Request for Comments: 2025 Bell-Northern Research
Category: Standards Track October 1996
The Simple Public-Key GSS-API Mechanism (SPKM)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This specification defines protocols, procedures, and conventions to
be employed by peers implementing the Generic Security Service
Application Program Interface (as specified in RFCs 1508 and 1509)
when using the Simple Public-Key Mechanism.
Background
Although the Kerberos Version 5 GSS-API mechanism [KRB5] is becoming
well-established in many environments, it is important in some
applications to have a GSS-API mechanism which is based on a public-
key, rather than a symmetric-key, infrastructure. The mechanism
described in this document has been proposed to meet this need and to
provide the following features.
1) The SPKM allows both unilateral and mutual authentication
to be accomplished without the use of secure timestamps. This
enables environments which do not have access to secure time
to nevertheless have access to secure authentication.
2) The SPKM uses Algorithm Identifiers to specify various
algorithms to be used by the communicating peers. This allows
maximum flexibility for a variety of environments, for future
enhancements, and for alternative algorithms.
3) The SPKM allows the option of a true, asymmetric algorithm-
based, digital signature in the gss_sign() and gss_seal()
operations (now called gss_getMIC() and gss_wrap() in
[GSSv2]), rather than an integrity checksum based on a MAC
computed with a symmetric algorithm (e.g., DES). For some
environments, the availability of true digital signatures
supporting non-repudiation is a necessity.
4) SPKM data formats and procedures are designed to be as similar
to those of the Kerberos mechanism as is practical. This is
done for ease of implementation in those environments where
Kerberos has already been implemented.
For the above reasons, it is felt that the SPKM will offer
flexibility and functionality, without undue complexity or overhead.
Key Management
The key management employed in SPKM is intended to be as compatible
as possible with both X.509 [X.509] and PEM [RFC-1422], since these
represent large communities of interest and show relative maturity in
standards.
Acknowledgments
Much of the material in this document is based on the Kerberos
Version 5 GSS-API mechanism [KRB5], and is intended to be as
compatible with it as possible. This document also owes a great debt
to Warwick Ford and Paul Van Oorschot of Bell-Northern Research for
many fruitful discussions, to Kelvin Desplanque for implementation-
related clarifications, to John Linn of OpenVision Technologies for
helpful comments, and to Bancroft Scott of OSS for ASN.1 assistance.
1. Overview
The goal of the Generic Security Service Application Program
Interface (GSS-API) is stated in the abstract of [RFC-1508] as
follows:
"This Generic Security Service Application Program Interface (GSS-
API) definition provides security services to callers in a generic
fashion, supportable with a range of underlying mechanisms and
technologies and hence allowing source-level portability of
applications to different environments. This specification defines
GSS-API services and primitives at a level independent of
underlying mechanism and programming language environment, and is
to be complemented by other, related specifications:
- documents defining specific parameter bindings for particular
language environments;
- documents defining token formats, protocols, and procedures to
be implemented in order to realize GSS-API services atop
particular security mechanisms."
The SPKM is an instance of the latter type of document and is
therefore termed a "GSS-API Mechanism". This mechanism provides
authentication, key establishment, data integrity, and data
confidentiality in an on-line distributed application environment
using a public-key infrastructure. Because it conforms to the
interface defined by [RFC-1508], SPKM can be used as a drop-in
replacement by any application which makes use of security services
through GSS-API calls (for example, any application which already
uses the Kerberos GSS-API for security). The use of a public-key
infrastructure allows digital signatures supporting non-repudiation
to be employed for message exchanges, and provides other benefits
such as scalability to large user populations.
The tokens defined in SPKM are intended to be used by application
programs according to the GSS API "operational paradigm" (see [RFC-
1508] for further details):
The operational paradigm in which GSS-API operates is as follows.
A typical GSS-API caller is itself a communications protocol [or is
an application program which uses a communications protocol],
calling on GSS-API in order to protect its communications with
authentication, integrity, and/or confidentiality security
services. A GSS-API caller accepts tokens provided to it by its
local GSS-API implementation [i.e., its GSS-API mechanism] and
transfers the tokens to a peer on a remote system; that peer passes
the received tokens to its local GSS-API implementation for
processing.
This document defines two separate GSS-API mechanisms, SPKM-1 and
SPKM-2, whose primary difference is that SPKM-2 requires the
presence of secure timestamps for the purpose of replay detection
during context establishment and SPKM-1 does not. This allows
greater flexibility for applications since secure timestamps cannot
always be guaranteed to be available in a given environment.
2. Algorithms
A number of algorithm types are employed in SPKM. Each type, along
with its purpose and a set of specific examples, is described in this
section. In order to ensure at least a minimum level of
interoperability among various implementations of SPKM, one of the
integrity algorithms is specified as MANDATORY; all remaining
examples (and any other algorithms) may optionally be supported by a
given SPKM implementation (note that a GSS-conformant mechanism need
not support confidentiality). Making a confidentiality algorithm
mandatory may preclude exportability of the mechanism implementation;
this document therefore specifies certain algorithms as RECOMMENDED
(that is, interoperability will be enhanced if these algorithms are
included in all SPKM implementations for which exportability is not a
concern).
2.1 Integrity Algorithm (I-ALG):
Purpose:
This algorithm is used to ensure that a message has not been
altered in any way after being constructed by the legitimate
sender. Depending on the algorithm used, the application of
this algorithm may also provide authenticity and support non-
repudiation for the message.
Examples:
md5WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 4 -- imported from [PKCS1]
}
This algorithm (MANDATORY) provides data integrity and
authenticity and supports non-repudiation by computing an
RSA signature on the MD5 hash of that data. This is
essentially equivalent to md5WithRSA {1 3 14 3 2 3},
which is defined by OIW (the Open Systems Environment
Implementors' Workshop).
Note that since this is the only integrity/authenticity
algorithm specified to be mandatory at this time, for
interoperability reasons it is also stipulated that
md5WithRSA be the algorithm used to sign all context
establishment tokens which are signed rather than MACed --
see Section 3.1.1 for details. In future versions of this
document, alternate or additional algorithms may be
specified to be mandatory and so this stipulation on the
context establishment tokens may be removed.
DES-MAC OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithm(2) 10 -- carries length in bits of the MAC as
} -- an INTEGER parameter, constrained to
-- multiples of eight from 16 to 64
This algorithm (RECOMMENDED) provides integrity by computing
a DES MAC (as specified by [FIPS-113]) on that data.
md5-DES-CBC OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) integrity(3) md5-DES-CBC(1)
}
This algorithm provides data integrity by encrypting, using
DES CBC, the "confounded" MD5 hash of that data (see Section
3.2.2.1 for the definition and purpose of confounding).
This will typically be faster in practice than computing a
DES MAC unless the input data is extremely short (e.g., a
few bytes). Note that without the confounder the strength
of this integrity mechanism is (at most) equal to the
strength of DES under a known-plaintext attack.
sum64-DES-CBC OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) integrity(3) sum64-DES-CBC(2)
}
This algorithm provides data integrity by encrypting, using
DES CBC, the concatenation of the confounded data and the
sum of all the input data blocks (the sum computed using
addition modulo 2**64 - 1). Thus, in this algorithm,
encryption is a requirement for the integrity to be secure.
For comments regarding the security of this integrity
algorithm, see [Juen84, Davi89].
2.2 Confidentiality Algorithm (C-ALG):
Purpose:
This symmetric algorithm is used to generate the encrypted
data for gss_seal() / gss_wrap().
Example:
DES-CBC OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithm(2) 7 -- carries IV (OCTET STRING) as a parameter;
} -- this (optional) parameter is unused in
-- SPKM due to the use of confounding
This algorithm is RECOMMENDED.
2.3 Key Establishment Algorithm (K-ALG):
Purpose:
This algorithm is used to establish a symmetric key for use
by both the initiator and the target over the established
context. The keys used for C-ALG and any keyed I-ALGs (for
example, DES-MAC) are derived from this context key. As will
be seen in Section 3.1, key establishment is done within the
X.509 authentication exchange and so the resulting shared
symmetric key is authenticated.
Examples:
RSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 1 -- imported from [PKCS1] and [RFC-1423]
}
In this algorithm (MANDATORY), the context key is generated
by the initiator, encrypted with the RSA public key of the
target, and sent to the target. The target need not respond
to the initiator for the key to be established.
id-rsa-key-transport OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithm(2) 22 -- imported from [X9.44]
}
Similar to RSAEncryption, but source authenticating info.
is also encrypted with the target's RSA public key.
dhKeyAgreement OBJECT IDENTIFIER ::= {
iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
pkcs-3(3) 1
}
In this algorithm, the context key is generated jointly by
the initiator and the target using the Diffie-Hellman key
establishment algorithm. The target must therefore respond
to the initiator for the key to be established (so this
K-ALG cannot be used with unilateral authentication in
SPKM-2 (see Section 3.1)).
2.4 One-Way Function (O-ALG) for Subkey Derivation Algorithm:
Purpose:
Having established a context key using the negotiated K-ALG,
both initiator and target must be able to derive a set of
subkeys for the various C-ALGs and keyed I-ALGs supported over
the context. Let the (ordered) list of agreed C-ALGs be
numbered consecutively, so that the first algorithm (the
"default") is numbered "0", the next is numbered "1", and so
on. Let the numbering for the (ordered) list of agreed I-ALGs
be identical. Finally, let the context key be a binary string
of arbitrary length "M", subject to the following constraint:
L <= M <= U (where the lower limit "L" is the bit length of
the longest key needed by any agreed C-ALG or keyed I-ALG, and
the upper limit "U" is the largest bit size which will fit
within the K-ALG parameters).
For example, if DES and two-key-triple-DES are the negotiated
confidentiality algorithms and DES-MAC is the negotiated keyed
integrity algorithm (note that digital signatures do not use a
context key), then the context key must be at least 112 bits
long. If 512-bit RSAEncryption is the K-ALG in use then the
originator can randomly generate a context key of any greater
length up to 424 bits (the longest allowable RSA input
specified in [PKCS-1]) -- the target can determine the length
which was chosen by removing the padding bytes during the RSA
decryption operation. On the other hand, if dhKeyAgreement is
the K-ALG in use then the context key is the result of the
Diffie-Hellman computation (with the exception of the high-
order byte, which is discarded for security reasons), so that
its length is that of the Diffie-Hellman modulus, p, minus 8
bits.
The derivation algorithm for a k-bit subkey is specified as
follows:
rightmost_k_bits (OWF(context_key || x || n || s || context_key))
where
- "x" is the ASCII character "C" (0x43) if the subkey is
for a confidentiality algorithm or the ASCII character "I"
(0x49) if the subkey is for a keyed integrity algorithm;
- "n" is the number of the algorithm in the appropriate agreed
list for the context (the ASCII character "0" (0x30), "1"
(0x31), and so on);
- "s" is the "stage" of processing -- always the ASCII
character "0" (0x30), unless "k" is greater than the output
size of OWF, in which case the OWF is computed repeatedly
with increasing ASCII values of "stage" (each OWF output
being concatenated to the end of previous OWF outputs),
until "k" bits have been generated;
- "||" is the concatenation operation; and
- "OWF" is any appropriate One-Way Function.
Examples:
MD5 OBJECT IDENTIFIER ::= {
iso(1) member-body(2) US(840) rsadsi(113549)
digestAlgorithm(2) 5
}
This algorithm is MANDATORY.
SHA OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithm(2) 18
}
It is recognized that existing hash functions may not satisfy
all required properties of OWFs. This is the reason for
allowing negotiation of the O-ALG OWF during the context
establishment process (see Section 2.5), since in this way
future improvements in OWF design can easily be accommodated.
For example, in some environments a preferred OWF technique
might be an encryption algorithm which encrypts the input
specified above using the context_key as the encryption key.
2.5 Negotiation:
During context establishment in SPKM, the initiator offers a set of
possible confidentiality algorithms and a set of possible integrity
algorithms to the target (note that the term "integrity algorithms"
includes digital signature algorithms). The confidentiality
algorithms selected by the target become ones that may be used for
C-ALG over the established context, and the integrity algorithms
selected by the target become ones that may be used for I-ALG over
the established context (the target "selects" algorithms by
returning, in the same relative order, the subset of each offered
list that it supports). Note that any C-ALG and I-ALG may be used
for any message over the context and that the first confidentiality
algorithm and the first integrity algorithm in the agreed sets become
the default algorithms for that context.
The agreed confidentiality and integrity algorithms for a specific
context define the valid values of the Quality of Protection (QOP)
parameter used in the gss_getMIC() and gss_wrap() calls -- see
Section 5.2 for further details. If no response is expected from the
target (unilateral authentication in SPKM-2) then the algorithms
offered by the initiator are the ones that may be used over the
context (if this is unacceptable to the target then a delete token
must be sent to the initiator so that the context is never
established).
Furthermore, in the first context establishment token the initiator
offers a set of possible K-ALGs, along with the key (or key half)
corresponding to the first algorithm in the set (its preferred
algorithm). If this K-ALG is unacceptable to the target then the
target must choose one of the other K-ALGs in the set and send this
choice along with the key (or key half) corresponding to this choice
in its response (otherwise a delete token must be sent so that the
context is never established). If necessary (that is, if the target
chooses a 2-pass K-ALG such as dhKeyAgreement), the initiator will
send its key half in a response to the target.
Finally, in the first context establishment token the initiator
offers a set of possible O-ALGs (only a single O-ALG if no response
is expected). The (single) O-ALG chosen by the target becomes the
subkey derivation algorithm OWF to be used over the context.
In future versions of SPKM, other algorithms may be specified for any
or all of I-ALG, C-ALG, K-ALG, and O-ALG.
3. Token Formats
This section discusses protocol-visible characteristics of the SPKM;
it defines elements of protocol for interoperability and is
independent of language bindings per [RFC-1509].
The SPKM GSS-API mechanism will be identified by an Object Identifier
representing "SPKM-1" or "SPKM-2", having the value {spkm spkm-1(1)}
or {spkm spkm-2(2)}, where spkm has the value {iso(1) identified-
organization(3) dod(6) internet(1) security(5) mechanisms(5)
spkm(1)}. SPKM-1 uses random numbers for replay detection during
context establishment and SPKM-2 uses timestamps (note that for both
mechanisms, sequence numbers are used to provide replay and out-of-
sequence detection during the context, if this has been requested by
the application).
Tokens transferred between GSS-API peers (for security context
management and per-message protection purposes) are defined.
3.1. Context Establishment Tokens
Three classes of tokens are defined in this section: "Initiator"
tokens, emitted by calls to gss_init_sec_context() and consumed by
calls to gss_accept_sec_context(); "Target" tokens, emitted by calls
to gss_accept_sec_context() and consumed by calls to
gss_init_sec_context(); and "Error" tokens, potentially emitted by
calls to gss_init_sec_context() or gss_accept_sec_context(), and
potentially consumed by calls to gss_init_sec_context() or
gss_accept_sec_context().
Per RFC-1508, Appendix B, the initial context establishment token
will be enclosed within framing as follows:
InitialContextToken ::= [APPLICATION 0] IMPLICIT SEQUENCE {
thisMech MechType,
-- MechType is OBJECT IDENTIFIER
-- representing "SPKM-1" or "SPKM-2"
innerContextToken ANY DEFINED BY thisMech
} -- contents mechanism-specific
When thisMech is SPKM-1 or SPKM-2, innerContextToken is defined as
follows:
SPKMInnerContextToken ::= CHOICE {
req [0] SPKM-REQ,
rep-ti [1] SPKM-REP-TI,
rep-it [2] SPKM-REP-IT,
error [3] SPKM-ERROR,
mic [4] SPKM-MIC,
wrap [5] SPKM-WRAP,
del [6] SPKM-DEL
}
The above GSS-API framing shall be applied to all tokens emitted by
the SPKM GSS-API mechanism, including SPKM-REP-TI (the response from
the Target to the Initiator), SPKM-REP-IT (the response from the
Initiator to the Target), SPKM-ERROR, context-deletion, and per-
message tokens, not just to the initial token in a context
establishment exchange. While not required by RFC-1508, this enables
implementations to perform enhanced error-checking. The tag values
provided in SPKMInnerContextToken ("[0]" through "[6]") specify a
token-id for each token; similar information is contained in each
token's tok-id field. While seemingly redundant, the tag value and
tok-id actually perform different tasks: the tag ensures that
InitialContextToken can be properly decoded; tok-id ensures, among
other things, that data associated with the per-message tokens is
cryptographically linked to the intended token type. Every
innerContextToken also includes a context-id field; see Section 6 for
a discussion of both token-id and context-id information and their
use in an SPKM support function).
The innerContextToken field of context establishment tokens for the
SPKM GSS-API mechanism will contain one of the following messages:
SPKM-REQ; SPKM-REP-TI; SPKM-REP-IT; and SPKM-ERROR. Furthermore, all
innerContextTokens are encoded using ASN.1 BER (constrained, in the
interests of parsing simplicity, to the DER subset defined in
[X.509], clause 8.7).
The SPKM context establishment tokens are defined according to
[X.509] Section 10 and are compatible with [9798]. SPKM-1 (random
numbers) uses Section 10.3, "Two-way Authentication", when performing
unilateral authentication of the target to the initiator and uses
Section 10.4, "Three-way Authentication", when mutual authentication
is requested by the initiator. SPKM-2 (timestamps) uses Section
10.2, "One-way Authentication", when performing unilateral
authentication of the initiator to the target and uses Section 10.3,
"Two-way Authentication", when mutual authentication is requested by
the initiator.
The implication of the previous paragraph is that for SPKM-2
unilateral authentication no negotiation of K-ALG can be done (the
target either accepts the K-ALG and context key given by the
initiator or disallows the context). For SPKM-2 mutual or SPKM-1
unilateral authentication some negotiation is possible, but the
target can only choose among the one-pass K-ALGs offered by the
initiator (or disallow the context). Alternatively, the initiator
can request that the target generate and transmit the context key.
For SPKM-1 mutual authentication the target can choose any one- or
two-pass K-ALG offered by the initiator and, again, can be requested
to generate and transmit the context key.
It is envisioned that typical use of SPKM-1 or SPKM-2 will involve
mutual authentication. Although unilateral authentication is
available for both mechanisms, its use is not generally recommended.
3.1.1. Context Establishment Tokens - Initiator (first token)
In order to accomplish context establishment, it may be necessary
that both the initiator and the target have access to the other
partys public-key certificate(s). In some environments the initiator
may choose to acquire all certificates and send the relevant ones to
the target in the first token. In other environments the initiator
may request that the target send certificate data in its response
token, or each side may individually obtain the certificate data it
needs. In any case, however, the SPKM implementation must have the
ability to obtain certificates which correspond to a supplied Name.
The actual mechanism to be used to achieve this is a local
implementation matter and is therefore outside the scope of this
specification.
Relevant SPKM-REQ syntax is as follows (note that imports from other
documents are given in Appendix A):
SPKM-REQ ::= SEQUENCE {
requestToken REQ-TOKEN,
certif-data [0] CertificationData OPTIONAL,
auth-data [1] AuthorizationData OPTIONAL
-- see [RFC-1510] for a discussion of auth-data
}
CertificationData ::= SEQUENCE {
certificationPath [0] CertificationPath OPTIONAL,
certificateRevocationList [1] CertificateList OPTIONAL
} -- at least one of the above shall be present
CertificationPath ::= SEQUENCE {
userKeyId [0] OCTET STRING OPTIONAL,
-- identifier for user's public key
userCertif [1] Certificate OPTIONAL,
-- certificate containing user's public key
verifKeyId [2] OCTET STRING OPTIONAL,
-- identifier for user's public verification key
userVerifCertif [3] Certificate OPTIONAL,
-- certificate containing user's public verification key
theCACertificates [4] SEQUENCE OF CertificatePair OPTIONAL
} -- certification path from target to source
Having separate verification fields allows different key pairs
(possibly corresponding to different algorithms) to be used for
encryption/decryption and signing/verification. Presence of [0] or
[1] and absence of [2] and [3] implies that the same key pair is to
be used for enc/dec and verif/signing (note that this practice is not
typically recommended). Presence of [2] or [3] implies that a
separate key pair is to be used for verif/signing, and so [0] or [1]
must also be present. Presence of [4] implies that at least one of
[0], [1], [2], and [3] must also be present.
REQ-TOKEN ::= SEQUENCE {
req-contents Req-contents,
algId AlgorithmIdentifier,
req-integrity Integrity -- "token" is Req-contents
}
Integrity ::= BIT STRING
-- If corresponding algId specifies a signing algorithm,
-- "Integrity" holds the result of applying the signing procedure
-- specified in algId to the BER-encoded octet string which results
-- from applying the hashing procedure (also specified in algId) to
-- the DER-encoded octets of "token".
-- Alternatively, if corresponding algId specifies a MACing
-- algorithm, "Integrity" holds the result of applying the MACing
-- procedure specified in algId to the DER-encoded octets of
-- "token" (note that for MAC, algId must be one of the integrity
-- algorithms offered by the initiator with the appropriate subkey
-- derived from the context key (see Section 2.4) used as the key
-- input)
It is envisioned that typical use of the Integrity field for each of
REQ-TOKEN, REP-TI-TOKEN, and REP-IT-TOKEN will be a true digital
signature, providing unilateral or mutual authentication along with
replay protection, as required. However, there are situations in
which the MAC choice will be appropriate. One example is the case in
which the initiator wishes to remain anonymous (so that the first, or
first and third, token(s) will be MACed and the second token will be
signed). Another example is the case in which a previously
authenticated, established, and cached context is being re-
established at some later time (here all exchanged tokens will be
MACed).
The primary advantage of the MAC choice is that it reduces processing
overhead for cases in which either authentication is not required
(e.g., anonymity) or authentication is established by some other
means (e.g., ability to form the correct MAC on a "fresh" token in
context re-establishment).
Req-contents ::= SEQUENCE {
tok-id INTEGER (256), -- shall contain 0100(hex)
context-id Random-Integer, -- see Section 6.3
pvno BIT STRING, -- protocol version number
timestamp UTCTime OPTIONAL, -- mandatory for SPKM-2
randSrc Random-Integer,
targ-name Name,
src-name [0] Name OPTIONAL,
-- must be supplied unless originator is "anonymous"
req-data Context-Data,
validity [1] Validity OPTIONAL,
-- validity interval for key (may be used in the
-- computation of security context lifetime)
key-estb-set Key-Estb-Algs,
-- specifies set of key establishment algorithms
key-estb-req BIT STRING OPTIONAL,
-- key estb. parameter corresponding to first K-ALG in set
-- (not used if initiator is unable or unwilling to
-- generate and securely transmit key material to target).
-- Established key must satisfy the key length constraints
-- specified in Section 2.4.
key-src-bind OCTET STRING OPTIONAL
-- Used to bind the source name to the symmetric key.
-- This field must be present for the case of SPKM-2
-- unilateral authen. if the K-ALG in use does not provide
-- such a binding (but is optional for all other cases).
-- The octet string holds the result of applying the
-- mandatory hashing procedure MD5 (in MANDATORY I-ALG;
-- see Section 2.1) as follows: MD5(src || context_key),
-- where "src" is the DER-encoded octets of src-name,
-- "context-key" is the symmetric key (i.e., the
-- unprotected version of what is transmitted in
-- key-estb-req), and "||" is the concatenation operation.
}
-- The protocol version number (pvno) parameter is a BIT STRING which
-- uses as many bits as necessary to specify all the SPKM protocol
-- versions supported by the initiator (one bit per protocol
-- version). The protocol specified by this document is version 0.
-- Bit 0 of pvno is therefore set if this version is supported;
-- similarly, bit 1 is set if version 1 (if defined in the future) is
-- supported, and so on. Note that for unilateral authentication
-- using SPKM-2, no response token is expected during context
-- establishment, so no protocol negotiation can take place; in this
-- case, the initiator must set exactly one bit of pvno. The version
-- of REQ-TOKEN must correspond to the highest bit set in pvno.
-- The "validity" parameter above is the only way within SPKM for
-- the initiator to transmit desired context lifetime to the target.
-- Since it cannot be guaranteed that the initiator and target have
-- synchronized time, the span of time specified by "validity" is to
-- be taken as definitive (rather than the actual times given in this
-- parameter).
Random-Integer ::= BIT STRING
-- Each SPKM implementation is responsible for generating a "fresh"
-- random number for the purpose of context establishment; that is,
-- one which (with high probability) has not been used previously.
-- There are no cryptographic requirements on this random number
-- (i.e., it need not be unpredictable, it simply needs to be fresh).
Context-Data ::= SEQUENCE {
channelId ChannelId OPTIONAL, -- channel bindings
seq-number INTEGER OPTIONAL, -- sequence number
options Options,
conf-alg Conf-Algs, -- confidentiality. algs.
intg-alg Intg-Algs, -- integrity algorithm
owf-alg OWF-Algs -- for subkey derivation
}
ChannelId ::= OCTET STRING
Options ::= BIT STRING {
delegation-state (0),
mutual-state (1),
replay-det-state (2), -- used for replay det. during context
sequence-state (3), -- used for sequencing during context
conf-avail (4),
integ-avail (5),
target-certif-data-required (6)
-- used to request targ's certif. data
}
Conf-Algs ::= CHOICE {
algs [0] SEQUENCE OF AlgorithmIdentifier,
null [1] NULL
-- used when conf. is not available over context
} -- for C-ALG (see Section 5.2 for discussion of QOP)
Intg-Algs ::= SEQUENCE OF AlgorithmIdentifier
-- for I-ALG (see Section 5.2 for discussion of QOP)
OWF-Algs ::= SEQUENCE OF AlgorithmIdentifier
-- Contains exactly one algorithm in REQ-TOKEN for SPKM-2
-- unilateral, and contains at least one algorithm otherwise.
-- Always contains exactly one algorithm in REP-TOKEN.
Key-Estb-Algs ::= SEQUENCE OF AlgorithmIdentifier
-- to allow negotiation of K-ALG
A context establishment sequence based on the SPKM will perform
unilateral authentication if the mutual-req bit is not set in the
application's call to gss_init_sec_context(). SPKM-2 accomplishes
this using only SPKM-REQ (thereby authenticating the initiator to the
target), while SPKM-1 accomplishes this using both SPKM-REQ and
SPKM-REP-TI (thereby authenticating the target to the initiator).
Applications requiring authentication of both peers (initiator as
well as target) must request mutual authentication, resulting in
"mutual-state" being set within SPKM-REQ Options. In response to
such a request, the context target will reply to the initiator with
an SPKM-REP-TI token. If mechanism SPKM-2 has been chosen, this
completes the (timestamp-based) mutual authentication context
establishment exchange. If mechanism SPKM-1 has been chosen and
SPKM-REP-TI is sent, the initiator will then reply to the target with
an SPKM-REP-IT token, completing the (random-number-based) mutual
authentication context establishment exchange.
Other bits in the Options field of Context-Data are explained in
RFC-1508, with the exception of target-certif-data-required, which
the initiator sets to TRUE to request that the target return its
certification data in the SPKM-REP-TI token. For unilateral
authentication in SPKM-2 (in which no SPKM-REP-TI token is
constructed), this option bit is ignored by both initiator and
target.
3.1.2. Context Establishment Tokens - Target
SPKM-REP-TI ::= SEQUENCE {
responseToken REP-TI-TOKEN,
certif-data CertificationData OPTIONAL
-- included if target-certif-data-required option was
-- set to TRUE in SPKM-REQ
}
REP-TI-TOKEN ::= SEQUENCE {
rep-ti-contents Rep-ti-contents,
algId AlgorithmIdentifier,
rep-ti-integ Integrity -- "token" is Rep-ti-contents
}
Rep-ti-contents ::= SEQUENCE {
tok-id INTEGER (512), -- shall contain 0200 (hex)
context-id Random-Integer, -- see Section 6.3
pvno [0] BIT STRING OPTIONAL, -- prot. version number
timestamp UTCTime OPTIONAL, -- mandatory for SPKM-2
randTarg Random-Integer,
src-name [1] Name OPTIONAL,
-- must contain whatever value was supplied in REQ-TOKEN
targ-name Name,
randSrc Random-Integer,
rep-data Context-Data,
validity [2] Validity OPTIONAL,
-- validity interval for key (used if the target can only
-- support a shorter context lifetime than was offered in
-- REQ-TOKEN)
key-estb-id AlgorithmIdentifier OPTIONAL,
-- used if target is changing key estb. algorithm (must be
-- a member of initiators key-estb-set)
key-estb-str BIT STRING OPTIONAL
-- contains (1) the response to the initiators
-- key-estb-req (if init. used a 2-pass K-ALG), or (2) the
-- key-estb-req corresponding to the K-ALG supplied in
-- above key-estb-id, or (3) the key-estb-req corresponding
-- to the first K-ALG supplied in initiator's key-estb-id,
-- if initiator's (OPTIONAL) key-estb-req was not used
-- (target's key-estb-str must be present in this case).
-- Established key must satisfy the key length constraints
-- specified in Section 2.4.
}
The protocol version number (pvno) parameter is a BIT STRING which
uses as many bits as necessary to specify a single SPKM protocol
version offered by the initiator which is supported by the target
(one bit per protocol version); that is, the target sets exactly one
bit of pvno. If none of the versions offered by the initiator are
supported by the target, a delete token must be returned so that the
context is never established. If the initiator's pvno has only one
bit set and the target happens to support this protocol version, then
this version is used over the context and the pvno parameter of REP-
TOKEN can be omitted. Finally, if the initiator and target do have
one or more versions in common but the version of the REQ-TOKEN
received is not supported by the target, a REP-TOKEN must be sent
with a desired version bit set in pvno (and dummy values used for all
subsequent token fields). The initiator can then respond with a new
REQ-TOKEN of the proper version (essentially starting context
establishment anew).
3.1.3. Context Establishment Tokens - Initiator (second token)
Relevant SPKM-REP-IT syntax is as follows:
SPKM-REP-IT ::= SEQUENCE {
responseToken REP-IT-TOKEN,
algId AlgorithmIdentifier,
rep-it-integ Integrity -- "token" is REP-IT-TOKEN
}
REP-IT-TOKEN ::= SEQUENCE {
tok-id INTEGER (768), -- shall contain 0300 (hex)
context-id Random-Integer,
randSrc Random-Integer,
randTarg Random-Integer,
targ-name Name, -- the targ-name specified in REP-TI
src-name Name OPTIONAL,
-- must contain whatever value was supplied in REQ-TOKEN
key-estb-rep BIT STRING OPTIONAL
-- contains the response to targets key-estb-str
-- (if target selected a 2-pass K-ALG)
}
3.1.4. Error Token
The syntax of SPKM-ERROR is as follows:
SPKM-ERROR ::= SEQUENCE {
error-token ERROR-TOKEN,
algId AlgorithmIdentifier,
integrity Integrity -- "token" is ERROR-TOKEN
}
ERROR-TOKRN ::= SEQUENCE {
tok-id INTEGER (1024), -- shall contain 0400 (hex)
context-id Random-Integer
}
The SPKM-ERROR token is used only during the context establishment
process. If an SPKM-REQ or SPKM-REP-TI token is received in error,
the receiving function (either gss_init_sec_context() or
gss_accept_sec_context()) will generate an SPKM-ERROR token to be
sent to the peer (if the peer is still in the context establishment
process) and will return GSS_S_CONTINUE_NEEDED. If, on the other
hand, no context establishment response is expected from the peer
(i.e., the peer has completed context establishment), the function
will return the appropriate major status code (e.g., GSS_S_BAD_SIG)
along with a minor status of GSS_SPKM_S_SG_CONTEXT_ESTB_ABORT and all
context-relevant information will be deleted. The output token will
not be an SPKM-ERROR token but will instead be an SPKM-DEL token
which will be processed by the peer's gss_process_context_token().
If gss_init_sec_context() receives an error token (whether valid or
invalid), it will regenerate SPKM-REQ as its output token and return
a major status code of GSS_S_CONTINUE_NEEDED. (Note that if the
peer's gss_accept_sec_context() receives SPKM-REQ token when it is
expecting a SPKM-REP-IT token, it will ignore SPKM-REQ and return a
zero-length output token with a major status of
GSS_S_CONTINUE_NEEDED.)
Similarly, if gss_accept_sec_context() receives an error token
(whether valid or invalid), it will regenerate SPKM-REP-TI as its
output token and return a major status code of GSS_S_CONTINUE_NEEDED.
md5WithRsa is currently stipulated for the signing of context
establishment tokens. Discrepancies involving modulus bitlength can
be resolved through judicious use of the SPKM-ERROR token. The
context initiator signs REQ-TOKEN using the strongest RSA it supports
(e.g., 1024 bits). If the target is unable to verify signatures of
this length, it sends SPKM-ERROR signed with the strongest RSA that
it supports (e.g. 512).
At the completion of this exchange, both sides know what RSA
bitlength the other supports, since the size of the signature is
equal to the size of the modulus. Further exchanges can be made
(using successively smaller supported bitlengths) until either an
agreement is reached or context establishment is aborted because no
agreement is possible.
3.2. Per-Message and Context Deletion Tokens
Three classes of tokens are defined in this section: "MIC" tokens,
emitted by calls to gss_getMIC() and consumed by calls to
gss_verifyMIC(); "Wrap" tokens, emitted by calls to gss_wrap() and
consumed by calls to gss_unwrap(); and context deletion tokens,
emitted by calls to gss_init_sec_context(), gss_accept_sec_context(),
or gss_delete_sec_context() and consumed by calls to
gss_process_context_token().
3.2.1. Per-message Tokens - Sign / MIC
Use of the gss_sign() / gss_getMIC() call yields a token, separate
from the user data being protected, which can be used to verify the
integrity of that data as received. The token and the data may be
sent separately by the sending application and it is the receiving
application's responsibility to associate the received data with the
received token.
The SPKM-MIC token has the following format:
SPKM-MIC ::= SEQUENCE {
mic-header Mic-Header,
int-cksum BIT STRING
-- Checksum over header and data,
-- calculated according to algorithm
-- specified in int-alg field.
}
Mic-Header ::= SEQUENCE {
tok-id INTEGER (257),
-- shall contain 0101 (hex)
context-id Random-Integer,
int-alg [0] AlgorithmIdentifier OPTIONAL,
-- Integrity algorithm indicator (must
-- be one of the agreed integrity
-- algorithms for this context).
-- field not present = default id.
snd-seq [1] SeqNum OPTIONAL -- sequence number field.
}
SeqNum ::= SEQUENCE {
num INTEGER, -- the sequence number itself
dir-ind BOOLEAN -- a direction indicator
}
3.2.1.1. Checksum
Checksum calculation procedure (common to all algorithms -- note that
for SPKM the term "checksum" includes digital signatures as well as
hashes and MACs): Checksums are calculated over the data field,
logically prepended by the bytes of the plaintext token header (mic-
header). The result binds the data to the entire plaintext header,
so as to minimize the possibility of malicious splicing.
For example, if the int-alg specifies the md5WithRSA algorithm, then
the checksum is formed by computing an MD5 [RFC-1321] hash over the
plaintext data (prepended by the header), and then computing an RSA
signature [PKCS1] on the 16-byte MD5 result. The signature is
computed using the RSA private key retrieved from the credentials
structure and the result (whose length is implied by the "modulus"
parameter in the private key) is stored in the int-cksum field.
If the int-alg specifies a keyed hashing algorithm (for example,
DES-MAC or md5-DES-CBC), then the key to be used is the appropriate
subkey derived from the context key (see Section 2.4). Again, the
result (whose length is implied by int-alg) is stored in the int-
cksum field.
3.2.1.2. Sequence Number
It is assumed that the underlying transport layers (of whatever
protocol stack is being used by the application) will provide
adequate communications reliability (that is, non-malicious loss,
re-ordering, etc., of data packets will be handled correctly).
Therefore, sequence numbers are used in SPKM purely for security, as
opposed to reliability, reasons (that is, to avoid malicious loss,
replay, or re-ordering of SPKM tokens) -- it is therefore recommended
that applications request sequencing and replay detection over all
contexts. Note that sequence numbers are used so that there is no
requirement for secure timestamps in the message tokens. The
initiator's initial sequence number for the current context may be
explicitly given in the Context-Data field of SPKM-REQ and the
target's initial sequence number may be explicitly given in the
Context-Data field of SPKM-REP-TI; if either of these is not given
then the default value of 00 is to be used.
Sequence number field: The sequence number field is formed from the
sender's four-byte sequence number and a Boolean direction-indicator
(FALSE - sender is the context initiator, TRUE - sender is the
context acceptor). After constructing a gss_sign/getMIC() or
gss_seal/wrap() token, the sender's seq. number is incremented by 1.
3.2.1.3. Sequence Number Processing
The receiver of the token will verify the sequence number field by
comparing the sequence number with the expected sequence number and
the direction indicator with the expected direction indicator. If
the sequence number in the token is higher than the expected number,
then the expected sequence number is adjusted and GSS_S_GAP_TOKEN is
returned. If the token sequence number is lower than the expected
number, then the expected sequence number is not adjusted and
GSS_S_DUPLICATE_TOKEN, GSS_S_UNSEQ_TOKEN, or GSS_S_OLD_TOKEN is
returned, whichever is appropriate. If the direction indicator is
wrong, then the expected sequence number is not adjusted and
GSS_S_UNSEQ_TOKEN is returned.
Since the sequence number is used as part of the input to the
integrity checksum, sequence numbers need not be encrypted, and
attempts to splice a checksum and sequence number from different
messages will be detected. The direction indicator will detect
tokens which have been maliciously reflected.
3.2.2. Per-message Tokens - Seal / Wrap
Use of the gss_seal() / gss_wrap() call yields a token which
encapsulates the input user data (optionally encrypted) along with
associated integrity check quantities. The token emitted by
gss_seal() / gss_wrap() consists of an integrity header followed by a
body portion that contains either the plaintext data (if conf-alg =
NULL) or encrypted data (using the appropriate subkey specified in
Section 2.4 for one of the agreed C-ALGs for this context).
The SPKM-WRAP token has the following format:
SPKM-WRAP ::= SEQUENCE {
wrap-header Wrap-Header,
wrap-body Wrap-Body
}
Wrap-Header ::= SEQUENCE {
tok-id INTEGER (513),
-- shall contain 0201 (hex)
context-id Random-Integer,
int-alg [0] AlgorithmIdentifier OPTIONAL,
-- Integrity algorithm indicator (must
-- be one of the agreed integrity
-- algorithms for this context).
-- field not present = default id.
conf-alg [1] Conf-Alg OPTIONAL,
-- Confidentiality algorithm indicator
-- (must be NULL or one of the agreed
-- confidentiality algorithms for this
-- context).
-- field not present = default id.
-- NULL = none (no conf. applied).
snd-seq [2] SeqNum OPTIONAL
-- sequence number field.
}
Wrap-Body ::= SEQUENCE {
int-cksum BIT STRING,
-- Checksum of header and data,
-- calculated according to algorithm
-- specified in int-alg field.
data BIT STRING
-- encrypted or plaintext data.
}
Conf-Alg ::= CHOICE {
algId [0] AlgorithmIdentifier,
null [1] NULL
}
3.2.2.1: Confounding
As in [KRB5], an 8-byte random confounder is prepended to the data to
compensate for the fact that an IV of zero is used for encryption.
The result is referred to as the "confounded" data field.
3.2.2.2. Checksum
Checksum calculation procedure (common to all algorithms): Checksums
are calculated over the plaintext data field, logically prepended by
the bytes of the plaintext token header (wrap-header). As with
gss_sign() / gss_getMIC(), the result binds the data to the entire
plaintext header, so as to minimize the possibility of malicious
splicing.
The examples for md5WithRSA and DES-MAC are exactly as specified in
3.2.1.1.
If int-alg specifies md5-DES-CBC and conf-alg specifies anything
other than DES-CBC, then the checksum is computed according to
3.2.1.1 and the result is stored in int-cksum. However, if conf-alg
specifies DES-CBC then the encryption and the integrity are done as
follows. An MD5 [RFC-1321] hash is computed over the plaintext data
(prepended by the header). This 16-byte value is appended to the
concatenation of the "confounded" data and 1-8 padding bytes (the
padding is as specified in [KRB5] for DES-CBC). The result is then
CBC encrypted using the DES-CBC subkey (see Section 2.4) and placed
in the "data" field of Wrap-Body. The final two blocks of ciphertext
(i.e., the encrypted MD5 hash) are also placed in the int-cksum field
of Wrap-Body as the integrity checksum.
If int-alg specifies sum64-DES-CBC then conf-alg must specify DES-CBC
(i.e., confidentiality must be requested by the calling application
or SPKM will return an error). Encryption and integrity are done in
a single pass using the DES-CBC subkey as follows. The sum (modulo
2**64 - 1) of all plaintext data blocks (prepended by the header) is
computed. This 8-byte value is appended to the concatenation of the
"confounded" data and 1-8 padding bytes (the padding is as specified
in [KRB5] for DES-CBC). As above, the result is then CBC encrypted
and placed in the "data" field of Wrap-Body. The final block of
ciphertext (i.e., the encrypted sum) is also placed in the int-cksum
field of Wrap-Body as the integrity checksum.
3.2.2.3 Sequence Number
Sequence numbers are computed and processed for gss_wrap() exactly as
specified in 3.2.1.2 and 3.2.1.3.