Page 304 - DCAP103_Principle of operating system

Unit 9: System Security

9.4.2.1 Secret Key Cryptography Notes
With secret key cryptography, a single key is used for both encryption and decryption. As
shown in Figure 9.1, the sender uses the key (or some set of rules) to encrypt the plaintext and
sends the ciphertext to the receiver. The receiver applies the same key (or rule set) to decrypt
the message and recover the plaintext. Because a single key is used for both functions, secret
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 secret. The biggest difficulty with this approach, of course,
is the distribution of the key.
Secret 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.
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.
Block ciphers can operate in one of several modes; the following four are the most important:
• Electronic Codebook (ECB) mode is the simplest, most obvious application—the secret key
is used to encrypt the plaintext block to form a ciphertext block. Two identical plaintext
blocks, then, will always generate the same ciphertext block. Although this is the most
common mode of block ciphers, it is susceptible to a variety of brute-force attacks.

• Cipher Block Chaining (CBC) mode adds a feedback mechanism to the encryption scheme.
In CBC, the plaintext is exclusively-ORed (XORed) with the previous ciphertext block
prior to encryption. In this mode, two identical blocks of plaintext never encrypt to the
same ciphertext.

• Cipher Feedback (CFB) mode is a block cipher implementation as a self-synchronizing
stream cipher. CFB mode allows data to be encrypted in units smaller than the block size,
which might be useful in some applications such as encrypting interactive terminal input.
If we were using 1-byte CFB mode, for example, each incoming character is placed into
a shift register the same size as the block, encrypted, and the block transmitted. At the
receiving side, the ciphertext is decrypted and the extra bits in the block (i.e., everything
above and beyond the one byte) are discarded.

• Output Feedback (OFB) mode is a block cipher implementation conceptually similar to a
synchronous stream cipher. OFB prevents the same plaintext block from generating the
same ciphertext block by using an internal feedback mechanism that is independent of
both the plaintext and ciphertext bitstreams.

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