INTERNET-DRAFT R. Housley
Internet Engineering Task Force (IETF) Vigil Security
Intended Status: Proposed Standard
Expires: 11 November 2019 10 May 2019
Use of the HSS/LMS Hash-based Signature Algorithm
in the Cryptographic Message Syntax (CMS)
Abstract
This document specifies the conventions for using the the HSS/LMS
hash-based signature algorithm with the Cryptographic Message Syntax
(CMS). In addition, the algorithm identifier and public key syntax
are provided. The HSS/LMS algorithm is one form of hash-based
digital signature; it is described in RFC 8554.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. ASN.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Algorithm Considerations . . . . . . . . . . . . . . . . . 3
2. HSS/LMS Hash-based Signature Algorithm Overview . . . . . . . 4
2.1. Hierarchical Signature System (HSS) . . . . . . . . . . . 4
2.2. Leighton-Micali Signature (LMS) . . . . . . . . . . . . . 5
2.3. Leighton-Micali One-time Signature Algorithm (LM-OTS) . . 6
3. Algorithm Identifiers and Parameters . . . . . . . . . . . . . 7
4. HSS/LMS Public Key Identifier . . . . . . . . . . . . . . . . 8
5. Signed-data Conventions . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix: ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
This document specifies the conventions for using the HSS/LMS hash-
based signature algorithm with the Cryptographic Message Syntax (CMS)
[CMS] signed-data content type. The Leighton-Micali Signature (LMS)
system provides a one-time digital signature that is a variant of
Merkle Tree Signatures (MTS). The Hierarchical Signature System
(HSS) is built on top of the LMS system to efficiently scale for a
larger numbers of signatures. The HSS/LMS algorithm is one form of
hash-based digital signature, and it is described in [HASHSIG]. The
HSS/LMS signature algorithm can only be used for a fixed number of
signing operations. The number of signing operations depends upon
the size of the tree. The HSS/LMS signature algorithm uses small
public keys, and it has low computational cost; however, the
signatures are quite large. The HSS/LMS private key can be very
small when the signer is willing to perform additional computation at
signing time; alternatively, the private key can consume additional
memory and provide a faster signing time.
1.1. ASN.1
CMS values are generated using ASN.1 [ASN1-B], using the Basic
Encoding Rules (BER) and the Distinguished Encoding Rules (DER)
[ASN1-E].
1.2. Terminology
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.
1.3. Algorithm Considerations
There have been recent advances in cryptanalysis and advances in the
development of quantum computers. Each of these advances pose a
threat to widely deployed digital signature algorithms.
At Black Hat USA 2013, some researchers gave a presentation on the
current state of public key cryptography. They said: "Current
cryptosystems depend on discrete logarithm and factoring which has
seen some major new developments in the past 6 months" [BH2013]. Due
to advances in cryptanalysis, they encouraged preparation for a day
when RSA and DSA cannot be depended upon.
If large-scale quantum computers are ever built, these computers will
be able to break many of the public-key cryptosystems currently in
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use. A post-quantum cryptosystem [PQC] is a system that is secure
against quantum computers that have more than a trivial number of
quantum bits (qu-bits). It is open to conjecture when it will be
feasible to build such computers; however, RSA, DSA, ECDSA, and EdDSA
are all vulnerable if large-scale quantum computers come to pass.
The HSS/LMS signature algorithm does not depend on the difficulty of
discrete logarithm or factoring, as a result these algorithms are
considered to be post-quantum secure.
Hash-based signatures [HASHSIG] are currently defined to use
exclusively SHA-256 [SHS]. An IANA registry is defined so that other
hash functions could be used in the future. LM-OTS signature
generation prepends a random string as well as other metadata before
computing the hash value. The inclusion of the random value reduces
the chances of an attacker being able to find collisions, even if the
attacker has a large-scale quantum computer.
Today, RSA is often used to digitally sign software updates. This
means that the distribution of software updates could be compromised
if a significant advance is made in factoring or a large-scale
quantum computer is invented. The use of HSS/LMS hash-based
signatures to protect software update distribution, perhaps using the
format described in [FWPROT], will allow the deployment of software
that implements new cryptosystems.
2. HSS/LMS Hash-based Signature Algorithm Overview
Merkle Tree Signatures (MTS) are a method for signing a large but
fixed number of messages. An MTS system depends on a one-time
signature method and a collision-resistant hash function.
This specification makes use of the hash-based algorithm specified in
[HASHSIG], which is the Leighton and Micali adaptation [LM] of the
original Lamport-Diffie-Winternitz-Merkle one-time signature system
[M1979][M1987][M1989a][M1989b].
As implied by the name, the hash-based signature algorithm depends on
a collision-resistant hash function. The hash-based signature
algorithm specified in [HASHSIG] currently uses only the SHA-256 one-
way hash function [SHS], but it also establishes an IANA registry to
permit the registration of additional one-way hash functions in the
future.
2.1. Hierarchical Signature System (HSS)
The MTS system specified in [HASHSIG] uses a hierarchy of trees. The
Hierarchical N-time Signature System (HSS) allows subordinate trees
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to be generated when needed by the signer. Otherwise, generation of
the entire tree might take weeks or longer.
An HSS signature as specified in [HASHSIG] carries the number of
signed public keys (Nspk), followed by that number of signed public
keys, followed by the LMS signature as described in Section 2.2. The
public key for the top-most LMS tree is the public key of the HSS
system. The LMS private key in the parent tree signs the LMS public
key in the child tree, and the LMS private key in the bottom-most
tree signs the actual message. The signature over the public key and
the signature over the actual message are LMS signatures as described
in Section 2.2.
The elements of the HSS signature value for a stand-alone tree (a top
tree with no children) can be summarized as:
u32str(0) ||
lms_signature /* signature of message */
The elements of the HSS signature value for a tree with Nspk signed
public keys can be summarized as:
u32str(Nspk) ||
signed_public_key[0] ||
signed_public_key[1] ||
...
signed_public_key[Nspk-2] ||
signed_public_key[Nspk-1] ||
lms_signature /* signature of message */
where, as defined in Section 3.3 of [HASHSIG], a signed_public_key is
the lms_signature over the public key followed by the public key
itself. Note that Nspk is the number of levels in the hierarchy of
trees minus 1.
2.2. Leighton-Micali Signature (LMS)
Each tree in the system specified in [HASHSIG] uses the Leighton-
Micali Signature (LMS) system. LMS systems have two parameters. The
first parameter is the height of the tree, h, which is the number of
levels in the tree minus one. The [HASHSIG] specification supports
five values for this parameter: h=5; h=10; h=15; h=20; and h=25.
Note that there are 2^h leaves in the tree. The second parameter is
the number of bytes output by the hash function, m, which is the
amount of data associated with each node in the tree. The [HASHSIG]
specification supports only the SHA-256 hash function [SHS], with
m=32.
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The [HASHSIG] specification supports five tree sizes:
LMS_SHA256_M32_H5;
LMS_SHA256_M32_H10;
LMS_SHA256_M32_H15;
LMS_SHA256_M32_H20; and
LMS_SHA256_M32_H25.
The [HASHSIG] specification establishes an IANA registry to permit
the registration of additional hash functions and additional tree
sizes in the future.
The LMS public key is the string consists of four elements: the
lms_algorithm_type from the list above, the otstype to identify the
LM-OTS type as discussed in Section 2.3, the private key identifier
(I) as described in Section 5.3 of [HASHSIG], and the m-byte string
associated with the root node of the tree.
The LMS public key can be summarized as:
u32str(lms_algorithm_type) || u32str(otstype) || I || T[1]
An LMS signature consists of four elements: the number of the leaf
(q) associated with the LM-OTS signature, an LM-OTS signature as
described in Section 2.3, a typecode indicating the particular LMS
algorithm, and an array of values that is associated with the path
through the tree from the leaf associated with the LM-OTS signature
to the root. The array of values contains the siblings of the nodes
on the path from the leaf to the root but does not contain the nodes
on the path itself. The array for a tree with height h will have h
values. The first value is the sibling of the leaf, the next value
is the sibling of the parent of the leaf, and so on up the path to
the root.
The four elements of the LMS signature value can be summarized as:
u32str(q) ||
ots_signature ||
u32str(type) ||
path[0] || path[1] || ... || path[h-1]
2.3. Leighton-Micali One-time Signature Algorithm (LM-OTS)
Merkle Tree Signatures (MTS) depend on a one-time signature method.
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[HASHSIG] specifies the use of the LM-OTS. An LM-OTS has five
parameters.
n - The number of bytes associated with the hash function.
[HASHSIG] supports only SHA-256 [SHS], with n=32.
H - A preimage-resistant hash function that accepts byte strings
of any length, and returns an n-byte string.
w - The width in bits of the Winternitz coefficients. [HASHSIG]
supports four values for this parameter: w=1; w=2; w=4; and
w=8.
p - The number of n-byte string elements that make up the LM-OTS
signature.
ls - The number of left-shift bits used in the checksum function,
which is defined in Section 4.4 of [HASHSIG].
The values of p and ls are dependent on the choices of the parameters
n and w, as described in Appendix B of [HASHSIG].
The [HASHSIG] specification supports four LM-OTS variants:
LMOTS_SHA256_N32_W1;
LMOTS_SHA256_N32_W2;
LMOTS_SHA256_N32_W4; and
LMOTS_SHA256_N32_W8.
The [HASHSIG] specification establishes an IANA registry to permit
the registration of additional variants in the future.
Signing involves the generation of C, an n-byte random value.
The LM-OTS signature value can be summarized as the identifier of the
LM-OTS variant, the random value, and a sequence of hash values that
correspond to the elements of the public key as described in Section
4.5 of [HASHSIG]:
u32str(otstype) || C || y[0] || ... || y[p-1]
3. Algorithm Identifiers and Parameters
The algorithm identifier for an HSS/LMS hash-based signatures is:
id-alg-hss-lms-hashsig OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) alg(3) 17 }
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When this object identifier is used for a HSS/LMS signature, the
AlgorithmIdentifier parameters field MUST be absent (that is, the
parameters are not present; the parameters are not set to NULL).
The signature value is a large OCTET STRING. The signature format is
designed for easy parsing. Each format includes a counter and type
codes that indirectly providing all of the information that is needed
to parse the value during signature validation.
The signature value identifies the hash function used in the HSS/LMS
tree. In [HASHSIG] only the SHA-256 hash function [SHS] is
supported, but it also establishes an IANA registry to permit the
registration of additional hash functions in the future.
4. HSS/LMS Public Key Identifier
The AlgorithmIdentifier for an HSS/LMS public key uses the id-alg-
hss-lms-hashsig object identifier, and the parameters field MUST be
absent.
When this AlgorithmIdentifier appears in the SubjectPublicKeyInfo
field of an X.509 certificate [RFC5280], the certificate key usage
extension MAY contain digitalSignature, nonRepudiation, keyCertSign,
and cRLSign; however, it MUST NOT contain other values.
pk-HSS-LMS-HashSig PUBLIC-KEY ::= {
IDENTIFIER id-alg-hss-lms-hashsig
KEY HSS-LMS-HashSig-PublicKey
PARAMS ARE absent
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign } }
HSS-LMS-HashSig-PublicKey ::= OCTET STRING
Note that the id-alg-hss-lms-hashsig algorithm identifier is also
referred to as id-alg-mts-hashsig. This synonym is based on the
terminology used in an early draft of the document that became
[HASHSIG].
The public key value is an OCTET STRING. Like the signature format,
it is designed for easy parsing. The value is the number of levels
in the public key, L, followed by the LMS public key.
The HSS/LMS public key value can be summarized as:
u32str(L) || lms_public_key
Note that the public key for the top-most LMS tree is the public key
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of the HSS system. When L=1, the HSS system is a single tree.
5. Signed-data Conventions
As specified in [CMS], the digital signature is produced from the
message digest and the signer's private key. The signature is
computed over different value depending on whether signed attributes
are absent or present. When signed attributes are absent, the
HSS/LMS signature is computed over the content. When signed
attributes are present, a hash is computed over the content using the
same hash function that is used in the HSS/LMS tree, and then a
message-digest attribute is constructed with the resulting hash
value, and then DER encode the set of signed attributes, which MUST
include a content-type attribute and a message-digest attribute, and
then the HSS/LMS signature is computed over the output of the DER-
encode operation. In summary:
IF (signed attributes are absent)
THEN HSS_LMS_Sign(content)
ELSE message-digest attribute = Hash(content);
HSS_LMS_Sign(DER(SignedAttributes))
When using [HASHSIG], the fields in the SignerInfo are used as
follows:
digestAlgorithm MUST contain the one-way hash function used to in
the HSS/LMS tree. In [HASHSIG], SHA-256 is the only supported
hash function, but other hash functions might be registered in
the future. For convenience, the AlgorithmIdentifier for
SHA-256 from [PKIXASN1] is repeated here:
mda-sha256 DIGEST-ALGORITHM ::= {
IDENTIFIER id-sha256
PARAMS TYPE NULL ARE preferredAbsent }
id-sha256 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
country(16) us(840) organization(1) gov(101) csor(3)
nistAlgorithms(4) hashalgs(2) 1 }
signatureAlgorithm MUST contain id-alg-hss-lms-hashsig, and the
algorithm parameters field MUST be absent.
signature contains the single HSS signature value resulting from
the signing operation as specified in [HASHSIG].
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6. Security Considerations
Implementations MUST protect the private keys. Compromise of the
private keys may result in the ability to forge signatures. Along
with the private key, the implementation MUST keep track of which
leaf nodes in the tree have been used. Loss of integrity of this
tracking data can cause an one-time key to be used more than once.
As a result, when a private key and the tracking data are stored on
non-volatile media or stored in a virtual machine environment, care
must be taken to preserve confidentiality and integrity.
When generating a LMS key pair, an implementation MUST generate each
key pair independently of all other key pairs in the HSS tree.
An implementation MUST ensure that a LM-OTS private key is used to
generate a signature only one time, and ensure that it cannot be used
for any other purpose.
The generation of private keys relies on random numbers. The use of
inadequate pseudo-random number generators (PRNGs) to generate these
values can result in little or no security. An attacker may find it
much easier to reproduce the PRNG environment that produced the keys,
searching the resulting small set of possibilities, rather than brute
force searching the whole key space. The generation of quality
random numbers is difficult, and [RFC4086] offers important guidance
in this area.
The generation of hash-based signatures also depends on random
numbers. While the consequences of an inadequate pseudo-random
number generator (PRNGs) to generate these values is much less severe
than the generation of private keys, the guidance in [RFC4086]
remains important.
When computing signatures, the same hash function SHOULD be used to
compute the message digest of the content and the signed attributes,
if they are present.
7. IANA Considerations
SMI Security for S/MIME Module Identifier (1.2.840.113549.1.9.16.0)
registry, change the reference for value 64 to point to this
document.
In the SMI Security for S/MIME Algorithms (1.2.840.113549.1.9.16.3)
registry, change the description for value 17 to
"id-alg-hss-lms-hashsig" and change the reference to point to this
document.
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Also, add the following note to the registry:
Value 17, "id-alg-hss-lms-hashsig", is also referred to as
"id-alg-mts-hashsig".
8. References
8.1. Normative References
[ASN1-B] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, 2015.
[ASN1-E] ITU-T, "Information technology -- ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, 2015.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
.
[HASHSIG] McGrew, D., Curcio, M., and S. Fluhrer, "Leighton-Micali
Hash-Based Signatures", RFC 8554, April 2019,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, .
[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,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
RFC 2119 Key Words", BCP 14, RFC 8174, DOI
10.17487/RFC8174, May 2017, .
[SHS] National Institute of Standards and Technology (NIST),
FIPS Publication 180-3: Secure Hash Standard, October
2008.
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8.2. Informative References
[BH2013] Ptacek, T., T. Ritter, J. Samuel, and A. Stamos, "The
Factoring Dead: Preparing for the Cryptopocalypse", August
2013.
[CMSASN1] Hoffman, P. and J. Schaad, "New ASN.1 Modules for
Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
DOI 10.17487/RFC5911, June 2010, .
[CMSASN1U] Schaad, J. and S. Turner, "Additional New ASN.1 Modules
for the Cryptographic Message Syntax (CMS) and the Public
Key Infrastructure Using X.509 (PKIX)", RFC 6268, DOI
10.17487/RFC6268, July 2011, .
[FWPROT] Housley, R., "Using Cryptographic Message Syntax (CMS) to
Protect Firmware Packages", RFC 4108, DOI
10.17487/RFC4108, August 2005, .
[LM] Leighton, T. and S. Micali, "Large provably fast and
secure digital signature schemes from secure hash
functions", U.S. Patent 5,432,852, July 1995.
[M1979] Merkle, R., "Secrecy, Authentication, and Public Key
Systems", Stanford University Information Systems
Laboratory Technical Report 1979-1, 1979.
[M1987] Merkle, R., "A Digital Signature Based on a Conventional
Encryption Function", Lecture Notes in Computer Science
crypto87, 1988.
[M1989a] Merkle, R., "A Certified Digital Signature", Lecture Notes
in Computer Science crypto89, 1990.
[M1989b] Merkle, R., "One Way Hash Functions and DES", Lecture Notes
in Computer Science crypto89, 1990.
[PKIXASN1] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
DOI 10.17487/RFC5912, June 2010, .
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[PQC] Bernstein, D., "Introduction to post-quantum
cryptography", 2009.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, .
Appendix: ASN.1 Module
```
MTS-HashSig-2013
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
id-smime(16) id-mod(0) id-mod-mts-hashsig-2013(64) }
DEFINITIONS IMPLICIT TAGS ::= BEGIN
EXPORTS ALL;
IMPORTS
PUBLIC-KEY, SIGNATURE-ALGORITHM, SMIME-CAPS
FROM AlgorithmInformation-2009 -- RFC 5911 [CMSASN1]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) } ;
--
-- Object Identifiers
--
id-alg-hss-lms-hashsig OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) alg(3) 17 }
id-alg-mts-hashsig OBJECT IDENTIFIER ::= id-alg-hss-lms-hashsig
--
-- Signature Algorithm and Public Key
--
sa-HSS-LMS-HashSig SIGNATURE-ALGORITHM ::= {
IDENTIFIER id-alg-hss-lms-hashsig
PARAMS ARE absent
PUBLIC-KEYS { pk-HSS-LMS-HashSig }
SMIME-CAPS { IDENTIFIED BY id-alg-hss-lms-hashsig } }
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pk-HSS-LMS-HashSig PUBLIC-KEY ::= {
IDENTIFIER id-alg-hss-lms-hashsig
KEY HSS-LMS-HashSig-PublicKey
PARAMS ARE absent
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign } }
HSS-LMS-HashSig-PublicKey ::= OCTET STRING
--
-- Expand the signature algorithm set used by CMS [CMSASN1U]
--
SignatureAlgorithmSet SIGNATURE-ALGORITHM ::=
{ sa-HSS-LMS-HashSig, ... }
--
-- Expand the S/MIME capabilities set used by CMS [CMSASN1]
--
SMimeCaps SMIME-CAPS ::=
{ sa-HSS-LMS-HashSig.&smimeCaps, ... }
END
``````
Acknowledgements
Many thanks to Scott Fluhrer, Jonathan Hammell, Panos Kampanakis,
John Mattsson, Jim Schaad, Sean Turner, and Daniel Van Geest for
their careful review and comments.
Author's Address
Russ Housley
Vigil Security, LLC
516 Dranesville Road
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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```