INTERNET-DRAFT R. Housley
Intended Status: Proposed Standard Vigil Security
Expires: 21 September 2016 21 March 2016
Use of the Hash-based Merkle Tree Signature (MTS) Algorithm
in the Cryptographic Message Syntax (CMS)
<draft-housley-cms-mts-hash-sig-04>
Abstract
This document specifies the conventions for using the Merkle Tree
Signatures (MTS) digital signature algorithm with the Cryptographic
Message Syntax (CMS). The MTS algorithm is one form of hash-based
digital signature.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. MTS Digital Signature Algorithm . . . . . . . . . . . . . 3
1.2. LM-OTS One-time Signature Algorithm . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Algorithm Identifiers and Parameters . . . . . . . . . . . . . 4
3. Signed-data Conventions . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4.1. Implementation Security Considerations . . . . . . . . . . 6
4.2. Algorithm Security Considerations . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . . 7
7. Informative References . . . . . . . . . . . . . . . . . . . . 7
Appendix: ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
This document specifies the conventions for using the Merkle Tree
Signatures (MTS) digital signature algorithm with the Cryptographic
Message Syntax (CMS) [CMS] signed-data content type. The MTS
algorithm is one form of hash-based digital signature that can only
be used for a fixed number of signatures. The MTS algorithm is
described in [HASHSIG]. The MTS algorithm uses small private and
public keys, and it has low computational cost; however, the
signatures are quite large.
CMS values are generated using ASN.1 [ASN1-02], using the Basic
Encoding Rules (BER) and the Distinguished Encoding Rules (DER).
1.1. MTS Digital Signature Algorithm
Merkle Tree Signatures (MTS) are a method for signing a large but
fixed number of messages. An MTS system is an N-time signature
system, meaning that the private key can be used to generate at most
N signatures.
An MTS system uses two cryptographic components: a one-time signature
method and a collision-resistant hash function. Each MTS
public/private key pair is associated with a k-way tree. Each leaf
of the tree can be used to generate a one-time signature (OTS), which
can be used to securely sign exactly one message, but cannot securely
sign more than one.
This specification makes use of the MTS 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]. It makes use of the LM-OTS one-time
signature scheme and the SHA-256 [SHS] one-way hash function.
An LMS system has two parameters. The height of the tree, h, which
is the number of levels in the tree minus one. The [HASHSIG]
specification supports three values for this parameter: h=20; h=10;
and h=5. The number of bytes associated with each node in the tree,
n, is defined by the hash function. The [HASHSIG] specification
supports two hash functions: SHA-256 [SHS], with n=32; and
SHA-256-16, which is the same as SHA-256, except that the hash result
is truncated to 16 bytes, with n=16. Note that there are 2^h leaves
in the tree.
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Six tree sizes are specified in [HASHSIG]:
lms_sha256_n32_h20;
lms_sha256_n32_h10;
lms_sha256_n32_h5;
lms_sha256_n16_h20;
lms_sha256_n16_h10; and
lms_sha256_n16_h5.
An LMS signature consists of three things: a typecode indicating the
particular LMS algorithm, an LM-OTS signature, 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.
1.2. LM-OTS One-time Signature Algorithm
Merkle Tree Signatures (MTS) depend on a LM-OTS one-time signature
method. An LM-OTS has four parameters.
n - The number of bytes associated with the hash function, which
is the same as the LMS parameter. The [HASHSIG]
specification supports two hash functions: SHA-256 [SHS],
with n=32; and SHA-256-16, with n=16.
w - The the Winternitz parameter. The [HASHSIG] specification
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.
The values of p and ls are dependent on the choices of the parameters
n and w, as described in Appendix A of [HASHSIG].
Eight LM-OTS variants are defined in [HASHSIG]:
LMOTS_SHA256_N32_W1;
LMOTS_SHA256_N32_W2;
LMOTS_SHA256_N32_W4;
LMOTS_SHA256_N32_W8;
LMOTS_SHA256_N16_W1;
LMOTS_SHA256_N16_W2;
LMOTS_SHA256_N16_W4; and
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LMOTS_SHA256_N16_W8.
1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [KEYWORDS].
2. Algorithm Identifiers and Parameters
The algorithm identifier for an MTS signature is id-alg-mts-hashsig:
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }
id-alg OBJECT IDENTIFIER ::= { id-smime 3 }
id-alg-mts-hashsig OBJECT IDENTIFIER ::= { id-alg 17 }
When the id-alg-mts-hashsig algorithm identifier is used for a
signature, the AlgorithmIdentifier parameters field MUST be absent.
The first 4 bytes of the signature value contains the
mls_algorithm_type as defined in Section 5.5 of [HASHSIG]. This type
tells how to parse the remaining parts of the signature value, which
is composed of an LM-OTS signature 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 first 4 bytes of the LM-OTS signature value contains the
ots_algorithm_type as defined in Section 4.10 of [HASHSIG]. This
type is followed by n*p bytes of signature value.
The signature format is designed for easy parsing. Each format
starts with a 4-byte enumeration value that indicates all of the
details of the signature algorithm, indirectly providing all of the
information that is needed to parse the value during signature
validation.
3. Signed-data Conventions
digestAlgorithms SHOULD contain the one-way hash function used to
compute the message digest on the eContent value. Since the hash-
based signature algorithms all depend on SHA-256, it is strongly
RECOMMENDED that SHA-256 also be used to compute the message digest
on the content.
Further, the same one-way hash function SHOULD be used to compute the
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message digest on both the eContent and the signedAttributes value if
signedAttributes exist. Again, since the hash-based signature
algorithms all depend on SHA-256, it is strongly RECOMMENDED that
SHA-256 be used.
signatureAlgorithm MUST contain id-alg-mts-hashsig. The algorithm
parameters field MUST be absent.
signature contains the single value resulting from the signing
operation as specified in [HASHSIG].
4. Security Considerations
4.1. Implementation 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 maintain a counter
value that indicates which leaf nodes in the tree have been used.
Loss of integrity of this counter can cause an one-time key to be
used more than once. As a result, when a private key and an
associated counter value are stored on non-volatile media or stored
in a virtual machine environment, care must be taken to preserve
these properties.
An implementation must ensure that a LDWM private key is used only
one time, and ensure that the LDWM private key 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. RFC 4086 [RANDOM] offers important
guidance in this area.
When computing signatures, the same hash function SHOULD be used for
all operations. This reduces the number of failure points in the
signature process.
4.2. Algorithm Security Considerations
At Black Hat USA 2013, some researchers gave a presentation on the
current sate 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].
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They encouraged preparation for a day when RSA and DSA cannot be
depended upon.
A post-quantum cryptosystem is a system that is secure against
quantum computers that have more than a trivial number of quantum
bits. It is open to conjecture whether it is feasible to build such
a machine. RSA, DSA, and ECDSA are not post-quantum secure.
The LM-OTP one-time signature and LMS do not depend on discrete
logarithm or factoring, and these algorithms are considered to be
post-quantum secure.
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 quantum computer
is invented. The use of MTS signatures to protect software update
distribution, perhaps using the format described in [FWPROT], will
allow the deployment of software that implements new cryptosystems.
5. IANA Considerations
{{ RFC Editor: Please remove this section prior to publication. }}
This document has no actions for IANA.
6. Normative References
[ASN1-02] ITU-T, "ITU-T Recommendation X.680, X.681, X.682, and
X.683", ITU-T X.680, X.681, X.682, and X.683, 2002.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<http://www.rfc-editor.org/info/rfc5652>.
[HASHSIG] McGrew, D., and M. Curcio, "Hash-Based Signatures", Work
in progress. <draft-mcgrew-hash-sigs-03>
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
[SHS] National Institute of Standards and Technology (NIST),
FIPS Publication 180-3: Secure Hash Standard, October
2008.
7. Informative References
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[BH2013] Ptacek, T., T. Ritter, J. Samuel, and A. Stamos, "The
Factoring Dead: Preparing for the Cryptopocalypse", August
2013. <https://media.blackhat.com/us-13/us-13-Stamos-The-
Factoring-Dead.pdf>
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[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, <http://www.rfc-
editor.org/info/rfc5911>.
[FWPROT] Housley, R., "Using Cryptographic Message Syntax (CMS) to
Protect Firmware Packages", RFC 4108, DOI
10.17487/RFC4108, August 2005, <http://www.rfc-
editor.org/info/rfc4108>.
[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, <http://www.rfc-
editor.org/info/rfc5912>.
[PQC] Bernstein, D., "Introduction to post-quantum
cryptography", 2009.
<http://www.pqcrypto.org/www.springer.com/cda/content/
document/cda_downloaddocument/9783540887010-c1.pdf>
[RANDOM] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, <http://www.rfc-
editor.org/info/rfc4086>.
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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 EXPLICIT TAGS ::= BEGIN
EXPORTS ALL;
IMPORTS
SIGNATURE-ALGORITHM PUBLIC-KEY
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) }
mda-sha256
FROM PKIX1-PSS-OAEP-Algorithms-2009 -- RFC 5912 [PKIXASN1]
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-rsa-pkalgs-02(54) } ;
--
-- Object Identifiers
--
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }
id-alg OBJECT IDENTIFIER ::= { id-smime 3 }
id-alg-mts-hashsig OBJECT IDENTIFIER ::= { id-alg 17 }
--
-- Signature Algorithm and Public Key
--
sa-MTS-HashSig SIGNATURE-ALGORITHM ::= {
IDENTIFIER id-alg-mts-hashsig
HASHES { mda-sha256, ... }
PUBLIC-KEYS { pk-MTS-HashSig } }
pk-MTS-HashSig PUBLIC-KEY ::= {
IDENTIFIER id-alg-mts-hashsig
KEY MTS-HashSig-PublicKey }
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MTS-HashSig-PublicKey ::= OCTET STRING
HashSignatureAlgs SIGNATURE-ALGORITHM ::= {
sa-MTS-HashSig, ... }
END
Author's Address
Russ Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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