Secure Inter-Domain Routing M. Reynolds
Internet-Draft IPSw
Intended status: Standards Track S. Kent
Expires: October 2, 2013 BBN
M. Lepinski
BBN
Apr 5, 2013
Local Trust Anchor Management for the Resource Public Key Infrastructure
<draft-ietf-sidr-ltamgmt-08.txt>
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Abstract
This document describes a facility to enable a relying party (RP) to
manage trust anchors (TAs) in the context of the Resource Public Key
Infrastructure (RPKI). It is common in RP software (not just in the
RPKI) to allow an RP to import TA material in the form of self-signed
certificates. However, this approach to incorporating TAs is
potentially dangerous. (These self-signed certificates rarely
incorporate any extensions that impose constraints on the scope of
the imported public keys, and the RP is not able to impose such
constraints.) The facility described in this document allows an RP to
impose constraints on such TAs. Because this mechanism is designed to
operate in the RPKI context, the most important constraints are the
Internet Number Resources (INRs) expressed via RFC 3779 extensions.
These extentions bind address spaces and/or autonomous system (AS)
numbers to entities. The primary motivation for the facility described
in this document is to enable an RP to ensure that INR information
that it has acquired via some trusted channel is not overridden by the
information acquired from the RPKI repository system or by the putative
TAs that the RP imports. Specifically, the mechanism allows an RP to
specify a set of overriding bindings between public key identifiers and
INR data. These bindings take precedence over any conflicting bindings
acquired by the putative TAs and the certificates downloaded from the
RPKI repository system. This mechanism is designed for local use by an RP,
but any entity that is accorded administrative control over a set of RPs
may use this mechanism to convey its view of the RPKI to RPs within its
jurisdiction. The means by which this latter use case is effected is
outside the scope of this document.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Overview of Certificate Processing . . . . . . . . . . . . . . 5
2.1 Target Certificate Processing . . . . . . . . . . . . . . . 5
2.2 Perforation . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 TA Re-parenting . . . . . . . . . . . . . . . . . . . . . . 6
2.4 Paracertificates . . . . . . . . . . . . . . . . . . . . . 6
3 Format of the constraints file . . . . . . . . . . . . . . . . 8
3.1 Relying party subsection . . . . . . . . . . . . . . . . . 8
3.2 Flags subsection . . . . . . . . . . . . . . . . . . . . . 8
3.3 Tags subsection . . . . . . . . . . . . . . . . . . . . . . 9
3.3.1 Xvalidity_dates tag . . . . . . . . . . . . . . . . . . 10
3.3.2 Xcrldp tag . . . . . . . . . . . . . . . . . . . . . . 10
3.3.3 Xcp tag . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.4 Xaia tag . . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Blocks subsection . . . . . . . . . . . . . . . . . . . . . 12
4 Certificate Processing Algorithm . . . . . . . . . . . . . . . 13
4.1 Proofreading algorithm . . . . . . . . . . . . . . . . . . 14
4.2 TA processing algorithm . . . . . . . . . . . . . . . . . . 15
4.2.1 Preparatory processing (stage 0) . . . . . . . . . . . 16
4.2.2 Target processing (stage 1) . . . . . . . . . . . . . . 17
4.2.3 Ancestor processing (stage 2) . . . . . . . . . . . . . 18
4.2.4 Tree processing (stage 3) . . . . . . . . . . . . . . . 19
4.2.5 TA re-parenting (stage 4) . . . . . . . . . . . . . . . 20
4.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . 21
5 Implications for Path Discovery . . . . . . . . . . . . . . . . 21
5.1 Two answers . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2 One answer . . . . . . . . . . . . . . . . . . . . . . . . 22
5.3 No answer . . . . . . . . . . . . . . . . . . . . . . . . . 22
6 Implications for Revocation . . . . . . . . . . . . . . . . . . 22
6.1 No state bits set . . . . . . . . . . . . . . . . . . . . . 23
6.2 ORIGINAL state bit set . . . . . . . . . . . . . . . . . . 23
6.3 PARA state bit set . . . . . . . . . . . . . . . . . . . . 23
6.4 Both ORIGINAL and PARA state bits set . . . . . . . . . . . 24
7 Security Considerations . . . . . . . . . . . . . . . . . . . . 24
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 24
9 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
10 References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1 Normative References . . . . . . . . . . . . . . . . . . . 24
10.2 Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Appendix A: Sample Constraints File . . . . . . . . . . . . . . . 26
Appendix B: Optional Sorting Algorithm for Ancestor Processing . . 27
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1 Introduction
The Resource Public Key Infrastructure (RPKI) [RFC6480] is a
PKI in which certificates are issued to facilitate management of
Internet Resource Numbers (INRs). Such resources are expressed in
the form of X.509v3 "resource" certificates with extensions defined
by RFC 3779 [RFC6487]. Validation of a resource certificate is
preceded by path discovery. In a PKI path discovery is effected by
constructing a certificate path between a target certificate and a
trust anchor (TA). No IETF standards define how to construct a
certificate path; commonly such paths are based on a bottom-up
search using Subject/Issuer name matching, but top-down and
meet-in-the-middle approaches may also be employed [RFC4158]. In
contrast, path validation is top-down, as defined by [RFC5280].
In the RPKI, certificates can be acquired in various ways, but the
default is a top-down tree walk as described in [RFC6481],
initialized via a Trust Anchor Locator [RFC6490]. Note that the
process described there is not path discovery per sem but the
collecting of certificates to populate a local cache. Thus, the
common, bottom-up path discovery approach is not inconsistent
with these RFCs. Morevoer, a bottom-up path discovery approach
is more general, accommodating certificates that might be
acquired by other means, i.e., not from an RPKI repository. There
are circumstances under which an RP may wish to override the INR
specifications obtained through the RPKI distributed repository
system [RFC6481]. This document describes a mechanism by which
an RP may override any conflicting information expressed via
putative TAs and the certificates downloaded from the RPKI
repository system. Thus the algorithms described in this document
adopt a bottom-up path discovery approach.
To effect this local control, this document calls for a relying party
to specify a set of bindings between public key identifiers and
INRs through a text file known as a constraints file. The constraints
expressed in this file then take precedence over any competing claims
expressed by resource certificates acquired from the distributed
repository system. (The means by which a relying party acquires the
key identifier and the RFC 3779 extension data used to populate the
constraints file is outside the scope of this document.) The relying
party also may use a local publication point (the root of a local
directory tree that is made available as if it were a remote
repository) as a source of certificates and CRLs (and other RPKI
signed objects, e.g., ROAs and manifests) that do not appear in the
RPKI repository system.
In order to allow reuse of existing, standard path validation
mechanisms, the RP-imposed constraints are realized by having the RP
itself represented as the only TA known in the local certificate
validation context. To ensure that all RPKI certificates can be
validated relative to this TA, this RP TA certificate must contain
all-encompassing resource allocations, i.e. 0/0 for IPv4, 0::/0 for
IPv6 and 0-4294967295 for AS numbers. Thus, a conforming
implementation of this mechanism must be able to cause a self-signed
certification authority (CA) certificate to be created with a locally
generated key pair. It also must be able to issue CA certificates
subordinate to this TA. Finally, a conforming implementation of this
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mechanism must process the constraints file and modify certificates
as needed in order to enforce the constraints asserted in the file.
The remainder of this document describes in detail the types of
certificate modification that may occur, the syntax and semantics of
the constraints file, and the implications of certificate modification
on path discovery and revocation.
1.1 Terminology
It is assumed that the reader is familiar with the terms and concepts
described in "Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile" [RFC5280] and "X.509
Extensions for IP Addresses and AS Identifiers" [RFC3779].
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 RFC 2119.
2 Overview of Certificate Processing
The fundamental aspect of the facility described in this document is
one of certificate modification. The constraints file, described in
more detail in the next section, contains assertions about INRs
that are to be specially processed. As a result of this processing,
certificates in the local copy of the RPKI repository are transformed
into new certificates satisfying the INR constraints so specified.
This enables the RP to override conflicting assertions about resource
holdings as acquired from the RPKI repository system. Three forms of
certificate modification can occur. (Every certificate is digitally
signed and thus cannot be modified without "breaking" its signature.
In the context of this document we assume that certificates that are
modified have been validated previously. Thus the content can be
modified, locally, without the need to preserve the integrity of the
signature. These modified certificates are referred to as
paracertificates (see section 2.4 below).)
2.1 Target Certificate Processing
If a certificate is acquired from the RPKI repository system and its
Subject key identifier (SKI) is listed in the constraints file, it
will be reissued directly under the RP TA certificate, with (possibly)
modified RFC 3779 extensions. (The SKI is used as a compact reference
to the public key in a target certificate.) The modified extensions
will include any RFC 3779 data expressed in the constraints file. Other
certificate fields may also be modified to maintain consistency. (These
fields are enumerated in Table 1, and discussed in Section 3.3.) In
Section 4.2, target certificate processing corresponds to stage one of
the algorithm. (When a target certificate is re-parented, all
subordinate signed products will still be valid, unless the set of
INRs in the targeted certificate is reduced.)
2.2 Perforation
When a target certificate is re-issued directly under the RP's TA, its
INRs MUST be removed from all of its parent (CA) certificates. (If
these INRs were not removed, then conflicting assertions about INRs
could arise and undermine the authority of the RP TA.) Thus, every
certificate acquired from the RPKI repository MUST be examined to
determine if it contains an RFC 3779 extension that intersects the
resource data in the constraints file. If there is an intersection
the certificate will be reissued directly under the RP TA, with
modified RFC 3779 extensions. We refer to the process of modifying
the RFC 3779 extension in an affected certificate as "perforation"
(because the process will create "holes" in these extensions). The
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modified extensions will exclude any RFC 3779 data expressed in the
constraints file. In the certificate processing algorithm described
in Section 4.2, perforation corresponds to stage two of the algorithm
("ancestor processing") and also to stage three of the algorithm
("tree processing").
2.3 TA Re-parenting
All valid, self-signed certificates offered as TAs in the public RPKI
certificate hierarchy, e.g., self-signed certificates issued by IANA
or RIRs, will be re-issued under the RP TA certificate. This processing
is done even though all but one of these certificates might not
intersect any resources specified in the constraints file. We refer to
this reissuance as "re-parenting" since the issuer (parent) of the
certificate has been changed. The issuer name is changed from that of
the certificate subject (this is a self-signed certificate) to that of
the RP TA. In the certificate processing algorithm described in Section
4.2, TA re-parenting corresponds to stage four of the algorithm. (In
a more generic PKI context, re-parenting enables an RP to insert
extensions in these certificates to impose constraings on path
processing in a fashion consistent with RFC 5280. In this fashion an
RP can impose name constraints, policy constraints, etc.)
2.4 Paracertificates
If a certificate is subject to any of the three forms of processing
just described, that certificate will be referred to as an "original"
certificate and the processed (output) certificate will be referred
to as a paracertificate. When an original certificate is transformed
into a paracertificate all the fields and extensions from the
original certificate will be retained, except as indicated in Table
1, below.
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Original Certificate Field Action
Version unchanged
Serial number created per note A
Signature replaced if needed
with RP's signing alg
Issuer replaced with RP's name
Validity dates replaced per note B
Subject unchanged
Subject public key info unchanged
Extensions
Subject key identifier unchanged
Key usage unchanged
Basic constraints unchanged
CRL distribution points replaced per note B
Certificate policy replaced per note B
Authority info access replaced per note B
Authority key ident replaced with RP's
IP address block modified as described
AS number block modified as described
Subject info access unchanged
All other extensions unchanged
Signature Algorithm same as above
Signature value new
Table 1 Certificate Field Modifications
Note A. The serial number will be created by concatenating the
current time (the number of seconds since Jan 1, 1970) with a count
of the certificates created in the current run. Because all
paracertificates are issued directly below the RP TA, this algorithm
ensures serial number uniqueness.
Note B. These fields are derived (as described in Section 3.3 below)
from parameters in the constraints file (if present); otherwise, they
take on values from the certificates from which the paracertificates
are derived.
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3 Format of the constraints file
This section describes the syntax of the constraints file. (The
syntax has been defined to enable creation and distribution of
constraint files to a set of RPs, by an authorized third party.)
The model described below is nominal; implementations need not match
all details of this model as presented, but the external behavior of
implementations MUST correspond to the externally observable
characteristics of this model in order to be compliant. It is
RECOMMENDED that the syntax described herein be supported, to
facilitate interoperability between creators and comsumers of
constraints files.
The constraints file consists of four logical subsections: the
replying party subsection, the flags subsection, the tags subsection
and the blocks subsection. The relying party subsection and the
blocks subsection are REQUIRED and MUST be present; the flags and
tags subsections are OPTIONAL. Each subsection is described in more
detail below. Note that the semicolon (;) character acts as the
comment character, to enable annotating constraints files. All
characters from a semicolon to the end of that line are ignored. In
addition, lines consisting only of whitespace are ignored. The
subsections MUST occur in the order indicated. An example constraints
file is given in Appendix A.
3.1 Relying party subsection
The relying party subsection is a REQUIRED subsection of the
constraints file. It MUST be the first subsection of the constraints
file, and it MUST consist of two lines of the form:
(RECOMMENDED)
PRIVATEKEYMETHOD value [ ... value ]
TACERTIFICATE value
The first line provides a pointer (including an access method) to
the RP's private key. This line consists of the string literal
PRIVATEKEYMETHOD, followed by one or more whitespace delimited string
values. These values are passed to the certificate processing
algorithm as described below. Note that this entry, as for all
entries in the constraints file, is case sensitive.
The second line of this subsection consists of the string literal
TACERTIFICATE, followed by exactly one string value. This value
is the name of a file containing the relying party's TA certificate.
The file name is passed to the certificate processing algorithm as
described below.
3.2 Flags subsection
The flags subsection of the constraints file is an OPTIONAL
subsection. If present it MUST immediately follow the relying party
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subsection. The flags subsection consists of one or more lines of the
form
CONTROL flagname booleanvalue
Each such line is referred to as a control line. Each control line
MUST contain exactly three whitespace delimited strings. The first
string MUST be the literal CONTROL. The second string MUST be one of
the following three literals:
resource_nounion
intersection_always
treegrowth
The third string denotes a Boolean value, and MUST be one of the
literals TRUE or FALSE. Control flags influence the global operation
of the certificate processing algorithm; the semantics of the flags
is described in Section 4.2. Note that each flag has a default value,
so that if the corresponding CONTROL line does not appear in the
constraints file, the algorithm flag is considered to take the
corresponding default value. The default value for each flag is FALSE.
Thus, if any flag is not named in a control line it takes the value
FALSE. If the flags subsection is absent, all three flags assume the
default value FALSE.
3.3 Tags subsection
The tags subsection is an OPTIONAL subsection in the constraints
file. If present it MUST immediately follow the relying party
subsection (if the flags subsection is absent) or the flags
subsection (if it is present). The tags subsection consists of one or
more lines of the form
TAG tagname tagvalue [ ... tagvalue ]
Each such line is referred to as a tag line. Each tag line MUST
consist of at least three whitespace delimited string values, the
first of which must be the literal TAG. The second string value gives
the name of the tag, and subsequent string(s) give the value(s) of
the tag. The tag name MUST be one of the following four string
literals:
Xvalidity_dates
Xcrldp
Xcp
Xaia
The purpose of the tag lines is to provide an indication of the means
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by which paracertificate fields, specifically those indicated above
under "Note B", of Table 1are constructed. Each tag has a default, so
that if the corresponding tag line is not present in the constraints
file, the default behavior is used when constructing the
paracertificates. The syntax and semantics of each tag line is
described next.
Note that the tag lines are considered to be global; the action of
each tag line (or the default action, if that tag line is not
present) applies to all paracertificates that are created as part of
the certificate processing algorithm.
3.3.1 Xvalidity_dates tag
This tag line is used to control the value of the notBefore and
notAfter fields in paracertificates. If this tag line is specified
and there is a single tagvalue which is the literal string C, the
paracertificate validity interval is copied from the original
certificate validity interval from which it is derived. If this tag
is specified and there is a single tagvalue which is the literal
string R, the paracertificate validity interval is copied from the
validity interval of the RP's TA certificate. If this tag is specified
and the tagvalue is neither of these literals, then exactly two
tagvalues MUST be specified. Each must be a Generalized Time string
of the form YYYYMMDDHHMMSSZ. The first tagvalue is assigned to the
notBefore field and the second tagvalue is assigned to the notAfter
field. It MUST be the case that the tagvalues can be parsed as valid
Generalized Time strings such that notBefore is less than notAfter,
and also such that notAfter represents a time in the future (i.e.,
the paracertificate has not already expired).
If this tag line is not present in the constraints file the default
behavior is to copy the validity interval from the original
certificate to the corresponding paracertificate.
3.3.2 Xcrldp tag
This tag line is used to control the value of the CRL distribution
point extension in paracertificates. If this tag line is specified
and there is a single tagvalue that is the string literal C, the
CRLDP of the paracertificate is copied from the CRLDP of the original
certificate from which it is derived. If this tag line is specified
and there is a single tagvalue that is the string literal R, the
CRLDP of the paracertificate is copied from the CRLDP of the RP's TA
certificate. If this tag line is specified and there is a single
tagvalue that is not one of these two reserved literals, or if
there is more than one tagvalue, then each tagvalue is interpreted as
a URI that will be placed in the CRLDP sequence in the
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paracertificate.
If this tag line is not present in the constraints file the default
behavior is to copy the CRLDP from the original certificate into the
corresponding paracertificate.
3.3.3 Xcp tag
This tag line is used to control the value of the policyQualifierId
field in paracertificates. If this tag line is specified there MUST
be exactly one tagvalue. If the tagvalue is the string literal C, the
paracertificate value is copied from the value in the corresponding
original certificate. If the tagvalue is the string literal R, the
paracertificate value is copied from the value in the RP's top level
TA certificate. If the tagvalue is the string literal D, the
paracertificate value is set to the default OID. If the tagvalue is
not one of these reserved string literals, then the tagvalue MUST be
an OID specified using the standard dotted notation. The value in the
paracertificate's policyQualifierId field is set to this OID. Note
the RFC 5280 specifies that only a single policy may be specified in
a certificate, so only a single tagvalue is permitted in this tag
line, even though the CertificatePolicy field is an ASN.1 sequence.
If this tag line is not specified the default behavior is to use the
default OID in creating the paracertificate.
This option permits the RP to convert a value of the
policyQualifierId field in a certificate (that would not be in
conformance with the RPKI CP) to a conforming value in the
paracertificate. This conversion enables use of RPKI validation
software that checks the policy field against that specified in the
RPKI CP [RFC6484].
3.3.4 Xaia tag
This tag line is used to control the value of the Authority
Information Access (AIA) extension in the paracertificate. If this
tag line is present then it MUST have exactly one tagvalue. If this
tagvalue is the string literal C, then the AIA field in the
paracertificate is copied from the AIA field in the original
certificate from which it is derived. If this tag line is present and
the tagvalue is not the reserved string literal, then the tagvalue
MUST be a URI. This URI is set as the AIA extension of the
paracertificates that are created.
If this tag line is not specified the default behavior is to use copy
the AIA field from the original certificate to the AIA field of the
paracertificate.
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3.4 Blocks subsection
The blocks subsection is a REQUIRED subsection of the constraints
file. If the tags subsection is present, the blocks subsection MUST
appear immediately after it. This MUST be the last subsection in
the constraints file. The blocks subsection consists of one or more
blocks, known as target blocks. A target block is used to specify an
association between a certificate (identified by an SKI) and a set
of resource assertions. Each target block contains four regions, an SKI
region, an IPv4 region, an IPv6 region and an AS number region. All
regions MUST be present in a target block.
The SKI region contains a single line beginning with the string
literal SKI and followed by forty hexadecimal characters giving the
subject key identifier of a certificate, known as the target
certificate. The hex character string MAY contain embedded whitespace
or colon characters (included to improve readability), which are
ignored. The IPv4 region consists of a line containing only the
string literal IPv4. This line is followed by zero or more lines
containing IPv4 prefixes in the format described in RFC 3779. The
IPv6 region consists of a line containing only the string literal
IPv6, followed by zero or more lines containing IPv6 prefixes using
the format described in RFC 3513. (The presence of the IPv4 and IPv6
literals is to simplify parsing of the constraints file.) Finally,
the AS number region consists of a line containing only the string
literal AS#, followed by zero or more lines containing AS numbers
(one per line). The AS numbers are specified in decimal notation as
recommended in RFC 5396. A target block is terminated by either the
end of the constraints file, or by the beginning of the next target
block, as signaled by its opening SKI region line. An example target
block is shown below. (The indentation used below is employed to
improve readability and is not required.) See also the complete
constraints file example in Appendix A. Note that whitespace, as
always, is ignored.
SKI 00:12:33:44:00:BA:BA:DE:EB:EE:00:99:88:77:66:55:44:33:22:11
IPv4
10.2.3/24
10.8/16
IPv6
1:2:3:4:5:6/112
AS#
123
567
The blocks subsection MUST contain at least one target block. Note
that it is OPTIONAL that the SKI refer to a certificate that is known
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or resolvable within the context of the local RPKI repository. Also,
there is no REQUIRED or implied ordering of target blocks within the
block subsection. Since blocks may occur in any order, the outcome of
processing a constraints file may depend on the order in which target
blocks occur within the constraints file. The next section of this
document contains a detailed description of the certificate processing
algorithm.
4 Certificate Processing Algorithm
The section describes the certificate processing algorithm by which
paracertificates are created from original certificates in the
local RPKI repository. For the purposes of describing this algorithm,
it will be assumed that certificates are persistently associated
with state (or metadata) information. This state information is
nominally represented by an array of named bits associated with each
certificate. No specific implementation of this functionality is
mandated by this document. Any implementation that provides the
indicated functionality is acceptable, and need not actually consist
of a bit field associated with each certificate.
The following state bits used in certificate processing are
NOCHAIN
ORIGINAL
PARA
TARGET
If the NOCHAIN bit is set, this indicates that a full path between
the given certificate and a TA has not yet been discovered. If the
ORIGINAL bit is set, this indicates that the certificate in question
has been processed by some part of the processing algorithm described
in Section 4.2. If it was processed as part of stage one processing,
as described in section 4.2.2, the TARGET bit also will be set.
Finally, every paracertificate will have the PARA bit set.
At the beginning of algorithm processing each certificate in the
local RPKI repository has the ORIGINAL, PARA and TARGET bits clear.
If a certificate has a complete, validated path to a TA, or is itself
a TA, then that certificate will have the NOCHAIN bit clear,
otherwise it will have the NOCHAIN bit set. As the certificate
processing algorithm proceeds, the metadata state of original
certificates may change. In addition, since the certificate
processing algorithm may also be creating paracertificates, it is
responsible for actively setting or clearing the state of these four
bits on those paracertificates.
The certificate processing algorithm consists of two sub-algorithms:
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"proofreading" and "TA processing". Conceptually, the proofreading
algorithm performs syntactic checks on the constraints file,
while the TA processing algorithm performs the actual certificate
transformation processing. If the proofreading algorithm does not
succeed in parsing the constraints file, the TA processing-
algorithm is not executed. Note also that if the constraints file is
not present, neither algorithm is executed and the local RPKI
repository is not modified. Each of the constituent algorithms will
now be described in detail.
4.1 Proofreading algorithm
The proofreading algorithm checks the constraints file for syntactic
errors, e.g., missing REQUIRED subsections, or malformed addresses.
Implementation of this algorithm is OPTIONAL. If it is implemented,
the following text defines correct operation for the algorithm.
The proofreading algorithms performs a set of heuristic checks, such
as checking for prefixes that are too large (e.g., larger than /8).
The proofreading algorithm also SHOULD examine resource regions (IPv4,
IPv6 and AS# regions) within the blocks subsection, and reorder such
resources within a region in ascending numeric order. On encountering
any error the proofreading algorithm SHOULD provide an error message
indicating the line on which the error occurred as well as informative
text that is sufficiently descriptive as to allow the user to identify
and correct the error. An implementation of the proofreading algorithm
MUST NOT assume that it has access to the local RPKI repository (even
read-only access). An implementation of the proofreading algorithm MUST
NOT alter the local RPKI repository in any way; it also MUST NOT change
any of the metadata associated with certificates in that repository.
(Recall that the processing described here is creating a copy of that
local repository.) For simplicity the remainder of this document
assumes that the proofreading algorithm produces a transformed output
file. This file contains the same syntactic information as the text
version of the constraints file.
The proofreading algorithm performs the following syntactic checks on
the constraints file:
- verifies the presence of the REQUIRED relying party subsection
and the REQUIRED blocks subsection.
- verifies the order of the two, three or four subsections as
stated above.
- verifies that the relying party subsection conforms to the
specification given in Section 3.1 above.
- verifies that, if present, the tags and flags subsections conform
to the specifications in Sections 3.2 and 3.3 above.
After these checks have been performed, the proofreading algorithm
then checks the blocks subsection:
- splits the blocks subsection into constituent target blocks, as
delimited by the SKI region line(s)
- verifies that at least one target block is present
- verifies that each SKI region line contains exactly forty hexadecimal
digits and contains no additional characters other than whitespace or
colon characters.
For each target the proofreading algorithm:
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- verifies the presence of the IPv4, IPv6 and AS# regions, and
verifies that at least one such resource is present.
- verifies that, for each IPv4 prefix, IPv6 prefix and autonomous
system number given, that the indicated resource is syntactically
valid according to the appropriate RFC definition, as described in
Section 3.4.
- verifies that no IPv4 resource has a prefix larger than /8.
- optionally performing reordering within each of the three resource
regions so that stated resources occur in ascending numerical order.
(If the proofreading algorithm has performed any reordering of
information it MAY overwrite the constraints file. If it does so,
however, it MUST preserve all information contained within the file,
including information that is not parsed (such as comments). If the
proofreading algorithm has performed any reordering of information
but has not overwritten the constraints file, it MAY produce a
transformed output file, as described above. If the proofreading
algorithm has performed any reordering of information, but has
neither overwritten the constraints file nor produced a transformed
output file, it MUST provide an error message to the user indicating
what reordering was performed.)
4.2 TA processing algorithm
The TA processing algorithm acts on the constraints file (as processed
by the proofreading algorithm) and the contents of the local RPKI
repository to produce paracertificates for the purpose of enforcing
the resource allocations as expressed in the constraints file. The
TA processing algorithm operates in five stages, a preparatory stage
(stage 0), target processing (stage 1), ancestor processing (stage 2),
tree processing (state 3) and TA re-parenting (stage 4). Conceptually,
during the preparatory stage the proofreader output file is read and a
set of internal RP, tag and flag variables are set based on the contents
of that file. (If the constraint file has not specified one or more of
the tags and/or flags, those tags and flags are set to default
values.) During target processing all certificates specified by a
target block are processed, and the resources for those certificates
are (potentially) expanded; for each target found a new
paracertificate is manufactured with its various fields set, as shown
in Table 1, using the values of the internal variables set in the
preparatory stage and also, of course, the fields of the original
certificate (and, potentially, fields of the RP's TA certificate). In
stage 2 (ancestor) processing, all ancestors of the each target
certificate are found, and the claimed resources are then removed
(perforated). A new paracertificate with these diminished resources
is crafted, with its fields generated based on internal variable
settings, original certificate field values, and, potentially, the
fields of the RP's TA certificate. In tree processing (stage 3), the
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entire local RPKI repository is searching for any other certificates
that have resources that intersect a target resource, and that were
not otherwise processed during a preceding stage. Perforation is
again performed for any such intersecting certificates, and
paracertificates created as in stage 2. In the fourth (last) stage,
TA re-parenting, any TA certificates in the local RPKI
repository that have not already been processed are now re-parented
under the RP's TA certificate. This transformation creates
paracertificates; however, these paracertificates may have RFC 3779
resources that were not altered during algorithm processing. The
final output of algorithm processing will be threefold:
- the metadata information on some (original) certificates in the
repository MAY be altered.
- paracertificates will be created, with the appropriate metadata,
and entered into the repository.
- the TA processing algorithm SHOULD produce a human readable
log of its actions, indicating which paracertificates were created
and why. The remainder of this section describes the processing
stages of the algorithm in detail.
4.2.1 Preparatory processing (stage 0)
During preparatory processing, the output of the proofreader
algorithm, is read. Internal variables are set corresponding to each tag
and flag, if present, or to their defaults, if absent. Internal variables
are set corresponding to the PRIVATEKEYMETHOD value string(s) and the
TACERTIFICATE string. The TA processing algorithm is queried to
determine if it supports the indicated private key access
methodology. This query is performed in an implementation-specific
manner. In particular, an implementation is free to vacuously return
success to this query. The TA processing algorithm next uses the
value string for the TACERTIFICATE to locate this certificate,
again in an implementation-specific manner. The certificate in
question may already be present in the local RPKI repository, or it
may be located elsewhere. The implementation is free to create the
top level certificate at this time, and then assign to this
newly-created certificate the name indicated. It is necessary only
that, at the conclusion of this processing, a valid trust anchor
certificate for the relying party has been created or otherwise
obtained.
Some form of access to the RP's private key and top level certificate
are required for subsequent correct operation of the algorithm.
Therefore, stage 0 processing MUST terminate if one or both
conditions are not satisfied. In the error case, the implementation
SHOULD provide an error message of sufficient detail that the user
can correct the error(s). If stage 0 processing does not succeed, no
further stages of TA processing are executed.
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4.2.2 Target processing (stage 1)
During target processing, the TA processing algorithm reads all
target blocks in the proofreader output file. It then processes each
target block in the order specified in the file. In the description
that follows, except where noted, the operation of the algorithm on
a single target block will be described. Note, however, that all
stage 1 processing is executed before any processing in subsequent
stages is performed.
The algorithm first obtains the SKI region of the target block. It
then locates (in an implementation-dependent manner) the certificate
identified by the SKI. Note that this search is performed only
against (original) certificates, not against paracertificates.
If more than one original certificate is found matching this SKI,
there are two possible scenarios. If a resource holder has two
certificates issued by the same CA, with overlapping validity
intervals and the same key, but distinct subject names (typically,
by virtue of the SerialNumber parts being different), then these
two certificates are both considered to be (distinct) targets, and
are both processed. If, however, a resource holder has certificates
issued by two different CAs, containing different resources, but
using the same key, there is no unambiguous method to decide which
of the certificates is intended as the target. In this latter case
the algorithm MUST issue a warning to that effect, mark the target
block in question as unavailable for processing by subsequent stages
and proceed to the next target block. If no certificate is found
then the algorithm SHOULD issue a warning to that effect and proceed
to process the next target block.
If a single (original) certificate is found matching the indicated SKI,
then the algorithm takes the following actions. First, it sets the
ORIGINAL state bit for the certificate found. Second, it sets the
TARGET state bit for the certificate found. Third, it extracts the
INRs from the certificate. If the global resource_nounion flag is TRUE,
the algorithm compares the extracted certificate INRs with the INRs
specified in the constraints file. If the two resource sets are
different, the algorithm SHOULD issue a warning noting the difference.
An output resource set is then formed that is identical to the resource
set extracted from the certificate. If, however, the resource_nounion
flag is FALSE, then the output resource set is calculated by forming
the union of the resources extracted from the certificate and the
resources specified for this target block in the constraints file. A
paracertificate is then constructed according to Table 1, using fields
from the original certificate, the tags that had been set during
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stage 0, and, if necessary, fields from the RP's TA certificate. The
INR resources of the paracertificate are equated to the derived output
resource set. The PARA state bit is set for the newly created
paracertificate.
4.2.3 Ancestor processing (stage 2)
The goal of ancestor processing is to discover all ancestors of a
target certificate and remove from those ancestors the resources
specified in the target blocks corresponding to the targets being
processed. Note that it is possible that, for a given chain from a
target certificate to a trust anchor, another target might be
encountered. This is handled by removing all the target resources of
all descendants. The set of all targets that are descendants of the
given certificate is formed. The union of all the target resources of
the corresponding target blocks is computed, and this union in then
removed from the shared ancestor.
In detail, the algorithm is as follows. First, all (original) target
certificates processed during stage 1 processing are collected.
Second, any collected certificates that have the NOCHAIN state bit set
are eliminated from the collection. (Note that, as a result of
eliminating such certificates, the resulting collection may be empty,
in which case this stage of algorithm processing terminates, and
processing advances to stage 3.) Next, an implementation MAY sort the
collection. The optional sorting algorithm is described in Appendix
B. Note that all stage 2 processing is completed before any stage 3
processing.
Two levels of nested iteration are performed. The outer iteration is
effected over all certificates in the collection; the inner iteration
is over all ancestors of the designated certificate being processed.
The first certificate in the collection is chosen, and a resource set
R is initialized based on the resources of the target block for that
certificate (since the certificate is in the collection, it must be a
target certificate, and thus correspond to a target block). The
parent of the certificate is then located using ordinary path
discovery over original certificates only. The ancestor's certificate
resources A are then extracted. These resources are then perforated
with respect to R. That is, an output set of resources is created by
forming the intersection I of A and R, and then taking the set
difference A - I as the output resources. A paracertificate is then
created containing resources that are these output resources, and
containing other fields and extensions from the original certificate
(and possibly the RP's TA certificate) according to the procedure
given in Table 1. The PARA state bit is set on this paracertificate
and the ORIGINAL state bit is set on A. If A is also a target
certificate, as indicated by its TARGET state bit being set, then
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there will already have been a paracertificate created for it. This
previous paracertificate is destroyed in favor of the newly created
paracertificate. In this case also, the set R is augmented by adding
into it the set of resources of the target block for A. The algorithm
then proceeds to process the parent of A. This inner iteration
continues until the self-signed certificate at the root of the path
is encountered and processed. The outer iteration then continues by
clearing R and proceeding to the next certificate in the target
collection.
Note that ancestor processing has the potential for order dependency,
as mentioned earlier in this document. If sorting is not implemented,
or if the sorting algorithm fails to completely process the
collection of target certificates because the allotted maximum number
of iterations has been realized, it may be the case that an ancestor
of a certificate logically occurs before that certificate in the
collection. Whenever an existing paracertificate is replaced by a
newly created paracertificate during ancestor processing, the
algorithm SHOULD alert the user, and SHOULD log sufficient detail
such that the user is able to determine which resources were
perforated from the original certificate in order to create the (new)
paracertificate.
In addition, implementations MUST provide for conflict detection and
notification during ancestor processing. During ancestor processing
a certificate may be encountered two or more times and the
modifications dictated by the ancestor processing algorithm may be
in conflict. If this situation arises the algorithm MUST refrain
from processing that certificate. Further, the implementation MUST
present the user with an error message that contains enough detail
so that the user can locate those directives in the constraints file
that are creating the conflict. For example, during one stage of the
processing algorithm it may be directed that resources R1 be added to
a certificate C, while during a different stage of the processing
algorithm it may be directed that resources R2 be removed from
certificate C. If the resource sets R1 and R2 have a non-empty
intersection, that is a conflict.
4.2.4 Tree processing (stage 3)
The goal of tree processing is to locate other certificates
containing INRs that conflict with the resources allocated to a
target, by virtue of the INRs specified in the constraints file.
The certificates processed are not ancestors of any target. The
algorithm used is described below.
First, all target certificates are collected. Second, all target
certificates that have the NOCHAIN state bit set are eliminated
from this collection. Third, if the intersection_always
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global flag is set, target blocks that occur in the constraints
file, but that did not correspond to a certificate in the local
repository, are added to the collection. In tree processing,
unlike ancestor processing, this collection is not sorted. An
iteration is now performed over each certificate (or set of target
block resources) in the collection. Note that the collection may be
empty, in which case this stage of algorithm processing terminates,
and processing advances to stage 4. Note also that all stage 3
processing is performed before any stage 4 processing.
Given a certificate or target resource block, each top level original
TA certificate is examined. If that TA certificate has an
intersection with the target block resources, then the certificate is
perforated with respect to those resources. A paracertificate is
created based on the contents of the original certificate (and
possibly the RP's TA certificate, as indicated in Table 1) using the
perforated resources. The ORIGINAL state bit is set on the original
certificate processed in this manner, and the PARA state bit is set
on the paracertificate just created. An inner iteration then begins
on the descendants of the original certificate just processed. There
are two ways in which this iteration may proceed. If the treegrowth
global flag is clear, then examination of the children proceeds until
all children are exhausted, or until one child is found with
intersecting resources. If the treegrowth global flag is set, all
children are examined. If a transfer of resources is in process,
more than one child may possess intersecting resources. In this case,
it is RECOMMENDED that the treegrowth flag be set. The inner iteration
proceeds until all descendants have been examined and no further
intersecting resources are found. The outer iteration then continues
with the next certificate or target resource block in the collection.
Note that unlike ancestor processing, there is no concept of a
potentially cumulating resource collection R; only the resources
in the target block are used for perforation.
4.2.5 TA re-parenting (stage 4)
In the final stage of TA algorithm processing, all TA certificates
(other than the RP's TA certificate) that have not already been
processed are now processed. At this stage all unprocessed TA
certificates have no intersection with any target resource blocks.
As such, in creating the corresponding paracertificates, the output
resource set is identical to the input resource set. Other
transformations as described in Table 1 are performed. The original
TA certificates have the ORIGINAL state bit set; the newly created
paracertificates have the PARA state bit set. Note that once stage
four processing is completely, only a single TA certificate will
remain in an unprocessed state, namely the relying party's own
TA certificate.
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4.3 Discussion
The algorithm described in this document effectively creates two
coexisting certificate hierarchies: the original certificate
hierarchy and the paracertificate hierarchy. Original
certificates are not removed during any of the processing described
in the previous section. Some original certificates may move from
having no state bits set (or only the NOCHAIN state bit set) to
having one or both of the ORIGINAL and TARGET state bits set. In
addition, the NOCHAIN state bit will still be set if it was set
before any processing. The paracertificate hierarchy, however, is
intended to supersede the original hierarchy for ROA validation.
The presence of two hierarchies has implications for path
discovery, and for revocation.
If one thinks of a certificate as being "named" by its SKI, then
there can now be two certificates with the same name, an original
certificate and a paracertificate. The next two sections discuss the
implications of this duality in detail. Before proceeding, it is
worth noting that even without the existence of the paracertificate
hierarchy, cases may exist in which two or more original
certificates have the same SKI. As noted earlier, in Section
4.2.2, these cases may be subdivided into the case in which such
certificates are distinguishable by virtue of having different
subject names, but identical issuers and resource sets, versus all
other cases. In the distinguishable case, the path discovery
algorithm treats the original certificates as separate certificates,
and processes them separately. In all other cases, the original
certificates should be treated as indistinguishable, and path
validation should fail.
5 Implications for Path Discovery
Path discovery proceeds from a child certificate C by asking for a
parent certificate P such that the AKI of C is equal to the SKI of P.
With one hierarchy this question would produce at most one answer.
With two hierarchies, the original certificate hierarchy and the
paracertificate hierarchy, the question may produce two answers, one
answer, or no answer. Each of these cases is considered in turn.
5.1 Two answers
If two paths are discovered, it SHOULD be the case that one of the
matches is a certificate with the ORIGINAL state bit set and the
PARA state bit clear, while the other match inversely has the
ORIGINAL state bit clear and the PARA state bit set. If any other
combination of ORIGINAL and PARA state bits obtains, the path
discovery algorithm MUST alert the user. In addition, the path
discovery algorithm SHOULD refrain from attempting to make a
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choice as to which of the two certificates is the putative parent. In
the no-error case, with the state bits are as indicated, the
certificate with the PARA state bit set is chosen as the parent P.
Note this means, in effect, that all children of the original
certificate have been re-parented under the paracertificate.
5.2 One answer
If the matching certificate has neither the ORIGINAL state bit set
nor the PARA state bit set, this certificate is the parent. If the
matching certificate has the PARA state bit set but the ORIGINAL
state bit not set, this certificate is the parent. (This situation
would arise, for example, if the original certificate had been
revoked by its issuer but the paracertificate had not been revoked by
the RP.) If the matching certificate has the ORIGINAL state bit set
but the PARA state bit not set, this is not an error but it is a
situation in which path discovery MUST be forced to fail. The parent
P MUST be set to NULL, and the NOCHAIN state bit must be set on C and
all its descendants; the user SHOULD be warned. Even if the RP has
revoked the paracertificate, the original certificate MAY persist.
Forcing path discovery to unsuccessfully terminate is a reflection of
the RP's preference for path discovery to fail as opposed to using
the original hierarchy. Finally, if the matching certificate has both
the ORIGINAL and PARA state bits set, this is an error. The parent P
MUST be set to NULL, and the user MUST be warned.
5.3 No answer
This situation occurs when C has no parent in either the original
hierarchy or the paracertificate hierarchy. In this case the parent P
is NULL and path discovery terminates unsuccessfully. The NOCHAIN
state bit must be set on C and all its descendants.
6 Implications for Revocation
In a standard implementation of revocation in a PKI, a valid CRL
names a (sibling) certificate by serial number. That certificate is
revoked and is purged from the local RPKI repository. The original
certificate hierarchy and the paracertificate hierarchy created by
applying the algorithms described above are closely related. It
can thus be asked how revocation is handled in the presence of these
two hierarchies. In particular do changes in one of the hierarchies
trigger corresponding changes in the other hierarchy. There are four
cases based on the state of the ORIGINAL and PARA bits. These are
discussed in the subsections below. It should be noted that the
existence of two hierarchies presents a particular challenge with
respect to revocation. If a CRL arrives and is processed, that
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processing can result in the descrution of one of the path chains.
In the case of a single hierarchy this would mean that certain objects
would fail to validate. In the presence of two hierarchies, however,
a CRL revocation may force the preferred path to be destroyed. If
the RP later determines that the CRL revocation should not have
occurred, he is faced with an undesirable situation: the deprecated
path will be discovered. In order to prevent this outcome, an RP
MUST be able to configure one or more additional repository URIs
in support of local trust anchor management.
6.1 No state bits set
If the CRL names a certificate that has neither the ORIGINAL state
bit set nor the PARA state bit set, revocation proceeds normally. All
children of the revoked certificate have their state modified so that
the NOCHAIN state bit is set.
6.2 ORIGINAL state bit set
If the CRL names a certificate with the ORIGINAL state bit set and
the PARA state bit clear, then this certificate is revoked as usual.
If this original certificate also has the TARGET state bit set, then
the corresponding paracertificate (if it exists) is not revoked; if
this original certificate has the TARGET state bit clear, then the
corresponding paracertificate is revoked as well. Note that since all
the children of the original certificate have been re-parented to be
children of the corresponding paracertificate, as described above,
the revocation algorithm MUST NOT set the NOCHAIN state bit on these
children unless the paracertificate is also revoked. Note also that
if the original certificate is revoked but the paracertificate is not
revoked, the paracertificate retains its PARA state bit. This is to
ensure that path discovery proceeds preferentially through the
paracertificate hierarchy, as described above.
6.3 PARA state bit set
If the CRL names a certificate with the PARA state bit set and the
ORIGINAL state bit clear, this CRL must have been issued, perforce,
by the RP itself. This is because all the paracertificates are
children of the RP's TA certificate. (Recall that a TA is not revoked
via a CRL; it is merely removed from the repository.) The
paracertificate is revoked and all children of the paracertificate
have the NOCHAIN state bit set. No action is taken on the
corresponding original certificate; in particular, its ORIGINAL state
bit is not cleared.
Note that the serial numbers of paracertificates are synthesized
according to the procedure given in Table 1, rather than being
assigned by an algorithm under the control of the (original) issuer.
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6.4 Both ORIGINAL and PARA state bits set
This is an error. The revocation algorithm MUST alert the user and
take no further action.
'
7 Security Considerations
The goal of the algorithm described in this document is to enable an
RP to impose its own view of the RPKI, which is intrinsically a
ecurity function. An RP using a constraints file is trusting the
assertions made in that file. Errors in the constraints file used
by an RP can undermine the security offered by the RPKI, to that RP.
In particular, since the paracertificate hierarchy is intended to
trump the original certificate hierarchy for the purposes of path
discovery, an improperly constructed paracertificate hierarchy could
validate ROAs that would otherwise be invalid. It could also
declare as invalid ROAs that would otherwise be valid. As a result,
an RP must carefully consider the security implications of the
constraints file being used, especially if the file is provided by
a third party.
8 IANA Considerations
[Note to IANA, to be removed prior to publication: there are no IANA
considerations stated in this version of the document.]
9 Acknowledgements
The authors would like to acknowledge the significant contributions
of Charles Gardiner, who was the original author of an internal
version of this document, and who contributed significantly to its
evolution into the current version.
10 References
10.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3513] Hinden, R., and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779, June 2004.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
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Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5396] Huston, G., and G. Michaelson, "Textual Representation of
Autonomous System (AS) Numbers", RFC 5396, December 2008.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, February 2012.
[RFC6481] Huston, G., Loomans, R., and G. Michaelson, "A Profile
for Resource Certificate Policy Structure", RFC 6481,
Feburary 2012.
[RFC6487] Huston, G., Michaelson, G., and R. Loomans, "A Profile
for X.509 PKIX Resource Certificates", RFC 6487, February
2012.
10.2 Informative References
None.
Authors' Addresses
Stephen Kent
Raytheon BBN Technologies
10 Moulton St.
Cambridge, MA 02138
Email: kent@bbn.com
Matthew Lepinski
Raytheon BBN Technologies
10 Moulton St.
Cambridge, MA 02138
Email: mlepinsk@bbn.com
Mark C. Reynolds
Island Peak Software
328 Virginia Road
Concord, MA 01742
Email: mcr@islandpeaksoftware.com
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Appendix A: Sample Constraints File
;
; Sample constraints file for TBO LTA Test Corporation.
;
; TBO manages its own local (10.x.x.x) address space
; via the target blocks in this file.
;
;
; Relying party subsection. TBO uses ssh-agent as
; a software cryptographic agent.
;
PRIVATEKEYMETHOD OBO(ssh-agent)
TACERTIFICATE tbomaster.cer
;
; Flags subsection
;
; Always use the resources in this file to augment
; certificate resources.
; Always process resource conflicts in the tree, even
; if the target certificate is missing.
; Always search the entire tree.
;
CONTROL resource_nounion FALSE
CONTROL intersection_always TRUE
CONTROL treegrowth TRUE
;
; Tags subsection
;
; Copy the original cert's validity dates.
; Use the default policy OID.
; Use our own CRLDP.
; Use our own AIA.
;
TAG Xvalidity_dates C
TAG Xcp D
TAG Xcrldp rsync://tbo_lta_test.com/pub/CRLs
TAG Xaia rsync://tbo_lta_test.com/pub/repos
;
; Block subsection
;
Reynolds, et al Expires October 2, 2013 [Page 26]
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;
; First block: TBO Corporate
;
; Resource Holder: TBO Corporation
SKI 00112233445566778899998877665544332211
IPv4
10.2.3/24
10.8/16
IPv6
2001:db8::/32
AS#
60123
5507
;
; Second block: TBO LTA Test Enforcement Division
;
; Resource Holder: TBO Corporation
SKI 653420AF758421CF600029FF857422AA6833299F
IPv4
10.2.8/24
10.47/16
IPv6
AS#
60124
;
; Third block: TBO LTA Test Acceptance Corporation
; Quality financial services since sometime
; late yesterday.
;
; Resource Holder: TBO Acceptance Corporation
SKI 19:82:34:90:8b:a0:9c:ef:00:af:a0:98:23:09:82:4b:ef:ab:98:09
IPv4
10.3.3/24
IPv6
AS#
60125
; End of TBO constraints file
Appendix B: Optional Sorting Algorithm for Ancestor Processing
Sorting is performed in an effort to eliminate any order dependencies
in ancestor processing, as described in section 4.2.3 of this
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document. The sorting algorithm does this by rearranging the
processing of certificates such that if A is an ancestor of B, B is
processed before A. The sorting algorithm is an OPTIONAL part of
ancestor processing. Sorting proceeds as follows. The collection
created at the beginning of ancestor processing is traversed and any
certificate in the collection that is visited as a result of path
discovery is temporarily marked. After the traversal, all unmarked
certificates are moved to the beginning of the collection. The
remaining marked certificates are unmarked, and a traversal again
performed through this sub-collection of previously marked
certificates. The sorting algorithm proceeds iteratively until all
certificates have been sorted or until a predetermined fixed number
of iterations has been performed. (Eight is suggested as a munificent
value for the upper bound, since the number of sorting steps need not
be any greater than the maximum depth of the tree.) Finally, the
ancestor processing algorithm is applied in turn to each certificate
in the remaining sorted collection. If the sorting algorithm fails to
converge, that is if the maximum number of iterations has been
reached and unsorted certificates remain, the implementation SHOULD
warn the user.
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