Hash-based Signatures: State and Backup Management
draft-wiggers-hbs-state-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Thom Wiggers , Kaveh Bashiri , Stefan Kölbl , Jim Goodman , Stavros Kousidis | ||
| Last updated | 2024-02-19 | ||
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draft-wiggers-hbs-state-00
Network Working Group T. Wiggers
Internet-Draft PQShield
Intended status: Informational K. Bashiri
Expires: 22 August 2024 BSI
S. Kölbl
Google
J. Goodman
Crypto4A Technologies
S. Kousidis
BSI
19 February 2024
Hash-based Signatures: State and Backup Management
draft-wiggers-hbs-state-00
Abstract
Stateful Hash-Based Signature Schemes (S-HBS) such as LMS, HSS, XMSS
and XMSS^MT combine Merkle trees with One-Time Signatures (OTS) to
provide signatures that are resistant against attacks using large-
scale quantum computers. Unlike conventional stateless digital
signature schemes, S-HBS have a state to keep track of which OTS keys
have been used, as double-signing with the same OTS key allows
forgeries.
This document provides guidance and documents security considerations
for the operational and technical aspects of deploying systems that
rely on S-HBS. Management of the state of the S-HBS, including any
handling of redundant key material, is a sensitive topic, and we
discuss some approaches to handle the associated challenges. We also
describe the challenges that need to be resolved before certain
approaches should be considered.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/hbs-guidance/draft-hbs-state.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2024 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/
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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
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
2.1. Specific terminology in the context of S-HBS . . . . . . 4
2.1.1. State management . . . . . . . . . . . . . . . . . . 4
2.1.2. Backup management . . . . . . . . . . . . . . . . . . 5
2.1.3. Key export, key import and key transfer . . . . . . . 5
3. Operational Considerations . . . . . . . . . . . . . . . . . 6
4. Requirements for secure state management . . . . . . . . . . 8
5. Potential Solutions . . . . . . . . . . . . . . . . . . . . . 9
5.1. Multiple Public Keys (SP-800-208) . . . . . . . . . . . . 9
5.2. Distributed Multi-trees (SP-800-208) . . . . . . . . . . 10
5.3. Sectorization . . . . . . . . . . . . . . . . . . . . . . 10
5.4. Key/State Transfer . . . . . . . . . . . . . . . . . . . 11
5.5. Key Rotation . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Variable-length Signature Chains . . . . . . . . . . . . 12
5.7. Pre-assigning States . . . . . . . . . . . . . . . . . . 13
5.8. Time-based State Management . . . . . . . . . . . . . . . 13
6. Backup management beyond NIST SP-800-208 . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
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9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Stateful Hash-Based Signature Schemes (S-HBS) such as LMS, HSS, XMSS
and XMSS^MT combine Merkle trees with One-Time Signatures (OTS) in
order to provide digital signature schemes that remain secure even
when large-scale quantum computers become available. Their theoretic
security is well understood and depends only on the security of the
underlying hash function. As such, they can serve as an important
building block for quantum-resistant information and communication
technology. S-HBS are specified in [RFC8391], [RFC8554], and NIST
[SP-800-208].
The private key of an S-HBS is a finite collection of OTS keys and an
associated data structure which keeps track of which OTS keys have
been used. This data structure is typically a simple counter and
often called an index; we refer to it as the *state* of the private
key. Each S-HBS private key can be used to sign a finite number of
messages, and the state must be updated with each generated
signature.
One must not reuse any OTS key that is part of an S-HBS private key.
If an attacker is able to obtain signatures for two different
messages created using the same OTS key, it is computationally
feasible for that attacker to create forgeries [BH16]. As noted in
[MCGREW] and [ETSI-TR-103-692], extreme care should be taken in order
to avoid the risk that an OTS key will be reused accidentally.
The statefulness of S-HBS leads to significant challenges in
practice:
* Implementers must ensure that each creation of a signature updates
the state correctly.
* If the S-HBS private key is distributed to multiple signers at the
same time, implementers must ensure that they never use the same
OTS key. This may require synchronization between all signers.
* If key backups are required, implementers must ensure that any
backup mechanism can not lead to re-using a previously used OTS
key.
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These issues lead to new requirements that most developers will not
be familiar with and that require careful handling in practice. Due
to these challenges, most applications should use stateless
alternatives like SLH-DSA [FIPS205] instead. However, if
performance, signature or key sizes of stateless alternatives are
prohibitive, and the specific use case allows a very tight control of
the signing environment, using S-HBS may be an appropriate solution.
It seems likely that in many scenarios, this will only be possible
when using purpose-designed hardware, such as hardware-security
modules.
The purpose of this document is to present, recall, and discuss
various strategies for a correct state and backup management for
S-HBS.
2. Conventions and Definitions
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.
2.1. Specific terminology in the context of S-HBS
In this subsection we specify certain notions which are important in
the context of S-HBS.
2.1.1. State management
In this document _state management_ refers to the handling and
implementation of the state of the private key.
This includes mechanisms, which aim:
* to securely update the state after each signature,
* to set up S-HBS with a split state and/or private key, so that
signatures can be generated from either part without risk of state
reuse,
* to enable effective but secure handling of private key and state
backup material,
* to guarantee the availability of both the private key and its
state across the lifetime of the key.
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Note that in particular implementations of S-HBS, or in alternative
signature mechanisms such as, e.g., puncturable schemes [BSW16], the
state and private key might be inseparable. However, even in these
scenarios, this document's guidance should still apply.
2.1.2. Backup management
In order to mitigate failure of, e.g., devices storing key material
and to facilitate other types of disaster recovery, backups of
private keys and their associated states SHOULD be considered as part
of a security architecture.
In this document, _backup management_ refers to all mechanisms
surrounding the goal to guarantee the availability of the private key
and state, but with special care to avoid state reuse by rolling back
to a state in which already-used OTS keys are still available.
These mechanisms include procedures and protocols, which aim:
* to securely store this private key and state material outside the
in-use signing device,
* to import an externally stored private key and state to a newly
initiated signing device,
* allow practicing with backup recovery and to ensure backups are
valid.
Backup management can be viewed as a more specific type of state
management; we make this distinction to clarify the aims of our
recommendations.
2.1.3. Key export, key import and key transfer
As part of state and backup management, we will discuss mechanisms to
export, import or transfer private key and state material. In order
to avoid misunderstandings we now specify these notions more
precisely.
key export mechanism of exporting secret data, which yields
(partial) private key and state material, from the signing device
to external storage. This external storage may be given in
digital or non-digital form.
key import mechanism of importing secret data, which loads (partial)
private key and state material, from external storage to the
signing device.
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key transfer direct and cryptographically protected migration of
private key and state material from one signing device to another
signing device.
Systems and architectures relying on key transfer are generally
expected to require fewer operational and manually-executed steps and
checks to avoid state reuse.
Note that, at times, secure variants of the aforementioned primitives
may be required (e.g., securely importing/exporting the key). In
these situations cryptographic mechanisms SHOULD be utilized to
provide assurances related to the confidentiality (e.g., utilizing
symmetric/asymmetric encryption mechanisms) and/or integrity/
authenticity (e.g., utilizing digital signatures, hash functions, and
keyed message authentication codes) of the associated operations.
3. Operational Considerations
An important aspect of the evaluation of various hash-based signature
state and backup management options is to consider the operational
costs associated with the option(s) being evaluated. In the past, a
traditional trust infrastructure solution could utilize
straightforward archival procedures to make copies of the keys, which
could then be distributed geographically to ensure their availability
and deliver a sufficiently resilient solution, all the while
enforcing whatever security protocols and procedures were required.
Unfortunately, stateful hash-based signatures introduce an additional
constraint in that they need to ensure the state is never re-used.
Hence, archival procedures used for traditional trust infrastructures
MUST be amended/redesigned to be used as viable options.
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One of the most problematic aspects of providing a long-lived
resilient solution is simply managing the physical media on which the
keys/state are stored externally (i.e., outside of the signing
device) throughout the course of their lifetime. Physical media/
devices degrade over time, and the more complex the media/device, the
more likely it is to fail at some point in time (e.g., data stored on
a CD vs. data stored on a USB drive vs. data stored in a Hardware
Security Module). Combine that fact with the long lifetimes
associated with stateful hash-based signature keys (e.g., 10-20+
years) and the difficulties associated with transferring keys between
devices, and one finds them self with a perplexing set of challenges
that needs to be accounted for in any state selection process of a
proper state and backup management solution. Compounding these
complexities is the fact any resilient state management system SHOULD
also provide some means to verify the integrity of these long lived
backups to ensure they will be valid when they are required, and to
ensure the operators know how to execute the necessary recovery
procedure(s).
Similarly, many of the prescribed state management options require a
high degree of human involvement which means one SHOULD consider the
costs associated with training the human element to ensure processes
and procedures are adhered to and failures caught early and corrected
before a catastrophic loss of security can occur (e.g., accidentally
instantiating multiple instances of a stateful hash-based signature
key/state). Note that training is not a fixed one-time cost either
as long lifetimes will necessitate succession planning amongst the
human element, and training of each successive generation of
participants. Mechanisms also SHOULD be put in place to mitigate the
ever-present insider threat via mechanisms such as M-of-N controls,
ensuring least-privileges amongst participants, and enforcing a
segregation of duties to ensure multiple parties are required to
collude to undermine a solution's security. Note that the
segregation of duties MUST persist across successive generations to
ensure participants do not acquire multiple roles over time, thereby
undermining the intended segregation.
Lastly, costs associated with any external dependencies required by a
particular solution (e.g., access to a public ledger or transparency
log, providing accurate time references and synchronization
mechanisms, access to attestation facilities, etc.) MUST be
accounted for as well, particularly if a system is operating in an
offline mode that makes delivering these additional capabilities all
the more complicated and expensive.
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4. Requirements for secure state management
A system deploying S-HBS SHOULD fulfill certain requirements to allow
securely handling the state. The system MUST ensure that no two
signing operations can ever be issued from the same state. In
addition, the generation of a signature and update of the state
SHOULD appear to be an _atomic transaction_. This means that the
system MUST NOT release a signature without irrevocably and correctly
updating the state.
These requirements impose significant restrictions on the underlying
technical approach and a careful implementation of how the state will
be updated or synchronized. The abstraction layers of modern systems
can make it particularly difficult to guarantee that no two versions
of the same state are present. The main concerns here are
* how the actual storage for the state is implemented,
* how it is modified,
* how an accidental/intentional failure/glitch might affect the
state security, and
* cloning.
A system may have a version of the private key stored in non-volatile
memory (e.g. a disk) and will load it into volatile memory (e.g.
RAM) while processing. Here, an implementer MUST ensure that these
are always perfectly synchronized [MCGREW]. This can be particularly
challenging if there are additional abstraction layers present in the
system, like additional caches which may affect reading/writing the
state and its potential existence in multiple locations.
Cloning is another concern, as it can easily lead to re-using the
same state. This can happen for instance if a process which issues a
signing operation is forked, and no proper synchronization is
enforced in the implementation to guarantee correct state update.
Virtual machine (VM) cloning is another potential security risk here,
as both backing up a VM into non-volatile memory or live cloning of a
VM can easily lead to a state re-use [MCGREW]. With users shifting
workloads to cloud service providers, the issue of VM cloning may
become more prevalent.
Using dedicated cryptographic hardware is RECOMMENDED to enforce
these requirements, ensure correct behavior and handle the complexity
of state management. In particular, this enables implementing
rollback resistant counters which can be difficult to achieve in a
software-only fashion.
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On the verifier side, no state management is required. However, the
verifier needs to trust the signer to not have re-used the state. A
verifier MAY want to check that no state re-use happened in the past
by a signer, before accepting a signature.
In practice, this can be done if the verifier has access to all
signatures issued by the signer. As the signatures contain the index
of the OTS key used, detecting if an index was used more than once
becomes trivial. In practice, such a (public) data structure which
contains all signatures MAY already be present in some use cases
(e.g. certificate transparency [RFC9162]) or could be built. It is
worth noting that while trusting the signer to not re-use the state
is a strong assumption, other signature schemes like ECDSA introduce
similar assumptions for the verifier, by requiring the signer to
never re-use the nonce.
5. Potential Solutions
A variety of potential solutions have been proposed both within the
[SP-800-208] specification, as well as from external sources. This
section describes a number of approaches and their potential
advantages/disadvantages.
5.1. Multiple Public Keys (SP-800-208)
The [SP-800-208] proposes generating multiple S-HBS keypairs and
configuring devices and clients to accept signatures created by any
of these keys.
This negatively impacts one of the advantages of using S-HBS by
increasing the public key footprint within the client, which can be
problematic if it has limited public key storage capacity.
[SP-800-208] addresses this concern by suggesting using a mechanism
such as that proposed in [RFC8649] to update the stored public key by
having the current key endorse the next key that is to be installed.
Unfortunately, for many constrained devices the public key is
embedded in immutable ROM or fuses due to security reasons, so it
cannot be updated in this manner.
The proposal of using multiple S-HBS keypairs for a single instance
also generates questions as to how to establish that approach in
existing public key infrastructures. For example, issueing multiple
certificates adds the storage needs of the certificate material to
the public key footprint. In order to alternatively issue multiple
public keys encoded inside a single certificate one would need a
standardized format if interoperability is a concern.
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5.2. Distributed Multi-trees (SP-800-208)
The [SP-800-208] also proposes creating multiple S-HBS keys across
multiple cryptographic modules using a distributed multi-tree
approach that is a variant of the standard hyper-tree based S-HBS
schemes HSS and XMSS^MT. In this approach trees are instantiated on
a root device (HSM_root), as well as one or more subordinate devices
(HSM_(sub[i])), and the root tree is used to sign the root nodes of
the subordinate trees to synthesize a multi-level S-HBS key. The
root device is only ever used to sign subordinate device root nodes,
while the subordinate device(s) is(are) used to sign messages. This
is relatively straightforward to do using HSS, and [SP-800-208]
describes the necessary algorithmic modifications when using XMSS^MT.
One drawback of this approach is the increased signature size as an
additional OTS needs to be generated, effectively doubling the
overall signature size. Another concern is the single point of
failure nature of relying on the root tree module to sign all of the
subordinate trees; if the root tree device fails then no new
subordinate trees can be signed. [SP-800-208] suggested that as many
subordinate trees as possible be generated during the initial root
key generation and subordinate-signing procedure. Unfortunately,
this can incur a large capital expenditure to procure all of the
necessary devices, many of which may not be used for a long period of
time, if at all. The subordinate tree root node signing process MUST
also be carefully managed to ensure top level trees are only ever
used to sign the root nodes of trusted/approved subordinate trees to
ensure that no malicious signing request is accepted, which would
effectively give a rogue entity the ability to generate valid
signatures, thereby undermining the security of the entire system.
The [SP-800-208] also suggests combining distributed multi-trees with
multiple root public keys as a means to mitigate some of the concerns
regarding having a single point of failure in the root tree.
However, even if a system operator does everything right, use cases
with exceptionally long lifetimes of 10-20+ years (e.g., automotive
and aerospace/satellite applications) will require system operators
to rely on devices well beyond their expected lifetimes of 5-10
years, which may constitute an unacceptable business risk.
5.3. Sectorization
Distributed multi-trees attempt to partition a S-HBS signing space
amongst multiple cryptographic modules by breaking up the signing
space along the boundaries of the subordinate trees generated during
the multi-tree key generation process. An alternative approach would
be to use only a single tree, and partition its signature space along
some power-of-2 less than the total number of leaves in the tree
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(e.g., 2^s for a tree of height h > s), creating N = 2^(h-s)
partitions or sectors, which are instantiated as N height-s Merkle
trees whose root nodes are considered interior nodes of the overall
height-h Merkle tree. Hence, there is no additional OTS required to
sign their root nodes; their values are used as-is in the underlying
S-HBS scheme's tree ascent mechanism, yielding a common public key
(i.e., root node) for all sectors. Care MUST be taken to ensure that
each sector uses the same root tree identifier (i.e., the "I" value
for HSS/LMS and "root" value for XMSS/XMSS^MT).
Each of the N sectors can be generated with a unique seed value to
cryptographically isolate the sectors from each other, though this
REQUIRES that the path information for each sector's root node (i.e.,
all off-path nodes between the sector root node and the top level
node value) be stored with each sector's private key at key
generation time since a sector will not know the seed information
required to compute any of the other sectors' root nodes during the
tree ascent phase of a signature generation operation. During
signature generation the signer appends the stored path information
to the path information it computes to ascend from the leaf OTS to
the sector's root node (which it can compute given that it knows its
own seed value).
Hence, sectorized key generation results in a single public key value
and 2^(h-s) private key values, each capable of generating 2^s
signatures, after which the sectorized key is exhausted.
In addition to avoiding an increased signature size; when unique
seeds are utilized sectorization breaks a given S-HBS key/state into
multiple independent fragments that can be managed as independent
objects. As a result, system operators MAY distribute sectors to
multiple cryptographic devices, allowing for performance scaling, and
resiliency/availability, while only requiring them to manage the
uniqueness of each sector instead of having to co-ordinate state
usage between devices since in this scenario a sector cannot generate
signatures from another sector's signature space.
5.4. Key/State Transfer
S-HBS key/state transfer between cryptographic modules entails having
a means to migrate one instance of a S-HBS key/state on a source
device to a separate destination device, while ensuring that any copy
of the key/state is deleted from the source device.
This capability may help alleviate the aforementioned concern
regarding operating devices beyond their expected lifetimes by
allowing operators to migrate S-HBS key/state to a newer device when
the original device begins to approach its end-of-life. However, it
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still leaves the operator vulnerable to having the source device fail
before the key/state can be transferred, effectively causing the loss
of the key/state. Hence, it will not be of much help addressing the
single point of failure issue identified for root trees, but may be
useful for managing subordinate trees.
A more elaborate variant of key transfer, going beyond what
[SP-800-208] allows, can be found described in Section 6 where key
transfer is accomplished using a two-step export and import process
with hash-based transfer validation to yield a more robust transfer
mechanism.
5.5. Key Rotation
Key rotation, such as that defined in [RFC8649], would generate new
S-HBS keys on an as-needed basis, and provide a means to transition
the system on to using this new S-HBS key, while generating the next
key in the chain in preparation of a future rotation/update.
However, this just shifts the problem to the PKI and certificate
handling.
Key rotation is not foolproof since in most use cases it will require
redundancy to ensure there is at least one S-HBS signing key
available to attest to newly generated keys. In addition, for many
applications the device keys cannot be updated due to engineering
constraints or security reasons.
5.6. Variable-length Signature Chains
A variant of the key rotation approach is to simply have an available
signing tree endorse a new subordinate tree when it is about to
become exhausted (e.g., use its final OTS to sign the root node of a
new subordinate tree, creating a {n+1}-layer multi-tree from a {n}-
layer multi-tree). This process can in theory be repeated as many
times as necessary. However, this entails having a multi-tree scheme
with a variable number of levels, and hence, variable length
signatures.
In addition to departing quite significantly from the current S-HBS
specifications and [SP-800-208], this approach has a number of
significant challenges on both the engineering and operational
fronts. Firstly, the variable length nature of the signature can
lead to variable length verification of signatures, which may cause
significant issues for use cases with strict time constraints (e.g.,
secure booting of a semiconductor device). From an operational
perspective, the ability of a subordinate tree to sign either
messages or new subordinate trees leads to severe security
implications as the rigor around authorizing those two types of
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operations will vary dramatically, leading to either a much more
onerous message signing operation, or a much more risky subordinate
tree signing operation. This may put the system operator in an
untenable situation where no users are satisfied with the resulting
solution, and hence, SHOULD NOT be considered as a viable solution.
5.7. Pre-assigning States
In some applications, individual one-time signatures (or states) can
be pre-assigned to the to-be-signed objects. This may for example be
possible if the signed objects are monotonically increasingly
numbered. One example of such a use case may be software signing.
This solution basically externalizes the state management to the to-
be signed messages.
Expanding on the given example, for software that is released with
strictly increasing, simple single-position version numbers (i.e.,
versions 1, 2, 3...), this can be trivially implemented. As versions
have a one-to-one correspondence to an S-HBS signing state, operators
MUST ensure that versions can only be minted a single time. This MAY
require skipping version numbers if a release process failed, to
avoid double-signing.
This scheme can be adapted to more complicated release schemes: for
example, minor update-releases 1.0 to 1.99 can be accommodated by
assigning signatures 1-100 for these version numbers, while release
2.0-2.99 would get signatures 101-200. The assignments MUST be fixed
as the scheme is set up, and operators SHOULD take into account that
they are strictly limiting the number of update releases. In the
described solution to state management, one MUST move up a major
release number after 99 minor releases, even if this would break,
e.g., semantic versioning conventions.
A variant of pre-assigning signatures is doing this on the basis of
time, which we describe in the next section.
5.8. Time-based State Management
As a variant of pre-assigning one-time signatures based on external
counters, it is in theory possible to base the selection of one-time
signature indexes on the current date and time. For example, if a
given S-HBS instance offers 1024 total signatures, they could be
broken up into 8 groups of 128 OTS instances each, with the first 128
allowed to be used in the first time window, the second 128 in the
second time window, and so on, until the signature space is
effectively exhausted after 8 time windows. Note that a time-based
approach to state management will "waste" any OTS keys that are
unused in past time windows. One MUST NOT attempt to use these keys
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after the time window has gone by.
Any time-based approach has a very strict reliance on accurate time-
keeping and synchronization of clocks. In particular, we identify
that at least the following engineering-related challenges need to be
considered:
* Signing devices MUST have accurate timekeeping (which is a very
challenging engineering problem [XKCD1883], [XKCD2867],
[TIMEFALSEHOODS]).
* Time on signing devices MUST NOT be allowed to ever move
backwards, as this can cause double-signing.
* Within time windows, signers MUST track the number of signatures
produced to ensure it does not exceed the number allowed within
the window.
* Signing devices MUST still operate consistently with the
requirements of state keeping for S-HBS: the signature index
within a time window SHOULD still appear to be updated atomically,
and signatures MUST NOT be released before state changes have been
recorded.
* A system SHOULD be robust against exhaustion of the number of
signatures available in a time window, as in this case it is
REQUIRED to wait until the next time window starts before new
messages can be signed.
* Time on signing devices SHOULD NOT be allowed to be moved forward
maliciously or accidentally, which would allow for a simple
denial-of-service attack by skipping over portions of the
signature space.
* If a signing device needs to be replaced, the replacement device
MUST be set up with its time in sync with or ahead of the device
it is to replace. This implies the current time on signing
devices SHOULD be continuously recorded.
* Rate limiting MAY need to be considered, as exhausting the
available signatures in a given time window may otherwise be easy.
* It MAY be necessary for signers to keep a separate clock for time-
based state management, and one for not necessarily monotonically
increasing "wall-time", e.g., if signed artifacts are expected to
be time-stamped with real-world time.
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If these concerns can not be sufficiently addressed, time-based state
management as described in this paragraph SHOULD NOT be used. Note
that this list of concerns is not exhaustive, and other, unmentioned,
concerns may also be relevant to the security of a time-based
solution.
Time-based systems can be backed up by simply recording the private
keys and the configuration of the time windows. In case of loss of a
signing device, a time-based state management system can be recovered
by using this information to bring online a new device in the next
time window. This approach MAY also be used as a recovery mechanism
in the case of (suspected) state consistency problems during a time
window. However, the operator MUST NOT allow new signatures to be
produced before the new time window starts, unless they know the
exact state at which the previous device became unavailable and are
able to set up the new device accordingly. Waiting until the start
of the next time window avoids double signing, as the OTS keys
assigned to future time windows are guaranteed to have not yet been
used. However, this might incur significant downtime of the signing
systems. Downtime MAY be avoided by forcibly moving the signing
device to the next time window by incrementing its clock; however,
this induced clock drift will then need to be accounted for in the
future. If clock drift is to be avoided, this approach SHOULD
account for availability considerations.
6. Backup management beyond NIST SP-800-208
In this section, an alternative backup mechanism for S-HBS is
presented in a generic form, which makes the strategy applicable for
both multi-tree instances XMSS^MT and HSS. However, following the
same arguments as in Section 5.3, with minor modifications, the
presented strategy is also applicable for single-tree instances such
as XMSS and LMS.
The strategy presented in this section builds upon the multi-tree
variant approach from [SP-800-208], and aims to mitigate its
limitations described in Section 5.2. Thus, it is assumed that
already a top-level Merkle tree (for signing the root-nodes of sub-
trees) and several bottom-level Merkle trees (for signing messages)
are initiated. These bottom-level trees MAY be implemented on
different hardware modules in order to obtain redundancy and improve
availability. Let R be the number of these already initiated bottom-
level trees. Let h_0 be the height of the top-level-tree. It is
assumed that R + 1 is strictly smaller than 2^(h_0), the number of
leaves of the top-level tree.
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In this new strategy, after the completed key generation procedure
from the multi-tree variant approach from [SP-800-208], further
bottom-level trees are generated, one by one, in one of the hardware
modules. These new bottom-level trees are each generated from a
different seed, which is chosen uniformly at random. For the sake of
clarity, let us introduce some notation:
* S denotes the number of these newly generated bottom-level trees.
Note that at most 2^(h_0) - R new bottom-level trees can be
generated, i.e. S is lower or equal to 2^(h_0) - R. In the
following we suppose that S is _strictly smaller_ than 2^(h_0) -
R.
* I_new denotes the set of indices that belong to these newly
generated bottom-level trees, i.e. I_new = {R, R+1, ..., R+S-1}.
For each new bottom-level tree, after it has been generated, the
following steps MUST be performed:
* sign the corresponding root node with an unused OTS key from the
top-level tree,
* securely _key export_ (as decribed in Section 2.1.3) the seed,
which was used to generate the bottom-level tree,
* export the signature of the root node, the corresponding OTS key
index and finally the hash of the seed, using appropriate domain
separation (i.e. ensuring there is no domain overlap with the
hashes in the S-HBS scheme, and the hash of the seed includes the
public key and leaf index to mitigate multi-target attacks),
* irreversibly delete the seed and the bottom-level tree from the
hardware module.
The newly generated bottom-level trees (i.e. those bottom-level
trees, whose indices belong to I_new) are only used in order to
guarantee availability in the _worst-case scenario_, where at the
same time both
* none of the R bottom-level Merkle trees (which were generated
according to the multi-tree variant approach from [SP-800-208])
are available for signing messages and
* the top-level Merkle tree (which is used for signing the root-
nodes of sub-trees) is also not available any more.
This scenario may, for example, happen if all hardware modules are
broken at the same time.
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As soon as this worst-case scenario occurs, the newly generated
bottom-level trees (i.e. those bottom-level trees, whose indices
belong to I_new) need to be initiated in order to ensure
availability. In order to do this the following steps MUST be
performed:
* initiate a new hardware module
* securely _key import_ (as decribed in Section 2.1.3) the first
unused seed into this hardware module
* generate the bottom-level tree corresponding to the seed
* irreversibly delete the seed from the backup medium
* perform a correctness check by letting the hardware module output
the hash of the seed
Now this bottom-level tree can be used to sign messages. As soon as
no more OTS on the bottom-level tree are available or as soon as the
hardware module is broken, the above steps with a new seed from the
backup medium can be repeated.
Note that the resulting signatures generated from these backed up
seeds do not require any special processing on the verifier side.
The signature stored alongside the backed up seed, and the signature
generated from the bottom-level trees created from the backed up seed
can be combined to match the format of a signature over the complete
tree.
7. Security Considerations
Further security considerations, which are not already covered in
this document, are given in [SP-800-208], [MCGREW], [FIPS205],
[RFC8391] and [RFC8554].
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. 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>.
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[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>.
9.2. Informative References
[BH16] Bruinderink, L. and A. Hülsing, "Oops, I did it again –
Security of One-Time Signatures under Two-Message
Attacks.", 2016, <https://eprint.iacr.org/2016/1042.pdf>.
[BSW16] Bellare, M., Stepanovss, I., and B. Waters, "New Negative
Results on Differing-Inputs Obfuscation", n.d.,
<https://link.springer.com/
chapter/10.1007/978-3-662-49896-5_28>.
[ETSI-TR-103-692]
European Telecommunications Standards Institute (ETSI),
"State management for stateful authentication mechanisms",
November 2021, <https://www.etsi.org/deliver/
etsi_tr/103600_103699/103692/01.01.01_60/
tr_103692v010101p.pdf>.
[FIPS205] National Institute of Standards and Technology, "FIPS 205
(draft): Stateless Hash-Based Digital Signature Standard",
Federal Information Processing Standards , 24 August 2023,
<https://doi.org/10.6028/NIST.FIPS.205.ipd>.
[HBSX509] Bashiri, K., Fluhrer, S., Gazdag, S., Van Geest, D., and
S. Kousidis, "Internet X.509 Public Key Infrastructure:
Algorithm Identifiers for Hash-based Signatures", n.d.,
<https://www.ietf.org/archive/id/draft-gazdag-x509-hash-
sigs-02.html>.
[MCGREW] McGrew, D., Kampanakis, P., Fluhrer, S., Gazdag, S.,
Butin, D., and J. Buchmann, "State Management for Hash-
Based Signatures", 2 November 2016,
<https://tubiblio.ulb.tu-darmstadt.de/id/eprint/101633>.
[RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
RFC 8391, DOI 10.17487/RFC8391, May 2018,
<https://www.rfc-editor.org/rfc/rfc8391>.
[RFC8411] Schaad, J. and R. Andrews, "IANA Registration for the
Cryptographic Algorithm Object Identifier Range",
RFC 8411, DOI 10.17487/RFC8411, August 2018,
<https://www.rfc-editor.org/rfc/rfc8411>.
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[RFC8554] McGrew, D., Curcio, M., and S. Fluhrer, "Leighton-Micali
Hash-Based Signatures", RFC 8554, DOI 10.17487/RFC8554,
April 2019, <https://www.rfc-editor.org/rfc/rfc8554>.
[RFC8649] Housley, R., "Hash Of Root Key Certificate Extension",
RFC 8649, DOI 10.17487/RFC8649, August 2019,
<https://www.rfc-editor.org/rfc/rfc8649>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://www.rfc-editor.org/rfc/rfc9162>.
[SP-800-208]
Cooper, D., Apon, D., Dang, Q., Davidson, M., Dworkin, M.,
and C. Miller, "Recommendation for Stateful Hash-Based
Signature Schemes", NIST Special Publication 800-208 ,
October 2020, <https://doi.org/10.6028/NIST.SP.800-208>.
[TIMEFALSEHOODS]
Visée, T., "Falsehoods programmers believe about time",
n.d., <https://gist.github.com/timvisee/
fcda9bbdff88d45cc9061606b4b923ca>.
[XKCD1883] Munroe, R., "xkcd: Timezones", n.d.,
<https://xkcd.com/1883/>.
[XKCD2867] Munroe, R., "xkcd: DateTime", n.d.,
<https://xkcd.com/2867/>.
Acknowledgments
This document was inspired by discussions at the 2nd Oxford Post-
Quantum Cryptography Summit 2023.
We gratefully acknowledge Melissa Azouaoui for her input to this
document.
The abstract and the introduction are based on the introduction in
[HBSX509]. Thanks go to the authors of this document. "Copying
always makes things easier and less error-prone" - [RFC8411].
Contributors
* Jeff Andersen (Google)
* Bruno Couillard (Crypto4A Technologies)
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Authors' Addresses
Thom Wiggers
PQShield
Email: thom@thomwiggers.nl
Kaveh Bashiri
BSI
Germany
Email: kaveh.bashiri.ietf@gmail.com
Stefan Kölbl
Google
Switzerland
Email: kste@google.com
Jim Goodman
Crypto4A Technologies
Canada
Email: jimg@crypto4a.com
Stavros Kousidis
BSI
Germany
Email: kousidis.ietf@gmail.com
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