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Crypto Forum Research Group                                    D. McGrew
Internet-Draft                                                 M. Curcio
Intended status: Informational                                S. Fluhrer
Expires: April 9, 2018                                     Cisco Systems
                                                         October 6, 2017


                         Hash-Based Signatures
                       draft-mcgrew-hash-sigs-08

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 one-time 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 side-channel
   attacks.  Unlike most other signature systems, hash-based signatures
   would still be secure even if it proves feasible for an attacker to
   build a quantum computer.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 9, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions Used In This Document . . . . . . . . . . . .   4
   2.  Interface . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Notation  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Data Types  . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Operators . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Functions . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.3.  Strings of w-bit elements . . . . . . . . . . . . . .   6
     3.2.  Security string . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Typecodes . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  LM-OTS One-Time Signatures  . . . . . . . . . . . . . . . . .   9
     4.1.  Parameters  . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Parameter Sets  . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Private Key . . . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Public Key  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.5.  Checksum  . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.6.  Signature Generation  . . . . . . . . . . . . . . . . . .  13
     4.7.  Signature Verification  . . . . . . . . . . . . . . . . .  14
   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 . . . . . . . . . . . . . . . . . . .  21
     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 . . . . . . . . . . . . . . . . . . . . .  29
   11. Intellectual Property . . . . . . . . . . . . . . . . . . . .  30
     11.1.  Disclaimer . . . . . . . . . . . . . . . . . . . . . . .  30
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  31
     12.1.  Stateful signature algorithm . . . . . . . . . . . . . .  32
     12.2.  Security of LM-OTS Checksum  . . . . . . . . . . . . . .  33



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   13. Comparison with other work  . . . . . . . . . . . . . . . . .  34
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  34
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     15.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Appendix A.  Pseudorandom Key Generation  . . . . . . . . . . . .  36
   Appendix B.  LM-OTS 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

   One-time signature systems, and general purpose signature systems
   built out of one-time 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 moderatley slow key generation (in comparison with RSA
   and ECDSA).  In recent years there has been interest in these systems
   because of their post-quantum security and their suitability for
   compact verifier implementations.

   This note describes the Leighton and Micali adaptation [USPTO5432852]
   of the original Lamport-Diffie-Winternitz-Merkle one-time 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 One-Time
   Signature (OTS) system can be used to sign one message securely, but
   cannot securely sign more than one.  An N-time signature system can
   be used to sign N or fewer messages securely.  A Merkle tree
   signature scheme is an N-time signature system that uses an OTS
   system as a component.

   In this note we describe the Leighton-Micali 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 one-time signature scheme
   incorporate in LMS as LM-OTS.  This note is structured as follows.
   Notation is introduced in Section 3.  The LM-OTS signature system is



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   described in Section 4, and the LMS and HSS N-time signature systems
   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 changes made to this document over previous
   versions is listed in Section 9.  The IANA registry for these
   signature systems is described in Section 10.  Intellectual Property
   issues are discussed in Section 11.  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 N-time 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.






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3.  Notation

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 non-negative 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 non-negative integer; the identity
   u32str(strTou32(S)) = S holds for any four-byte 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
   right-shift 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.



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   The i^th element in an array A is denoted as A[i].

3.1.2.  Functions

   If r is a non-negative 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 base-2 logarithm of r

3.1.3.  Strings of w-bit 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 w-bit 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, w-bit value, if S
   is interpreted as a sequence of w-bit values.  That is,

       coef(S, i, w) = (2^w - 1) AND
                       ( byte(S, floor(i * w / 8)) >>
                         (8 - (w * (i % (8 / w)) + w)) )



















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   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 four-bit 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 w-bit values in S, then coef(S, i, w) is undefined, and
   an attempt to compute that value should raise an error.

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 will appear in fixed locations in the
   value we compute the hash of, and so we list where in the hash these
   fields would be present.  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 0-15
      of the hash.

      r - in the LMS N-time 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 32-bit (four byte) unsigned integer in network




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      byte order.  Either r or q (depending on the domain separation
      parameter) will be bytes 16-19 of the hash.

      q - in the LMS N-time signature scheme, each LM-OTS 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 LM-OTS public/private key pair.  This value is
      represented as a 32-bit (four byte) unsigned integer in network
      byte order.  Either r or q (depending on the domain separation
      parameter) will be bytes 16-19 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 LM-OTS algorithm

         D_MESG = 0x8181 when computing the hash of the message in the
         LM-OTS 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

         i = a value between 0 and 264; this is used in the LM-OTS
         scheme, when either computing the iterations of the Winternitz
         chain, or when using the suggested LM-OTS private key
         generation process.  The value is also the index of the LM-OTS
         private key element upon which H is being applied.  It is
         represented as a 16-bit (two byte) unsigned integer in network
         byte order.

      j - in the LM-OTS scheme, j is the iteration number used when the
      private key element is being iteratively hashed.  It is
      represented as an 8-bit (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 n-byte 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 22 to 21+n of the hash.





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3.3.  Typecodes

   A typecode is an unsigned integer that is associated with a
   particular data format.  The format of the LM-OTS, 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 four-byte unsigned integers in network byte order;
   equivalently, they are XDR enumerations (see Section 7).

4.  LM-OTS One-Time Signatures

   This section defines LM-OTS 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 one-time signatures;
   each private key MUST be used at most one time to sign any given
   message.

   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 N-time signature system, the LM-
   OTS key generation, signing, and verification algorithms all take as
   input a diversification parameter q (which is used as part of the
   security string, as listed in Section 3.2.  When the LM-OTS signature
   system is used outside of an N-time signature system, this value
   SHOULD be set to the all-zero 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 (in bits) of the Winternitz coefficients; it is a
      member of the set { 1, 2, 4, 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
      Cksm (defined in Section 4.5).

      H : a second-preimage-resistant cryptographic hash function that
      accepts byte strings of any length, and returns an n-byte string.



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   For more background on the cryptographic security requirements on H,
   see the Section 12.

   The value of n is determined by the hash function selected for use as
   part of the LM-OTS 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^w-1 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.

4.2.  Parameter Sets

   To fully describe a LM-OTS signature method, the parameters n and w,
   as well as the function H, MUST be specified.  This section defines
   several LM-OTS methods, each of which is identified by a name.  The
   values for p and ls are provided as a convenience; the formal method
   of computing them is given in Appendix B.

   The value of w describes a space/time trade-off; increasing the value
   of w will cause the signature to shrink (by decreasing the value of
   p) while increasing the amount of time needed to perform operations
   with it (generate the public key, generate and verify the signature);
   in general, the LM-OTS signature is n*p bytes long, and public key
   generation will take p*(2^w-1)+1 hash computations (and signature
   generation and verification will take about half that on average).

           +---------------------+--------+----+---+-----+----+
           | 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].





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4.3.  Private Key

   The format of the LM-OTS private key is an internal matter to the
   implementation, and this document does not attempt to define it.  One
   possibility is that the private key may consist of a typecode
   indicating the particular LM-OTS algorithm, an array x[] containing p
   n-byte 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. retrieve the value p from the type, and the value I the 16 byte
     identifier of the LMS public/private keypair that this LM-OTS private
     key will be used with

  2. set type to the typecode of the algorithm

  3. set n and p according to the typecode and Table 1

  4. compute the array x as follows:
     for ( i = 0; i < p; i = i + 1 ) {
       set x[i] to a uniformly random n-byte string
     }

  5. return u32str(type) || I || u32str(q) || x[0] || x[1] || ... || x[p-1]

   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
   MUST match that of the LM-OTS algorithm.  Appendix A provides an
   example of a pseudorandom method for computing the LM-OTS private
   key.

4.4.  Public Key

   The LM-OTS 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.









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   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[p-1])

   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
   n-byte string that is input to that function, and the value sum is a
   16-bit unsigned integer:

   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 left-shift 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.








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4.6.  Signature Generation

   The LM-OTS 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 w-bit values, and
   using each of the w-bit 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.

   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 n-byte 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[p-1]

   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.








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4.7.  Signature Verification

   In order to verify a message with its signature (an array of n-byte
   strings, denoted as y), the receiver must "complete" the chain of
   iterations of H using the w-bit 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.

   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 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 pubtype, return INVALID

     c. set n and p according to the pubtype 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[p-1] = 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[p-1])

  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 one-time 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 m-byte value,
   where m is the size of the hash function.  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 second-preimage-resistant cryptographic hash function that
      accepts byte strings of any length, and returns an m-byte 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
   LM-OTS 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

   The format of the LMS private key is an internal matter to the
   implementation, and this document does not attempt to define it.  One
   possibility is that it may consist of an array OTS_PRIV[] of 2^h LM-



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   OTS private keys, and the leaf number q of the next LM-OTS private
   key that has not yet been used.  The q^th element of OTS_PRIV[] is
   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] = LM-OTS 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 LM-OTS 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 balanced binary tree, and the nodes of
   that tree are indexed from 1 to 2^(h+1)-1.  Each node is associated
   with an m-byte string, and the string for the r^th node is denoted as
   T[r] and is defined as

   T[r]=/ H(I||u32str(r)||u16str(D_LEAF)||OTS_PUB[r-2^h])   if r >= 2^h,
        \ H(I||u32str(r)||u16str(D_INTR)||T[2*r]||T[2*r+1]) otherwise.

   The LMS public key is the string u32str(type) || u32str(otstype) ||
   I || T[1].  Section 7 specifies the format of the type variable.  The
   value otstype is the parameter set for the LM-OTS public/private
   keypairs used.  The value I is the private key identifier, 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 LM-OTS signature, as
      a four-byte unsigned integer in network byte order,

      an LM-OTS signature, and

      a typecode indicating the particular LMS algorithm,

      an array of h m-byte values that is associated with the path
      through the tree from the leaf associated with the LM-OTS
      signature to the root.

   Symbolically, the signature can be represented as u32str(q) ||
   ots_signature || u32str(type) || path[0] || path[1] || ... ||
   path[h-1].  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 LM-OTS signature of the message using
   the leaf number of the next unused LM-OTS 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 LM-OTS 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 issue this tries to prevent is a
   scenario where a) we generate a signature, using one LM-OTS private
   key, and release it to the application, b) before we update the
   nonvolatile memory, we crash, and c) we reboot, and generate a second
   signature using the same LM-OTS private key; with two different
   signatures using the same LM-OTS private key, someone could
   potentially generate a forged signature of a third message.

   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



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   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
   node-caching 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 LM-OTS signature
   verification algorithm (Algorithm 4b) to compute the LM-OTS public
   key from the LM-OTS 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. set m according to pubtype, based on Table 2

        c. if the public key is not exactly 20 + m bytes
           long, return INVALID

        d. I = next 16 bytes of the public key

        e. T[1] = next m bytes of the public key

     3. compute the candidate LMS root value Tc from the signature,
        message, identifier and pubtype using Algorithm 6b.

     4. 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




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  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)

    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[h-1] = next m bytes of signature

  3. Kc = candidate public key computed by applying Algorithm 4b
     to the signature ots_signature, the message, and the
     identifiers I, q

  4. 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(I||u32str(node_num/2)||u16str(D_INTR)||path[i]||tmp)
       else:
         tmp = H(I||u32str(node_num/2)||u16str(D_INTR)||tmp||path[i])
       node_num = node_num/2
       i = i + 1
     }
     Tc = tmp




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  5. return Tc

6.  Hierarchical signatures

   In scenarios where it is necessary to minimize the time taken by the
   public key generation process, a Hierarchical N-time Signature System
   (HSS) can be used.  Leighton and Micali describe a scheme in which an
   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 L-1 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 N-time 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, ... , L-1.



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   These key pairs, and their identifiers, MUST be generated
   independently.

   The public key of the HSS scheme 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[L-1].  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[L-1] is exhausted, then determine the smallest integer d
      such that all of the private keys prv[d], prv[d+1], ... , prv[L-1]
      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 L-1 must be
      regenerated during the signature generation process, as follows.
      For i from d to L-1, 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[i-1], and
      sig[i-1] is set to the resulting value.

      The message is signed with prv[L-1], and the value sig[L-1] 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 = L-1 denotes the number of signed public
      keys.  Then the HSS signature is u32str(Nspk) ||
      signed_pub_key[0] || ... || signed_pub_key[Nspk-1] || 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[Nspk-1] || sig[Nspk]



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   which is equivalent to

   u32str(Nspk) || sig[0] || pub[1] || ... || sig[Nspk-1] || 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:

      Nspk = strTou32(first four bytes of S)
      if Nspk+1 is not equal to the number of levels L in pub:
         return INVALID
      for (i = 0; i < Nspk; i = i + 1) {
         siglist[i] = next LMS signature parsed from S
         publist[i] = next LMS public key parsed from S
      }
      siglist[Nspk] = next LMS signature parsed from S

      key = pub
      for (i = 0; i < Nspk; 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[Nspk])


   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



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   follows:


   /* one-time 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 */
   };




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   /* hash based signatures (hbs) */

   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:



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      void;     /* error condition */
   };

   /* 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
   variable-length 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 LM-OTS, 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 follow-on 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).




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   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 SHA-256
   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 (SHA-512/256 and the various SHA-3 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 32-bit
   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 non-negative integer
   "sum", whose width was chosen to be an integer number of w-bit 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 16-bit non-negative 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 left-shift operation.

9.  History

   This is the eigth version of this draft.  It has the following
   changes from previous versions:

   Version 07

      Corrected the LMS public key format specification in section 5.3;
      the format listed in section 7 was correct.

      Corrected the LMS public key generation algorithm in appendix C.
      The hashes listed in section 5.3 were correct.

      Corrected the HSS signature verification algorithm in section 6.3.

      Miscellaneous clarifications and typo corrections.

   Version 06

      Modified the order of the values that were hashed to make it
      easier to prove security.



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      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 hash-based signatures.

   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 Merkle-Damgard.

      The Hierarchical Signature Scheme was generalized slightly so that
      it can use more than two levels.






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      Several points of confusion were corrected; these had resulted
      from incomplete or inconsistent changes from the Merkle approach
      of the earlier draft to the Leighton-Micali 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 Leighton-Micali
   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:

      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 LM-OTS registry is as follows.













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         +----------------------+-----------+--------------------+
         | 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.

          +--------------------+-----------+--------------------+
          | 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.






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   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.

   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 SHA-256 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[p-1]
      I || u32str(q) || u16str(D_MESG) || C || message
      I || u32str(r) || u16str(D_LEAF) || OTS_PUB[r-2^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



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   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
   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 will not be a four-way 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 effect on the security of the system.

12.1.  Stateful signature algorithm

   The LMS signature system, like all N-time signature systems, requires
   that the signer maintain state across different invocations of the
   signing algorithm, to ensure that none of the component one-time
   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
   non-volatile 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 non-volatile 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



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   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 on-drive
   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
   non-volatile 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 LM-OTS Checksum

   To show the security of LM-OTS 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 cover), 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 (p-1), inclusive,

      b' = coef(c', k, w),




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      b = coef(c, k, w), and

      b' < b

   Due to the one-way 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.

   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 allows a single-level
   tree in a simple way (as described in Section 6.2).

14.  Acknowledgements

   Thanks are due to Chirag Shroff, Andreas Huelsing, Burt Kaliski, Eric
   Osterweil, Ahmed Kosba, Russ Housley, Philip Lafrance, Alexander
   Truskovsky, Mark Peruzel 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, and Bruno Couillard and Jim Goodman for an
   especially thorough review.

15.  References

15.1.  Normative References

   [FIPS180]  National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS 180-4, 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,
              <https://www.rfc-editor.org/info/rfc2119>.




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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,
              <https://www.rfc-editor.org/info/rfc2434>.

   [RFC3979]  Bradner, S., Ed., "Intellectual Property Rights in IETF
              Technology", RFC 3979, DOI 10.17487/RFC3979, March 2005,
              <https://www.rfc-editor.org/info/rfc3979>.

   [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
              Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
              2006, <https://www.rfc-editor.org/info/rfc4506>.

   [RFC4879]  Narten, T., "Clarification of the Third Party Disclosure
              Procedure in RFC 3979", RFC 4879, DOI 10.17487/RFC4879,
              April 2007, <https://www.rfc-editor.org/info/rfc4879>.

   [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 hash-based
              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 hash-based signature
              standard", Security Standardization Research (SSR)
              Conference
              http://www.cs.umd.edu/~jkatz/papers/HashBasedSigs-
              SSR16.pdf, 2016.

   [Merkle79]
              Merkle, R., "Secrecy, Authentication, and Public Key
              Systems", Stanford University Information Systems
              Laboratory Technical Report 1979-1, 1979.

   [SPHINCS]  Bernstein, D., Hopwood, D., Hulsing, A., Lange, T.,
              Niederhagen, R., Papachristadoulou, L., Schneider, M.,
              Schwabe, P., and Z. Wilcox-O'Hearn, "SPHINCS: Practical
              Stateless Hash-Based 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, "XMSS-a
              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 m-byte 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 LM-OTS private key,

      x_q denotes the x array of private elements in the LM-OTS private
      key with leaf number q,

      j is an index of the private key element, and

      H is the hash function used in LM-OTS.

   The elements of the LM-OTS 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 m-byte 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 LM-OTS.  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.  LM-OTS Parameter Options

   The LM-OTS one time signature method uses several internal
   parameters, which are a function of the selected parameter set.
   These internal parameters set:

      p - This is the number of independent Winternitz chains used in
      the signature; it will be the number of w-bit digits needed to
      hold the n-bit hash (u in the below equations), along with the
      number of digits needed to hold the checksum (v in the below
      equations)

      ls - This is the size of the shift needed to move the checksum so
      that it appears in the checksum digits

   The parameters ls, and p are computed as follows:

     u = ceil(8*n/w)
     v = ceil((floor(lg((2^w - 1) * u)) + 1) / w)
     ls = 16 - (v * w)
     p = u + v

   Here u and v represent the number of w-bit fields required to contain
   the hash of the message and the checksum byte strings, respectively.
   And as the value of p is the number of w-bit 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.  The value 16 in the ls computation of
   ls corresponds to the 16 bits value used for the sum variable in
   Algorithm 2 in Section 4.5

   A table illustrating various combinations of n and w with the
   associated values of u, v, ls, and p is provided in Table 5.



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   +---------+------------+-----------+-----------+-------+------------+
   |   Hash  | Winternitz |   w-bit   |   w-bit   |  Left |   Total    |
   |  Length | Parameter  |  Elements |  Elements | Shift | Number of  |
   |    in   |    (w)     |  in Hash  |     in    |  (ls) |   w-bit    |
   |  Bytes  |            |    (u)    |  Checksum |       |  Elements  |
   |   (n)   |            |           |    (v)    |       |    (p)     |
   +---------+------------+-----------+-----------+-------+------------+
   |    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 < 2^h; i = i + 1 ) {
       r = i + num_lmots_keys;
       temp = H(I || u32str(r) || u16str(D_LEAF) || OTS_PUBKEY[i])
       j = i;
       while (j % 2 == 1) {
         r = (r - 1)/2;
         j = (j-1) / 2;
         left_side = pop(data stack);
         temp = H(I || u32str(r) || u16str(D_INTR) || left_side || temp)
       }
       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 h-1 elements, that is, a total of
   (h-1)*n bytes; for the currently defined parameter sets, this will
   never be more than 768 bytes of data.

Appendix D.  Example Implementation

   Two example implementations can be found online at
   http://github.com/davidmcgrew/hash-sigs/ and at
   http://github.com/cisco/hash-sigs.

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



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               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
   7025-2 Kit Creek Road
   Research Triangle Park, NC  27709-4987
   USA

   Email: micurcio@cisco.com



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   Scott Fluhrer
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA
   USA

   Email: sfluhrer@cisco.com












































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