciphertext in a stream cipher.

Figure 8.5: Symmetric-key Cryptography

Stream ciphers come in several flavors but two are worth mentioning here. Self-synchronizing
stream ciphers calculate each bit in the keystream as a function of the previous n bits in the
keystream. It is termed “self-synchronizing” because the decryption process can stay synchronized
with the encryption process merely by knowing how far into the n-bit keystream it is.
One problem is error propagation; a garbled bit in transmission will result in n garbled bits at
the receiving side. Synchronous stream ciphers generate the keystream in a fashion independent
of the message stream but by using the same keystream generation function at sender and
receiver. While stream ciphers do not propagate transmission errors, they are, by their nature,
periodic so that the keystream will eventually repeat.

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