Problem - Kevin Tafuro

assume that you always need it, then figure out if your design somehow provides it.
Generally, that will be the case if you use a hash function in a component of a MAC
that requires a nonce, and not true otherwise (however, see Recipe 6.8).
As a result, you should consider MD5 to have, at best, 64 bits of strength. In fact,
considering the weaknesses inherent in MD5, you should assume that, in practice,
MD5’s strength is less than that. 64 bits of security is on the borderline of what is
breakable. (It may or may not be possible for entities with enough resources to bruteforce
64 bits in a reasonable time frame.)
Table 6-1 lists popular cryptographic hash functions and compares important properties
of those functions. Note that the two MDC-2 constructs we detail are covered
by patent restrictions until August 28, 2004, but everything else in this list is widely
believed to be patent-free.
When comparing speeds, times were measured in x86 cycles per byte processed
(lower numbers are better), though results will vary slightly from place to place.
Implementations used for speed testing were either the default OpenSSLimplementation
(when available); the implementation in this book using OpenSSLversions of
the underlying cryptographic primitives; or, when neither of those two were available,
a reference implementation from the Web (in particular, for the last three SHA
algorithms). In many cases, implementations of each algorithm exist that are more
efficient, but we believe that our testing strategy should give you a reasonable idea of
relative speeds between algorithms.
Table 6-1. Cryptographic hash functions and their properties
Algorithm Digest size
Davies-Meyer-AES-
128
128 bits (same length
as cipher block size)
Security
confidence
256 | Chapter 6: Hashes and Message Authentication
Small message
speed (64
bytes), in cycles
per byte a
Large message
speed (8K), in
cycles per byte
a All timing values are best cases based on our empirical testing, and assume that the data being processed is already in cache. Do not expect
that you’ll quite be able to match these speeds in practice.
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
Uses
block
cipher
Good 46.7 cpb 57.8 cpb Yes
MD2 128 bits Good to low 392 cpb 184 cpb No
MD4 128 bits Insecure 32 cpb 5.8 cpb No
MD5 128 bits Very low, may be
insecure
40.9 cpb 7.7 cpb No
MDC-2-AES-128 256 bits Very high 93 cpb 116 cpb Yes
MDC-2-DES 128 bits Good 444 cpb 444 cpb Yes
RIPEMD-160 160 bits High 62.2 cpb 20.6 cpb No
SHA1 160 bits High 53 cpb 15.9 cpb No
SHA-256 256 bits Very high 119 cpb 116 cpb No
SHA-384 384 bits Very high 171 cpb 166 cpb No
SHA-512 512 bits Very high 171 cpb 166 cpb No