Crypto Forum Research Group D. McGrew
InternetDraft M. Curcio
Intended status: Informational S. Fluhrer
Expires: December 22, 2017 Cisco Systems
June 20, 2017
HashBased Signatures
draftmcgrewhashsigs07
Abstract
This note describes a digital signature system based on cryptographic
hash functions, following the seminal work in this area of Lamport,
Diffie, Winternitz, and Merkle, as adapted by Leighton and Micali in
1995. It specifies a onetime signature scheme and a general
signature scheme. These systems provide asymmetric authentication
without using large integer mathematics and can achieve a high
security level. They are suitable for compact implementations, are
relatively simple to implement, and naturally resist sidechannel
attacks. Unlike most other signature systems, hashbased signatures
would still be secure even if it proves feasible for an attacker to
build a quantum computer.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used In This Document . . . . . . . . . . . . 4
2. Interface . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Data Types . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Operators . . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Strings of wbit elements . . . . . . . . . . . . . . 6
3.2. Security string . . . . . . . . . . . . . . . . . . . . . 7
3.3. Functions . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Typecodes . . . . . . . . . . . . . . . . . . . . . . . . 8
4. LMOTS OneTime Signatures . . . . . . . . . . . . . . . . . 8
4.1. Parameters . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Parameter Sets . . . . . . . . . . . . . . . . . . . . . 10
4.3. Private Key . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Public Key . . . . . . . . . . . . . . . . . . . . . . . 11
4.5. Checksum . . . . . . . . . . . . . . . . . . . . . . . . 11
4.6. Signature Generation . . . . . . . . . . . . . . . . . . 12
4.7. Signature Verification . . . . . . . . . . . . . . . . . 13
5. Leighton Micali Signatures . . . . . . . . . . . . . . . . . 15
5.1. Parameters . . . . . . . . . . . . . . . . . . . . . . . 16
5.2. LMS Private Key . . . . . . . . . . . . . . . . . . . . . 16
5.3. LMS Public Key . . . . . . . . . . . . . . . . . . . . . 17
5.4. LMS Signature . . . . . . . . . . . . . . . . . . . . . . 18
5.4.1. LMS Signature Generation . . . . . . . . . . . . . . 18
5.5. LMS Signature Verification . . . . . . . . . . . . . . . 19
6. Hierarchical signatures . . . . . . . . . . . . . . . . . . . 20
6.1. Key Generation . . . . . . . . . . . . . . . . . . . . . 21
6.2. Signature Generation . . . . . . . . . . . . . . . . . . 22
6.3. Signature Verification . . . . . . . . . . . . . . . . . 23
7. Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9. History . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
11. Intellectual Property . . . . . . . . . . . . . . . . . . . . 30
11.1. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . 30
12. Security Considerations . . . . . . . . . . . . . . . . . . . 30
12.1. Stateful signature algorithm . . . . . . . . . . . . . . 32
12.2. Security of LMOTS Checksum . . . . . . . . . . . . . . 33
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13. Comparison with other work . . . . . . . . . . . . . . . . . 33
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
15.1. Normative References . . . . . . . . . . . . . . . . . . 34
15.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. Pseudorandom Key Generation . . . . . . . . . . . . 36
Appendix B. LMOTS Parameter Options . . . . . . . . . . . . . . 37
Appendix C. An iterative algorithm for computing an LMS public
key . . . . . . . . . . . . . . . . . . . . . . . . 38
Appendix D. Example Implementation . . . . . . . . . . . . . . . 39
Appendix E. Test Cases . . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
Onetime signature systems, and general purpose signature systems
built out of onetime signature systems, have been known since 1979
[Merkle79], were well studied in the 1990s [USPTO5432852], and have
benefited from renewed attention in the last decade. The
characteristics of these signature systems are small private and
public keys and fast signature generation and verification, but large
signatures and relatively slow key generation. In recent years there
has been interest in these systems because of their postquantum
security and their suitability for compact verifier implementations.
This note describes the Leighton and Micali adaptation [USPTO5432852]
of the original LamportDiffieWinternitzMerkle onetime signature
system [Merkle79] [C:Merkle87][C:Merkle89a][C:Merkle89b] and general
signature system [Merkle79] with enough specificity to ensure
interoperability between implementations.
A signature system provides asymmetric message authentication. The
key generation algorithm produces a public/private key pair. A
message is signed by a private key, producing a signature, and a
message/signature pair can be verified by a public key. A OneTime
Signature (OTS) system can be used to sign at most one message
securely, but cannot securely sign more than one. An Ntime
signature system can be used to sign N or fewer messages securely. A
Merkle tree signature scheme is an Ntime signature system that uses
an OTS system as a component.
In this note we describe the LeightonMicali Signature (LMS) system,
which is a variant of the Merkle scheme, and a Hierarchical Signature
System (HSS) built on top of it that can efficiently scale to larger
numbers of signatures. We denote the onetime signature scheme
incorporate in LMS as LMOTS. This note is structured as follows.
Notation is introduced in Section 3. The LMOTS signature system is
described in Section 4, and the LMS and HSS Ntime signature systems
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are described in Section 5 and Section 6, respectively. Sufficient
detail is provided to ensure interoperability. The public formats
are described in Section 7. The rationale for design decisions are
given in Section 8. The IANA registry for these signature systems is
described in Section 10. Security considerations are presented in
Section 12.
1.1. Conventions Used In This Document
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 [RFC2119].
2. Interface
The LMS signing algorithm is stateful; it modifies and updates the
private key as a side effect of generating a signature. Once a
particular value of the private key is used to sign one message, it
MUST NOT be used to sign another.
The key generation algorithm takes as input an indication of the
parameters for the signature system. If it is successful, it
returns both a private key and a public key. Otherwise, it
returns an indication of failure.
The signing algorithm takes as input the message to be signed and
the current value of the private key. If successful, it returns a
signature and the next value of the private key, if there is such
a value. After the private key of an Ntime signature system has
signed N messages, the signing algorithm returns the signature and
an indication that there is no next value of the private key that
can be used for signing. If unsuccessful, it returns an
indication of failure.
The verification algorithm takes as input the public key, a
message, and a signature, and returns an indication of whether or
not the signature and message pair are valid.
A message/signature pair are valid if the signature was returned by
the signing algorithm upon input of the message and the private key
corresponding to the public key; otherwise, the signature and message
pair are not valid with probability very close to one.
3. Notation
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3.1. Data Types
Bytes and byte strings are the fundamental data types. A single byte
is denoted as a pair of hexadecimal digits with a leading "0x". A
byte string is an ordered sequence of zero or more bytes and is
denoted as an ordered sequence of hexadecimal characters with a
leading "0x". For example, 0xe534f0 is a byte string with a length
of three. An array of byte strings is an ordered set, indexed
starting at zero, in which all strings have the same length.
Unsigned integers are converted into byte strings by representing
them in network byte order. To make the number of bytes in the
representation explicit, we define the functions u8str(X), u16str(X),
and u32str(X), which take a nonnegative integer X as input and
return one, two, and four byte strings, respectively. We also make
use of the function strTou32(S), which takes a four byte string S as
input and returns a nonnegative integer; the identity
u32str(strTou32(S)) = S holds for any fourbyte string S.
3.1.1. Operators
When a and b are real numbers, mathematical operators are defined as
follows:
^ : a ^ b denotes the result of a raised to the power of b
* : a * b denotes the product of a multiplied by b
/ : a / b denotes the quotient of a divided by b
% : a % b denotes the remainder of the integer division of a by b
+ : a + b denotes the sum of a and b
 : a  b denotes the difference of a and b
The standard order of operations is used when evaluating arithmetic
expressions.
When B is a byte and i is an integer, then B >> i denotes the logical
rightshift operation. Similarly, B << i denotes the logical left
shift operation.
If S and T are byte strings, then S  T denotes the concatenation of
S and T. If S and T are equal length byte strings, then S AND T
denotes the bitwise logical and operation.
The i^th element in an array A is denoted as A[i].
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3.1.2. Strings of wbit elements
If S is a byte string, then byte(S, i) denotes its i^th byte, where
byte(S, 0) is the leftmost byte. In addition, bytes(S, i, j) denotes
the range of bytes from the i^th to the j^th byte, inclusive. For
example, if S = 0x02040608, then byte(S, 0) is 0x02 and bytes(S, 1,
2) is 0x0406.
A byte string can be considered to be a string of wbit unsigned
integers; the correspondence is defined by the function coef(S, i, w)
as follows:
If S is a string, i is a positive integer, and w is a member of the
set { 1, 2, 4, 8 }, then coef(S, i, w) is the i^th, wbit value, if S
is interpreted as a sequence of wbit values. That is,
coef(S, i, w) = (2^w  1) AND
( byte(S, floor(i * w / 8)) >>
(8  (w * (i % (8 / w)) + w)) )
For example, if S is the string 0x1234, then coef(S, 7, 1) is 0 and
coef(S, 0, 4) is 1.
S (represented as bits)
+++++++++++++++++
 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0
+++++++++++++++++
^

coef(S, 7, 1)
S (represented as fourbit values)
+++++
 1  2  3  4 
+++++
^

coef(S, 0, 4)
The return value of coef is an unsigned integer. If i is larger than
the number of wbit values in S, then coef(S, i, w) is undefined, and
an attempt to compute that value should raise an error.
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3.2. Security string
To improve security against attacks that amortize their effort
against multiple invocations of the hash function, Leighton and
Micali introduce a "security string" that is distinct for each
invocation of that function. Whenever this process computes a hash,
the string being hashed will start with a string formed from the
below fields. These fields are:
I  a 16 byte identifier for the LMS public/private key pair. It
MUST be chosen uniformly at random, or via a pseudorandom process,
at the time that a key pair is generated, in order to minimize the
probability that any specific value of I be used for a large
number of different LMS private keys. This is always bytes 015
of the hash.
r  in the LMS Ntime signature scheme, the node number r
associated with a particular node of a hash tree is used as an
input to the hash used to compute that node. This value is
represented as a 32bit (four byte) unsigned integer in network
byte order. Either r or q (depending on the domain separate
parameter) will be bytes 1619 of the hash.
q  in the LMS Ntime signature scheme, each LMOTS signature is
associated with the leaf of a hash tree, and q is set to the leaf
number. This ensures that a distinct value of q is used for each
distinct LMOTS public/private key pair. This value is
represented as a 32bit (four byte) unsigned integer in network
byte order. Either r or q (depending on the domain separate
parameter) will be bytes 1619 of the hash.
D  a domain separation parameter, which is a two byte identifier
that takes on different values in the different contexts in which
the hash function is invoked. D occurs in bytes 20, 21 of the
hash, and takes on the following values:
D_PBLC = 0x8080 when computing the hash of all of the iterates
in the LMOTS algorithm
D_MESG = 0x8181 when computing the hash of the message in the
LMOTS algorithms
D_LEAF = 0x8282 when computing the hash of the leaf of an LMS
tree
D_INTR = 0x8383 when computing the hash of an interior node of
an LMS tree
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i = a value between 0 and 264; this is used in the LMOTS
scheme, when either computing the iterations of the Winternitz
chain, or when using the suggested LMOTS private key
generation process. The value is also the index of the LMOTS
private key element upon which H is being applied. It is
represented as a 16bit (two byte) unsigned integer in network
byte order.
j  in the LMOTS scheme, j is the iteration number used when the
private key element is being iteratively hashed. It is
represented as an 8bit (one byte) unsigned integer and is present
if D is a value between 0 and 264. If present, it occurs at byte
22 of the hash.
C  an nbyte randomizer that is included with the message
whenever it is being hashed to improve security. C MUST be chosen
uniformly at random, or via a pseudorandom process. It is present
if D=D_MESG, and it occurs at bytes 2253 of the hash.
3.3. Functions
If r is a nonnegative real number, then we define the following
functions:
ceil(r) : returns the smallest integer larger than r
floor(r) : returns the largest integer smaller than r
lg(r) : returns the base2 logarithm of r
3.4. Typecodes
A typecode is an unsigned integer that is associated with a
particular data format. The format of the LMOTS, LMS, and HSS
signatures and public keys all begin with a typecode that indicates
the precise details used in that format. These typecodes are
represented as fourbyte unsigned integers in network byte order;
equivalently, they are XDR enumerations (see Section 7).
4. LMOTS OneTime Signatures
This section defines LMOTS signatures. The signature is used to
validate the authenticity of a message by associating a secret
private key with a shared public key. These are onetime signatures;
each private key MUST be used at most one time to sign any given
message.
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As part of the signing process, a digest of the original message is
computed using the cryptographic hash function H (see Section 4.1),
and the resulting digest is signed.
In order to facilitate its use in an Ntime signature system, the LM
OTS key generation, signing, and verification algorithms all take as
input a diversification parameter q. When the LMOTS signature
system is used outside of an Ntime signature system, this value
SHOULD be set to the allzero value.
4.1. Parameters
The signature system uses the parameters n and w, which are both
positive integers. The algorithm description also makes use of the
internal parameters p and ls, which are dependent on n and w. These
parameters are summarized as follows:
n : the number of bytes of the output of the hash function
w : the width (number of bits) of the Winternitz coefficients; it
is a member of the set { 1, 2, 4, 8 }
p : the number of nbyte string elements that make up the LMOTS
signature
ls : the number of leftshift bits used in the checksum function
Cksm (defined in Section 4.5).
H : a secondpreimageresistant cryptographic hash function that
accepts byte strings of any length, and returns an nbyte string.
For more background on the cryptographic security requirements on H,
see the Section 12.
The value of n is determined by the functions selected for use as
part of the LMOTS algorithm; the choice of this value has a strong
effect on the security of the system. The parameter w determines the
length of the Winternitz chains computed as a part of the OTS
signature (which involve 2^w1 invocations of the hash function); it
has little effect on security. Increasing w will shorten the
signature, but at a cost of a larger computation to generate and
verify a signature. The values of p and ls are dependent on the
choices of the parameters n and w, as described in Appendix B. A
table illustrating various combinations of n, w, p, and ls is
provided in Table 1.
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4.2. Parameter Sets
To fully describe a LMOTS signature method, the parameters n and w,
as well as the function H, MUST be specified. This section defines
several LMOTS methods, each of which is identified by a name. The
values for p and ls are provided as a convenience.
+++++++
 Name  H  n  w  p  ls 
+++++++
 LMOTS_SHA256_N32_W1  SHA256  32  1  265  7 
      
 LMOTS_SHA256_N32_W2  SHA256  32  2  133  6 
      
 LMOTS_SHA256_N32_W4  SHA256  32  4  67  4 
      
 LMOTS_SHA256_N32_W8  SHA256  32  8  34  0 
+++++++
Table 1
Here SHA256 denotes the NIST standard hash function [FIPS180].
4.3. Private Key
The LMOTS private key consists of a typecode indicating the
particular LMOTS algorithm, an array x[] containing p nbyte
strings, and the 16 byte string I and the 4 byte string q. This
private key MUST be used to sign (at most) one message. The
following algorithm shows pseudocode for generating a private key.
Algorithm 0: Generating a Private Key
1. set type to the typecode of the algorithm
2. set n and p according to the typecode and Table 1
3. compute the array x as follows:
for ( i = 0; i < p; i = i + 1 ) {
set x[i] to a uniformly random nbyte string
}
4. return u32str(type)  I  u32str(q)  x[0]  x[1]  ...  x[p1]
An implementation MAY use a pseudorandom method to compute x[i], as
suggested in [Merkle79], page 46. The details of the pseudorandom
method do not affect interoperability, but the cryptographic strength
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MUST match that of the LMOTS algorithm. Appendix A provides an
example of a pseudorandom method for computing LMOTS private key.
4.4. Public Key
The LMOTS public key is generated from the private key by
iteratively applying the function H to each individual element of x,
for 2^w  1 iterations, then hashing all of the resulting values.
The public key is generated from the private key using the following
algorithm, or any equivalent process.
Algorithm 1: Generating a One Time Signature Public Key From a
Private Key
1. set type to the typecode of the algorithm
2. set the integers n, p, and w according to the typecode and Table 1
3. determine x, I and q from the private key
4. compute the string K as follows:
for ( i = 0; i < p; i = i + 1 ) {
tmp = x[i]
for ( j = 0; j < 2^w  1; j = j + 1 ) {
tmp = H(I  u32str(q)  u16str(i)  u8str(j)  tmp)
}
y[i] = tmp
}
K = H(I  u32str(q)  u16str(D_PBLC)  y[0]  ...  y[p1])
5. return u32str(type)  I  u32str(q)  K
The public key is the value returned by Algorithm 1.
4.5. Checksum
A checksum is used to ensure that any forgery attempt that
manipulates the elements of an existing signature will be detected.
The security property that it provides is detailed in Section 12.
The checksum function Cksm is defined as follows, where S denotes the
nbyte string that is input to that function, and the value sum is a
16bit unsigned integer:
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Algorithm 2: Checksum Calculation
sum = 0
for ( i = 0; i < (n*8/w); i = i + 1 ) {
sum = sum + (2^w  1)  coef(S, i, w)
}
return (sum << ls)
Because of the leftshift operation, the rightmost bits of the result
of Cksm will often be zeros. Due to the value of p, these bits will
not be used during signature generation or verification.
4.6. Signature Generation
The LMOTS signature of a message is generated by first prepending
the LM key identifier I, the LM leaf identifier q, the value D_MESG
and the randomizer C to the message, then computing the hash, and
then concatenating the checksum of the hash to the hash itself, then
considering the resulting value as a sequence of wbit values, and
using each of the wbit values to determine the number of times to
apply the function H to the corresponding element of the private key.
The outputs of the function H are concatenated together and returned
as the signature. The pseudocode for this procedure is shown below.
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Algorithm 3: Generating a One Time Signature From a Private Key and a
Message
1. set type to the typecode of the algorithm
2. set n, p, and w according to the typecode and Table 1
3. determine x, I and q from the private key
4. set C to a uniformly random nbyte string
5. compute the array y as follows:
Q = H(I  u32str(q)  u16str(D_MESG)  C  message)
for ( i = 0; i < p; i = i + 1 ) {
a = coef(Q  Cksm(Q), i, w)
tmp = x[i]
for ( j = 0; j < a; j = j + 1 ) {
tmp = H(I  u32str(q)  u16str(i)  u8str(j)  tmp)
}
y[i] = tmp
}
6. return u32str(type)  C  y[0]  ...  y[p1]
Note that this algorithm results in a signature whose elements are
intermediate values of the elements computed by the public key
algorithm in Section 4.4.
The signature is the string returned by Algorithm 3. Section 7
specifies the typecode and more formally defines the encoding and
decoding of the string.
4.7. Signature Verification
In order to verify a message with its signature (an array of nbyte
strings, denoted as y), the receiver must "complete" the chain of
iterations of H using the wbit coefficients of the string resulting
from the concatenation of the message hash and its checksum. This
computation should result in a value that matches the provided public
key.
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Algorithm 4a: Verifying a Signature and Message Using a Public Key
1. if the public key is not at least four bytes long, return INVALID
2. parse pubtype, I, q, and K from the public key as follows:
a. pubtype = strTou32(first 4 bytes of public key)
b. if pubtype is not equal to sigtype, return INVALID
c. if the public key is not exactly 24 + n bytes long,
return INVALID
c. I = next 16 bytes of public key
d. q = strTou32(next 4 bytes of public key)
e. K = next n bytes of public key
3. compute the public key candidate Kc from the signature,
message, and the identifiers I and q obtained from the
public key, using Algorithm 4b. If Algorithm 4b returns
INVALID, then return INVALID.
4. if Kc is equal to K, return VALID; otherwise, return INVALID
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Algorithm 4b: Computing a Public Key Candidate Kc from a Signature,
Message, Signature Typecode Type , and identifiers I, q
1. if the signature is not at least four bytes long, return INVALID
2. parse sigtype, C, and y from the signature as follows:
a. sigtype = strTou32(first 4 bytes of signature)
b. if sigtype is not equal to Type, return INVALID
c. set n and p according to the sigtype and Table 1; if the
signature is not exactly 4 + n * (p+1) bytes long, return INVALID
d. C = next n bytes of signature
e. y[0] = next n bytes of signature
y[1] = next n bytes of signature
...
y[p1] = next n bytes of signature
3. compute the string Kc as follows
Q = H(I  u32str(q)  u16str(D_MESG)  C  message)
for ( i = 0; i < p; i = i + 1 ) {
a = coef(Q  Cksm(Q), i, w)
tmp = y[i]
for ( j = a; j < 2^w  1; j = j + 1 ) {
tmp = H(I  u32str(q)  u16str(i)  u8str(j)  tmp)
}
z[i] = tmp
}
Kc = H(I  u32str(q)  u16str(D_PBLC)  z[0]  z[1]  ...  z[p1])
4. return Kc
5. Leighton Micali Signatures
The Leighton Micali Signature (LMS) method can sign a potentially
large but fixed number of messages. An LMS system uses two
cryptographic components: a onetime signature method and a hash
function. Each LMS public/private key pair is associated with a
perfect binary tree, each node of which contains an mbyte value.
Each leaf of the tree contains the value of the public key of an LM
OTS public/private key pair. The value contained by the root of the
tree is the LMS public key. Each interior node is computed by
applying the hash function to the concatenation of the values of its
children nodes.
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Each node of the tree is associated with a node number, an unsigned
integer that is denoted as node_num in the algorithms below, which is
computed as follows. The root node has node number 1; for each node
with node number N < 2^h, its left child has node number 2*N, while
its right child has node number 2*N+1. The result of this is that
each node within the tree will have a unique node number, and the
leaves will have node numbers 2^h, (2^h)+1, (2^h)+2, ...,
(2^h)+(2^h)1. In general, the j^th node at level L has node number
2^L + j. The node number can conveniently be computed when it is
needed in the LMS algorithms, as described in those algorithms.
5.1. Parameters
An LMS system has the following parameters:
h : the height (number of levels  1) in the tree, and
m : the number of bytes associated with each node.
H : a secondpreimageresistant cryptographic hash function that
accepts byte strings of any length, and returns an mbyte string.
H SHOULD be the same as in Section 4.1, but MAY be different.
There are 2^h leaves in the tree. The hash function used within the
LMS system SHOULD be the same as the hash function used within the
LMOTS system used to generate the leaves.
+++++
 Name  H  m  h 
+++++
 LMS_SHA256_M32_H5  SHA256  32  5 
    
 LMS_SHA256_M32_H10  SHA256  32  10 
    
 LMS_SHA256_M32_H15  SHA256  32  15 
    
 LMS_SHA256_M32_H20  SHA256  32  20 
    
 LMS_SHA256_M32_H25  SHA256  32  25 
+++++
Table 2
5.2. LMS Private Key
An LMS private key consists of an array OTS_PRIV[] of 2^h LMOTS
private keys, and the leaf number q of the next LMOTS private key
that has not yet been used. The q^th element of OTS_PRIV[] is
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generated using Algorithm 0 with the identifiers I, q. The leaf
number q is initialized to zero when the LMS private key is created.
The process is as follows:
Algorithm 5: Computing an LMS Private Key.
1. determine h and m from the typecode and Table 2.
2. compute the array OTS_PRIV[] as follows:
for ( q = 0; q < 2^h; q = q + 1) {
OTS_PRIV[q] = LMOTS private key with identifiers I, q
}
3. q = 0
An LMS private key MAY be generated pseudorandomly from a secret
value, in which case the secret value MUST be at least m bytes long,
be uniformly random, and MUST NOT be used for any other purpose than
the generation of the LMS private key. The details of how this
process is done do not affect interoperability; that is, the public
key verification operation is independent of these details.
Appendix A provides an example of a pseudorandom method for computing
an LMS private key.
5.3. LMS Public Key
An LMS public key is defined as follows, where we denote the public
key associated with the i^th LMOTS private key as OTS_PUB[i], with i
ranging from 0 to (2^h)1. Each instance of an LMS public/private
key pair is associated with a perfect binary tree, and the nodes of
that tree are indexed from 1 to 2^(h+1)1. Each node is associated
with an mbyte string, and the string for the r^th node is denoted as
T[r] and is defined as
T[r]=/ H(Iu32str(r)u16str(D_LEAF)OTS_PUB[r2^h]) if r >= 2^h,
\ H(Iu32str(r)u16str(D_INTR)T[2*r]T[2*r+1]) otherwise.
The LMS public key is the string u32str(type)  I  T[1].
Section 7 specifies the format of the type variable. The value I is
the private key identifier (whose length is denoted by the parameter
set), and is the value used for all computations for the same LMS
tree. The value T[1] can be computed via recursive application of
the above equation, or by any equivalent method. An iterative
procedure is outlined in Appendix C.
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5.4. LMS Signature
An LMS signature consists of
the number q of the leaf associated with the LMOTS signature, as
a fourbyte unsigned integer in network byte order,
an LMOTS signature, and
a typecode indicating the particular LMS algorithm,
an array of h mbyte values that is associated with the path
through the tree from the leaf associated with the LMOTS
signature to the root.
Symbolically, the signature can be represented as u32str(q) 
ots_signature  u32str(type)  path[0]  path[1]  ... 
path[h1]. Section 7 specifies the typecode and more formally
defines the format. 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 themselves. 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.
5.4.1. LMS Signature Generation
To compute the LMS signature of a message with an LMS private key,
the signer first computes the LMOTS signature of the message using
the leaf number of the next unused LMOTS private key. The leaf
number q in the signature is set to the leaf number of the LMS
private key that was used in the signature. Before releasing the
signature, the leaf number q in the LMS private key MUST be
incremented, to prevent the LMOTS private key from being used again.
If the LMS private key is maintained in nonvolatile memory, then the
implementation MUST ensure that the incremented value has been stored
before releasing the signature.
The array of node values in the signature MAY be computed in any way.
There are many potential time/storage tradeoffs that can be applied.
The fastest alternative is to store all of the nodes of the tree and
set the array in the signature by copying them. The least storage
intensive alternative is to recompute all of the nodes for each
signature. Note that the details of this procedure are not important
for interoperability; it is not necessary to know any of these
details in order to perform the signature verification operation.
The internal nodes of the tree need not be kept secret, and thus a
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nodecaching scheme that stores only internal nodes can sidestep the
need for strong protections.
Several useful time/storage tradeoffs are described in the 'Small
Memory LM Schemes' section of [USPTO5432852].
5.5. LMS Signature Verification
An LMS signature is verified by first using the LMOTS signature
verification algorithm (Algorithm 4b) to compute the LMOTS public
key from the LMOTS signature and the message. The value of that
public key is then assigned to the associated leaf of the LMS tree,
then the root of the tree is computed from the leaf value and the
array path[] as described in Algorithm 6 below. If the root value
matches the public key, then the signature is valid; otherwise, the
signature fails.
Algorithm 6: LMS Signature Verification
1. if the public key is not at least four bytes long, return
INVALID
2. parse pubtype, I, and T[1] from the public key as follows:
a. pubtype = strTou32(first 4 bytes of public key)
b. if the public key is not exactly 4 + LenI + m bytes
long, return INVALID
c. I = next LenI bytes of the public key
d. T[1] = next m bytes of the public key
6. compute the candidate LMS root value Tc from the signature,
message, identifier and pubtype using Algorithm 6b.
7. if Tc is equal to T[1], return VALID; otherwise, return INVALID
Algorithm 6b: Computing an LMS Public Key Candidate from a Signature,
Message, Identifier, and algorithm typecode
1. if the signature is not at least eight bytes long, return INVALID
2. parse sigtype, q, ots_signature, and path from the signature as
follows:
a. q = strTou32(first 4 bytes of signature)
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b. otssigtype = strTou32(next 4 bytes of signature)
c. if otssigtype is not the OTS typecode from the public key, return INVALID
d. set n, p according to otssigtype and Table 1; if the
signature is not at least 12 + n * (p + 1) bytes long, return INVALID
e. ots_signature = bytes 8 through 8 + n * (p + 1)  1 of signature
f. sigtype = strTou32(4 bytes of signature at location 8 + n * (p + 1))
f. if sigtype is not the LM typecode from the public key, return INVALID
g. set m, h according to sigtype and Table 2
h. if q >= 2^h or the signature is not exactly 12 + n * (p + 1) + m * h bytes long, return INVALID
i. set path as follows:
path[0] = next m bytes of signature
path[1] = next m bytes of signature
...
path[h1] = next m bytes of signature
5. Kc = candidate public key computed by applying Algorithm 4b
to the signature ots_signature, the message, and the
identifiers I, q
6. compute the candidate LMS root value Tc as follows:
node_num = 2^h + q
tmp = H(I  u32str(node_num)  u16str(D_LEAF)  Kc)
i = 0
while (node_num > 1) {
if (node_num is odd):
tmp = H(Iu32str(node_num/2)u16str(D_INTR)path[i]tmp)
else:
tmp = H(Iu32str(node_num/2)u16str(D_INTR)tmppath[i])
node_num = node_num/2
i = i + 1
}
Tc = tmp
7. return Tc
6. Hierarchical signatures
In scenarios where it is necessary to minimize the time taken by the
public key generation process, a Hierarchical Ntime Signature System
(HSS) can be used. Leighton and Micali describe a scheme in which an
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LMS public key is used to sign a second LMS public key, which is then
distributed along with the signatures generated with the second
public key [USPTO5432852]. This hierarchical scheme, which we
describe in this section, uses an LMS scheme as a component. HSS, in
essence, utilizes a tree of LMS trees, in which the HSS public key
contains the public key of the LMS tree at the root, and an HSS
signature is associated with a path from the root of the HSS tree to
one of its leaves. Compared to LMS, HSS has a much reduced public
key generation time, as only the root tree needs to be generated
prior to the distribution of the HSS public key.
Each level of the hierarchy is associated with a distinct LMS public
key, private key, signature, and identifier. The number of levels is
denoted L, and is between one and eight, inclusive. The following
notation is used, where i is an integer between 0 and L1 inclusive,
and the root of the hierarchy is level 0:
prv[i] is the LMS private key of the i^th level,
pub[i] is the LMS public key of the i^th level (which includes the
identifier I as well as the key value K),
sig[i] is the LMS signature of the i^th level,
In this section, we say that an Ntime private key is exhausted when
it has generated N signatures, and thus it can no longer be used for
signing.
HSS allows L=1, in which case the HSS public key and signature
formats are essentially the LMS public key and signature formats,
prepended by a fixed field. Since HSS with L=1 has very little
overhead compared to LMS, all implementations MUST support HSS in
order to maximize interoperability.
6.1. Key Generation
When an HSS key pair is generated, the key pair for each level MUST
have its own identifier I.
To generate an HSS private and public key pair, new LMS private and
public keys are generated for prv[i] and pub[i] for i=0, ... , L1.
These key pairs, and their identifiers, MUST be generated
independently. All of the information of the leaf level L1,
including the private key, MUST NOT be stored in nonvolatile memory.
Letting Nnv denote the lowest level for which prv[Nnv] is stored in
nonvolatile memory, there are Nnv nonvolatile levels, and LNnv
volatile levels. For security, Nnv should be as close to one as
possible (see Section 12.1).
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The public key of the HSS scheme is consists of the number of levels
L, followed by pub[0], the public key of the top level.
The HSS private key consists of prv[0], ... , prv[L1]. The values
pub[0] and prv[0] do not change, though the values of pub[i] and
prv[i] are dynamic for i > 0, and are changed by the signature
generation algorithm.
6.2. Signature Generation
To sign a message using the private key prv, the following steps are
performed:
If prv[L1] is exhausted, then determine the smallest integer d
such that all of the private keys prv[d], prv[d+1], ... , prv[L1]
are exhausted. If d is equal to zero, then the HSS key pair is
exhausted, and it MUST NOT generate any more signatures.
Otherwise, the key pairs for levels d through L1 must be
regenerated during the signature generation process, as follows.
For i from d to L1, a new LMS public and private key pair with a
new identifier is generated, pub[i] and prv[i] are set to those
values, then the public key pub[i] is signed with prv[i1], and
sig[i1] is set to the resulting value.
The message is signed with prv[L1], and the value sig[L1] is set
to that result.
The value of the HSS signature is set as follows. We let
signed_pub_key denote an array of octet strings, where
signed_pub_key[i] = sig[i]  pub[i+1], for i between 0 and Nspk
1, inclusive, where Nspk = L1 denotes the number of signed public
keys. Then the HSS signature is u32str(Nspk) 
signed_pub_key[0]  ...  signed_pub_key[Nspk1]  sig[Nspk].
Note that the number of signed_pub_key elements in the signature
is indicated by the value Nspk that appears in the initial four
bytes of the signature.
In the specific case of L=1, the format of an HSS signature is
u32str(0)  sig[0]
In the general case, the format of an HSS signature is
u32str(Nspk)  signed_pub_key[0]  ...  signed_pub_key[Nspk1]  sig[Nspk]
which is equivalent to
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u32str(Nspk)  sig[0]  pub[1]  ...  sig[Nspk1]  pub[Nspk]  sig[Nspk]
6.3. Signature Verification
To verify a signature sig and message using the public key pub, the
following steps are performed:
The signature S is parsed into its components as follows:
L' = strTou32(first four bytes of S)
if L' is not equal to the number of levels L in pub:
return INVALID
for (i = 0; i < L; i = i + 1) {
siglist[i] = next LMS signature parsed from S
publist[i] = next LMS public key parsed from S
}
siglist[L1] = next LMS signature parsed from S
key = pub
for (i = 0; i < L; i = i + 1) {
sig = siglist[i]
msg = publist[i]
if (lms_verify(msg, key, sig) != VALID):
return INVALID
key = msg
return lms_verify(message, key, siglist[L1])
Since the length of an LMS signature cannot be known without parsing
it, the HSS signature verification algorithm makes use of an LMS
signature parsing routine that takes as input a string consisting of
an LMS signature with an arbitrary string appended to it, and returns
both the LMS signature and the appended string. The latter is passed
on for further processing.
7. Formats
The signature and public key formats are formally defined using the
External Data Representation (XDR) [RFC4506] in order to provide an
unambiguous, machine readable definition. For clarity, we also
include a private key format as well, though consistency is not
needed for interoperability and an implementation MAY use any private
key format. Though XDR is used, these formats are simple and easy to
parse without any special tools. An illustration of the layout of
data in these objects is provided below. The definitions are as
follows:
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/* onetime signatures */
enum ots_algorithm_type {
lmots_reserved = 0,
lmots_sha256_n32_w1 = 1,
lmots_sha256_n32_w2 = 2,
lmots_sha256_n32_w4 = 3,
lmots_sha256_n32_w8 = 4
};
typedef opaque bytestring32[32];
struct lmots_signature_n32_p265 {
bytestring32 C;
bytestring32 y[265];
};
struct lmots_signature_n32_p133 {
bytestring32 C;
bytestring32 y[133];
};
struct lmots_signature_n32_p67 {
bytestring32 C;
bytestring32 y[67];
};
struct lmots_signature_n32_p34 {
bytestring32 C;
bytestring32 y[34];
};
union ots_signature switch (ots_algorithm_type type) {
case lmots_sha256_n32_w1:
lmots_signature_n32_p265 sig_n32_p265;
case lmots_sha256_n32_w2:
lmots_signature_n32_p133 sig_n32_p133;
case lmots_sha256_n32_w4:
lmots_signature_n32_p67 sig_n32_p67;
case lmots_sha256_n32_w8:
lmots_signature_n32_p34 sig_n32_p34;
default:
void; /* error condition */
};
/* hash based signatures (hbs) */
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enum hbs_algorithm_type {
hbs_reserved = 0,
lms_sha256_n32_h5 = 5,
lms_sha256_n32_h10 = 6,
lms_sha256_n32_h15 = 7,
lms_sha256_n32_h20 = 8,
lms_sha256_n32_h25 = 9,
};
/* leighton micali signatures (lms) */
union lms_path switch (hbs_algorithm_type type) {
case lms_sha256_n32_h5:
bytestring32 path_n32_h5[5];
case lms_sha256_n32_h10:
bytestring32 path_n32_h10[10];
case lms_sha256_n32_h15:
bytestring32 path_n32_h15[15];
case lms_sha256_n32_h20:
bytestring32 path_n32_h20[20];
case lms_sha256_n32_h25:
bytestring32 path_n32_h25[25];
default:
void; /* error condition */
};
struct lms_signature {
unsigned int q;
ots_signature lmots_sig;
lms_path nodes;
};
struct lms_key_n32 {
ots_algorithm_type ots_alg_type;
opaque I[16];
opaque K[32];
};
union hbs_public_key switch (hbs_algorithm_type type) {
case lms_sha256_n32_h5:
case lms_sha256_n32_h10:
case lms_sha256_n32_h15:
case lms_sha256_n32_h20:
case lms_sha256_n32_h25:
lms_key_n32 z_n32;
default:
void; /* error condition */
};
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/* hierarchical signature system (hss) */
struct hss_public_key {
unsigned int L;
hbs_public_key pub;
};
struct signed_public_key {
hbs_signature sig;
hbs_public_key pub;
}
struct hss_signature {
signed_public_key signed_keys<7>;
hbs_signature sig_of_message;
};
Many of the objects start with a typecode. A verifier MUST check
each of these typecodes, and a verification operation on a signature
with an unknown type, or a type that does not correspond to the type
within the public key MUST return INVALID. The expected length of a
variablelength object can be determined from its typecode, and if an
object has a different length, then any signature computed from the
object is INVALID.
8. Rationale
The goal of this note is to describe the LMOTS, LMS and HSS
algorithms following the original references and present the modern
security analysis of those algorithms. Other signature methods are
out of scope and may be interesting followon work.
We adopt the techniques described by Leighton and Micali to mitigate
attacks that amortize their work over multiple invocations of the
hash function.
The values taken by the identifier I across different LMS public/
private key pairs are chosen randomly in order to improve security.
The analysis of this method in [Fluhrer17] shows that we do not need
uniqueness to ensure security; we do need to ensure that we don't
have a large number of private keys that use the same I value. By
randomly selecting 16 byte I values, the chance that, out of 2^64
private keys, 4 or more of them will use the same I value is
negligible (that is, has probability less than 2^128).
The reason this size was selected was to optimize the Winternitz hash
chain operation. With the current settings, the value being hashed
is exactly 55 bytes long (for a 32 byte hash function), which SHA256
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can hash in a single hash compression operation. Other hash
functions may be used in future specifications; all the ones that we
will be likely to support (SHA512/256 and the various SHA3 hashes)
would work well with a 16 byte I value.
The signature and public key formats are designed so that they are
relatively easy to parse. Each format starts with a 32bit
enumeration value that indicates the details of the signature
algorithm and provides all of the information that is needed in order
to parse the format.
The Checksum Section 4.5 is calculated using a nonnegative integer
"sum", whose width was chosen to be an integer number of wbit fields
such that it is capable of holding the difference of the total
possible number of applications of the function H as defined in the
signing algorithm of Section 4.6 and the total actual number. In the
case that the number of times H is applied is 0, the sum is (2^w  1)
* (8*n/w). Thus for the purposes of this document, which describes
signature methods based on H = SHA256 (n = 32 bytes) and w = { 1, 2,
4, 8 }, the sum variable is a 16bit nonnegative integer for all
combinations of n and w. The calculation uses the parameter ls
defined in Section 4.1 and calculated in Appendix B, which indicates
the number of bits used in the leftshift operation.
9. History
This is the seventh version of this draft. It has the following
changes from previous versions:
Version 06
Modified the order of the values that were hashed to make it
easier to prove security.
Decreased the size of the I LMS public key identifier to 16 bytes.
Version 05
Clarified the L=1 specific case.
Extended the parameter sets to include an H=25 option
A large number of corrections and clarifications
Added a comparison to XMSS and SPHINCS, and citations to those
algorithms and to the recent Security Standardization Research
2016 publications on the security of LMS and on the state
management in hashbased signatures.
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Version 04
Specified that, in the HSS method, the I value was computed from
the I value of the parent LM tree. Previous versions had the I
value extracted from the public key (which meant that all LM trees
of a particular level and public key used the same I value)
Changed the length of the I field based on the parameter set. As
noted in the Rationale section, this allows an implementation to
compute SHA256 n=32 based parameter sets significantly faster.
Modified the XDR of an HSS signature not to use an array of LM
signatures; LM signatures are variable length, and XDR doesn't
support arrays of variable length structures.
Changed the LMS registry to be in a consistent order with the LM
OTS parameter sets. Also, added LMS parameter sets with height 15
trees
Previous versions
In Algorithms 3 and 4, the message was moved from the initial
position of the input to the function H to the final position, in
the computation of the intermediate variable Q. This was done to
improve security by preventing an attacker that can find a
collision in H from taking advantage of that fact via the forward
chaining property of MerkleDamgard.
The Hierarchical Signature Scheme was generalized slightly so that
it can use more than two levels.
Several points of confusion were corrected; these had resulted
from incomplete or inconsistent changes from the Merkle approach
of the earlier draft to the LeightonMicali approach.
This section is to be removed by the RFC editor upon publication.
10. IANA Considerations
The Internet Assigned Numbers Authority (IANA) is requested to create
two registries: one for OTS signatures, which includes all of the LM
OTS signatures as defined in Section 3, and one for LeightonMicali
Signatures, as defined in Section 4. Additions to these registries
require that a specification be documented in an RFC or another
permanent and readily available reference in sufficient detail that
interoperability between independent implementations is possible.
Each entry in the registry contains the following elements:
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a short name, such as "LMS_SHA256_M32_H10",
a positive number, and
a reference to a specification that completely defines the
signature method test cases that can be used to verify the
correctness of an implementation.
Requests to add an entry to the registry MUST include the name and
the reference. The number is assigned by IANA. Submitters SHOULD
have their requests reviewed by the IRTF Crypto Forum Research Group
(CFRG) at cfrg@ietf.org. Interested applicants that are unfamiliar
with IANA processes should visit http://www.iana.org.
The numbers between 0xDDDDDDDD (decimal 3,722,304,989) and 0xFFFFFFFF
(decimal 4,294,967,295) inclusive, will not be assigned by IANA, and
are reserved for private use; no attempt will be made to prevent
multiple sites from using the same value in different (and
incompatible) ways [RFC2434].
The LMOTS registry is as follows.
++++
 Name  Reference  Numeric Identifier 
++++
 LMOTS_SHA256_N32_W1  Section 4  0x00000001 
   
 LMOTS_SHA256_N32_W2  Section 4  0x00000002 
   
 LMOTS_SHA256_N32_W4  Section 4  0x00000003 
   
 LMOTS_SHA256_N32_W8  Section 4  0x00000004 
++++
Table 3
The LMS registry is as follows.
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++++
 Name  Reference  Numeric Identifier 
++++
 LMS_SHA256_M32_H5  Section 5  0x00000005 
   
 LMS_SHA256_M32_H10  Section 5  0x00000006 
   
 LMS_SHA256_M32_H15  Section 5  0x00000007 
   
 LMS_SHA256_M32_H20  Section 5  0x00000008 
   
 LMS_SHA256_M32_H25  Section 5  0x00000009 
++++
Table 4
An IANA registration of a signature system does not constitute an
endorsement of that system or its security.
11. Intellectual Property
This draft is based on U.S. patent 5,432,852, which issued over
twenty years ago and is thus expired.
11.1. Disclaimer
This document is not intended as legal advice. Readers are advised
to consult with their own legal advisers if they would like a legal
interpretation of their rights.
The IETF policies and processes regarding intellectual property and
patents are outlined in [RFC3979] and [RFC4879] and at
https://datatracker.ietf.org/ipr/about.
12. Security Considerations
The hash function H MUST have second preimage resistance: it must be
computationally infeasible for an attacker that is given one message
M to be able to find a second message M' such that H(M) = H(M').
The security goal of a signature system is to prevent forgeries. A
successful forgery occurs when an attacker who does not know the
private key associated with a public key can find a message and
signature that are valid with that public key (that is, the Signature
Verification algorithm applied to that signature and message and
public key will return VALID). Such an attacker, in the strongest
case, may have the ability to forge valid signatures for an arbitrary
number of other messages.
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LMS is provably secure in the random oracle model, where the hash
compression function is considered the random oracle, as shown by
[Fluhrer17]. Corollary 1 of that paper states:
If we have no more than 2^64 randomly chosen LMS private keys,
allow the attacker access to a signing oracle and a SHA256 hash
compression oracle, and allow a maximum of 2^120 hash compression
computations, then the probability of an attacker being able to
generate a single forgery against any of those LMS keys is less
than 2^129.
The format of the inputs to the hash function H have the property
that each invocation of that function has an input that is repeated
by a small bounded number of other inputs (due to potential repeats
of the I value), and in particular, will vary somewhere in the first
23 bytes of the value being hashed. This property is important for a
proof of security in the random oracle model. The formats used
during key generation and signing are
I  u32str(q)  u16str(i)  u8str(j)  tmp
I  u32str(q)  u16str(D_PBLC)  y[0]  ...  y[p1]
I  u32str(q)  u16str(D_MESG)  C  message
I  u32str(r)  u16str(D_LEAF)  OTS_PUB[r2^h]
I  u32str(r)  u16str(D_INTR)  T[2*r]  T[2*r+1]
I  u32str(q)  u16str(j)  u8str(0xff)  SEED
Each hash type listed is distinct; at locations 20, 21 of each hash,
there exists either a fixed value D_PBLC, D_MESG, D_LEAF, D_INTR, or
a 16 bit value (i or j). These fixed values are distinct from each
other, and large (over 32768), while the 16 bit values are small
(currently no more than 265; possibly being slightly larger if larger
hash functions are supported; hence the hash invocations with i/j
will not collide any of the D_PBLC, D_MESG, D_LEAF, D_INTR hashes.
The only other collision possibility is the Winternitz chain hash
colliding with the recommended pseudorandom key generation process;
here, at location 22, the Winternitz chain function has the value
u8str(j), where j is a value between 0 and 254, while location 22 of
the recommended pseudorandom key generation process has value 255.
For the Winternitz chaining function, D_PBLC, and D_MESG, the value
of I  u32str(q) is distinct for each LMS leaf (or equivalently, for
each q value). For the Winternitz chaining function, the value of
u16str(i)  u8str(j) is distinct for each invocation of H for a
given leaf. For D_PBLC and D_MESG, the input format is used only
once for each value of q, and thus distinctness is assured. The
formats for D_INTR and D_LEAF are used exactly once for each value of
r, which ensures their distinctness. For the recommended
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pseuddorandom key generation process, for a given value of I, q and j
are distinct for each invocation of H.
The value of I is chosen uniformly at random from the set of all 128
bit strings. If 2^64 public keys are generated (and hence 2^64
random I values), there is a nontrivial probability of a duplicate
(which would imply duplicate prefixes. However, there will be an
extremely high probability there there will not be a fourway
collision (that is, any I value used for four distinct LMS keys;
probability < 2^132), and hence the number of repeats for any
specific prefix will be limited to 3. This can be shown (in
[Fluhrer17]) to have only a limited ebffect on the security of the
system.
12.1. Stateful signature algorithm
The LMS signature system, like all Ntime signature systems, requires
that the signer maintain state across different invocations of the
signing algorithm, to ensure that none of the component onetime
signature systems are used more than once. This section calls out
some important practical considerations around this statefulness.
In a typical computing environment, a private key will be stored in
nonvolatile media such as on a hard drive. Before it is used to
sign a message, it will be read into an application's Random Access
Memory (RAM). After a signature is generated, the value of the
private key will need to be updated by writing the new value of the
private key into nonvolatile storage. It is essential for security
that the application ensure that this value is actually written into
that storage, yet there may be one or more memory caches between it
and the application. Memory caching is commonly done in the file
system, and in a physical memory unit on the hard disk that is
dedicated to that purpose. To ensure that the updated value is
written to physical media, the application may need to take several
special steps. In a POSIX environment, for instance, the O_SYNC flag
(for the open() system call) will cause invocations of the write()
system call to block the calling process until the data has been to
the underlying hardware. However, if that hardware has its own
memory cache, it must be separately dealt with using an operating
system or device specific tool such as hdparm to flush the ondrive
cache, or turn off write caching for that drive. Because these
details vary across different operating systems and devices, this
note does not attempt to provide complete guidance; instead, we call
the implementer's attention to these issues.
When hierarchical signatures are used, an easy way to minimize the
private key synchronization issues is to have the private key for the
second level resident in RAM only, and never write that value into
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nonvolatile memory. A new second level public/private key pair will
be generated whenever the application (re)starts; thus, failures such
as a power outage or application crash are automatically
accommodated. Implementations SHOULD use this approach wherever
possible.
12.2. Security of LMOTS Checksum
To show the security of LMOTS checksum, we consider the signature y
of a message with a private key x and let h = H(message) and
c = Cksm(H(message)) (see Section 4.6). To attempt a forgery, an
attacker may try to change the values of h and c. Let h' and c'
denote the values used in the forgery attempt. If for some integer j
in the range 0 to u, where u = ceil(8*n/w) is the size of the range
that the checksum value can over), inclusive,
a' = coef(h', j, w),
a = coef(h, j, w), and
a' > a
then the attacker can compute F^a'(x[j]) from F^a(x[j]) = y[j] by
iteratively applying function F to the j^th term of the signature an
additional (a'  a) times. However, as a result of the increased
number of hashing iterations, the checksum value c' will decrease
from its original value of c. Thus a valid signature's checksum will
have, for some number k in the range u to (p1), inclusive,
b' = coef(c', k, w),
b = coef(c, k, w), and
b' < b
Due to the oneway property of F, the attacker cannot easily compute
F^b'(x[k]) from F^b(x[k]) = y[k].
13. Comparison with other work
The eXtended Merkle Signature Scheme (XMSS) [XMSS] is similar to HSS
in several ways. Both are stateful hash based signature schemes, and
both use a hierarchical approach, with a Merkle tree at each level of
the hierarchy. XMSS signatures are slightly shorter than HSS
signatures, for equivalent security and an equal number of
signatures.
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HSS has several advantages over XMSS. HSS operations are roughly
four times faster than the comparable XMSS ones, when SHA256 is used
as the underlying hash. This occurs because the hash operation done
as a part of the Winternitz iterations dominates performance, and
XMSS performs four compression function invocations (two for the PRF,
two for the F function) where HSS need only perform one.
Additionally, HSS is somewhat simpler, and it admits a singlelevel
tree in a simple way (as described in Section 6.2).
Another advantage of HSS is the fact that it can use a stateless
hashbased signature scheme in its nonvolatile levels, while
continuing to use LMS in its volatile levels, and thus realize a
hybrid stateless/stateful scheme as described in [STMGMT]. While we
conjecture that hybrid schemes will offer lower computation times and
signature sizes than purely stateless schemes, the details are
outside the scope of this note. HSS is therefore amenable to future
extensions that will enable it to be used in environments in which a
purely stateful scheme would be too brittle.
SPHINCS [SPHINCS] is a purely stateless hash based signature scheme.
While that property benefits security, its signature sizes and
generation times are an order of magnitude (or more) larger than
those of HSS, making it more difficult to adopt in some practical
scenarios.
14. Acknowledgements
Thanks are due to Chirag Shroff, Andreas Huelsing, Burt Kaliski, Eric
Osterweil, Ahmed Kosba, Russ Housley and Philip Lafrance for
constructive suggestions and valuable detailed review. We especially
acknowledge Jerry Solinas, Laurie Law, and Kevin Igoe, who pointed
out the security benefits of the approach of Leighton and Micali
[USPTO5432852] and Jonathan Katz, who gave us security guidance.
15. References
15.1. Normative References
[FIPS180] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS 1804, March 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
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[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434,
DOI 10.17487/RFC2434, October 1998,
.
[RFC3979] Bradner, S., Ed., "Intellectual Property Rights in IETF
Technology", RFC 3979, DOI 10.17487/RFC3979, March 2005,
.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, .
[RFC4879] Narten, T., "Clarification of the Third Party Disclosure
Procedure in RFC 3979", RFC 4879, DOI 10.17487/RFC4879,
April 2007, .
[USPTO5432852]
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.
15.2. Informative References
[C:Merkle87]
Merkle, R., "A Digital Signature Based on a Conventional
Encryption Function", Lecture Notes in Computer
Science crypto87vol, 1988.
[C:Merkle89a]
Merkle, R., "A Certified Digital Signature", Lecture Notes
in Computer Science crypto89vol, 1990.
[C:Merkle89b]
Merkle, R., "One Way Hash Functions and DES", Lecture
Notes in Computer Science crypto89vol, 1990.
[Fluhrer17]
Fluhrer, S., "Further analysis of a proposed hashbased
signature standard",
EPrint http://eprint.iacr.org/2017/553.pdf, 2017.
[Grover96]
Grover, L., "A fast quantum mechanical algorithm for
database search", 28th ACM Symposium on the Theory of
Computing p. 212, 1996.
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[Katz16] Katz, J., "Analysis of a proposed hashbased signature
standard", Security Standardization Research (SSR)
Conference http://www.cs.umd.edu/~jkatz/papers/
HashBasedSigsSSR16.pdf, 2016.
[Merkle79]
Merkle, R., "Secrecy, Authentication, and Public Key
Systems", Stanford University Information Systems
Laboratory Technical Report 19791, 1979.
[SPHINCS] Bernstein, D., Hopwood, D., Hulsing, A., Lange, T.,
Niederhagen, R., Papachristadoulou, L., Schneider, M.,
Schwabe, P., and Z. WilcoxO'Hearn, "SPHINCS: Practical
Stateless HashBased Signatures.", Annual International
Conference on the Theory and Applications of Cryptographic
Techniques Springer., 2015.
[STMGMT] McGrew, D., Fluhrer, S., Kampanakis, P., Gazdag, S.,
Butin, D., and J. Buchmann, "State Management for Hash
based Signatures.", Security Standardization Resarch (SSR)
Conference 224., 2016.
[XMSS] Buchmann, J., Dahmen, E., and . Andreas Hulsing, "XMSSa
practical forward secure signature scheme based on minimal
security assumptions.", International Workshop on Post
Quantum Cryptography Springer Berlin., 2011.
Appendix A. Pseudorandom Key Generation
An implementation MAY use the following pseudorandom process for
generating an LMS private key.
SEED is an mbyte value that is generated uniformly at random at
the start of the process,
I is LMS key pair identifier,
q denotes the LMS leaf number of an LMOTS private key,
x_q denotes the x array of private elements in the LMOTS private
key with leaf number q,
j is an index of the private key element, and
H is the hash function used in LMOTS.
The elements of the LMOTS private keys are computed as:
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x_q[j] = H(I  u32str(q)  u16str(j)  u8str(0xff)  SEED).
This process stretches the mbyte random value SEED into a (much
larger) set of pseudorandom values, using a unique counter in each
invocation of H. The format of the inputs to H are chosen so that
they are distinct from all other uses of H in LMS and LMOTS. A
careful reader will note that this is similar to the hash we perform
when iterating through the Winternitz chain; however in that chain,
the iteration index will vary between 0 and 254 maximum (for W=8),
while the corresponding value in this formula is 255. This algorithm
is included in the proof of security in [Fluhrer17] and hence this
method is safe when used within the LMS system; however any other
cryptographical secure method of generating private keys would also
be safe.
Appendix B. LMOTS Parameter Options
A table illustrating various combinations of n and w with the
associated values of u, v, ls, and p is provided in Table 5.
The parameters u, v, ls, and p are computed as follows:
u = ceil(8*n/w)
v = ceil((floor(lg((2^w  1) * u)) + 1) / w)
ls = (number of bits in sum)  (v * w)
p = u + v
Here u and v represent the number of wbit fields required to contain
the hash of the message and the checksum byte strings, respectively.
The "number of bits in sum" is defined according to Section 4.5. And
as the value of p is the number of wbit elements of
( H(message)  Cksm(H(message)) ), it is also equivalently the
number of byte strings that form the private key and the number of
byte strings in the signature.
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+++++++
 Hash  Winternitz  wbit  wbit  Left  Total 
 Length  Parameter  Elements  Elements  Shift  Number of 
 in  (w)  in Hash  in  (ls)  wbit 
 Bytes   (u)  Checksum   Elements 
 (n)    (v)   (p) 
+++++++
 16  1  128  8  8  137 
      
 16  2  64  4  8  68 
      
 16  4  32  3  4  35 
      
 16  8  16  2  0  18 
      
 32  1  256  9  7  265 
      
 32  2  128  5  6  133 
      
 32  4  64  3  4  67 
      
 32  8  32  2  0  34 
+++++++
Table 5
Appendix C. An iterative algorithm for computing an LMS public key
The LMS public key can be computed using the following algorithm or
any equivalent method. The algorithm uses a stack of hashes for
data. It also makes use of a hash function with the typical
init/update/final interface to hash functions; the result of the
invocations hash_init(), hash_update(N[1]), hash_update(N[2]), ... ,
hash_update(N[n]), v = hash_final(), in that order, is identical to
that of the invocation of H(N[1]  N[2]  ...  N[n]).
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Generating an LMS Public Key From an LMS Private Key
for ( i = 0; i < num_lmots_keys; i = i + 1 ) {
r = i + num_lmots_keys;
temp = H(I  OTS_PUBKEY[i]  u32str(r)  D_LEAF)
j = i;
while (j % 2 == 1) {
r = (r  1)/2; j = (j1) / 2;
left_size = pop(data stack);
temp = H(I  left_side  temp  u32str(r)  D_INTR)
}
push temp onto the data stack
}
public_key = pop(data stack)
Note that this pseudocode expects that all 2^h leaves of the tree
have equal depth; that is, num_lmots_keys to be a power of 2. The
maximum depth of the stack will be h1 elements, that is, a total of
(h1)*n bytes; for the currently defined parameter sets, this will
never be more than 768 bytes of data.
Appendix D. Example Implementation
An example implementation can be found online at
http://github.com/davidmcgrew/hashsigs/.
Appendix E. Test Cases
This section provides test cases that can be used to verify or debug
an implementation. This data is formatted with the name of the
elements on the left, and the value of the elements on the right, in
hexadecimal. The concatenation of all of the values within a public
key or signature produces that public key or signature, and values
that do not fit within a single line are listed across successive
lines.
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Test Case 1 Public Key

HSS public key
levels 00000002

LMS type 00000005 # LM_SHA256_M32_H5
LMOTS type 00000004 # LMOTS_SHA256_N32_W8
I 61a5d57d37f5e46bfb7520806b07a1b8
K 50650e3b31fe4a773ea29a07f09cf2ea
30e579f0df58ef8e298da0434cb2b878


Test Case 1 Message

Message 54686520706f77657273206e6f742064 The powers not d
656c65676174656420746f2074686520 elegated to the 
556e6974656420537461746573206279 United States by
2074686520436f6e737469747574696f  the Constitutio
6e2c206e6f722070726f686962697465 n, nor prohibite
6420627920697420746f207468652053 d by it to the S
74617465732c20617265207265736572 tates, are reser
76656420746f20746865205374617465 ved to the State
7320726573706563746976656c792c20 s respectively, 
6f7220746f207468652070656f706c65 or to the people
2e0a ..

Test Case 1 Signature

HSS signature
Nspk 00000001
sig[0]:

LMS signature
q 00000005

LMOTS signature
LMOTS type 00000004 # LMOTS_SHA256_N32_W8
C d32b56671d7eb98833c49b433c272586
bc4a1c8a8970528ffa04b966f9426eb9
y[0] 965a25bfd37f196b9073f3d4a232feb6
9128ec45146f86292f9dff9610a7bf95
y[1] a64c7f60f6261a62043f86c70324b770
7f5b4a8a6e19c114c7be866d488778a0
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y[2] e05fd5c6509a6e61d559cf1a77a970de
927d60c70d3de31a7fa0100994e162a2
y[3] 582e8ff1b10cd99d4e8e413ef469559f
7d7ed12c838342f9b9c96b83a4943d16
y[4] 81d84b15357ff48ca579f19f5e71f184
66f2bbef4bf660c2518eb20de2f66e3b
y[5] 14784269d7d876f5d35d3fbfc7039a46
2c716bb9f6891a7f41ad133e9e1f6d95
y[6] 60b960e7777c52f060492f2d7c660e14
71e07e72655562035abc9a701b473ecb
y[7] c3943c6b9c4f2405a3cb8bf8a691ca51
d3f6ad2f428bab6f3a30f55dd9625563
y[8] f0a75ee390e385e3ae0b906961ecf41a
e073a0590c2eb6204f44831c26dd768c
y[9] 35b167b28ce8dc988a3748255230cef9
9ebf14e730632f27414489808afab1d1
y[10] e783ed04516de012498682212b078105
79b250365941bcc98142da13609e9768
y[11] aaf65de7620dabec29eb82a17fde35af
15ad238c73f81bdb8dec2fc0e7f93270
y[12] 1099762b37f43c4a3c20010a3d72e2f6
06be108d310e639f09ce7286800d9ef8
y[13] a1a40281cc5a7ea98d2adc7c7400c2fe
5a101552df4e3cccfd0cbf2ddf5dc677
y[14] 9cbbc68fee0c3efe4ec22b83a2caa3e4
8e0809a0a750b73ccdcf3c79e6580c15
y[15] 4f8a58f7f24335eec5c5eb5e0cf01dcf
4439424095fceb077f66ded5bec73b27
y[16] c5b9f64a2a9af2f07c05e99e5cf80f00
252e39db32f6c19674f190c9fbc506d8
y[17] 26857713afd2ca6bb85cd8c107347552
f30575a5417816ab4db3f603f2df56fb
y[18] c413e7d0acd8bdd81352b2471fc1bc4f
1ef296fea1220403466b1afe78b94f7e
y[19] cf7cc62fb92be14f18c2192384ebceaf
8801afdf947f698ce9c6ceb696ed70e9
y[20] e87b0144417e8d7baf25eb5f70f09f01
6fc925b4db048ab8d8cb2a661ce3b57a
y[21] da67571f5dd546fc22cb1f97e0ebd1a6
5926b1234fd04f171cf469c76b884cf3
y[22] 115cce6f792cc84e36da58960c5f1d76
0f32c12faef477e94c92eb75625b6a37
y[23] 1efc72d60ca5e908b3a7dd69fef02491
50e3eebdfed39cbdc3ce9704882a2072
y[24] c75e13527b7a581a556168783dc1e975
45e31865ddc46b3c957835da252bb732
y[25] 8d3ee2062445dfb85ef8c35f8e1f3371
af34023cef626e0af1e0bc017351aae2
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y[26] ab8f5c612ead0b729a1d059d02bfe18e
fa971b7300e882360a93b025ff97e9e0
y[27] eec0f3f3f13039a17f88b0cf808f4884
31606cb13f9241f40f44e537d302c64a
y[28] 4f1f4ab949b9feefadcb71ab50ef27d6
d6ca8510f150c85fb525bf25703df720
y[29] 9b6066f09c37280d59128d2f0f637c7d
7d7fad4ed1c1ea04e628d221e3d8db77
y[30] b7c878c9411cafc5071a34a00f4cf077
38912753dfce48f07576f0d4f94f42c6
y[31] d76f7ce973e9367095ba7e9a3649b7f4
61d9f9ac1332a4d1044c96aefee67676
y[32] 401b64457c54d65fef6500c59cdfb69a
f7b6dddfcb0f086278dd8ad0686078df
y[33] b0f3f79cd893d314168648499898fbc0
ced5f95b74e8ff14d735cdea968bee74

LMS type 00000005 # LM_SHA256_M32_H5
path[0] d8b8112f9200a5e50c4a262165bd342c
d800b8496810bc716277435ac376728d
path[1] 129ac6eda839a6f357b5a04387c5ce97
382a78f2a4372917eefcbf93f63bb591
path[2] 12f5dbe400bd49e4501e859f885bf073
6e90a509b30a26bfac8c17b5991c157e
path[3] b5971115aa39efd8d564a6b90282c316
8af2d30ef89d51bf14654510a12b8a14
path[4] 4cca1848cf7da59cc2b3d9d0692dd2a2
0ba3863480e25b1b85ee860c62bf5136

LMS public key
LMS type 00000005 # LM_SHA256_M32_H5
LMOTS type 00000004 # LMOTS_SHA256_N32_W8
I d2f14ff6346af964569f7d6cb880a1b6
K 6c5004917da6eafe4d9ef6c6407b3db0
e5485b122d9ebe15cda93cfec582d7ab

final_signature:

LMS signature
q 0000000a

LMOTS signature
LMOTS type 00000004 # LMOTS_SHA256_N32_W8
C 0703c491e7558b35011ece3592eaa5da
4d918786771233e8353bc4f62323185c
y[0] 95cae05b899e35dffd71705470620998
8ebfdf6e37960bb5c38d7657e8bffeef
y[1] 9bc042da4b4525650485c66d0ce19b31
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7587c6ba4bffcc428e25d08931e72dfb
y[2] 6a120c5612344258b85efdb7db1db9e1
865a73caf96557eb39ed3e3f426933ac
y[3] 9eeddb03a1d2374af7bf771855774562
37f9de2d60113c23f846df26fa942008
y[4] a698994c0827d90e86d43e0df7f4bfcd
b09b86a373b98288b7094ad81a0185ac
y[5] 100e4f2c5fc38c003c1ab6fea479eb2f
5ebe48f584d7159b8ada03586e65ad9c
y[6] 969f6aecbfe44cf356888a7b15a3ff07
4f771760b26f9c04884ee1faa329fbf4
y[7] e61af23aee7fa5d4d9a5dfcf43c4c26c
e8aea2ce8a2990d7ba7b57108b47dabf
y[8] beadb2b25b3cacc1ac0cef346cbb90fb
044beee4fac2603a442bdf7e507243b7
y[9] 319c9944b1586e899d431c7f91bcccc8
690dbf59b28386b2315f3d36ef2eaa3c
y[10] f30b2b51f48b71b003dfb08249484201
043f65f5a3ef6bbd61ddfee81aca9ce6
y[11] 0081262a00000480dcbc9a3da6fbef5c
1c0a55e48a0e729f9184fcb1407c3152
y[12] 9db268f6fe50032a363c9801306837fa
fabdf957fd97eafc80dbd165e435d0e2
y[13] dfd836a28b354023924b6fb7e48bc0b3
ed95eea64c2d402f4d734c8dc26f3ac5
y[14] 91825daef01eae3c38e3328d00a77dc6
57034f287ccb0f0e1c9a7cbdc828f627
y[15] 205e4737b84b58376551d44c12c3c215
c812a0970789c83de51d6ad787271963
y[16] 327f0a5fbb6b5907dec02c9a90934af5
a1c63b72c82653605d1dcce51596b3c2
y[17] b45696689f2eb382007497557692caac
4d57b5de9f5569bc2ad0137fd47fb47e
y[18] 664fcb6db4971f5b3e07aceda9ac130e
9f38182de994cff192ec0e82fd6d4cb7
y[19] f3fe00812589b7a7ce51544045643301
6b84a59bec6619a1c6c0b37dd1450ed4
y[20] f2d8b584410ceda8025f5d2d8dd0d217
6fc1cf2cc06fa8c82bed4d944e71339e
y[21] ce780fd025bd41ec34ebff9d4270a322
4e019fcb444474d482fd2dbe75efb203
y[22] 89cc10cd600abb54c47ede93e08c114e
db04117d714dc1d525e11bed8756192f
y[23] 929d15462b939ff3f52f2252da2ed64d
8fae88818b1efa2c7b08c8794fb1b214
y[24] aa233db3162833141ea4383f1a6f120b
e1db82ce3630b3429114463157a64e91
y[25] 234d475e2f79cbf05e4db6a9407d72c6
McGrew, et al. Expires December 22, 2017 [Page 43]
InternetDraft HashBased Signatures June 2017
bff7d1198b5c4d6aad2831db61274993
y[26] 715a0182c7dc8089e32c8531deed4f74
31c07c02195eba2ef91efb5613c37af7
y[27] ae0c066babc69369700e1dd26eddc0d2
16c781d56e4ce47e3303fa73007ff7b9
y[28] 49ef23be2aa4dbf25206fe45c20dd888
395b2526391a724996a44156beac8082
y[29] 12858792bf8e74cba49dee5e8812e019
da87454bff9e847ed83db07af3137430
y[30] 82f880a278f682c2bd0ad6887cb59f65
2e155987d61bbf6a88d36ee93b6072e6
y[31] 656d9ccbaae3d655852e38deb3a2dcf8
058dc9fb6f2ab3d3b3539eb77b248a66
y[32] 1091d05eb6e2f297774fe6053598457c
c61908318de4b826f0fc86d4bb117d33
y[33] e865aa805009cc2918d9c2f840c4da43
a703ad9f5b5806163d7161696b5a0adc

LMS type 00000005 # LM_SHA256_M32_H5
path[0] d5c0d1bebb06048ed6fe2ef2c6cef305
b3ed633941ebc8b3bec9738754cddd60
path[1] e1920ada52f43d055b5031cee6192520
d6a5115514851ce7fd448d4a39fae2ab
path[2] 2335b525f484e9b40d6a4a969394843b
dcf6d14c48e8015e08ab92662c05c6e9
path[3] f90b65a7a6201689999f32bfd368e5e3
ec9cb70ac7b8399003f175c40885081a
path[4] 09ab3034911fe125631051df0408b394
6b0bde790911e8978ba07dd56c73e7ee
Authors' Addresses
David McGrew
Cisco Systems
13600 Dulles Technology Drive
Herndon, VA 20171
USA
Email: mcgrew@cisco.com
Michael Curcio
Cisco Systems
70252 Kit Creek Road
Research Triangle Park, NC 277094987
USA
Email: micurcio@cisco.com
McGrew, et al. Expires December 22, 2017 [Page 44]
InternetDraft HashBased Signatures June 2017
Scott Fluhrer
Cisco Systems
170 West Tasman Drive
San Jose, CA
USA
Email: sfluhrer@cisco.com
McGrew, et al. Expires December 22, 2017 [Page 45]