Security Working Group IPsec Working Group
INTERNET-DRAFT J. Hughes, Editor
June 1996
Expires in Six months
Combined DES-CBC, HMAC and Replay Prevention Security Transform
<draft-ietf-ipsec-esp-des-md5-02.txt>
Status of this Memo
This document is a submission to the IETF Internet Protocol Security
(IPSEC) Working Group. Comments are solicited and should be addressed
to the working group mailing list (ipsec@tis.com) or to the editor.
This document is an Internet-Draft. Internet Drafts are working
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Distribution of this memo is unlimited.
Abstract
This draft describes a combination of privacy, authentication,
integrity and replay prevention into a single packet format.
This document is the result of significant work by several major
contributors and the IPsec working group as a whole. These
contributors, cited later in this document, provided many of the key
technical details summarized in this document.
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Discussion
This draft allows a combination of MD5 and DES-CBC. In addition to
privacy, the goal of this transform is to ensure that the packet is
authentic, can not be modified in transit, or replayed.
For the purpose of this RFC, the terms conformance and compliance are
synonymous.
The claims of privacy, integrity, authentication, and replay
prevention are made in this draft. A good general text describing the
methods and algorithm are in [Schneier95].
Privacy is provided by DES-CBC [FIPS-46] [FIPS-46-1] [FIPS-74]
[FIPS-81].
Integrity is provided by HMAC [Krawczyk96].
Authentication is provided since only the source and destination know
the HMAC key. If the HMAC is correct, it proves that it must have
been added by the source.
Replay prevention is provided by the combination of a constantly
increasing count, the SPI and the HMAC key. The integrity of the
replay field is provided by the HMAC.
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Packet Format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---
| Security Parameters Index (SPI) | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ---
| Replay Prevention Field (count) | | ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | | |
~ Payload Data ~ | |
| |HMAC |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DES
| | Padding (0-7 bytes) | | CBC
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | Pad Length | Payload Type | v |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--- |
| | |
~ HMAC digest ~ |
| | v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
Security Parameters Index
This field is negotiated at key setup and shall not be 0 [RFC-1825]
Replay Prevention
Replay Prevention is an unsigned 32 bit incrementing counter starting
at a value of 1.
The key (K, as described in a later section) must be changed
frequently enough so that the counter is not allowed to wrap; in
other words, the key must be changed before (2^32)-2 packets are
transmitted using this key. For a given SPI, counter wrapping shall
be considered to be a replay attack. (While a wrap is a replay
attack, there is always the possibility that a packet can get
duplicated, so the presence of a single or small number of duplicate
packets is not an absolute indication of a replay attack.)
The receiver must verify that for a given SPI the packets received
have non-repeating (non-duplicate) counter values. This can be
implemented as a simple increasing count test or the receiver may
choose to accept out-of-order packets as long as it is guaranteed
that packets can be received only once. For example, an
implementation can use a sliding receive window, the size of which is
an implementation detail. If such a receive window is supported, the
receiver must ensure that it will accept packets within the current
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window only once, and reject any packets it receives with a value
that is less than the lower bound of the window.
An example may allow the most recent 32 packets to be allowed to
arrive out of order. That is, these 32 packets can arrive in any
sequence relative to each other except that these packets are
guaranteed to arrive only once. Appendix A has actual code that
implement a 32 packet replay window and a test routine. The purpose
of this routine is to show how it could be implemented.
Payload
The payload contains data that is described by the payload type
field. This field is an integral number of bytes in length; the
following padding and pad length fields will help provide alignment
to a double word boundary.
Padding
The padding (pad bytes and pad length field) is used to align the
following "payload type" field to end on a double word boundary (when
counting from the start of the replay field).
Padding bytes may be initialized with random data.
At a minimum, the number of pad bytes added must be enough to align
the payload type field on the next appropriate boundary. However, the
sender may choose to include additional padding, provided that the
alignment is maintained. In total, the sender can add 0-255 bytes of
padding.
Pad Length
The pad length field indicates the number of pad bytes immediately
preceding it. The range of valid values is 0-255, where a value of
zero indicates that the byte immediately preceding the pad length
field is the last byte of the payload.
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Payload Type
Describes what the payload is. The values are described in:
ftp://ftp.isi.edu/in-notes/iana/assignments/protocol-numbers
HMAC Digest
The HMAC digest is a 128 bit residue described in [Krawczyk96]. This
covers the SPI, replay, payload, padding, pad length, payload type.
HMAC is a keyed algorithm and shall use the HMAC key as described in
the section on keys.
Encryption Transform Procedure
CBC chaining with an IV_key is used.
IV_key count|x1 x2 x3
| | | |
|-------(+) --------(+) --------(+)
| | | | |
------- | ------- | --------
k--| DES | | k--| DES | | k--| DES |
------- | ------- | --------
| | | | |
|-----| |-----| |----...
| | |
y1 y2 y3
Where count is the Replay counter. x1, x2, x3 are the plaintext (x1
is 32 bits, all others are 64 bits). y1, y2, y3 are the ciphertext.
The process of this transformation is comprised of the following 3
steps.
1. Taking the data and encapsulating it with the SPI, count, pad,
pad length, and payload type.
2. Calculating the HMAC using the HMAC_key and creating the digest
from the SPI, count, data, pad, pad length, and payload type and
placing the result into the HMAC digest field.
3. Encrypting the count, data, pad, pad length, payload type, and
HMAC digest using DES and the appropriate DES_key_ and IV_key.
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(Note that the first DES block is a combination of the count and
the first word of plaintext.)
Decryption Transform Procedure
CBC chaining with an IV_key is used.
IV_key y1 y2 y3
| | | |
| |------ |------ |----...
| | | | | |
| ------- | ------- | --------
| k--| DES | | k--| DES | | k--| DES |
| ------- | ------- | --------
| | | | | |
|-------(+) |-------(+) |-------(+)
| | |
(count|x1) x2 x3
The process of this transformation is comprised of the following 4
steps.
1. (Optional step) Decrypt the first bock of data using the
appropriate DES_key and IV_key and then do a quick "sanity check"
of the count. If the count has decreased below the window or has
increased by more than 65k, then it is safe to discard this packet
as either a replay, non-authentic or too old. If the count is
within 65K, then the probability that the packet is authentic is
65535/65536. (The following replay check and HMAC check are both
still required).
2. Decrypt the count (if not already done), data, pad, pad length,
and payload type using DES and the appropriate DES_key_ and
IV_key.
3. Calculate the HMAC using the HMAC_key and create the digest
from the SPI, count, data, pad, pad length, and payload type and
checking the result at digest at the end of the packet. If the
digest is incorrect, discard the packet.
4. Check the count using the window algorithm discussed above. If
the packet is duplicate or too old, discard the packet.
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Key Material
The key K is provided by the key management layer. This key is used
to derive the symmetric keys, they are:
DES_Key_I is the DES key for traffic from the initiator ->
responder.
DES_Key_R is the DES key for traffic from the responder ->
initiator.
HMAC_Key is the key for the HMAC Algorithm (This is the same for
both directions, but since these are encrypted by different DES
keys, then this is not a problem.)
IV_key is used to stop code book attacks on the first block.
The vertical bar symbol "|" is used to denote concatenation of bit
strings.
MD5(x|y) denotes the result of applying the MD5 function to the
concatenated bit strings x and y.
Truncate(x,n) denotes the result of truncating x to its first n bits.
DES_Key_I = Truncate(MD5( D_Pad_I | K ),64)
DES_Key_R = Truncate(MD5( D_Pad_R | K ),64)
IV_Key = Truncate(MD5( I_Pad | K ),64)
HMAC_Key = MD5( H_Pad | K )
where each _Pad_is 512 bit string.
D_Pad_I = 0x5C repeated 64 times.
D_Pad_R = 0x3A repeated 64 times.
I_Pad = 0xAC repeated 64 times.
H_Pad = 0x53 repeated 64 times.
(Implementation note, The 16 byte intermediate residuals can be
precalculated from these constants and stored to reduce processing
overhead).
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Security Considerations
The ESP-DES-HMAC-RP transform described in this draft is immune to
the [Bellovin96] attacks. (AH [RFC-1826], in some modes, can also
provide immunity to these attack.)
The implications of the size of K can be found in [Blaze96].
References
[Bellovin96] Bellovin, S., "Problem Areas for the IP Security
Protocols", AT&T Research,
ftp://ftp.research.att.com/dist/smb/badesp.ps, July, 1996.
[FIPS-46] US National Bureau of Standards, "Data Encryption
Standard", Federal Information Processing Standard (FIPS) Publication
46, January 1977.
[FIPS-46-1] US National Bureau of Standards, "Data Encryption
Standard", Federal Information Processing Standard (FIPS) Publication
46-1, January 1988.
[FIPS-74] US National Bureau of Standards, "Guidelines for
Implementing and Using the Data Encryption Standard", Federal
Information Processing Standard (FIPS) Publication 74, April 1981.
[FIPS-81] US National Bureau of Standards, "DES Modes of Operation"
Federal Information Processing Standard (FIPS) Publication 81,
December 1980.
[Krawczyk96] Krawczyk, H., Bellare, M., Canetti, R., "HMAC-MD5:
Keyed-MD5 for Message Authentication", work-in-progress,
http://info.internet.isi.edu:80/in-drafts/files/draft-ietf-ipsec-
hmac-md5-00.txt, March, 1996
[Maughan96] Maughan, D., Schertler, M. Internet Security Association
and Key Management Protocol (ISAKMP), work-in-progress,
http://info.internet.isi.edu:80/in-drafts/files/draft-ietf-ipsec-
isakmp-04.txt, February, 1996
[Orman96] Orman, H., "The Oakley Key Determination Protocol", work-
in-progress, http://info.internet.isi.edu:80/in-drafts/files/draft-
ietf-ipsec-oakley-00.txt, February, 1996.
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[RFC-1825] Atkinson, R, "Security Architecture for the Internet
Protocol", ftp://ds.internic.net/rfc/rfc1825.txt, August 1995.
[RFC-1826] Atkinson, R, "IP Authentication Header",
ftp://ds.internic.net/rfc/rfc1826.txt, August 1995.
[Schneier95] Schneier, B., "Applied Cryptography Second Edition",
John Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7
[Blaze96] Blaze M., Diffie, W., Rivest, R., Schneier, B., Shimomura,
T., Thompson, E., Wiener, M., "Minimal Key Lengths for Symmetric
Ciphers to Provide Adequate Commercial Security",
http://theory.lcs.mit.edu/~rivest/bsa-final-report.ascii, January,
1996
Acknowledgements
This document is the result of significant work by several major
contributors. They include (in alphabetical order):
Robert W. Baldwin
<baldwin@rsa.com>
RSA Labs.
Kevin Kingdon
<kevin@rsa.com>
RSA Labs
Hugo Krawczyk
<hugo@watson.ibm.com>
IBM Corporation
Perry Metzger
<perry@piermont.com>
Piermont Information Services
Phil Rogaway
<rogaway@cs.ucdavis.edu>
University of California at Davis
Bill Simpson
<bill.simpson@um.cc.umich.edu>
Computer Systems Consulting Services
Hughes [Page 9]
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David A Wagner
<daw@cs.berkeley.edu>
University of California at Berkeley
In addition, the contributions of the entire IPSEC Working Group are
acknowledged.
The IPsec working group can be contacted through the chairs:
Ran Atkinson
<rja@cisco.com>
Cisco Systems
Paul Lambert
<PALAMBER@us.oracle.com>
Oracle Corporation
Editor's Address
James P. Hughes
<hughes@network.com>
Network Systems Corporation
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Appendix A
This is a routine that implements a 32 packet window. This is
intended on being an implementation sample.
#include <stdio.h>
#include <stdlib.h>
typedef unsigned long u_long;
enum {
ReplayWindowSize = 32
};
u_long bitmap = 0; /* session state - must be 32 bits */
u_long lastSeq = 0; /* session state */
/* Returns 0 if packet disallowed, 1 if packet permitted */
int ChkReplayWindow(u_long seq);
int ChkReplayWindow(u_long seq) {
u_long diff;
if (seq == 0) return 0; /* first == 0 or wrapped */
if (seq > lastSeq) { /* new larger sequence number */
diff = seq - lastSeq;
if (diff < ReplayWindowSize) { /* In window */
bitmap = (bitmap << diff) | 1; /* set bit for this packet */
} else bitmap = 1; /* This packet has a "way larger" */
lastSeq = seq;
return 1; /* larger is good */
}
diff = lastSeq - seq;
if (diff >= ReplayWindowSize) return 0; /* too old or wrapped */
if (bitmap & (1 << diff)) return 0; /* this packet already seen */
bitmap |= (1 << diff); /* mark as seen */
return 1; /* out of order but good */
}
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char string_buffer[512];
#define STRING_BUFFER_SIZE sizeof(string_buffer)
int main() {
int result;
u_long last, current, bits;
printf("Input initial state (bits in hex, last msgnum):\n");
if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) exit(0);
sscanf(string_buffer, "%lx %lu", &bits, &last);
if (last != 0)
bits |= 1;
bitmap = bits;
lastSeq = last;
printf("bits:%08lx last:%lu\n", bitmap, lastSeq);
printf("Input value to test (current):\n");
while (1) {
if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) break;
sscanf(string_buffer, "%lu", ¤t);
result = ChkReplayWindow(current);
printf("%-3s", result ? "OK" : "BAD");
printf(" bits:%08lx last:%lu\n", bitmap, lastSeq);
}
return 0;
}
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