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[Chapter 6] 6.5 Message Digests and Digital Signatures
MD2 was designed by Ronald Rivest and published in RFC 1319. There are no known weaknesses in it, but it is very
slow. To create a faster message-digest, Rivest developed MD4, which was published in Internet RFCS 1186 and
1320. The MD4 algorithm was designed to be fast, compact, and optimized for machines with "little-endian"
architectures.
Some potential attacks against MD4 were published in the cryptographic literature, so Dr. Rivest developed the MD5
algorithm, published in RFC 1321.[21] It was largely a redesign of MD4, and includes one more round of internal
operations and several significant algorithmic changes. Because of the changes, MD5 is somewhat slower than MD4.
However, it is more widely accepted and used than the MD4 algorithm.
[21] Internet RFCs are a form of open standards documents. They can be downloaded or mailed, and they
describe a common set of protocols and data structures for interpretability.
As of early 1996, significant flaws have been discovered in MD4. As a result, the algorithm should not be used.
6.5.4.2 SHA
The Secure Hash Algorithm was developed by NIST with some assistance by the NSA. The algorithm appears to be
closely related to the MD4 algorithm, except that it produces an output of 160 bits instead of 128. Analysis of the
algorithm reveals that some of the differences from the MD4 algorithm are similar in purpose to the improvements
added to the MD5 algorithm (although different in nature).
6.5.4.3 HAVAL
The HAVAL algorithm is a modification of the MD5 algorithm, developed by Yuliang Zheng, Josef Pieprzyk, and
Jennifer Seberry. It can be modified to produce output hash values of various lengths, from 92 bits to 256. It also has
an adjustable number of "rounds" (application of the internal algorithm). The result is that HAVAL can be made to
run faster than MD5, although there may be some corresponding decrease in the strength of the output. Alternatively,
HAVAL can be tuned to produce larger and potentially more secure hash codes.[22]
[22] You should note that merely having longer hash values does not necessarily make a message digest
algorithm more secure.
6.5.4.4 SNEFRU
SNEFRU was designed by Ralph Merkle to produce either 128-bit or 256-bit hash codes. The algorithm can also be
run with a variable number of "rounds" of the internal algorithm. However, analysis by several cryptographers has
shown that SNEFRU has weaknesses that can be exploited, and that you can find arbitrary messages that hash to a
given 128-bit value if the 4-round version is used. Dr. Merkle currently recommends that only 8-round SNEFRU be
used, but this algorithm is significantly slower than the MD5 or HAVAL algorithms.
6.5.5 Other Codes
For the sake of completeness, we will describe two other types of "signature" functions.
6.5.5.1 Checksums
A checksum is a function that is calculated over an input to determine if that input has been corrupted. Most often,
checksums are used to verify that data communications over a modem or network link have not undergone "bit-rot," or
random changes from noise. They may also be built into storage controllers to perform checks on data moved to and
from media: if a checksum doesn't agree with the data, then there may be a problem on the disk or tape.
Checksums are usually calculated as simple linear or polynomial functions over their input, and result in small values
(16 or 32 bits). CRC polynomials, or cyclic-redundancy checksums, are a particular form of checksum that are
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