Internet DRAFT - draft-cel-nfsv4-hash-tree-interchange-format
draft-cel-nfsv4-hash-tree-interchange-format
Network File System Version 4 C. Lever, Ed.
Internet-Draft Oracle
Intended status: Standards Track 4 May 2022
Expires: 5 November 2022
Attestation of File Content using an X.509 Certificate
draft-cel-nfsv4-hash-tree-interchange-format-03
Abstract
This document describes a compact open format for transporting and
storing an abbreviated form of a cryptographically signed hash tree.
Receivers use this representation to reconstitute the hash tree and
verify the integrity of file content protected by that tree.
An X.509 certificate encapsulates and protects the hash tree metadata
and provides cryptographic provenance. Therefore this document
updates the Internet X.509 certificate profile specified in RFC 5280.
Note
Discussion of this draft occurs on the NFSv4 working group mailing
list (nfsv4@ietf.org), archived at
https://mailarchive.ietf.org/arch/browse/nfsv4/. Working Group
information is available at https://datatracker.ietf.org/wg/nfsv4/
about/.
Submit suggestions and changes as pull requests at
https://github.com/chucklever/i-d-hash-tree-interchange-format.
Instructions are on that page.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 November 2022.
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Copyright Notice
Copyright (c) 2022 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 (https://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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Combining These Solutions . . . . . . . . . . . . . . . . 4
1.2. Efficient Content Verification . . . . . . . . . . . . . 4
1.3. Related Work . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
3. Hash Tree Metadata . . . . . . . . . . . . . . . . . . . . . 6
4. File Provenance Certificates . . . . . . . . . . . . . . . . 7
4.1. New Certificate Fields . . . . . . . . . . . . . . . . . 7
4.1.1. Root Hash . . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. Divergence Factor . . . . . . . . . . . . . . . . . . 8
4.1.3. Tree Height . . . . . . . . . . . . . . . . . . . . . 8
4.1.4. Block Size . . . . . . . . . . . . . . . . . . . . . 8
4.1.5. Salt Value . . . . . . . . . . . . . . . . . . . . . 8
4.2. Extended Key Usage Values . . . . . . . . . . . . . . . . 8
4.3. Validating Certificates and their Signatures . . . . . . 9
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. X.509 Certificate Vulnerabilities . . . . . . . . . . . . 9
6.2. Hash Tree Collisions and Pre-Image Attacks . . . . . . . 10
6.3. File Content Vulnerabilities . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. Object Identifiers for Hash Tree Metadata . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Linear hashing is a common technique for protecting the integrity of
data. A fixed-size hash, or digest, is computed over the bytes in a
data set using a deterministic and collision-resistant algorithm. An
example of such an algorithm is [FIPS.180-4].
Filesystem designers often employ this technique to protect the
integrity of both individual files and filesystem metadata. For
instance, to protect an individual file's integrity, the filesystem
computes a digest from the beginning of its content to its end. The
filesystem then stores that digest along with the file content. The
integrity of that digest can be further protected by
cryptographically signing it. The filesystem recomputes the digest
when the file is retrieved and compares the locally-computed digest
with the saved digest to verify the file content.
Over time, linear hashing has proven to be an inadequate fit with the
way filesystems manage file content. A content verifier must read
the entire file to validate its digest. Reading whole files is not
onerous for small files, but reading a large file every time its
digest needs verification quickly becomes costly.
Filesystems read files from persistent storage in small pieces
(blocks) on demand to manage large files efficiently. When memory is
short, the system evicts these data blocks and then reads them again
when needed later. There is no physical guarantee that a subsequent
read of a particular block will give the same result as an earlier
one. Thus the initial verification of a file's becomes stale,
sometimes quickly.
To address this shortcoming, some have turned to hash trees
[Merkle88]. A hash tree leaf node contains the linear hash of a
portion of the protected content. Interior nodes in a hash tree
contain hashes of the nodes below them, up to the root node which
stores a hash of everything in the tree. Validating a leaf node
means validating only the portion of the file content protected by
that node and its parents in the hash tree.
Hash trees present a new challenge, however. Even when signed, a
single linear hash is the same size no matter how much content it
protects. The size of a hash tree, however, increases
logarithmically with the size of the content it protects.
Transporting and storing a hash tree can therefore be unwieldy. It
is particularly a problem for legacy storage formats that do not have
mechanisms to handle extensive amounts of variably-sized metadata.
Software distribution and packaging formats might not be flexible
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enough to transport this possibly large amount of integrity data.
Backup mechanisms such as tar or rsync might be unable to handle
variably-sized metadata per file.
Moreover, we can readily extend network file storage protocols to
exchange a hash tree associated with every file. However, to support
such extensions, file servers and the ecosystems where they run must
be updated to manage and store this metadata. Thus it is not merely
an issue of enriching a file storage protocol to handle a new
metadata type.
1.1. Combining These Solutions
The root hash of a hash tree is itself a fixed-size piece of metadata
similar to a linear hash. The only disadvantage is that a verifier
must reconstitute the hash tree using the root hash and the file
content. However, if the verifier caches each tree on local trusted
storage, that is as good as storing the whole tree. The verifier can
then use the locally cached tree to validate portions of the file it
protects without reading each file repeatedly from remote or
untrusted durable storage.
To further insulate a root hash from unwanted change, an attestor can
protect it with a cryptographic signature. This cryptographic
protection then additionally covers the entire hash tree and the file
content it protects.
This integrity protection is independent of the file's storage format
and its underlying durable media. The file (and the root hash that
protects it) can be copied, transmitted over networks, or backed up
and restored while it remains protected end-to-end.
1.2. Efficient Content Verification
We now have a small fixed-size piece of metadata that can protect
potentially huge files. The trade-off is that the verifier must
reconstitute the hash tree during file installation or on-demand.
File systems or remote filesystem clients can store or cache
reconstituted trees in:
* Volatile or non-volatile memory
* A secure database
* A private directory on a local filesystem
* A named attribute or stream associated with the file
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An easily accessible copy of a file's hash tree enables frequent
verification of file content. Frequent verification protects that
content against unwanted changes due to local storage or copying
errors, malicious activity, or data retention issues. When
verification is truly efficient, it can take place as often as during
every application read operation without a significant impact on
throughput.
The current document's unique contribution is the use of an X.509 v3
certificate to encapsulate the representation of a hash tree. The
purpose of encapsulation is to enable the hash tree metadata to be
exchanged and recognized broadly in the Internet community.
Therefore each certificate has to:
* Cryptographically protect the integrity of the hash tree metadata
* Bind the hash tree metadata to the authenticated identity of the
file content's attestor
* Provide for a broadly-supported standard set of cryptographic
algorithms
* Represent the hash tree data in a commonly recognized format that
is independent of storage media
Lastly, we note that a standard representation of hash tree metadata
enables opportunities for hardware offload of content verification.
1.3. Related Work
Granted in 1982, expired US patent 4309569 [Merkle82] covers the
construction of a tree of digests. Initially, these "Merkle trees"
helped improve the security of digital signatures. Later they were
used in storage integrity applications such as [Mykletun06]. They
have also found their way into other domains. [RFC6962], published
in 2013, uses Merkle trees to manage log auditing, for example.
A Tiger tree is a form of a hash tree often used by P2P protocols to
verify a file's integrity while in transit. The Tree Hash EXchange
format [THEX03] enables the transmission of whole Tiger trees in an
XML format. The current document proposes similar usage where a
sidecar hash tree protects file content but reduces the integrity
metadata's size.
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2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Hash Tree Metadata
Reconstituting a hash tree (as opposed to building a more generic
directed graph of hashes) requires the protected content, a basic set
of metadata, and an understanding of how to use the metadata to
reconstitute the hash tree:
* The algorithm used to compute the tree's digests
* The divergence factor (defined as one for a hash list and two for
binary hash trees)
* The tree height (from root to the lowest leaf node)
* The block size covered by each leaf node in the tree
* An optional salt value
More research might be needed to cover recent innovations in hash
tree construction; in particular, the use of prefixes to prevent
second pre-image attacks.
The digest algorithm used to construct the hash tree MUST match the
digest algorithm used to sign the certificate. Thus if SHA-2 is used
to construct the hash tree, the certificate signature is created with
SHA-2. The verifier then uses SHA-2 when validating the certificate
signature and reconstituting the hash tree. The object identifiers
for the supported algorithms and the methods for encoding public key
materials (public key and parameters) are specified in [RFC3279],
[RFC4055], and [RFC4491].
The block size value of the tree is specified in octets. For
example, if the block size is 4096, then each leaf node of the hash
tree digests 4096 octets of the protected file (aligned on 4096-octet
boundaries).
The internal nodes are digests constructed from the hashes of two
adjacent child nodes up to the root node (further detail needed
here). The tree's height is the distance, counted in nodes, from the
root to the lowest leaf node.
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The leaf nodes are ordered (left to right) by the file offset of the
block they protect. Thus, the left-most leaf node represents the
first block in the file, and the right-most leaf node represents the
final block in the file.
An explanation of the salt value goes here.
Further, when computing each digest, an extra byte might be prefixed
to the pre-digested content to reduce the possibility of a second-
preimage attack.
4. File Provenance Certificates
| RFC Editor: In the following subsections, please replace the
| letters II once IANA has allocated this value. Furthermore,
| please remove this Editor's Note before this document is
| published.
X.509 certificates are specified in [X.509]. The current document
extends the Internet X.509 certificate profile specified in [RFC5280]
to represent file content protected by hash tree metadata.
File provenance certificates are end-entity certificates. The
certificate's signature identifies the attestor and cryptographically
protects the hash tree metadata.
The Subject field MUST be an empty sequence. The SubjectAltName list
carries a filename and the root hash, encoded in a new otherName
type-ID, shown below. The current document requests allocation of
this new type-ID on the id-on arc, defined in [RFC7299]. The
following subsections describe how the fields in this new type-ID are
used.
id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) }
id-on OBJECT IDENTIFIER ::= { id-pkix 8 }
id-on-fileContentAttestation OBJECT IDENTIFIER ::= { id-on II }
FileContentAttestation ::= SEQUENCE {
treeRootDigest OCTET STRING,
treeDivergenceFactor INTEGER (1..2),
treeHeight INTEGER,
treeBlockSize INTEGER,
treeSaltValue OCTET STRING
}
4.1. New Certificate Fields
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4.1.1. Root Hash
The root digest field stores the digest that appears at the root of
the represented Merkle tree. The digest appears as a hexadecimal
integer.
4.1.2. Divergence Factor
The value in the tree divergence factor field represents the maximum
number of children nodes each node has in the represented Merkle
tree. A value of two, for example, means each node (except the leaf
nodes) has no more than two children.
4.1.3. Tree Height
The tree height field stores the distance from the represented Merkle
tree's root node to its lowest leaf node. A value of one, for
example, means the tree has a single level at the root.
4.1.4. Block Size
The block size field contains the number of file content bytes
represented by each digest (node) in the Merkle tree. A typical
value is 4096, meaning each node in the tree contains a digest of up
to 4096 bytes, starting on 4096-byte boundaries.
4.1.5. Salt Value
The tree salt value is a hexadecimal integer combined with the digest
values in some way that I have to look up. If the tree salt value is
zero, salting is not to be used when reconstituting the represented
Merkle tree.
4.2. Extended Key Usage Values
Section 4.2.1.12 of [RFC5280] specifies the extended key usage X.509
certificate extension. This extension, which may appear in end-
entity certificates, indicates one or more purposes for which the
certified public key may be used in addition to or in place of the
basic purposes indicated in the key usage extension.
The inclusion of the codeSigning value (id-kp-codeSigning) indicates
that the certificate has been issued for the purpose of allowing the
holder to verify the integrity and provenance of file content.
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4.3. Validating Certificates and their Signatures
When validating a certificate containing hash tree metadata,
validation MUST include the verification rules per [RFC5280].
The validator reconstitutes a hash tree using the presented file
content and the hash tree metadata in the certificate. If the root
hash of the reconstituted hash tree does not match the value
contained in the treeRootHash, then the validation fails.
5. Implementation Status
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs.
Please note that the listing of any individual implementation here
does not imply endorsement by the IETF. Furthermore, no effort has
been spent to verify the information presented here that was supplied
by IETF contributors. This is not intended as, and must not be
construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
There are no known implementations of the X.509 certificate
extensions described in the current document.
6. Security Considerations
It is important to note the narrow meaning of the digital signature
in X.509 certificates as defined in this document. That signature
connotes that the data content of the certificate has not changed
since the certificate was signed, and it identifies the signer
cryptographically. The signature does not confer any meaning or
guarantees other than the integrity of the certificate's data
content.
6.1. X.509 Certificate Vulnerabilities
The file content and hash tree can be unpacked and then resigned by
someone who participates in the same web of trust as the original
content creator. Verifiers should consult appropriate certificate
revocation databases as part of validating attestor signatures to
mitigate this form of attack.
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6.2. Hash Tree Collisions and Pre-Image Attacks
A typical attack against digest algorithms is a collision attack.
The usual mitigation for this form of attack is choosing a hash
algorithm known to be strong. Implementers SHOULD choose amongst
digest algorithms that are known to be resistant to pre-image
attacks. See [RFC4270] for a discussion of attacks on digest
algorithms typically used in Internet protocols.
Hash trees are subject to a particular type of collision attack
called a "second pre-image attack". Digest values in intermediate
nodes in a hash tree are generated from lower nodes. Executing a
collision attack to replace a subtree with content that hashes to the
same value does not change the root hash value and is more manageable
than replacing all of a file's content. This kind of attack can
occur independently of the strength of the tree's hash algorithm.
The tree height is included in the signed metadata to mitigate this
form of attack.
6.3. File Content Vulnerabilities
There are two broad categories of attacks on mechanisms that protect
the integrity of file content:
Overt corruption An attacker makes the file's content dubious or
unusable (depending on the end system's security policies) by
corrupting either the file's content or its protective metadata in
a detectable manner.
Silent corruption An attacker alters the file's content and its
protective metadata in synchrony such that any changes remain
undetected.
The goal of the current document's mechanism is to turn as many
instances of the latter as possible into the former, which are more
likely to identify corrupted content before it is consumed.
7. IANA Considerations
| RFC Editor: In the following subsections, please replace RFC-
| TBD with the RFC number assigned to this document, and please
| replace II with the number assigned to this new type-ID.
| Furthermore, please remove this Editor's Note before this
| document is published.
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7.1. Object Identifiers for Hash Tree Metadata
Following the "Specification Required" policy as defined in
Section 4.6 of [RFC8126], the author of the current document requests
several new type-ID OIDs on the id-on arc defined in Section 2 of
[RFC7299]. The registry for this arc is maintained at the following
URL: https://www.iana.org/assignments/smi-numbers/smi-
numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.8
Following [RFC5280], the current document requests newly-defined
objects in the following subsections using 1988 ASN.1 notation.
id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) }
id-on OBJECT IDENTIFIER ::= { id-pkix 8 }
id-on-fileContentAttestation OBJECT IDENTIFIER ::= { id-on II }
IANA should use the current document (RFC-TBD) as the reference for
these new entries.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April
2002, <https://www.rfc-editor.org/rfc/rfc3279>.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
DOI 10.17487/RFC4055, June 2005,
<https://www.rfc-editor.org/rfc/rfc4055>.
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[RFC4491] Leontiev, S., Ed. and D. Shefanovski, Ed., "Using the GOST
R 34.10-94, GOST R 34.10-2001, and GOST R 34.11-94
Algorithms with the Internet X.509 Public Key
Infrastructure Certificate and CRL Profile", RFC 4491,
DOI 10.17487/RFC4491, May 2006,
<https://www.rfc-editor.org/rfc/rfc4491>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[X.509] International Telephone and Telegraph Consultative
Committee, "ITU-T X.509 - Information technology - The
Directory: Public-key and attribute certificate
frameworks.", January 2019.
8.2. Informative References
[FIPS.180-4]
Dang, Q., "Secure Hash Standard", National Institute of
Standards and Technology report,
DOI 10.6028/nist.fips.180-4, July 2015,
<https://doi.org/10.6028/nist.fips.180-4>.
[Merkle82] Merkle, R., "Method of providing digital signatures",
January 1982.
[Merkle88] Merkle, R., "A Digital Signature Based on a Conventional
Encryption Function", Advances in Cryptology - CRYPTO
'87 pp. 369-378, DOI 10.1007/3-540-48184-2_32, 1988,
<https://doi.org/10.1007/3-540-48184-2_32>.
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[Mykletun06]
Mykletun, E., Narasimha, M., and G. Tsudik,
"Authentication and integrity in outsourced databases",
ACM Transactions on Storage Vol. 2, pp. 107-138,
DOI 10.1145/1149976.1149977, May 2006,
<https://doi.org/10.1145/1149976.1149977>.
[RFC4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270,
DOI 10.17487/RFC4270, November 2005,
<https://www.rfc-editor.org/rfc/rfc4270>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/rfc/rfc6962>.
[RFC7299] Housley, R., "Object Identifier Registry for the PKIX
Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
<https://www.rfc-editor.org/rfc/rfc7299>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/rfc/rfc7942>.
[THEX03] Chapweske, J. and G. Mohr, "Tree Hash EXchange format
(THEX)", January 2003, <http://www.nuke24.net/docs/2003/
draft-jchapweske-thex-02.html>.
Acknowledgments
The editor is grateful to Bill Baker, Eric Biggers, James Bottomley,
Russ Housley, Benjamin Kaduk, Rick Macklem, Greg Marsden, Paul Moore,
Martin Thomson, and Mimi Zohar for their input and support.
Finally, special thanks to Transport Area Directors Martin Duke and
Zaheduzzaman Sarker, NFSV4 Working Group Chairs David Noveck and
Brian Pawlowski, and NFSV4 Working Group Secretary Thomas Haynes for
their guidance and oversight.
Author's Address
Charles Lever (editor)
Oracle Corporation
United States of America
Email: chuck.lever@oracle.com
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