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[Chapter 6] 6.4 Common Cryptographic Algorithms
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Output Feedback (OFB)
Each mode has particular advantages in some circumstances, such as when transmitting text over a noisy channel,
or when it is necessary to decrypt only a portion of a file. The following provides a brief discussion of these four
methods; consult FIPS PUB 81 or a good textbook on cryptography for details.
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ECB Mode. In electronic code book (ECB) mode, each block of the input is encrypted using the same key,
and the output is written as a block. This method performs simple encryption of a message, a block at a
time. This method may not indicate when portions of a message have been inserted or removed. It works
well with noisy transmission channels - alteration of a few bits will corrupt only a single 64-bit block.
CBC Mode. In cipher block chaining (CBC) mode, the plaintext is first XOR'ed with the encrypted value
of the previous block. Some known value (usually referred to as the initialization vector, or IV) is used for
the first block. The result is then encrypted using the key. Unlike ECB mode, long runs of repeated
characters in the plaintext will be masked in the output. CBC mode is the default mode for Sun
Microsystems' des program.
CFB Mode. In cipher feedback (CFB) mode, the output is fed back into the mechanism. After each block is
encrypted, part of it is shifted into a shift register. The contents of this shift register are encrypted with the
user's key value using (effectively) ECB mode, and this output is XOR'd with the data stream to produce
the encrypted result. This method is self synchronizing, and enables the user to decrypt only a portion of a
large database by starting a fixed distance before the start of the desired data.
OFB Mode. In output feedback (OFB) mode, the output is also fed back into the mechanism. A register is
initialized with some known value (again, the IV). This register is then encrypted with (effectively) ECB
mode using the user's key. The result of this is used as the key to encrypt the data block (using an XOR
operation), and it is also stored back into the register for use on the next block. The algorithm effectively
generates a long stream of key bits that can be used to encrypt/decrypt communication streams, with good
tolerance for small bit errors in the transmission. This mode is almost never used in UNIX-based systems.
All of these modes require that byte and block boundaries remain synchronized between the sender and recipient.
If information is inserted or removed from the encrypted data stream, it is likely that all of the following data
from the point of modification can be rendered unintelligible.
6.4.4.3 DES strength
Ever since DES was first proposed as a national standard, some people have been suspicious of the algorithm.
DES was based on a proprietary encryption algorithm developed by IBM called Lucifer, which IBM had
submitted to the National Bureau of Standards (NBS)[16] for consideration as a national cryptographic standard.
But whereas Lucifer had a key that was 112 bits long, the DES key was shortened to 56 bits at the request of the
National Security Agency. The NSA also requested that certain changes be made in the algorithm's S-boxes.
Many people suspected that NSA had intentionally weakened the Lucifer algorithm, so the final standard adopted
by NBS would not pose a threat to the NSA's ongoing intelligence collection activities. But nobody had any
proof.
[16] NBS later became the National Institute of Standards and Technology.
Today the DES is more than 20 years old, and the algorithm is definitely showing its age. Recently Michael
Weiner, a researcher at Bell Northern Research, published a paper detailing how to build a machine capable of
decrypting messages encrypted with the DES by conducting an exhaustive key search. Such a machine could be
built for a few million dollars, and could break any DES-encrypted message in about a day. We can reasonably
assume that such machines have been built by both governments and private industry.
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