INTERNET-DRAFT R. Housley
Intended Status: Proposed Standard Vigil Security
Expires: 2 September 2017 2 March 2017
Use of the Hash-based Merkle Tree Signature (MTS) Algorithm
in the Cryptographic Message Syntax (CMS)
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. ASN.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. MTS Digital Signature Algorithm Overview . . . . . . . . . . . 3
2.1. Hierarchical Signature System (HSS) . . . . . . . . . . . 3
2.2. Leighton-Micali Signature (LMS) . . . . . . . . . . . . . 4
2.3. Leighton-Micali One-time Signature Algorithm (LM-OTS) . . 5
3. Algorithm Identifiers and Parameters . . . . . . . . . . . . . 6
4. Signed-data Conventions . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Implementation Security Considerations . . . . . . . . . . 7
5.2. Algorithm Security Considerations . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . . 8
8. Informative References . . . . . . . . . . . . . . . . . . . . 8
Appendix: ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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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.
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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [KEYWORDS].
2. MTS Digital 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 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 one-way hash function [SHS].
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
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 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. Each signed public key is represented by the hash value
at the root of the tree, and the signature over that public key is an
LMS signature as described in Section 2.2.
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The elements of the HSS signature value for a stand-alone tree can be
summarized as:
u32str(0) ||
lms_signature_on_message
The elements of the HSS signature value for a tree with Nspk levels
can be summarized as:
u32str(Nspk) ||
lms_signature_on_public_key[0] || public_key[1] ||
lms_signature_on_public_key[1] || public_key[2] ||
...
lms_signature_on_public_key[Nspk-2] || public_key[Nspk-1] ||
lms_signature_on_public_key[Nspk-1] || public_key[Nspk] ||
lms_signature_on_message
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=24.
Note that there are 2^h leaves in the tree. The second parameter is
the number of bytes output by the hash function, n, which the amount
of data associated with each node in the tree. The [HASHSIG]
specification supports only the SHA-256 hash function [SHS], with
n=32.
Five tree sizes are specified in [HASHSIG]:
LMS_SHA256_M32_H5;
LMS_SHA256_M32_H10;
LMS_SHA256_M32_H15;
LMS_SHA256_M32_H20; and
LMS_SHA256_M32_H24.
An LMS signature consists of four elements: a typecode indicating the
particular LMS algorithm, the number of the leaf associated with the
LM-OTS signature, an LM-OTS signature as described in Section 2.3,
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.
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The four elements of the LMS signature value can be summarized as:
u32str(type) ||
u32str(q) ||
ots_signature ||
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.
[HASHSIG] specifies the use of the LM-OTS. An LM-OTS has five
parameters.
n - The number of bytes associated with the hash function, which
is the same as the LMS parameter. [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.
The values of p and ls are dependent on the choices of the parameters
n and w, as described in Appendix A of [HASHSIG].
Four LM-OTS variants are defined in [HASHSIG]:
LMOTS_SHA256_N32_W1;
LMOTS_SHA256_N32_W2;
LMOTS_SHA256_N32_W4; and
LMOTS_SHA256_N32_W8.
Signing involves the generation of C, an n-byte random value.
The LM-OTS signature value can be summarized as:
u32str(type) || C || y[0] || ... || y[p-1]
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3. Algorithm Identifiers and Parameters
The algorithm identifier for an MTS signature is id-alg-mts-hashsig:
id-alg-mts-hashsig OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) alg(3) 17 }
When the id-alg-mts-hashsig algorithm identifier is used for a
signature, the AlgorithmIdentifier parameters field MUST be absent
(that is, the parameters are not present; the parameters are not set
to NULL).
The signature values 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 first 4
octets of the signature value contains the number of signed public
keys (Nspk) in the HSS. The first 4 octets of each LMS signature
value contains type code, which tells how to parse the remaining
parts of the signature value. The first 4 octets of each LM-OTS
signature value contains type code, which tells how to parse the
remaining parts of the signature value.
4. Signed-data Conventions
As specified in [CMS], the digital signature is produced from the
message digest and the signer's private key. If signed attributes
are absent, then the message digest is the hash of the content. If
signed attributes are present, then the hash of the content is placed
in the message-digest attribute, the set of signed attributes is DER
encoded, and the message digest is the hash of the encoded
attributes. In summary:
IF (signed attributes are absent)
THEN md = Hash(content)
ELSE message-digest attribute = Hash(content);
md = Hash(DER(SignedAttributes))
Sign(md)
When using [HASHSIG], the fields in the SignerInfo are used as
follows:
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.
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Further, the same one-way hash function SHOULD be used to
compute the message digest on both the eContent and the
signedAttributes value if signedAttributes are present. 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 HSS signature value resulting from
the signing operation as specified in [HASHSIG].
5. Security Considerations
5.1. 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 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.
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. RFC 4086 [RANDOM] offers important
guidance in this area.
When computing signatures, the same hash function SHOULD be used for
all operations. In this specification, only SHA-256 is used. Using
only SHA-256 reduces the number of possible failure points in the
signature process.
5.2. 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
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seen some major new developments in the past 6 months" [BH2013].
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 when it will be feasible to build
such a machine. RSA, DSA, and ECDSA are not post-quantum secure.
The LM-OTP one-time signature, LMS, and HSS do not depend on discrete
logarithm or factoring, as a result 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.
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgements
Many thanks to Panos Kampanakis, Jim Schaad, and Sean Turner for
their careful review and comments.
8. 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., M. Curcio, and S. Fluhrer, "Hash-Based
Signatures", Work in progress.
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[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, .
[SHS] National Institute of Standards and Technology (NIST),
FIPS Publication 180-3: Secure Hash Standard, October
2008.
9. 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.
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[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, .
[PQC] Bernstein, D., "Introduction to post-quantum
cryptography", 2009.
[RANDOM] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, .
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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 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) }
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-alg-mts-hashsig OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) alg(3) 17 }
--
-- Signature Algorithm and Public Key
--
sa-MTS-HashSig SIGNATURE-ALGORITHM ::= {
IDENTIFIER id-alg-mts-hashsig
PARAMS ARE absent
HASHES { mda-sha256 }
PUBLIC-KEYS { pk-MTS-HashSig }
SMIME-CAPS { IDENTIFIED BY id-alg-mts-hashsig } }
pk-MTS-HashSig PUBLIC-KEY ::= {
IDENTIFIER id-alg-mts-hashsig
KEY MTS-HashSig-PublicKey
PARAMS ARE absent
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign } }
MTS-HashSig-PublicKey ::= OCTET STRING
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--
-- Expand the signature algorithm set used by CMS [CMSASN1U]
--
SignatureAlgorithmSet SIGNATURE-ALGORITHM ::=
{ sa-MTS-HashSig, ... }
--
-- Expand the S/MIME capabilities set used by CMS [CMSASN1]
--
SMimeCaps SMIME-CAPS ::= { sa-MTS-HashSig.&smimeCaps, ... }
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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