Crypto Forum Research Group D. McGrew Internet-Draft M. Curcio Intended status: Informational S. Fluhrer Expires: May 4, 2017 Cisco Systems October 31, 2016 Hash-Based Signatures draft-mcgrew-hash-sigs-05 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on May 4, 2017. Copyright Notice Copyright (c) 2016 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 McGrew, et al. Expires May 4, 2017 [Page 1]

Internet-Draft Hash-Based Signatures October 2016 (http://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 . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Data Types . . . . . . . . . . . . . . . . . . . . . . . 4 3.1.1. Operators . . . . . . . . . . . . . . . . . . . . . . 5 3.1.2. Strings of w-bit elements . . . . . . . . . . . . . . 6 3.2. Security string . . . . . . . . . . . . . . . . . . . . . 7 3.3. Functions . . . . . . . . . . . . . . . . . . . . . . . . 8 3.4. Typecodes . . . . . . . . . . . . . . . . . . . . . . . . 8 4. LM-OTS One-Time Signatures . . . . . . . . . . . . . . . . . 8 4.1. Parameters . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Hashing Functions . . . . . . . . . . . . . . . . . . . . 9 4.3. Signature Methods . . . . . . . . . . . . . . . . . . . . 9 4.4. Private Key . . . . . . . . . . . . . . . . . . . . . . . 10 4.5. Public Key . . . . . . . . . . . . . . . . . . . . . . . 11 4.6. Checksum . . . . . . . . . . . . . . . . . . . . . . . . 11 4.7. Signature Generation . . . . . . . . . . . . . . . . . . 12 4.8. 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 . . . . . . . . . . . . . . . . . . . . . . 17 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 . . . . . . . . . . . . . . . . . 22 7. Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 8. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9. History . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 11. Intellectual Property . . . . . . . . . . . . . . . . . . . . 29 11.1. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . 29 12. Security Considerations . . . . . . . . . . . . . . . . . . . 30 12.1. Stateful signature algorithm . . . . . . . . . . . . . . 31 McGrew, et al. Expires May 4, 2017 [Page 2]

Internet-Draft Hash-Based Signatures October 2016 12.2. Security of LM-OTS Checksum . . . . . . . . . . . . . . 32 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 14.1. Normative References . . . . . . . . . . . . . . . . . . 33 14.2. Informative References . . . . . . . . . . . . . . . . . 33 Appendix A. Pseudorandom Key Generation . . . . . . . . . . . . 34 Appendix B. LM-OTS Parameter Options . . . . . . . . . . . . . . 35 Appendix C. An iterative algorithm for computing an LMS public key . . . . . . . . . . . . . . . . . . . . . . . . 36 Appendix D. Example implementation . . . . . . . . . . . . . . . 36 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37 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 relatively slow key generation. 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. An example implementation is given in an appendix. 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 exactly 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 described in Section 4, and the LMS and HSS N-time signature systems McGrew, et al. Expires May 4, 2017 [Page 3]

Internet-Draft Hash-Based Signatures October 2016 are described in Section 5 and Section 6, respectively. Sufficient detail is provided to ensure interoperability. 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; 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. 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 McGrew, et al. Expires May 4, 2017 [Page 4]

Internet-Draft Hash-Based Signatures October 2016 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 functions strTou8(S), strTou16(S), and strTou32(S), which take a one, two, or four byte string S as input and return a nonnegative integer; these functions are such that u8str(strTou8(S)) = S, u16str(strTou16(S)) = S, and u32str(strTou326(S)) for all values of S that are in the appropriate range. 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. If A and B are bytes, then A AND B denotes the bitwise logical and operation. 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. The i^th byte string in an array A is denoted as A[i]. McGrew, et al. Expires May 4, 2017 [Page 5]

Internet-Draft Hash-Based Signatures October 2016 3.1.2. 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)) ) 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. McGrew, et al. Expires May 4, 2017 [Page 6]

Internet-Draft Hash-Based Signatures October 2016 3.2. Security string To improve security against attacks that amortize their effort against multiple invocations of the hash function H, Leighton and Micali introduce a "security string" that is distinct for each invocation of H. The following fields can appear in a security string: I - an identifier for the LMS public/private keypair. The length of this value varies based on the LMS parameter set and it MUST be chosen uniformly at random, or via a pseudorandom process, at the time that a key pair is generated, in order to ensure that it will be distinct from the identifier of any other LMS private key with probability close to one. D - a domain separation parameter, which is a single byte that takes on different values in the different algorithms in which H is invoked. D takes on the following values: D_ITER = 0x00 in the iterations of the LM-OTS algorithms D_PBLC = 0x01 when computing the hash of all of the iterates in the LM-OTS algorithm D_MESG = 0x02 when computing the hash of the message in the LM- OTS algorithms D_LEAF = 0x03 when computing the hash of the leaf of an LMS tree D_INTR = 0x04 when computing the hash of an interior node of an LMS tree D_I = 0x05 when computing the I value for a nonroot LM tree in the HSS system D_PRG = 0x06 in the recommended pseudorandom process for generating LMS private keys 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. 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 byte order. McGrew, et al. Expires May 4, 2017 [Page 7]

Internet-Draft Hash-Based Signatures October 2016 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 keypair. This value is represented as a 32-bit (four byte) unsigned integer in network byte order. i - in the LM-OTS one-time signature scheme, i is the index of the 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 one-time signature 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. 3.3. 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 3.4. 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 only 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 4.2), and the resulting digest is signed. McGrew, et al. Expires May 4, 2017 [Page 8]

Internet-Draft Hash-Based Signatures October 2016 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. 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 (number of 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 4.6). The value of n is determined by the functions 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. Hashing Functions The LM-OTS algorithm uses a hash function H that accepts byte strings of any length, and returns an n-byte string. 4.3. Signature Methods To fully describe a LM-OTS signature method, the parameters n and w, the length LenS of the security string S, as well as the function H, MUST be specified. This section defines several LM-OTS signature systems, each of which is identified by a name. Values for p and ls are provided as a convenience. McGrew, et al. Expires May 4, 2017 [Page 9]

Internet-Draft Hash-Based Signatures October 2016 +---------------------+--------+----+---+------+-----+----+ | Name | H | n | w | LenS | p | ls | +---------------------+--------+----+---+------+-----+----+ | LMOTS_SHA256_N32_W1 | SHA256 | 32 | 1 | 68 | 265 | 7 | | | | | | | | | | LMOTS_SHA256_N32_W2 | SHA256 | 32 | 2 | 68 | 133 | 6 | | | | | | | | | | LMOTS_SHA256_N32_W4 | SHA256 | 32 | 4 | 68 | 67 | 4 | | | | | | | | | | LMOTS_SHA256_N32_W8 | SHA256 | 32 | 8 | 68 | 34 | 0 | +---------------------+--------+----+---+------+-----+----+ Table 1 Here SHA256 denotes the NIST standard hash function [FIPS180]. SHA256-16 denotes the SHA256 hash function with its final output truncated to return the leftmost 16 bytes; that is, immediately after computing the SHA256 hash, the 32 bit hash output is truncated to be the leftmost 16 bytes. 4.4. Private Key The LM-OTS private key consists of a typecode indicating the particular LM-OTS algorithm, an array x[] containing p n-byte strings, and a LenS-bygte security string S. 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. if no security string S has been provided as input, then set S to a LenS-byte string generated uniformly at random 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) || S || 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 McGrew, et al. Expires May 4, 2017 [Page 10]

Internet-Draft Hash-Based Signatures October 2016 MUST match that of the LM-OTS algorithm. Appendix A provides an example of a pseudorandom method for computing LM-OTS private key. 4.5. 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. 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 and S from the private key 3. 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(S || tmp || u16str(i) || u8str(j) || D_ITER) } y[i] = tmp } K = H(S || y[0] || ... || y[p-1] || D_PBLC) 4. return u32str(type) || S || K The public key is the value returned by Algorithm 1. 4.6. 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: McGrew, et al. Expires May 4, 2017 [Page 11]

Internet-Draft Hash-Based Signatures October 2016 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. 4.7. Signature Generation The LM-OTS signature of a message is generated by first prepending the randomizer C and the security string S to the message, then appending D_MESG to the resulting string then computing its hash, 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. The identifier string I and diversification string q are the same as in Section 4.5. McGrew, et al. Expires May 4, 2017 [Page 12]

Internet-Draft Hash-Based Signatures October 2016 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 and S from the private key 3. set C to a uniformly random n-byte string 4. compute the array y as follows: Q = H(S || C || message || D_MESG ) 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(S || tmp || u16str(i) || u8str(j) || D_ITER) } y[i] = tmp } 5. 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 4.5. 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. 4.8. 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. McGrew, et al. Expires May 4, 2017 [Page 13]

Internet-Draft Hash-Based Signatures October 2016 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, S, 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 4 + LenS + n bytes long, return INVALID c. S = next LenS bytes of public key d. K = next n bytes of public key 3. compute the public key candidate Kc from the signature, message, and the security string S obtained from the public key, using Algorithm 4b. 4. if Kc is equal to K, return VALID; otherwise, return INVALID McGrew, et al. Expires May 4, 2017 [Page 14]

Internet-Draft Hash-Based Signatures October 2016 Algorithm 4b: Computing a Public Key Candidate Kc from a Signature, Message, Signature Typecode Type , and a Security String S 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 8 + 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(S || C || message || D_MESG) 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(S || tmp || u16str(i) || u8str(j) || D_ITER) } z[i] = tmp } Kc = H(S || z[0] || z[1] || ... || z[p-1] || D_PBLC) 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 n-byte 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. McGrew, et al. Expires May 4, 2017 [Page 15]

Internet-Draft Hash-Based Signatures October 2016 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, 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 jth 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. There are 2^h leaves in the tree. The parameter m MAY have any value in principle, but it SHOULD be equal to the parameter n from Section 4.1 so that the security level of LMS is comparable to that of LM-OTS. +--------------------+--------+----+----+ | 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 | +--------------------+--------+----+----+ Table 2 5.2. LMS Private Key An LMS private key consists of an array OTS_PRIV[] of 2^h LM-OTS private keys, and the leaf number q of the next LM-OTS private key that has not yet been used. The qth element of OTS_PRIV[] is generated using Algorithm 0 with the security string S = I || q. The leaf number q is initialized to zero when the LMS private key is created. The process is as follows: McGrew, et al. Expires May 4, 2017 [Page 16]

Internet-Draft Hash-Based Signatures October 2016 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) { S = I || q OTS_PRIV[q] = LM-OTS private key with security string S } 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 perfect 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 rth node is denoted as T[r] and is defined as T[r] = / H(I || OTS_PUB[r-2^h] || u32str(r) || D_LEAF) if r >= 2^h, \ H(I || T[2*r] || T[2*r+1] || u32str(r) || D_INTR) 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. 5.4. LMS Signature An LMS signature consists of a typecode indicating the particular LMS algorithm, McGrew, et al. Expires May 4, 2017 [Page 17]

Internet-Draft Hash-Based Signatures October 2016 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 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(type) || u32str(q) || ots_signature || 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 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 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]. McGrew, et al. Expires May 4, 2017 [Page 18]

Internet-Draft Hash-Based Signatures October 2016 5.5. LMS Signature Verification An LMS signature is verified by first using the LM-OTS signature verification algorithm 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. 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 McGrew, et al. Expires May 4, 2017 [Page 19]

Internet-Draft Hash-Based Signatures October 2016 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. sigtype = strTou32(first 4 bytes of signature) b. if pubtype is not equal to sigtype, return INVALID c. set m, h, and LenI according to sigtype and Table 2; d. q = strTou32(next 4 bytes of signature) e. otssigtype = strTou32(next 4 bytes of signature) f. set n and p according to otssigtype and Table 1; if the signature does string is not at least 12 + n * (p + 1) + m * h bytes long, return INVALID g. ots_signature = bytes 8 through 8 + n * (p + 1) of signature h. 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 5. Kc = candidate public key computed by applying Algorithm 4b to the signature ots_signature, the message, and the security string S = I || q 6. compute the candidate LMS root value Tc as follows: tmp = H(I || Kc || u32str(node_num) || D_LEAF) i = 0 node_num = 2^h + q while (node_num > 1) { if (node_num is odd): tmp = H(I || path[i] || tmp || u32str(node_num/2) || D_INTR) else: tmp = H(I || tmp || path[i] || u32str(node_num/2) || D_INTR) node_num = node_num/2 i = i + 1 7. return Tc McGrew, et al. Expires May 4, 2017 [Page 20]

Internet-Draft Hash-Based Signatures October 2016 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. It also makes use of an hbs_key_info structure, which contains the typecode of the LMS algorithm and the typecode of the LM-OTS algorithm. 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 two 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 ith level, pub[i] is the LMS public key of the ith level (which includes the identifier I as well as the key value K), sig[i] is the LMS signature of the ith 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. 6.1. Key Generation When an HSS keypair is generated, the keypair for each level has its own identifier. 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. These key pairs, and their identifiers, MUST be generated independently. All of the information of the leaf level L-1, including the private key, MUST NOT be stored in nonvolatile memory. Letting nv denote the lowest level for which prv[nv] is stored in nonvolatile memory, there are nv nonvolatile levels, and L-nv volatile levels. For security, nv should be as close to zero as possible (see Section 12.1). The public key of the HSS scheme is pub[1], the public key of the first level, followed by an array info[] containing the hbs_key_info structures for the remaining levels 1, ..., L-1. McGrew, et al. Expires May 4, 2017 [Page 21]

Internet-Draft Hash-Based Signatures October 2016 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 > 1, 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 one, then the HSS keypair is exhausted, and it MUST NOT generate any more signatures. Otherwise, the keypairs 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 strings, where signed_pub_key[i] = sig[i] || pub[i+1], for i between 0 and L-2, inclusive. Then the HSS signature is u32str(L-1) || signed_pub_key[0] || ... || signed_pub_key[L-2] || sig[L-1]. Note that the number of signed_pub_key elements in the signature is indicated by the value L-1 that appears in the initial four bytes of the signature. 6.3. Signature Verification To verify a signature sig and message using the public key pub, the following steps are performed: McGrew, et al. Expires May 4, 2017 [Page 22]

Internet-Draft Hash-Based Signatures October 2016 The signature S is parsed into its components as follows: L' = strToU32(first four bytes of S) for (i = 0; i < L'; i = i + 1) { siglist[0] = next LMS signature parsed from S publist[1] = next LMS public key parsed from S } siglist[L-1] = 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[L-1]) 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 futher 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: /* one-time signatures */ enum ots_algorithm_type { ots_reserved = 0, lmots_sha256_n32_w1 = 1, lmots_sha256_n32_w2 = 2, lmots_sha256_n32_w4 = 3, lmots_sha256_n32_w8 = 4 }; McGrew, et al. Expires May 4, 2017 [Page 23]

Internet-Draft Hash-Based Signatures October 2016 typedef opaque bytestring16[16]; 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 */ }; union ots_private_key switch (ots_algorithm_type type) { case lmots_sha256_n32_w1: case lmots_sha256_n32_w2: case lmots_sha256_n32_w4: case lmots_sha256_n32_w8: bytestring32 x32; default: void; /* error condition */ }; /* hash based signatures (hbs) */ McGrew, et al. Expires May 4, 2017 [Page 24]

Internet-Draft Hash-Based Signatures October 2016 enum hbs_algorithm_type { hbs_reserved = 0, lms_sha256_n32_h20 = 1, lms_sha256_n32_h10 = 2, lms_sha256_n32_h5 = 3, }; /* leighton mical signatures (lms) */ union lms_path switch (hbs_algorithm_type type) { case lms_sha256_n32_h20: bytestring32 path_n32_h20[20]; case lms_sha256_n32_h15: bytestring32 path_n32_h15[15]; case lms_sha256_n32_h10: bytestring32 path_n32_h10[10]; case lms_sha256_n32_h5: bytestring32 path_n32_h5[5]; 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[64]; opaque K[32]; }; union hbs_public_key switch (hbs_algorithm_type type) { case lms_sha256_n32_h20: case lms_sha256_n32_h10: case lms_sha256_n32_h5: lms_key_n32 z_n32; default: void; /* error condition */ }; /* hierarchical signature system (hss) */ struct hss_public_key { unsigned int levels; hbs_public_key pub; McGrew, et al. Expires May 4, 2017 [Page 25]

Internet-Draft Hash-Based Signatures October 2016 }; struct signed_public_key { hbs_signature sig; hbs_public_key pub; } struct hss_signature { signed_public_key signed_keys<L-1>; 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 and LMS 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 required to be distinct in order to improve security. That distinctness ensures the uniqueness of the inputs to H across all of those public/private key pair instances, which is important for provable security in the random oracle model. The length of I is set at 31 or 64 bytes so that randomly chosen values of I will be distinct with probability at least 1 - 1/2^128 as long as there are 2^60 or fewer instances of LMS public/private key pairs. The sizes of the parameters in the security string are such that, for n=16, the LM-OTS iterates a 55-byte value (that is, the string that is input to H() during the iteration over j during signature generation and verification is 55 bytes long). Thus, when SHA-256 is used as the function H, only a single invocation of its compression function is needed. McGrew, et al. Expires May 4, 2017 [Page 26]

Internet-Draft Hash-Based Signatures October 2016 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 4.6 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 4.7 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. A future version of this specification may support hash functions other than SHA-256. 9. History This is the fifth version of this draft. It has the following changes from previous versions: 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 implemetation to compute 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 paramter sets. Also, added LMS parameter sets with height 15 trees Previous versions McGrew, et al. Expires May 4, 2017 [Page 27]

Internet-Draft Hash-Based Signatures October 2016 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. 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_N32_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. McGrew, et al. Expires May 4, 2017 [Page 28]

Internet-Draft Hash-Based Signatures October 2016 +----------------------+-----------+--------------------+ | 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 | +--------------------+-----------+--------------------+ 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. McGrew, et al. Expires May 4, 2017 [Page 29]

Internet-Draft Hash-Based Signatures October 2016 12. Security Considerations 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. LM-OTS is provably secure in the random oracle model, as shown by Katz [Katz15]. From Theorem 8 of that reference: For any adversary attacking arbitrarily many instances of the one- time signature scheme, and making at most q hash queries, the probability with which the adversary is able to forge a signature with respect to any of the instances is at most q2^(1-8n). Here n is the number of bytes in the output of the hash function (as defined in Section 4.1). Thus, the security of the algorithms defined in this note can be roughly described as follows. For a security level of roughly 128 bits, even assuming that there are quantum computers that can compute the input to an arbitrary function with computational cost equivalent to the square root of the size of the domain of that function [Grover96], use n=32 by selecting an algorithm identifier with N32 in its name. The format of the inputs to H() have the property that each invocation of that function has an input that is distinct from all others, with high probability. This property is important for a proof of security in the random oracle model. The formats used during key generation and signing are S || tmp || u16str(i) || u8str(j) || D_ITER S || y[0] || ... || y[p-1] || D_PBLC S || C || message || D_MESG I || OTS_PUB[r-2^h] || u32str(r) || D_LEAF I || T[2*r] || T[2*r+1] || u32str(r) || D_INTR I || u32str(q) || x_q[j-1] || u16str(j) || D_PRG Because the suffixes D_ITER, D_PBLC, D_LEAF, D_INTR, and D_PRG are distinct, the input formats ending with different suffixes are all distinct. It remains to show the distinctness of the inputs for each suffix. McGrew, et al. Expires May 4, 2017 [Page 30]

Internet-Draft Hash-Based Signatures October 2016 The values of I and C are chosen uniformly at random from the set of all n*8 bit strings. For n=32, it is highly likely that each value of I and C will be distinct, even when 2^96 such values are chosen. For D_ITER, D_PBLC, and D_MESG, the value of S = I || u32str(q) is distinct for each LMS leaf (or equivalently, for each q value). For D_ITER, 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 S, 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 D_PRG, for a given value of I, q and j are distinct for each invocation of H (note that x_q[0] = SEED when j=0). 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 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 McGrew, et al. Expires May 4, 2017 [Page 31]

Internet-Draft Hash-Based Signatures October 2016 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 4.7). 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 (p-1), inclusive, b' = coef(c', k, w), 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. Acknowledgements Thanks are due to Chirag Shroff, Andreas Huelsing, Burt Kaliski, Eric Osterweil, Ahmed Kosba, and Russ Housley 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. McGrew, et al. Expires May 4, 2017 [Page 32]

Internet-Draft Hash-Based Signatures October 2016 14. References 14.1. 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, <http://www.rfc-editor.org/info/rfc2119>. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, DOI 10.17487/RFC2434, October 1998, <http://www.rfc-editor.org/info/rfc2434>. [RFC3979] Bradner, S., Ed., "Intellectual Property Rights in IETF Technology", BCP 79, RFC 3979, DOI 10.17487/RFC3979, March 2005, <http://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, <http://www.rfc-editor.org/info/rfc4506>. [RFC4879] Narten, T., "Clarification of the Third Party Disclosure Procedure in RFC 3979", BCP 79, RFC 4879, DOI 10.17487/RFC4879, April 2007, <http://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. 14.2. 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. McGrew, et al. Expires May 4, 2017 [Page 33]

Internet-Draft Hash-Based Signatures October 2016 [C:Merkle89b] Merkle, R., "One Way Hash Functions and DES", Lecture Notes in Computer Science crypto89vol, 1990. [Grover96] Grover, L., "A fast quantum mechanical algorithm for database search", 28th ACM Symposium on the Theory of Computing p. 212, 1996. [Katz15] Katz, J., "Analysis of a proposed hash-based signature standard", Contribution to IRTF http://www.cs.umd.edu/~jkatz/papers/HashBasedSigs.pdf, 2015. [Merkle79] Merkle, R., "Secrecy, Authentication, and Public Key Systems", Stanford University Information Systems Laboratory Technical Report 1979-1, 1979. 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 keypair 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, D_PRG is a diversification constant, and H is the hash function used in LM-OTS. The elements of the LM-OTS private keys are computed as follows: x_q[j] = / H(I || u32str(q) || SEED || u16str(j) || D_PRG) if j = 0, \ H(I || u32str(q) || x_q[j-1] || u16str(j) || D_PRG) otherwise This process stretches the m-byte random value SEED into a (much larger) set of pseudorandom values, using both output feedback and a unique counter in each invocation of H. The format of the inputs to McGrew, et al. Expires May 4, 2017 [Page 34]

Internet-Draft Hash-Based Signatures October 2016 H are chosen so that they are distinct from all other uses of H in LMS and LM-OTS. Appendix B. LM-OTS 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 w-bit 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 4.6. 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. +---------+------------+-----------+-----------+-------+------------+ | 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) | +---------+------------+-----------+-----------+-------+------------+ | 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 McGrew, et al. Expires May 4, 2017 [Page 35]

Internet-Draft Hash-Based Signatures October 2016 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 and a separate stack of integers to keep track of the level of the tree. 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]). Generating an LMS Public Key From an LMS Private Key for ( i = 0; i < num_lmots_keys; i = i + 2 ) { level = 0; for ( j = 0; j < 2; j = j + 1 ) { r = node_num push H(I || OTS_PUBKEY[i+j] || u32str(r) || D_LEAF) on data stack push level onto the integer stack } while ( height of the integer stack >= 2 ) { if level of the top 2 elements on the integer stack are equal { hash_init() siblings = "" repeat ( 2 ) { siblings = (pop(data stack) || siblings) level = pop(integer stack) } hash_update(siblings) r = node_num hash_update(I || u32str(r) || D_INTR) push hash_final() onto the data stack push (level + 1) onto the integer stack } } } public_key = pop(data stack) Note that this pseudocode expects that all 2^h leaves of the tree have equal depth. Neither stack ever contains more than h+1 elements. For typical parameters, these stacks will hold around 512 bytes of data. Appendix D. Example implementation McGrew, et al. Expires May 4, 2017 [Page 36]

Internet-Draft Hash-Based Signatures October 2016 --- Pending Revision --- 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 Scott Fluhrer Cisco Systems 170 West Tasman Drive San Jose, CA USA Email: sfluhrer@cisco.com McGrew, et al. Expires May 4, 2017 [Page 37]