Internet Engineering Task Force (IETF) R. Kisteleki Request for Comments: 7909 RIPE NCC Updates: 2622, 4012 B. Haberman Category: Standards Track JHU APL ISSN: 2070-1721 June 2016
Securing Routing Policy Specification Language (RPSL) Objects with Resource Public Key Infrastructure (RPKI) Signatures
This document describes a method that allows parties to electronically sign Routing Policy Specification Language objects and validate such electronic signatures. This allows relying parties to detect accidental or malicious modifications of such objects. It also allows parties who run Internet Routing Registries or similar databases, but do not yet have authentication (based on Routing Policy System Security) of the maintainers of certain objects, to verify that the additions or modifications of such database objects are done by the legitimate holder(s) of the Internet resources mentioned in those objects. This document updates RFCs 2622 and 4012 to add the signature attribute to supported RPSL objects.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Objects stored in resource databases, like the RIPE DB, are generally protected by an authentication mechanism: anyone creating or modifying an object in the database has to have proper authorization to do so, and therefore has to go through an authentication procedure (provide a password, certificate, email signature, etc.). However, for objects transferred between resource databases, the authentication is not guaranteed. This means that when a Routing Policy Specification Language (RPSL) object is downloaded from a database, the consumer can reasonably claim that the object is authentic if it was locally created, but cannot make the same claim for an object imported from a different database. Also, once such an object is downloaded from the database, it becomes a simple (but still structured) text file with no integrity protection. More importantly, the authentication and integrity guarantees associated with these objects do not always ensure that the entity that generated them is authorized to make the assertions implied by the data contained in the objects.
A potential use for resource certificates [RFC6487] is to use them to secure such (both imported and downloaded) database objects, by applying a digital signature over the object contents in lieu of methods such as Routing Policy System Security [RFC2725]. The signer of such signed database objects MUST possess a relevant resource certificate, which shows him/her as the legitimate holder of an Internet number resource. This mechanism allows the users of such database objects to verify that the contents are in fact produced by the legitimate holder(s) of the Internet resources mentioned in those objects. It also allows the signatures to cover whole RPSL objects, or just selected attributes of them. In other words, a digital signature created using the private key associated with a resource certificate can offer object security in addition to the channel security already present in most resource databases. Object security in turn allows such objects to be hosted in different databases and still be independently verifiable.
While the approach outlined in this document mandates the use of the Resource Public Key Infrastructure (RPKI) for certificate distribution, it is not dependent upon the RPKI for correct functionality. Equivalent functionality can be achieved with a more traditional Certification Authority (CA), using the extensions described in [RFC3779] within the certificates, and the appropriate trust anchor material to verify the digital signature.
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The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
When signing an RPSL object [RFC2622] [RFC4012], the input for the signature process is transformed into a sequence of strings of ASCII data. The approach is similar to the one used in Domain Key Identified Mail (DKIM) [RFC6376]. In the case of RPSL, the object to be signed closely resembles an SMTP header, so it seems reasonable to adapt DKIM's relevant features.
The digital signature associated with an RPSL object is itself a new attribute named "signature". It consists of mandatory and optional fields. These fields are structured in a sequence of name and value pairs, separated by a semicolon ";" and a whitespace. Collectively, these fields make up the value for the new "signature" attribute. The "name" part of such a component is always a single ASCII character that serves as an identifier; the value is an ASCII string the contents of which depend on the field type. Mandatory fields MUST appear exactly once, whereas optional fields MUST appear at most once.
Mandatory fields of the "signature" attribute:
o Version of the signature (field "v"): This field MUST be set to "rpkiv1" and MAY be the first field of the signature attribute to simplify the parsing of the attributes' fields. The signature format described in this document applies when the version field is set to "rpkiv1". All the rest of the signature attributes are defined by the value of the version field.
o Reference to the certificate corresponding to the private key used to sign this object (field "c"): The value of this field MUST be a URL of type "rsync" [RFC5781] or "http(s)" [RFC7230] that points to a specific resource certificate in an RPKI repository [RFC6481]. Any non URL-safe characters (including semicolon ";" and plus "+") must be URL encoded [RFC3986].
o Signature method (field "m"): What hash and signature algorithms were used to create the signature. This specification follows the algorithms defined in RFC 6485 [RFC6485]. The algorithms are referenced within the signature attribute by the ASCII names of the algorithms.
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o Time of signing (field "t"): The format of the value of this field MUST be in the Internet Date/Time ABNF format [RFC3339]. All times MUST be converted to Universal Coordinated Time (UTC), i.e., the ABNF time-offset is always "Z".
o The signed attributes (field "a"): This is a list of attribute names, separated by an ASCII "+" character (if more than one attribute is enumerated). The list must include any attribute at most once.
o The signature itself (field "b"): This MUST be the last field in the list. The signature is the output of the signature algorithm using the appropriate private key and the calculated hash value of the object as inputs. The value of this field is the digital signature in base64 encoding (Section 4 of [RFC4648]).
Optional fields of the "signature" attribute:
o Signature expiration time (field "x"): The format of the value of this field MUST be in the Internet Date/Time format [RFC3339]. All times MUST be represented in UTC.
One can look at an RPSL object as an (ordered) set of attributes, each having a "key: value" syntax. Understanding this structure can help in developing more flexible methods for applying digital signatures.
Some of these attributes are automatically added by the database, some are database-dependent, yet others do not carry operationally important information. This specification allows the maintainer of such an object to decide which attributes are important (signed) and which are not (not signed), from among all the attributes of the object; in other words, we define a way of including important attributes while excluding irrelevant ones. Allowing the maintainer of an object to select the attributes that are covered by the digital signature achieves the goals established in Section 1.
The type of the object determines the minimum set of attributes that MUST be signed. The signer MAY choose to sign additional attributes, in order to provide integrity protection for those attributes too.
When verifying the signature of an object, the verifier has to check whether the signature itself is valid, and whether all the specified attributes are referenced in the signature. If not, the verifier MUST reject the signature and treat the object as a regular, unsigned RPSL object.
Even if the signature over the object is valid according to the signature validation rules, it may not be relevant to the object; it also needs to cover the relevant Internet number resources mentioned in the object.
Therefore, the Internet number resources present in [RFC3779] extensions of the certificate referred to in the "c" field of the signature MUST cover the resources in the primary key of the object (e.g., value of the "aut-num:" attribute of an aut-num object, value of the "inetnum:" attribute of an inetnum object, values of "route:", and "origin:" attributes of a route object, etc.).
The validity time interval of a signature is the intersection of the validity time of the certificate used to verify the signature, the "not before" time specified by the "t" field of the signature, and the optional "not after" time specified by the "x" field of the signature.
When checking multiple signatures, these checks are individually applied to each signature.
The notion of canonicalization is essential to digital signature generation and validation whenever data representations may change between a signer and one or more signature verifiers. Canonicalization defines how one transforms a representation of data
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into a series of bits for signature generation and verification. The task of canonicalization is to make irrelevant differences in representations of the same object, which would otherwise cause signature verification to fail. Examples of this could be:
o data transformations applied by the databases that host these objects (such as notational changes for IPv4/IPv6 prefixes, automatic addition/modification of "changed" attributes, etc.)
o the difference of line terminators across different systems
This means that the destination database might change parts of the submitted data after it was signed, which would cause signature verification to fail. This document specifies strict canonicalization rules to overcome this problem.
The following steps MUST be applied in order to achieve canonicalized representation of an object, before the actual signature (verification) process can begin:
1. Comments (anything beginning with a "#") MUST be omitted.
3. A multi-line attribute MUST be converted into its single-line equivalent. This is accomplished by:
* Converting all line endings to a single blank space (ASCII code 32).
* Concatenating all lines into a single line.
* Replacing the trailing blank space with a single new line ("\n", ASCII code 10).
4. Numerical fields MUST be converted to canonical representations. These include:
* Date and time fields MUST be converted to UTC and MUST be represented in the Internet Date/Time format [RFC3339].
* AS numbers MUST be converted to ASPLAIN syntax [RFC5396].
* IPv6 addresses MUST be canonicalized as defined in [RFC5952].
* IPv4 addresses MUST be represented as the ipv4-address type defined by RPSL [RFC2622].
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* All IP prefixes (IPv4 and IPv6) MUST be represented in Classless Inter-Domain Routing (CIDR) notation [RFC4632].
5. All ranges, lists, or sets of numerical fields are represented using the appropriate RPSL attribute and each numerical element contained within those attributes MUST conform to the canonicalization rules in this document. The ordering of values within such fields MUST be maintained during database transfers.
6. The name of each attribute MUST be converted into lower case, and MUST be kept as part of the attribute line.
7. Tab characters ("\t", ASCII code 09) MUST be converted into spaces.
8. Multiple whitespaces MUST be collapsed into a single space (" ", ASCII code 32) character.
9. All line endings MUST be converted into a single new line ("\n", ASCII code 10) character, (thus avoiding CR vs. CRLF differences).
Given an RPSL object and corresponding certificate, in order to create the digital signature, the following steps MUST be performed:
1. Create a list of attribute names referring to the attributes that will be signed (contents of the "a" field). The minimum set of these attributes is determined by the object type; the signer MAY select additional attributes.
2. Arrange the selected attributes according to the selection sequence specified in the "a" field as above, omitting all attributes that will not be signed.
3. Construct the new "signature" attribute, with all its fields, leaving the value of the "b" field empty.
4. Apply canonicalization rules to the result (including the "signature" attribute).
5. Create the signature over the results of the canonicalization process (according to the signature and hash algorithms specified in the "m" field of the signature attribute).
6. Insert the base64-encoded value of the signature as the value of the "b" field.
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7. Append the resulting "signature" attribute to the original object.
In order to validate a signature over such an object, the following steps MUST be performed:
1. Verify the syntax of the "signature" attribute (i.e., whether it contains the mandatory and optional components and the syntax of these fields matches the specification as described in Section 2.1).
2. Fetch the certificate referred to in the "c" field of the "signature" attribute, and check its validity using the steps described in [RFC6487].
3. Extract the list of attributes that were signed using the signer from the "a" field of the "signature" attribute.
4. Verify that the list of signed attributes includes the minimum set of attributes for that object type.
5. Arrange the selected attributes according to the selection sequence provided in the value of the "a" field, omitting all unsigned attributes.
6. Replace the value of the signature field "b" of the "signature" attribute with an empty string.
7. Apply the canonicalization procedure to the selected attributes (including the "signature" attribute).
8. Check the validity of the signature using the signature algorithm specified in the "m" field of the signature attribute, the public key contained in the certificate mentioned in the "c" field of the signature, the signature value specified in the "b" field of the signature attribute, and the output of the canonicalization process.
4. Signed Object Types and Set of Signed Attributes
This section describes a list of object types that MAY be signed using this approach. For each object type, the set of attributes that MUST be signed for these object types (the minimum set noted in Section 3.3 is enumerated.
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This list generally excludes attributes that are used to maintain referential integrity in the databases that carry these objects, since these usually make sense only within the context of such a database, whereas the scope of the signatures is only one specific object. Since the attributes in the referred object (such as mnt-by, admin-c, tech-c, etc.) can change without any modifications to the signed object, signing such attributes could lead to a false sense of security in terms of the contents of the signed data; therefore, including such attributes should only be done in order to provide full integrity protection of the object itself.
The newly constructed "signature" attribute is always included in the list. The signature under construction MUST NOT include signature attributes that are already present in the object.
It should be noted that the approach defined in this document has a limitation in signing route objects. This document only supports a single signature per object. This means that it is not possible to properly sign route objects where one resource holder possesses the Autonomous System Number (ASN) and another resource holder possesses the referenced prefix. A future version of this specification may resolve this limitation.
For each signature, the extension described in RFC 3779 that appears in the certificate used to verify the signature MUST include a resource entry that is equivalent to, or covers (i.e., is "less specific" than) the following resources mentioned in the object the signature is attached to:
o For the as-block object type: the resource in the "as-block" attribute.
o For the aut-num object type: the resource in the "aut-num" attribute.
o For the inetnum object type: the resource in the "inetnum" attribute.
o For the route object type: the resource in the "route" or "origin" (or both) attributes.
5. Keys and Certificates Used for Signature and Verification
The certificate that is referred to in the signature (in the "c" field):
o MUST be an end-entity (i.e., non-CA) certificate
o MUST conform to the X.509 PKIX Resource Certificate profile [RFC6487]
o MUST have the extension described in RFC 3779 that covers the Internet number resource included in a signed attribute [RFC3779]
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The certificate generated will omit the Subject Information Access (SIA) extension mandated by RFC 6487 as that extension requires an rsync URI for the accessLocation form and RPSL currently does not support database access via rsync.
6. Security Considerations
RPSL objects stored in the Internet Routing Registry (IRR) databases are public, and as such there is no need for confidentiality. Each signed RPSL object can have its integrity and authenticity verified using the supplied digital signature and the referenced certificate.
Since the RPSL signature approach leverages X.509 extensions, the security considerations in [RFC3779] apply here as well. Additionally, implementers MUST follow the certificate validation steps described in RFC 6487.
The maintainer of an object has the ability to include attributes in the signature that are not included in the resource certificate used to create the signature. Potentially, a maintainer may include attributes that reference resources the maintainer is not authorized to use.
It should be noted that this digital signature does not preclude monkey-in-the-middle attacks where the adversary either intercepts RPSL object transfers, deletes the signature attribute, modifies the contents, or intercepts the transfer and drops the objects destined for the requester.
The authors would like to acknowledge the valued contributions from Jos Boumans, Tom Harrison, Steve Kent, Sandra Murphy, Magnus Nystrom, Alvaro Retana, Sean Turner, Geoff Huston, and Stephen Farrell in preparation of this document.