MSEC Working Group B. Weis
Internet-Draft S. Rowles
Intended status: Standards Track Cisco Systems
Expires: March 17, 2008 September 14, 2007
Updates to the Group Domain of Interpretation (GDOI)
draft-ietf-msec-gdoi-update-03
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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Abstract
This memo describes updates to the Group Domain of Interpretation
(GDOI) [RFC3547]. It provides clarification where the original text
is unclear. It also includes a discussion of algorithm agility
within GDOI, and proposes several new algorithm attribute values.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 4
2. Cryptographic Algorithm agility . . . . . . . . . . . . . . . 5
2.1. Phase 1 protocol . . . . . . . . . . . . . . . . . . . . . 5
2.2. GROUPKEY-PUSH message . . . . . . . . . . . . . . . . . . 5
2.3. IPsec TEK Distribution . . . . . . . . . . . . . . . . . . 6
2.4. Certificate Payload . . . . . . . . . . . . . . . . . . . 6
2.5. POP Payload . . . . . . . . . . . . . . . . . . . . . . . 6
3. RFC 3547 Clarification . . . . . . . . . . . . . . . . . . . . 7
3.1. SA Payload . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. SIG Payload . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. SEQ Payload . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. POP Payload . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. CERT Payload . . . . . . . . . . . . . . . . . . . . . . . 10
3.6. TEK Integrity Key Length . . . . . . . . . . . . . . . . . 10
3.7. KE Payload . . . . . . . . . . . . . . . . . . . . . . . . 10
3.8. Minimum defined attributes . . . . . . . . . . . . . . . . 12
3.9. Attribute behavour . . . . . . . . . . . . . . . . . . . . 12
4. GCKS and Group Member Authorization . . . . . . . . . . . . . 13
4.1. Authorization using the CERT/POP Payloads . . . . . . . . 13
4.2. Authorization through other methods . . . . . . . . . . . 13
5. New GDOI Attributes . . . . . . . . . . . . . . . . . . . . . 14
5.1. Signature Hash Algorithm . . . . . . . . . . . . . . . . . 14
5.2. Support of AH . . . . . . . . . . . . . . . . . . . . . . 14
5.3. Sender-Specific Attributes . . . . . . . . . . . . . . . . 16
5.3.1. SENDER_ID . . . . . . . . . . . . . . . . . . . . . . 17
5.3.1.1. GCKS Semantics . . . . . . . . . . . . . . . . . . 17
5.3.1.2. Group Member Semantics . . . . . . . . . . . . . . 18
6. New IPsec Security Association Attributes . . . . . . . . . . 19
6.1. Address Preservation . . . . . . . . . . . . . . . . . . . 19
6.2. SA Direction . . . . . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
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8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 28
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1. Introduction
The Group Domain of Interpretation (GDOI) is a group key management
protocol fitting into the Multicast Security Group Key Management
Architecture [RFC4046]. GDOI is used to disseminate policy and
corresponding secrets to a group of participants. GDOI is
implemented on hosts and intermediate systems to protect group IP
communication (e.g., IP multicast packets) by encapsulating them with
the IP Encapsulating Security Payload (ESP) [RFC4303] packets.
However, implementation experience has revealed some inconsistencies
in RFC 3547 needing clarification. It also defines some additional
GDOI algorithm attributes which are useful to GDOI applications.
Algorithm agility, the ability to add new algorithms to namespaces,
is an important consideration for any protocol. This memo analyzes
the state of algorithm agility within GDOI, and proposes some changes
based upon that analysis. In particular, methods for fully
supporting SHA-256 [FIPS.180-2.2002] as an alternative to theSHA-1
and MD5 hash algorithms are described.
The clarification and modifications in this memo retain backwards
compatibility with RFC 3547.
1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Cryptographic Algorithm agility
Algorithm agility was a goal during the development of GDOI, and RFC
3547 generally provides the ability to add new algorithms as
necessary. However, further analysis has shown that there are places
where algorithm agility is not complete. This section discusses the
use of cryptographic algorithms within GDOI, points out variances in
algorithm agility, and proposes clarifications without changing any
payload formats within the protocol.
Recent published attacks on the SHA-1 algorithm motivate its
replacement as a cryptographic algorithm. The ability for GDOI to
move to the SHA-256 algorithm is explicitly discussed. Later
sections on this document propose some enhancements to the GDOI
protocol to provide for an easier means of supporting this and
additional hash functions.
2.1. Phase 1 protocol
GDOI is a "phase 2" protocol protected by a "phase 1" protocol. The
Phase 1 protocol defined in RFC 3547 is an IKEv1 Phase 1 protocol
(Main Mode or Aggressive Mode). The Phase 1 protocol provides
confidentiality via an encryption cipher. It also provides message
integrity via a pseudo random function ("prf") (described in Section
4 of [RFC2409], which is usually a hash algorithm using the HMAC
[RFC2104] construction. IKEv1 negotiates which encryption ciphers
and hash algorithms are to be used.
IKEv1 cipher algorithms come from the "Encryption Algorithm" list in
the IANA IPsec registry [IPSEC-REG], and the hash algorithms come
from the "Hash Algorithm" list in the same registry. The IANA IPsec
registry currently includes the SHA2-256, which is intended to be the
SHA-256 algorithm.
In summary, there are no cryptographic algorithm agility issues with
the IKEv1 Phase 1 protocol when used as a GDOI "phase 1" protocol.
For a more detailed analysis of the use of hash algorithms in IKE and
IPsec, see RFC 4894 [RFC4894].
2.2. GROUPKEY-PUSH message
The GROUPKEY-PUSH message is protected by an encryption cipher for
confidentiality and a digital signature for message integrity. The
encryption cipher is described by the IANA GDOI registry as the
KEK_ALGORITHM attribute [GDOI-REG]. The digital signature comprises
both a hash algorithm defined by the GDOI SIG_HASH_ALGORITHM
attribute and a public key signature algorithm defined by the
SIG_ALGORITHM attribute. This memo adds the SHA-256 algorithm to the
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SIG_HASH_ALGORITHM attribute in a later section.
In summary, there are no cryptographic algorithm agility issues with
the GROUPKEY-PUSH message.
2.3. IPsec TEK Distribution
IPsec SAs are distributed by GDOI. An IPsec ESP SA can include an
encryption cipher for confidentiality and an algorithm for packet
authentication. The encryption ciphers are defined by the IPsec ESP
Transform Identifiers defined in the IANA ISAKMP registry
[ISAKMP-REG]. The packet authentication method is distributed via an
"Authentication Algorithm" SA attribute. SHA-256 may be chosen as
the authentication algorithm with HMAC-SHA2-256. Similarly, an IPsec
AH SA is defined by choosing AH_SHA2-256 as the "AH Transform
Identifier".
In summary, there are no cryptographic algorithm agility issues
during TEK distribution.
2.4. Certificate Payload
Messages in the GROUPKEY-PULL and GROUPKEY-PUSH protocols may include
a Certificate Payload (CERT). Certificate digital signatures, and
the algorithm agility thereof, are outside the scope of this memo.
2.5. POP Payload
The GDOI Proof of Possession (POP) payload may be included in the
GROUPKEY-PULL protocol. It contains a digital signature over the
hash of a set of identities and nonces, which provides the current
possession of the peer. The digital signature algorithm is defined
as the "POP Algorithm" in the IANA GDOI registry [GDOI-REG].
However, identity of the hash algorithm to create the signed data was
omitted in RFC 3547.
In summary, the POP Payload has an algorithm agility deficiency with
regards to the hash algorithm. This memo remedies this omission in a
clarification section below.
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3. RFC 3547 Clarification
Implementation experience of RFC 3547 has revealed a few areas of
text that are not sufficiently clear. This section provides
clarifying text for those areas.
3.1. SA Payload
The SA KEK payload includes the "POP Key Length" field, which is
meant to declare the length of a POP signature key. Unfortunately,
it is not clear as to whether this refers to the GCKS POP signature
key or all entities' POP signature keys. The intent of conveying the
length was to ensure that a group member has the capability to
validate the GCKS POP payload, so this memo specifies that the value
is taken to indicate the length of only the GCKS signature key.
Additionally, the units of this field are not explicitly specified in
RFC 3547. The value is a number representing the length of key in
bits. In the case of POP_ALG_RSA, the value represents the size of
the modulus.
The units of the SIG_KEY_LENGTH KEK attribute value was not
explicitly specified in RFC 3547. The value is a number representing
the length of the KEK encryption key in bits.
The GDOI_PROTO_IPSEC_ESP attribute is sometimes referred to by the
truncated name PROTO_IPSEC_ESP.
RFC 3547 explicitly specifies that if a KEK cipher requires an IV,
then the IV MUST precede the key in the KEK_ALGORITHM_KEY KD payload
attribute. However, it should be noted that this IV length is not
included in the KEK_KEY_LEN SA payload attribute sent in the SA
payload. The KEK_KEY_LEN includes only the actual length of the
cipher key.
The Group Controller/Key Server (GCKS) adds the KEK_KEY_LEN attribute
to the SA payload when distributing KEK policy to group members. The
group member verifies whether or not it has the capability of using a
cipher key of that size. If the cipher definition includes a fixed
key length (e.g., KEK_ALG_3DES), the group member can make its
decision solely using KEK_ALGORITHM attribute and does not need the
KEK_KEY_LEN attribute. Sending the KEK_KEY_LEN attribute in the SA
payload is OPTIONAL if the KEK cipher has a fixed key length.
3.2. SIG Payload
The GROUPKEY-PUSH message SIG payload is further clarified here; the
SIG payload is a signature of the entire GROUPKEY-PUSH message (not
including the SIG payload) before it's been encrypted. The HASH is
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taken over the string 'rekey', the GROUPKEY-PUSH HDR, SEQ, SA, KD,
and optionally the CERT payload. After the SIG payload is created
using the signature of the above hash, the current KEK encryption key
encrypts all the payloads following the GROUPKEY-PUSH HDR.
The SIG_ALGORITHM type of SIG_ALG_RSA does not specify which PKCS#1
[RFC3447] encoding method is employed. To match existing practice,
this memo requires that it be the EMSA-PKCS1-v1_5 encoding method.
3.3. SEQ Payload
Each GROUPKEY-PUSH message contains a sequence number, which provides
anti-replay protection for a KEK. Thus, the GCKS returns a SEQ
payload in the GROUPKEY-PULL exchange only if a KEK attribute also
exists in the SA payload.
A KEK sequence number is associated with a single SPI (i.e., the
single set of cookie pair values sent in a GROUPKEY-PUSH ISAKMP HDR).
When a new KEK is distributed by a GCKS, it contains a new SPI and
resets the sequence number.
When a SEQ payload is included in the GROUPKEY-PULL exchange, it
includes the most recently used sequence number for the group. At
the conclusion of a GROUPKEY-PULL exchange, the initiating group
member MUST NOT accept any rekey message with both the KEK attribute
SPI value and a sequence number less than or equal to the one
received during the GROUPKEY-PULL. When the first group member
initiates a GROUPKEY-PULL exchange, the GCKS provides a Sequence
Number of zero, since no GROUPKEY-PUSH messages have yet been sent.
Note the sequence number increments only with GROUPKEY-PUSH messages.
The GROUPKEY-PULL exchange distributes the current sequence number to
the group member.
The sequence number resets to one with a new KEK attribute, as
described in section 5.6 of RFC 3547: "Thus the first packet sent for
a given Rekey SA will have a Sequence Number of 1". The sequence
number increments with each successive rekey.
3.4. POP Payload
RFC 3547 defines the Proof of Possession (POP) payload, which
contains a digital signature over a hash. Some RFC 3547 text
erroneously describes it as a "prf()".
RFC 3547 omitted including a method of specifying the hash function
type used in the POP payload. As a result, the GCKS or group member
do not have a means by which to agree which hash algorithm should be
used. To remedy this omission without changing the protocol, this
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memo specifies that the hash algorithm passed in the
SIG_HASH_ALGORITHM MUST be also used as the POP hash algorithm.
The SIG_ALGORITHM type of SIG_ALG_RSA does not specify which PKCS#1
[RFC3447] encoding method is employed. To match existing practice,
this memo requires that it be the EMSA-PKCS1-v1_5 encoding method.
Meadows and Pavlovic have published a paper [MP04] describing a means
by which a rogue GDOI device (i.e., GCKS or group member) can gain
access to a group for which it is not a group member. The rogue
device perpetrates a man-in-the-middle attack, which can occur if the
following conditions are true:
1. The rogue GDOI participant convinces an authorized member of the
group (i.e., victim group member) that it is a GCKS for that
group, and it also convinces the GCKS (i.e., victim GCKS) of that
group it is an authorized group member.
2. The victim group member, victim GCKS, and rogue group member all
share IKEv1 authentication credentials.
3. The victim GCKS does not properly verify that the IKEv1
authentication credentials used to protect a GROUPKEY-PULL
protocol are authorized to be join the group.
The point of proof-of-possession is to prove that the owner of the
identity associated with the Phase 1 key is the same as the owner of
the key distributed in the CERT. This attack can be mitigated by
adding the Phase 1 identities into the hashed data. This memo
replaces the method of computing POP_HASH, which is:
POP_HASH = hash("pop" | IKE-Initiator-P1-ID | IKE-Responder-P1-ID |
Ni | Nr)
where the fields are hashed as follows:
o The string "pop" without a NULL termination character.
o The IKE Phase 1 identity of the GCKS as distributed in the
"identification Data" portion of the ID payload. Because the
length of the identity is variable, the length of the
Identification Data MUST be hashed as a four octet value with the
length located in the least significant bits (in big-endian
format). The length value is hashed before the data value.
o The IKE Phase 1 identity of the group member as distributed in the
"identification Data" portion of the ID payload. Because the
length of the identity is variable, the length of the
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Identification Data MUST be hashed as a four octet value with the
length located in the least significant bits (in big-endian
format). The length value is hashed before the data value.
o The initiator nonce Ni, as passed in the first GROUPKEY-PULL
message.
o The responder nonce Nr, as passed in the second GROUPKEY-PULL
message.
3.5. CERT Payload
Receivers of the POP payload need the sender's public key in order to
validate the POP. However the source of that public key is not
explicitly defined. For example, if the certificate passed in the
CERT payload is an attribute certificate (not containing a public
key) then no public key is available. To remedy this omission, this
memo specifies that the certificate passed in the CERT payload MUST
be an identity certificate (including a public key).
3.6. TEK Integrity Key Length
This length of an integrity key distributed within GDOI varies
according to the integrity algorithm. The SHA1 keys will consist of
160 bits, SHA256 keys will consist of 256 bits, and MD5 keys will
consist of 128 bits.
3.7. KE Payload
RFC 3547 provides an OPTIONAL additional protection for the KD
payload during a GROUPKEY-PULL exchange called Perfect Forward
Secrecy (PFS). If the GCKS and group member exchange KE payloads
containing Diffie-Hellman public keys, the GCKS encrypts the KD
payload with a secret obtained from the Diffie-Hellman shared number.
This encryption precedes the encryption of the entire GROUPKEY-PULL
message.
The purpose of PFS in GDOI is to more carefully protect the keying
material passed from the GCKS to the group member. If a passive
attacker captures the GROUPKEY-PULL exchange and performs an offline
attack of the IKE Phase 1 confidentiality keys, it may eventually
discover them. If PFS is not used, the attacker can immediately use
the recovered keys to decrypt data packets and GROUPKEY-PUSH
messages, either live or stored. Thus, the IKE Phase 1 keys are
critical to the long-term confidentiality of the group. PFS was
added as an additional mechanism to hinder a passive attacker by
requiring the attacker to perform an additional cryptanalysis to
recover the Diffie-Hellman shared number computed by the GCKS and
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group member.
RFC 3547 Section 3.2.1 says "The GCKS responder will xor the DH
secret with the KD payload and send it to the member Initiator, which
recovers the KD by repeating this operation as in the Oakley IEXTKEY
procedure [RFC2412]". However, the IEXTKEY procedure does not xor
the DH shared secret with an entire payload, and the DH shared secret
is not likely to be long enough to cover the entire payload.
Therefore, the following amended procedure MUST be used for PFS.
1. The GCKS and group member MUST derive an encryption key and IV
(if needed by the encryption algorithm mode) using the dhEphem
method described in Section 6.1.2.1 of [NIST.800-56A.2006].
2. The key derivation function MUST be the preferred key derivation
function described in Section 5.8.1 of [NIST.800-56A.2006]. The
"kdf" function MUST be algorithm defined in the group policy as
the SIG_HASH_ALGORITHM attribute. The "keydatalen" input will be
the number of bits necessary for the encryption algorithm plus
the number of bits needed by the algorithm mode (if any). The
following kdf "OtherInfo" values MUST be hashed:
* AlgorithmID: This value represents the encryption algorithm
with which the derived keying material will be used (i.e.,
KEK_ALGORITHM). The value is hashed as a two octet value with
the algorithm id located in the least significant bits (in
big-endian format).
* PartyUInfo: This value will be the IKE Phase 1 identity of the
GCKS as distributed in the "identification Data" portion of
the ID payload. Because the length of the identity is
variable, the length of the Identification Data MUST be hashed
as a four octet value with the length located in the least
significant bits (in big-endian format). The length value is
hashed before the data value.
* PartyVInfo: This value will be the IKE Phase 1 identity of the
group member as distributed in the "identification Data"
portion of the ID payload. Because the length of the identity
is variable, the length of the Identification Data MUST be
hashed as a four octet value with the length located in the
least significant bits (in big-endian format). The length
value is hashed before the data value.
3. The result of the hash will be used as input to the encryption
algorithm described in the KEK_ALGORITHM attribute. If the
algorithm mode requires an IV, the initial bits (in big-endian
format) of the derived keying material will be used as the IV.
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The subsequent bits are used as the encryption key, encrypting
the bytes of the KD payload as described in Section 5.4 of RFC
3547. Note that the length of the KD payload may be larger due
to cipher block padding. If so, the KD payload length must be
modified to reflect the actual length of the ciphertext.
3.8. Minimum defined attributes
Minimum attributes that must be sent as part of an SA KEK:
KEK_ALGORITHM, KEK_KEY_LENGTH (if the cipher definition includes a
variable length key), KEK_KEY_LIFETIME, SIG_HASH_ALGORITHM (except
for DSA based algorithms), SIG_ALGORITHM, and SIG_KEY_LENGTH.
RFC 3547 states that all mandatory IPsec DOI attributes are mandatory
in GDOI_PROTO_IPSEC_ESP. However, no such list of mandatory IPsec
DOI attributes can be found in RFC 2407. This memo requires that the
following attributes MUST be supported by an RFC 3547 implementation
supporting the GDOI_PROTO_IPSEC_ESP SA TEK: SA Life Type, SA Life
Duration, Encapsulation Mode, Authentication Algorithm (if the ESP
transform includes authentication).
3.9. Attribute behavour
An GDOI implementation MUST abort if it encounters and attribute or
capability that it does not understand.
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4. GCKS and Group Member Authorization
A GDOI group member SHOULD be configured with policy describing which
IKEv1 identities are authorized to act as GCKS for a group.
The following sections clarify the need for a GCKS to authorize group
members. Authorization is needed regardless of whether the CERT and
POP payloads are mandated in group policy.
4.1. Authorization using the CERT/POP Payloads
A GCKS conforming with RFC 3547 SHOULD perform authorization based on
the IKEv1 authentication credentials. When the CERT and POP payloads
are used for authorization, the GCKS and group member SHOULD verify
that the identify in the CERT payload is syntactically the same
identity as used in the IKEv1 authentication credentials. For the
authorization check to succeed, the two credentials are compared and
found to be identical. This stops a group member from authenticating
to the GCKS with its own credential, yet including another group
member's credentials and proof-of-possession in the CERT and POP
payloads.
4.2. Authorization through other methods
When the use of CERT and POP payloads are not mandated in group
policy, the GCKS SHOULD have a means of recognizing authorized group
members for each group, where the recognition is based on IKEv1
authentication credentials. For example, the GCKS may have a list of
authorized IKEv1 identifiers stored for each Group. The
authorization check SHOULD be made after receipt of the ID payload
containing a group id the group member is requesting to join.
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5. New GDOI Attributes
This section contains new attributes to be are defined as part of
GDOI.
5.1. Signature Hash Algorithm
RFC 3547 defines two signature hash algorithms (MD5 and SHA-1).
However, steady advances in technology have rendered both hash
algorithms to be weak when used as a signature hash algorithm.
The SHA-256 algorithm [FIPS.180-2.2002] has been made available by
NIST as a replacement for SHA-1, and is its preferred replacement for
both MD5 and SHA-1. A new value for the GDOI SIG_HASH_ALGORITHM
attribute is defined by this memo to represent the SHA-256 algorithm:
SIG_HASH_SHA256. Support for SIG_HASH_SHA256 is OPTIONAL.
5.2. Support of AH
RFC3547 only specifies data-security SAs for one security protocol,
IPsec ESP. Typically IPsec implementations use ESP and AH IPsec SAs.
This document extends the capability of GDOI to support both ESP and
AH. The GROUPKEY-PULL mechanism will establish IPsec ESP SAs and
IPsec AH SAs. The GROUPKEY-PUSH will refresh the IPsec ESP SAs and
the IPsec AH SAs. Support for AH [RFC4302] will come with the
introduction of a new SA_TEK Protocol-ID with the name
GDOI_PROTO_IPSEC_AH. Support for the GDOI_PROTO_IPSEC_AH SA TEK is
OPTIONAL.
The TEK Protocol-Specific payload for AH is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! Protocol ! SRC ID Type ! SRC ID Port !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
!SRC ID Data Len! SRC Identification Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! DST ID Type ! DST ID Port !DST ID Data Len!
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! DST Identification Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! Transform ID ! SPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! SPI ! RFC 2407 SA Attributes ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
The SAT Payload fields are defined as follows:
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o Protocol (1 octet) -- Value describing an IP protocol ID (e.g.,
UDP/TCP). A value of zero means that the Protocol field should be
ignored.
o SRC ID Type (1 octet) -- Value describing the identity information
found in the SRC Identification Data field. Defined values are
specified by the IPsec Identification Type section in the IANA
ISAKMP Registry [ISAKMP-REG].
o SRC ID Port (2 octets) -- Value specifying a port associated with
the source Id. A value of zero means that the SRC ID Port field
should be ignored.
o SRC ID Data Len (1 octet) -- Value specifying the length of the
SRC Identification Data field.
o SRC Identification Data (variable length) -- Value, as indicated
by the SRC ID Type. Set to three bytes of zero for multiple-
source multicast groups that use a common TEK for all senders.
o DST ID Type (1 octet) -- Value describing the identity information
found in the DST Identification Data field. Defined values are
specified by the IPsec Identification Type section in the IANA
ISAKMP Registry [ISAKMP-REG].
o DST ID Port (1 octet) -- Value describing an IP protocol ID (e.g.,
UDP/TCP). A value of zero means that the DST Id Port field should
be ignored.
o DST ID Port (2 octets) -- Value specifying a port associated with
the source Id. A value of zero means that the DST ID Port field
should be ignored.
o DST ID Data Len (1 octet) -- Value specifying the length of the
DST Identification Data field.
o DST Identification Data (variable length) -- Value, as indicated
by the DST ID Type.
o Transform ID (1 octet) -- Value specifying which AH transform is
to be used. The list of valid values is defined in the IPsec AH
Transform Identifiers section of the IANA ISAKMP Registry
[ISAKMP-REG].
o SPI (4 octets) -- Security Parameter Index for AH.
o RFC 2407 Attributes -- AH Attributes from Section 4.5 of
[RFC2407]. The GDOI supports all IPsec DOI SA Attributes for
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GDOI_PROTO_IPSEC_AH excluding the Group Description, which MUST
NOT be sent by a GDOI implementation and is ignored by a GDOI
implementation if received. The Authentication Algorithm
attribute of the IPsec DOI is group authentication in GDOI. The
following RFC 2407 attributes MUST be sent as part of a
GDOI_PROTO_IPSEC_AH attribute: SA Life Type, SA Life Duration,
Encapsulation Mode.
5.3. Sender-Specific Attributes
RFC 3547 provides for the distribution of policy in the GROUPKEY-PULL
exchange in an SA payload. Policy can define GROUPKEY-PUSH policy
(SA KEK) or traffic encryption policy (SA TEK) such as IPsec policy.
Additionally, there is a need to distribute sender-specific policy to
each group member that is irrespective of either the SA KEK or SA TEK
policy.
GDOI distributes this sender-specific policy in a new payload called
the SA Sender-Specific Attributes Payload (SA SSA). The SA SSA
payload follows any SA KEK payload, and is placed before any SA TEK
payloads. In the case that group policy does not include an SA KEK,
the SA Attribute Next Payload field in the SA payload MAY indicate
the SA SSA payload.
The SA SSA payload MUST NOT be a part of a GROUPKEY-PUSH message,
because distributing the same sender-specific policy to more than one
group member may reduce the security of the group. A group member
MUST NOT process an SA SSA payload present in a GROUPKEY-PUSH
message.
The SA SSA payload is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! Next Payload ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! Sender-Specific Attributes ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
The SA SSA payload fields are defined as follows:
o Next Payload (1 octet) -- Identifies the next payload for the
GROUPKEY-PULL or the GROUPKEY-PUSH message. The only valid next
payload type for this message is an SA TEK or zero to indicate
there are no more security association attributes.
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o RESERVED (1 octet) -- Must be zero.
o Payload Length (2 octets) -- Length of this payload, including the
SA SSA header and Sender-Specific Attributes.
o Group Attributes (variable) -- Contains sender-specific attributes
following the format defined in ISAKMP [RFC2408] section 3.3.
One attribute with the type of SENDER_ID is defined in this memo.
5.3.1. SENDER_ID
Several new AES counter-based modes of operation have been specified
for ESP [RFC3686],[RFC4106],[RFC4309],[RFC4543] and AH [RFC4543].
These AES counter-based modes require that no two senders in the
group ever send a packet with the same IV. This requirement can be
met using the method described in
[I-D.ietf-msec-ipsec-group-counter-modes], which requires each sender
to be allocated a unique Sender ID (SID). The SENDER_ID attribute is
used to distribute a SID to a group member during the GROUPKEY-PULL
message. Other algorithms with the same need may be defined in the
future; the sender MUST use the IV construction method described
above with those algorithms as well.
The SENDER_ID attribute value contains the following fields.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! SID Length ! SID Value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
o SID Length (1 octet) -- A natural number defining the number of
bits to be used in the SID field of the counter mode transform
nonce.
o SID Value (variable) -- The Sender ID value allocated to the group
member.
5.3.1.1. GCKS Semantics
The GCKS maintains a SID counter (SIDC). It is incremented each time
a SENDER_ID attribute is distributed to a group member.The first
group member to register is given the SID of 1.
Any group member registering will be given a new SID value, which
allows group members to act as a group sender when an older SID value
becomes unusable (as described in the next section).
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A GCKS MAY allocate multiple SID values in one SA SSA payload.
Allocating several SID values at the same time to a group member
expected to send at a high rate would obviate the need for the group
member to re-register as frequently.
If a GKCS allocates all SID values, it can no longer respond to GDOI
registrations. and must re-initialize the entire group. This is done
by issuing DELETE notifications for all ESP and AH SAs in a GDOI
rekey message, resetting the SIDC to zero, and creating new ESP and
AH SAs that match the group policy. When group members re-register,
the SIDs are allocated again beginning with the value 1 as described
above. Each re-registering group member will be given a new SID and
the new group policy.
5.3.1.2. Group Member Semantics
The SENDER_ID attribute value distributed to the group member MUST be
used by that group member as the Sender Identifier (SID) field
portion of the IV. The SID is used for all counter mode SAs
distributed by the GCKS to be used for communications sent as a part
of this group.
When the Sender-Specific IV (SSIV) field for any IPsec SA is
exhausted, the group member MUST no longer act as a sender using its
active SID. The group member SHOULD re-register, during which time
the GCKS will issue a new SID to the group member. The new SID
replaces the existing SID used by this group member, and also resets
the SSIV value to it's starting value. A group member MAY re-
register prior to the actual exhaustion of the SSIV field to avoid
dropping data packets due to the exhaustion of available SSIV values
combined with a particular SID value.
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6. New IPsec Security Association Attributes
The Multicast Extensions to RFC 4301 [I-D.ietf-msec-ipsec-extensions]
describes new attributes to an IPsec security association. These
attributes describe policy that a group key management system must
convey in order to support those extensions. The GDOI SA TEK payload
distributes IPsec policy using IPsec security association attributes
defined in [ISAKMP-REG]. This section defines how GDOI can convey
the new attributes as IPsec Security Association Attributes.
6.1. Address Preservation
In order for an IP multicast packet to be encapsulated such that it
will remain an IP multicast packet, the original IP addresses may
need to be retained. This requires a new IPsec SA attribute
describing which of the IP addresses are to be preserved.
Depending on group policy, several address preservation methods are
possible: no address preservation ("None"), preservation of the
original source address ("Source-Only"), preservation of the original
destination address ("Destination-Only"), or both addresses ("Source-
And-Destination"). If the attribute is not included in a GDOI SA TEK
payload then Source-And-Destination address preservation has been
defined for the SA TEK.
6.2. SA Direction
Depending on group policy, an IPsec SA may be required in one or both
directions. An IPsec SA used by multiple senders is required to be
installed in both the sending and receiving direction ("Symmetric"),
whereas an SA with a single sender need only be installed in the
receiving direction by receivers ("Receiver-Only") and in the sending
direction by the sender ("Sender-Only"). If the attribute is not
included in a GDOI SA TEK payload then the IPsec SA is treated as a
Symmetric IPsec SA.
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7. IANA Considerations
The GDOI SIG_HASH_ALGORITHM KEK Attribute [GDOI-REG] should be
assigned a new Algorithm Type value from the RESERVED space to
represent the SHA-256 hash algorithm as defined. The new algorithm
name should be SIG_HASH_SHA256.
A new GDOI KEK Attribute [GDOI-REG] is needed to represent the number
of seconds before a sender should transmit on an TEK. The attribute
has the name TEK_TRANSMIT_WAIT_PERIOD, and is a Variable type.
A new GDOI SA TEK type Protocol-ID type [GDOI-REG] should be assigned
from the RESERVED space. The new algorithm id should be called
GDOI_PROTO_IPSEC_AH, and refers to the IPsec AH encapsulation.
A new Next Payload Type [ISAKMP-REG] should be assigned. The new
type is called "SA SSA Payload (SSA)".
A new namespace should be created in the GDOI Payloads registry
[GDOI-REG] to describe SA SSA Payload Values. The following rules
apply to define the attributes in SA SSA Payload Values:
Attribute Type Value Type
---- ----- ----
RESERVED 0
SENDER_ID 1 V
Reserved to IANA 2-127
Private Use 128-255
A new IPSEC Security Association Attribute [ISAKMP-REG] defining the
preservation of IP addresses is needed. The attribute class is
called "Address Preservation", and it is a Basic type. The following
rules apply to define the values of the attribute:
Name Value
---- -----
Reserved 0
None 1
Source-Only 2
Destination-Only 3
Source-And-Destination 4
Reserved to IANA 5-61439
Private Use 61440-65535
A new IPSEC Security Association Attribute [ISAKMP-REG] defining the
SA direction is needed. The attribute class is called "SA
Direction", and it is a Basic type. The following rules apply to
define the values of the attribute:
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Name Value
---- -----
Reserved 0
Sender-Only 1
Receiver-Only 2
Symmetric 3
Reserved to IANA 4-61439
Private Use 61440-65535
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8. Security Considerations
This memo describes additional clarification and protocol updates to
the GDOI protocol. The security considerations in RFC 3547 remain
accurate, with the following additions.
o Several minor cryptographic hash algorithm agility issues are
resolved, and the stronger SHA-256 cryptographic hash algorithm is
added.
o Protocol analysis has revealed a man-in-the-middle attack when the
GCKS does not authorize group members based on their IKEv1
authentication credentials. This is true even when a CERT and POP
payloads are used for authorization. Although suggested as an
option in RFC 3547, a GDOI device (group member or GCKS) SHOULD
NOT accept an identity in a CERT payload that does not match the
IKEv1 identity used to authenticate the group member.
o Any SA TEK specicifying a counter-based mode of operation with
multiple senders MUST construct the IVs in each SA TEK according
to [I-D.ietf-msec-ipsec-group-counter-modes]. The SID MUST either
be pre-configured on all group members or distributed using the
SENDER_ID attribute in the SA SSA payload. However, use use of
the SENDER_ID attribute is RECOMMENDED.
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9. Acknowledgements
The authors are grateful to Catherine Meadows for her careful review
and suggestions for mitigating the man-in-the-middle attack she had
previously identified.
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10. References
10.1. Normative References
[FIPS.180-2.2002]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002, <http://
csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.
[I-D.ietf-msec-ipsec-extensions]
Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", draft-ietf-msec-ipsec-extensions-06 (work in
progress), July 2007.
[I-D.ietf-msec-ipsec-group-counter-modes]
McGrew, D. and B. Weis, "Using Counter Modes with
Encapsulating Security Payload (ESP) and Authentication
Header (AH) to Protect Group Traffic",
draft-ietf-msec-ipsec-group-counter-modes-00 (work in
progress), February 2007.
[NIST.800-56A.2006]
National Institute of Standards and Technology,
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", NIST 800-56A,
March 2006, <http://csrc.nist.gov/publications/nistpubs/
800-56A/sp800-56A_May-3-06.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
10.2. Informative References
[GDOI-REG]
Internet Assigned Numbers Authority, "Group Domain of
Interpretation (GDOI) Payload Type Values", IANA Registry,
December 2004,
<http://www.iana.org/assignments/gdoi-payloads>.
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[IPSEC-REG]
Internet Assigned Numbers Authority, "Internet Key
Exchange (IKE) Attributes IKE Attributes", IANA Registry,
December 2005,
<http://www.iana.org/assignments/ipsec-registry>.
[ISAKMP-REG]
Internet Assigned Numbers Authority, "Internet Security
Association and Key Management Protocol (ISAKMP)
Identifiers ISAKMP Attributes", IANA Registry,
January 2006,
<http://www.iana.org/assignments/isakmp-registry>.
[MP04] Meadows, C. and D. Pavlovic, "Deriving, Attacking, and
Defending the GDOI Protocol", ESORICS 2004 pp. 53-72,
September 2004.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2407] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[RFC2408] Maughan, D., Schneider, M., and M. Schertler, "Internet
Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.
[RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
"Multicast Security (MSEC) Group Key Management
Architecture", RFC 4046, April 2005.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
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Mode with IPsec Encapsulating Security Payload (ESP)",
RFC 4309, December 2005.
[RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message
Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
May 2006.
[RFC4894] Hoffman, P., "Use of Hash Algorithms in Internet Key
Exchange (IKE) and IPsec", RFC 4894, May 2007.
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Authors' Addresses
Brian Weis
Cisco Systems
170 W. Tasman Drive
San Jose, California 95134-1706
USA
Phone: +1-408-526-4796
Email: bew@cisco.com
Sheela Rowles
Cisco Systems
170 W. Tasman Drive
San Jose, California 95134-1706
USA
Phone: +1-408-527-7677
Email: srowles@cisco.com
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