Network Working Group P Karn [Qualcomm]
Internet Draft W A Simpson [DayDreamer]
expires in six months November 1997
Photuris: Extended Schemes and Attributes
draft-simpson-photuris-schemes-04.txt |
Status of this Memo
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Distribution of this memo is unlimited.
Abstract
Photuris is a session-key management protocol. Extensible Exchange-
Schemes are provided to enable future implementation changes without
affecting the basic protocol.
Additional authentication attributes are included for use with the IP
Authentication Header (AH).
Additional confidentiality attributes are included for use with the
IP Encapsulating Security Protocol (ESP).
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1. Additional Exchange-Schemes
The packet format and basic facilities are already defined for Pho-
turis [RFC-zzzz].
These optional Exchange-Schemes are specified separately, and no sin-
gle implementation is expected to support all of them.
This document defines the following values:
(3) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 3. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
An Exchange-Scheme Size of zero is invalid.
The Key-Generation-Function is "MD5 Hash".
The Privacy-Method is "Simple Masking".
The Validity-Method is "MD5-IPMAC Check". |
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
(4) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 2. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Scheme #2.
The Key-Generation-Function is "MD5 Hash".
The Privacy-Method is "DES-CBC over Mask".
The Validity-Method is "MD5-IPMAC Check". |
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
(5) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 5. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
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An Exchange-Scheme Size of zero is invalid.
The Key-Generation-Function is "MD5 Hash".
The Privacy-Method is "Simple Masking".
The Validity-Method is "MD5-IPMAC Check". |
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
(6) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 3. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Scheme #3.
The Key-Generation-Function is "MD5 Hash".
The Privacy-Method is "DES-CBC over Mask".
The Validity-Method is "MD5-IPMAC Check". |
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
(7) Implementation Optional. Any modulus (p) with a variable gen-
erator (g). When the Exchange-Scheme Size is non-zero, the
pair [g,p] is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes. Each is encoded in a separate |
Variable Precision Integer (VPI). The generator VPI is fol- |
lowed by (concatenated to) the modulus VPI, and the result is
nested inside the Exchange-Scheme Value field.
An Exchange-Scheme Size of zero is invalid.
The Key-Generation-Function is "MD5 Hash".
The Privacy-Method is "Simple Masking".
The Validity-Method is "MD5-IPMAC Check". |
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
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When more than one modulus is specified for a given kind of
Scheme, the Size of the modulus MUST be unique, independent of
the Size of the generator.
(8) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 2. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Schemes #2 and #4.
The Key-Generation-Function is "SHA1 Hash".
The Privacy-Method is "DES-EDE3-CBC over Mask".
The Validity-Method is "SHA1-IPMAC Check". |
This combination of features requires a modulus with at least
112-bits of cryptographic strength.
(10) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 5. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Scheme #5.
The Key-Generation-Function is "MD5 Hash".
The Privacy-Method is "DES-CBC over Mask".
The Validity-Method is "MD5-IPMAC Check". |
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
(12) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 3. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Schemes #3 and #6.
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The Key-Generation-Function is "SHA1 Hash".
The Privacy-Method is "DES-EDE3-CBC over Mask".
The Validity-Method is "SHA1-IPMAC Check". |
This combination of features requires a modulus with at least
112-bits of cryptographic strength.
(14) Implementation Optional. Any modulus (p) with a variable gen-
erator (g). When the Exchange-Scheme Size is non-zero, the
pair [g,p] is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes. Each is encoded in a separate |
Variable Precision Integer (VPI). The generator VPI is fol- |
lowed by (concatenated to) the modulus VPI, and the result is
nested inside the Exchange-Scheme Value field.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Scheme #7.
The Key-Generation-Function is "MD5 Hash".
The Privacy-Method is "DES-CBC over Mask".
The Validity-Method is "MD5-IPMAC Check". |
This combination of features requires a modulus with at least
64-bits of cryptographic strength.
When more than one modulus is specified for a given kind of
Scheme, the Size of the modulus MUST be unique, independent of
the Size of the generator.
(20) Implementation Optional. Any modulus (p) with a recommended
generator (g) of 5. When the Exchange-Scheme Size is non-zero,
the modulus is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Schemes #5 and #10.
The Key-Generation-Function is "SHA1 Hash".
The Privacy-Method is "DES-EDE3-CBC over Mask".
The Validity-Method is "SHA1-IPMAC Check". |
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This combination of features requires a modulus with at least
112-bits of cryptographic strength.
(28) Implementation Optional. Any modulus (p) with a variable gen-
erator (g). When the Exchange-Scheme Size is non-zero, the
pair [g,p] is contained in the Exchange-Scheme Value field in
the list of Offered-Schemes. Each is encoded in a separate |
Variable Precision Integer (VPI). The generator VPI is fol- |
lowed by (concatenated to) the modulus VPI, and the result is
nested inside the Exchange-Scheme Value field.
When the Exchange-Scheme Size field is zero, includes by refer-
ence all of the moduli specified in the list of Offered-Schemes
for Schemes #7 and #14.
The Key-Generation-Function is "SHA1 Hash".
The Privacy-Method is "DES-EDE3-CBC over Mask".
The Validity-Method is "SHA1-IPMAC Check". |
This combination of features requires a modulus with at least
112-bits of cryptographic strength.
When more than one modulus is specified for a given kind of
Scheme, the Size of the modulus MUST be unique, independent of
the Size of the generator.
2. Additional Key-Generation-Function
2.1. SHA1 Hash
SHA1 [FIPS-180-1] is used as a pseudo-random-function for generating
the key(s). The key(s) begin with the most significant bits of the
hash. SHA1 is iterated as needed to generate the requisite length of
key material.
When an individual key does not use all 160-bits of the last hash,
any remaining unused (least significant) bits of the last hash are
discarded. When combined with other uses of key generation for the
same purpose, the next key will begin with a new hash iteration.
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3. Additional Privacy-Methods
3.1. DES-CBC over Mask
As described in [RFC-zzzz] "Privacy-Key Computation", sufficient pri-
vacy-key material is generated to match the message length, beginning
with the next field after the SPI, and including the Padding. The
message is masked by XOR with the privacy-key.
Then, the Key-Generation-Function is iterated to generate a DES key.
The most significant 64-bits (8 bytes) of the generated hash are used
for the privacy-key, and the remainder are discarded. Although
extremely rare, the 64 weak, semi-weak, and possibly weak keys
[Schneier95, pages 280-282] are discarded. The Key-Generation-
Function is iterated until a valid key is obtained.
The least significant bit of each key byte is ignored (or set to par-
ity when the implementation requires).
Message encryption begins with the next field after the SPI, and con-
tinues to the end of the data indicated by the UDP Length.
3.2. DES-EDE3-CBC over Mask
This is "Triple DES" outer-CBC EDE encryption (and DED decryption)
with three 56-bit keys [KR96].
As described in [RFC-zzzz] "Privacy-Key Computation", sufficient pri-
vacy-key material is generated to match the message length, beginning
with the next field after the SPI, and including the Padding. The
message is masked by XOR with the privacy-key.
Then, the Key-Generation-Function is iterated (at least) three times
to generate the three DES keys. The most significant 64-bits (8
bytes) of each generated hash are used for each successive privacy-
key, and the remainder are discarded. Each key is examined sequen-
tially, in the order used for encryption. A key that is identical to
a previous key MUST be discarded. Although extremely rare, the 64
weak, semi-weak, and possibly weak keys [Schneier95, pages 280-282]
MUST be discarded. The Key-Generation-Function is iterated until a
valid key is obtained before generating the next key.
In all three keys, the least significant bit of each key byte is
ignored (or set to parity when the implementation requires).
Message encryption begins with the next field after the SPI, and con-
tinues to the end of the data indicated by the UDP Length.
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4. Additional Validity-Method
4.1. SHA1-IPMAC Check
As described in [RFC-zzzz] "Validity Verification", the Verification |
field value is the SHA1 [FIPS-180-1] hash over the concatenation of
SHA1( key, keyfill, data, datafill, key, sha1fill ) |
where the key is the computed verification-key. |
The keyfill and datafill use the same pad-with-length technique |
defined for sha1fill. This padding and length is implicit, and does
not appear in the datagram. -
The resulting Verification field is a 160-bit Variable Precision |
Integer (22 bytes including Size).
5. Additional Attributes
The attribute format and basic facilities are already defined for
Photuris [RFC-zzzz].
These optional attributes are specified separately, and no single
implementation is expected to support all of them.
This document defines the following values:
Use Type
I 4 SHA1-IPMAC Symmetric Identification |
X 6 SHA1-IPMAC Authentication |
E 8 DES-CBC Encryption
E 9 DES Decryption
E 11 XOR
A AH-only Attribute-Choice
E ESP-only Attribute-Choice
I Identity-Choice
X dependent on list location
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5.1. SHA1-IPMAC Symmetric Identification
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 4
Length 0
When selected as an Identity-Choice, the immediately following Iden-
tification field contains an unstructured Variable Precision Integer. |
Valid Identifications and symmetric secret-keys are preconfigured by
the parties.
There is no required format or content for the Identification value.
The value may be a number or string of any kind. See [RFC-zzzz] "Use
of Identification and Secrets" for details.
The symmetric secret-key (as specified) is selected based on the con- |
tents of the Identification field. All implementations MUST support
at least 62 bytes. The selected symmetric secret-key SHOULD provide
at least 80-bits of cryptographic strength.
As described in [RFC-zzzz] "Identity Verification", the Verification |
field value is the SHA1 [FIPS-180-1] hash over the concatenation of:
SHA1( key, keyfill, data, datafill, key, sha1fill ) |
where the key is the computed verification-key. |
The keyfill and datafill use the same pad-with-length technique |
defined for sha1fill. This padding and length is implicit, and does
not appear in the datagram. -
The resulting Verification field is a 160-bit Variable Precision |
Integer (22 bytes including Size).
For both [RFC-zzzz] "Identity Verification" and "Validity Verifica- |
tion", the verification-key is the SHA1 [FIPS-180-1] hash of the fol- |
lowing concatenated values: |
+ the symmetric secret-key, |
+ the computed shared-secret. |
For [RFC-zzzz] "Session-Key Computation", the symmetric secret-key is |
used directly as the generation-key. |
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The symmetric secret-key is used in calculations in the same fashion |
as [RFC-zzzz] "MD5-IPMAC Symmetric Identification".
5.2. SHA1-IPMAC Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 6
Length 0
May be selected as an AH or ESP Attribute-Choice, pursuant to
[RFC-1852] et sequitur. The selected Exchange-Scheme SHOULD provide
at least 80-bits of cryptographic strength.
As described in [RFC-zzzz] "Session-Key Computation", the most sig-
nificant 384-bits (48 bytes) of the Key-Generation-Function itera-
tions are used for the key.
Profile:
When negotiated with Photuris, the transform differs slightly from
[RFC-1852].
The form of the authenticated message is:
SHA1( key, keyfill, datagram, datafill, key, sha1fill )
where the key is the SPI session-key.
The additional datafill protects against the attack described in
[PO96]. This is also filled to the next 512-bit boundary, using
the same pad-with-length technique defined for SHA1. This padding
and length is implicit, and does not appear in the datagram. -
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5.3. DES-CBC Encryption
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 8
Length 0
May be selected as an ESP Attribute-Choice, pursuant to [RFC-1829] et
sequitur. The selected Exchange-Scheme SHOULD provide at least
56-bits of cryptographic strength.
As described in [RFC-zzzz] "Session-Key Computation", the most sig-
nificant 64-bits (8 bytes) of the Key-Generation iteration are used
for the key, and the remainder are discarded. Although extremely
rare, the 64 weak, semi-weak, and possibly weak keys [Schneier95,
pages 280-282] MUST be discarded. The Key-Generation-Function is
iterated until a valid key is obtained.
The least significant bit of each key byte is ignored (or set to par-
ity when the implementation requires).
Profile:
When negotiated with Photuris, the transform differs slightly from
[RFC-1829].
The 32-bit Security Parameters Index (SPI) field is followed by a
32-bit Sequence Number (SN).
The 64-bit CBC IV is generated from the 32-bit Security Parameters
Index (SPI) field followed by (concatenated with) the 32-bit
Sequence Number (SN) field. Then, the bit-wise complement of the
32-bit Sequence Number (SN) value is XOR'd with the first 32-bits
(SPI):
(SPI ^ -SN) || SN
The Padding values begin with the value 1, and count up to the |
number of padding bytes. For example, if the plaintext length is
41, the padding values are 1, 2, 3, 4, 5, 6 and 7, plus any addi- |
tional obscuring padding. |
The PadLength and PayloadType are not appended.
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After decryption, if the padding bytes are not the correct sequen- |
tial values, then the payload is discarded, and a "Decryption
Failed" error is indicated, as described in [RFC-xxxx].
5.4. DES Decryption
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 9
Length 0
May be selected as an ESP Attribute-Choice, pursuant to [RFC-1851] et
sequitur. The combination
"DES-CBC Encryption",
"DES Decryption",
"DES-CBC Encryption",
indicates "Triple DES" outer-CBC EDE encryption (and DED decryption)
with three keys [KR96]. The selected Exchange-Scheme SHOULD provide
at least 112-bits of cryptographic strength.
As described in [RFC-zzzz] "Session-Key Computation", the Key-
Generation-Function is iterated (at least) three times to generate
the three independent keys, in the order used for encryption. The
most significant 64-bits (8 bytes) of each iteration are used for
each successive key, and the remainder are discarded. Each key is
examined sequentially, in the order used for encryption. A key that
is identical to a previous key MUST be discarded. Although extremely
rare, the 64 weak, semi-weak, and possibly weak keys [Schneier95,
pages 280-282] MUST be discarded. The Key-Generation-Function is
iterated until a valid key is obtained before generating the next
key.
In all three keys, the least significant bit of each key byte is
ignored (or set to parity when the implementation requires).
Profile:
When negotiated with Photuris, the transform differs slightly from
[RFC-1851].
The 32-bit Security Parameters Index (SPI) field is followed by a
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32-bit Sequence Number (SN).
The 64-bit CBC IV is generated from the 32-bit Security Parameters
Index (SPI) field followed by (concatenated with) the 32-bit
Sequence Number (SN) field. Then, the bit-wise complement of the
32-bit Sequence Number (SN) value is XOR'd with the first 32-bits
(SPI):
(SPI ^ -SN) || SN
The Padding values begin with the value 1, and count up to the |
number of padding bytes. For example, if the plaintext length is
41, the padding values are 1, 2, 3, 4, 5, 6 and 7, plus any addi- |
tional obscuring padding. |
The PadLength and PayloadType are not appended.
After decryption, if the padding bytes are not the correct sequen- |
tial values, then the payload is discarded, and a "Decryption
Failed" error is indicated, as described in [RFC-xxxx].
5.5. XOR Whitening
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attribute 11
Length 0
May be selected as an ESP Attribute-Choice, pursuant to [RFC-yyyy] et
sequitur. The combination
"XOR",
"DES-CBC Encryption",
"XOR",
indicates "DESX" encryption with three keys [KR96]. The selected
Exchange-Scheme SHOULD provide at least 104-bits of cryptographic
strength.
As described in [RFC-zzzz] "Session-Key Computation", the Key-
Generation-Function is iterated (at least) three times to generate
the three independent keys, in the order used for encryption. The |
most significant bytes of each iteration are used for each successive
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key, and the remainder are discarded.
Note that this attribute may appear multiple times in the same ESP
attribute list, both before and after an encryption transform. For
example,
"XOR",
"DES-CBC Encryption",
"XOR",
"DES Decryption",
"XOR",
"DES-CBC Encryption",
"XOR",
would be one possible combination with Triple DES.
Security Considerations
The "whitening" of the plaintext before DES encryption is intended to
obscure the relation of the number of parties and SPIs active between
two IP nodes. The combination of a randomized secret mask with the
CBC IV generates a different initial encrypted block for every SPI
creation message.
This obscurement is less effective when the SPI and SPILT are invari-
ant or are not created for a particular exchange direction. The num-
ber of parties could be revealed by the number of exchanges with dif-
ferences in the initial encrypted blocks.
Acknowledgements
Phil Karn was principally responsible for the design of party privacy
protection, and provided much of the design rationale text (now
removed to a separate document).
William Simpson designed the packet formats, and additional Exchange-
Schemes, editing and formatting. All such mistakes are his responsi-
bity.
Use of encryption for privacy protection is also found in the Sta-
tion-To-Station authentication protocol [DOW92].
Bart Preneel and Paul C van Oorschot in [PO96] suggested adding
padding between the data and trailing key when hashing for authenti-
cation.
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Niels Provos developed the first implementation with multiple schemes
and multiple moduli per scheme (circa July 1997).
References
[DOW92] Whitfield Diffie, Paul C van Oorshot, and Michael J
Wiener, "Authentication and Authenticated Key Exchanges",
Designs, Codes and Cryptography, v 2 pp 107-125, Kluwer
Academic Publishers, 1992.
[FIPS-180-1]
"Secure Hash Standard", National Institute of Standards
and Technology, U.S. Department Of Commerce, April 1995.
Also known as: 59 Fed Reg 35317 (1994).
[KR96] Kaliski, B., and Robshaw, M., "Multiple Encryption:
Weighing Security and Performance", Dr. Dobbs Journal,
January 1996.
[PO96] Bart Preneel, and Paul C van Oorshot, "On the security of
two MAC algorithms", Advances in Cryptology -- Eurocrypt
'96, Lecture Notes in Computer Science 1070 (May 1996),
Springer-Verlag, pages 19-32.
[RFC-1829] Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC
Transform", July 1995.
[RFC-1850] Karn, P., Metzger, P., Simpson, W., "The ESP Triple DES
Transform", September 1995.
[RFC-1851] Metzger, P., Simpson, W., "IP Authentication using Keyed
SHA", September 1995.
[RFC-xxxx] Karn, P., and Simpson, W., "ICMP Security Failures Mes-
sages", draft-simpson-icmp-ipsec-fail-02.txt, work in
progress.
[RFC-yyyy] Simpson, W., Baldwin, R., "The ESP DES-XEX3-CBC Trans-
form", draft-ietf-ipsec-ciph-desx-00.txt, work in
progress.
[RFC-zzzz] Karn, P., and Simpson, W., "Photuris: Session Key Manage-
ment Protocol", draft-simpson-photuris-17.txt, work in |
progress.
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Contacts
Comments about this document should be discussed on the
photuris@adk.gr mailing list.
Questions about this document can also be directed to:
Phil Karn
Qualcomm, Inc.
6455 Lusk Blvd.
San Diego, California 92121-2779
karn@qualcomm.com
karn@unix.ka9q.ampr.org (preferred)
William Allen Simpson
DayDreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
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