RFC1810 - Report on MD5 Performance

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　　Network Working Group J. Touch
Request for Comments: 1810 ISI
Category: Informational June 1995

Report on MD5 Performance

Status of this Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Abstract

MD5 is an authentication algorithm, which has been proposed as the
default authentication option in IPv6. When enabled, the MD5
algorithm operates over the entire data packet, including header.
This RFCaddresses how fast MD5 can be implemented in software and
hardware, and whether it supports currently available IP bandwidth.
MD5 can be implemented in existing hardware technology at 256 Mbps,
and in software at 87 Mbps. These rates cannot support current IP
rates, e.g., 100 Mbps TCP and 130 Mbps UDP over ATM. If MD5 cannot
support existing network bandwidth using existing technology, it will
not scale as network speeds increase in the future. This RFCis
intended to alert the IP community about the performance limitations
of MD5, and to suggest that alternatives be considered for use in
high speed IP implementations.

Introduction

MD5 is an authentication algorithm, which has been proposed as one
authentication option in IPv6 [1]. RFC1321 describes the MD5
algorithm and gives a reference implementation [3]. When enabled,
the MD5 algorithm operates over the entire data packet, including
header (with dummy values for volatile fields). This RFCaddresses
how fast MD5 can be implemented in software and hardware, and whether
it supports currently available IP bandwidth.

This RFCconsiders the general issue of checksumming and security at
high speed in IPv6. IPv6 has no header checksum (which IPv4 has
[5]), but proposes an authentication digest over the entire body of
the packet (including header where volatile fields are zeroed) [1].
This RFCspecifically addresses the performance of that
authentication mechanism.

Measurements

The performance of MD5 was measured. The code was an optimized
version of the MD5 reference implementation from the RFC[3], and is
available for anonymous FTP [7]. The following are the results of
the performance test "md5 -t", modified to prohibit on-chip caching
of the data block:

87 Mbps DEC Alpha (190 Mhz)
33 Mbps HP 9000/720
48 Mbps IBM RS/6000 7006 (PPC 601 @80 Mhz)
31 Mbps Intel i486/66 NetBSD
44 Mbps Intel Pentium/90 NeXTStep
52 Mbps SGI/IP-20 IRIX 5.2
37 Mbps Sun SPARC-10/51, SPARC-20/50 SunOS 4.1.3
57 Mbps Sun SPARC-20/71 SunOS 4.1.3

These rates do not keep up with currently available IP bandwidth,
e.g., 100 Mbps TCP and 130 Mbps UDP over a Fore SBA-200 ATM host
interface in a Sun SPARC-20/71.

Values as high as 100 Mbps have been reported for the DEC Alpha (190
Mhz). These values reflect on-chip caching of the data. It is not
clear at this time whether in-memory, off-chip cache, or on-chip
cache performance measures are more relevant to IP performance.

Analysis of the MD5 Algorithm

The MD5 algorithm is a block-chained hashing algorithm. The first
block is hashed with an initial seed, resulting in a hash. The hash
is summed with the seed, and that result becomes the seed for the
next block. When the last block is computed, it's "next-seed' value
becomes the hash for the entire stream. Thus, the seed for block
depends on both the hash and the seed of its preceding block. As a
result, blocks cannot be hashed in parallel.

Each 16-word (64-byte) block is hashed via 64 basic steps, using a
4-word intermediate hash, and collapsing the intermediate hash at the
end. The 64 steps are 16 groups of 4 steps, one step per
intermediate hash word. This RFCuses the following notation (as
from RFC-1321 [3]):

A,B,C,D intermediate hash words
X[i] input data block
T[i] sine table lookup
<< i rotate i bits
F logical functions of 3 args

The subscripts to X, I, and << are fixed for each step, and are
omitted here. There are four different logical functions, also
omitted. Each 4-step group looks like:

A = B + ((A + F(B,C,D) + X[i] + T[i]) << i)
D = A + ((D + F(A,B,C) + X[i] + T[i]) << i)
C = D + ((C + F(D,A,B) + X[i] + T[i]) << i)
B = C + ((B + F(C,D,A) + X[i] + T[i]) << i)

Note that this has the general form shown below. Due to the
complexity of the function 'f', these equations cannot be transformed
into a less serial set.

A = f(D); B = f(A); C = f(B); D = f(C)

Each steps is composed of two table lookups, one rotation, a 3-
component logical operation, and 4 additions. The best
parallelization possible leaves F(x,y,z) to the last step, waiting as
long as possible for the result from the previous step. The
resulting tree is shown below.

(t0) B* C C D X T
| | | | | |
| | | | | |
\/ \/ \ /
t1 op op A + X T
\ / \ / | |
\ / \ / | |
\/ \/ \ /
t2 op + (t0) B* C C D A +
\ / | | | | \ /
\ / \ | | / \ /
\ / \\// \/
t3 + t1 op +
| \ /
| \ /
| \ /
t4 << B* t2 + B*
\ / \ /
\ / << /
\ / \ /
t5 + t3 +
| |
| |
| |
A** A**

Binary operation tree Optimized hardware tree

This diagram assumes that each operation takes one unit time. The
tree shows the items that depend on the previous step as B*, and the
item that the next step depends on as A**. Sequences of the binary
operation tree cannot be overlapped, but the optimized hardware tree
can (by one time step).

There are 4 steps processed per word of input, ignoring inter-block
processing. The speed of the overall algorithm depends on how fast
we can process these 4 steps, vs. the bandwidth of one word of input
being processed.

The binary tree takes 5 time units per step of the algorithm, and
permits at best 3-way parallelism (at time t1). In software, this
means it takes 5 * 4 = 20 instructions per word input. A computer
capable of M MIPS can support a data bandwidth of M/20 * 32 Mbps,
i.e., bits per second equal to 1.6x its MIPS rate. Therefore, a 100
MIPS machine can support a 160 Mbps stream.

Parallel software rate in Mbps = 1.6 * MIPS rate

This assumes that register reads and writes are overlapped with
computation entirely. Without any parallelism, there are 8
operations per step, and 4 steps per word, so 32 operations per word,
i.e., the data rate in Mbps would be identical to the MIPS rate:

Serial software rate in Mbps = MIPS rate

Predictions using SpecInt92 numbers as MIPS estimators can be
compared with measured rates [2]:

Spec- Predicted MD5
Int92 Upper-Bound Measured Machine
------------------------------------------------------------
122 122-195 87 Mbps DEC Alpha (190 Mhz)
48 48- 77 33 Mbps HP 9000/720
88 88-141 48 Mbps IBM RS/6000 7006 (PPC 601 @80 Mhz)
32 32- 51 31 Mbps Intel i486/33 NetBSD
90 90-144 44 Mbps Intel Pentium/90 NeXTStep
90 90-144 52 Mbps SGI/IP-20 IRIX 5.2
65 65-104 37 Mbps Sun SPARC-10/51 SunOS 4.1.3
126 126-202 57 Mbps Sun SPARC-20/71 SunOS 4.1.3

The hardware rate takes 3 time units per step, i.e. 3 * 4 = 12 time
units per word of input. Hardware capable of doing an operation
(e.g., 32-bit addition) in N nanoseconds can support a data bandwidth
of 32/12/N bps, i.e., 2/3N bps.

Hardware rate in Mbps = 8/3N * 1,000

For CMOS, an operation (32-bit addition, including register retrieval
and storage) costs about 5.2 ns (2.6 ns per add, 2 ns for latching)
[6]. There are 6 clocks through the most highly-parallelized
implementation, resulting in 31.2 ns per 32-bit word, or 256 Mbps
[6]. This will not keep pace with existing hardware, which is
capable of link speeds in excess of 622 Mbps (ATM).

By comparison, IPv4 uses the Internet Checksum [5]. This checksum
can be performed in 32-bit-wide units in excess of 1 Gbps in an
existing, low-cost PLD. The checksum can also be parallelized by
computing partial sums and reducing the result.

One Proposed Solution

There are several ways to increase the performance of the IPv6
authentication mechanism. One is to increase the hardware
performance of MD5 by slightly modifying the algorithm, the other is
to propose a replacement algorithm. This RFCdiscusses briefly the
modification of MD5 for high-speed hardware implementation.
Alternate algorithms, capable of 3.5x the speed of MD5, have been
discussed elsewhere [6].

MD5 uses block chaining to ensure sensitivity to block order. Block
chaining also prevents arbitrary parallelism, which can be as much a
benefit to the spoofer as to the user. MD5 can be slightly altered
to accommodate a higher bandwidth data rate. There should be a
predetermined finite number of blocks processed from independent
seeds, such that the I-th block is part of the "I mod K"-th chain.
The resulting K digests themselves form a message, which can be MD5-
encoded using a single-block algorithm. This idea was proposed
independently by the author and by Burt Kaliski of RSA.

The goal is to support finite parallelism to provide adequate
bandwidth at current processing rates, without providing arbitrary
power for spoofing. It would require further analysis to ensure that
it provides an adequate level of security.

For current technology and network bandwidth, a minimum of 4-way
parallel chaining would suffice, and 16-way chaining would be
preferable. This would support network bandwidth of 1 Gbps with 4-
way chaining, in CMOS hardware. The chaining parallelism should be a
multiple of 4-way, to generate a complete block of digests (4 words
per digest, 16 words per block). This modification is believed to
achieve the goals of MD5, without the penalties of implementation of
the current MD5 algorithm.

Security Considerations

This entire document addresses a mechanism for providing security in
IPv6. MD5 is the proposed default optional authentication mechanism
for IPv6 traffic. This RFCspecifically addresses the concern that
security mechanisms such as MD5 that cannot support high bandwidth
with available hardware will compromise their deployment, and
ultimately, the security of the systems they are intended to
maintain.

The IPv6 requirements document emphasizes that IPv6 implementations
should not compromise performance, compared to IPv4. This is
presumably despite IPv6's increased functionality. "Required
optional" components of IPv6 should be held to this same standard.
MD5 compromises performance, and so its use as a required default
option in IPv6 should be reconsidered.

The use of MD5 as the default to the required authentication option
may compromise security in high-bandwidth systems, because enabling
the option causes performance degradation, defeating its inclusion as
an IPv6 option. As a result, the authentication option may be
disabled entirely.

It is important to the use of authentication in high-performance
systems that an alternative mechanism be available in IPv6 from the
outset. This may require the specification of multiple "required"
authentication algorithms - one that's slower but believed strong,
and one that's faster but may inspire somewhat less confidence.

Conclusions

MD5 cannot be implemented in existing technology at rates in excess
of 256 Mbps in hardware, or 86 Mbps in software. MD5 is a proposed
authentication option in IPv6, a protocol that should support
existing networking technology, which is capable of 130 Mbps UDP.

As a result, MD5 cannot be used to support IP authentication in
existing networks at existing rates. Although MD5 will support
higher bandwidth in the future due to technological advances, these
will be offset by similar advances in networking. If MD5 cannot
support existing network bandwidth using existing technology, it will
not be able to scale as network speeds increase in the future. This
RFCproposes that MD5 be modified to support a 16-way block chaining,
in order to allow existing technology (CMOS hardware) to support
existing networking rates (1 Gbps). It further proposes that
alternatives to MD5 be considered for use in high-speed networks.

Acknowledgements

The author would like to thank Steve Kent at BBN, Burt Kaliski,
Victor Chang, and Steve Burnett at RSA, Ran Atkinson at the NRL, and
the HPCC Division at ISI for reviewing the contents of this document.
Burt independently suggested the block-parallel modification proposed
here.

References

[1] Atkinson, R., "IPv6 Authentication Header", Work in Progress,
Naval Research Lab, February 1995.

[2] DiMarco, J., "Spec Benchmark table, V. 4.12",
<ftp://ftp.cfd.toronto.edu/pub/spectable>.

[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, MIT LCS
& RSA Data Security, Inc., April 1992.

[4] Partridge, C., and F. Kastenholz, "Technical Criteria for
Choosing IP The Next Generation (IPng)", RFC1726, BBN Systems
and Technologies, FTP Software, December 1994.

[5] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification," STD 5, RFC791, USC/Information Sciences
Institute, September 1981.

[6] Touch, J., "Performance Analysis fo MD5," to appear in ACM
Sigcomm '95, Boston.

[7] Touch, J., Optimized MD5 software, <ftp://ftp.isi.edu/pub/hpcc-
papers/touch/md5-opt.tar>.

Author's Address

Joe Touch
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, CA 90292-6695
USA

Phone: +1 310-822-1511 x151
Fax: +1 310-823-6714

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