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Internet Research Task Force (IRTF) D. McGrew Request for Comments: 8554 M. Curcio Category: Informational S. Fluhrer ISSN: 2070-1721 Cisco Systems April 2019 Leighton-Micali Hash-Based Signatures 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 are naturally resistant to side- channel attacks. Unlike many other signature systems, hash-based signatures would still be secure even if it proves feasible for an attacker to build a quantum computer. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. This has been reviewed by many researchers, both in the research group and outside of it. The Acknowledgements section lists many of them. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Research Task Force (IRTF). The IRTF publishes the results of Internet-related research and development activities. These results might not be suitable for deployment. This RFC represents the consensus of the Crypto Forum Research Group of the Internet Research Task Force (IRTF). Documents approved for publication by the IRSG are not candidates for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8554.

Copyright Notice Copyright (c) 2019 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 (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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. CFRG Note on Post-Quantum Cryptography . . . . . . . . . 5 1.2. Intellectual Property . . . . . . . . . . . . . . . . . . 6 1.2.1. Disclaimer . . . . . . . . . . . . . . . . . . . . . 6 1.3. Conventions Used in This Document . . . . . . . . . . . . 6 2. Interface . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Data Types . . . . . . . . . . . . . . . . . . . . . . . 7 3.1.1. Operators . . . . . . . . . . . . . . . . . . . . . . 7 3.1.2. Functions . . . . . . . . . . . . . . . . . . . . . . 8 3.1.3. Strings of w-Bit Elements . . . . . . . . . . . . . . 8 3.2. Typecodes . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Notation and Formats . . . . . . . . . . . . . . . . . . 9 4. LM-OTS One-Time Signatures . . . . . . . . . . . . . . . . . 12 4.1. Parameters . . . . . . . . . . . . . . . . . . . . . . . 13 4.2. Private Key . . . . . . . . . . . . . . . . . . . . . . . 14 4.3. Public Key . . . . . . . . . . . . . . . . . . . . . . . 15 4.4. Checksum . . . . . . . . . . . . . . . . . . . . . . . . 15 4.5. Signature Generation . . . . . . . . . . . . . . . . . . 16 4.6. Signature Verification . . . . . . . . . . . . . . . . . 17 5. Leighton-Micali Signatures . . . . . . . . . . . . . . . . . 19 5.1. Parameters . . . . . . . . . . . . . . . . . . . . . . . 19 5.2. LMS Private Key . . . . . . . . . . . . . . . . . . . . . 20 5.3. LMS Public Key . . . . . . . . . . . . . . . . . . . . . 21 5.4. LMS Signature . . . . . . . . . . . . . . . . . . . . . . 22 5.4.1. LMS Signature Generation . . . . . . . . . . . . . . 23 5.4.2. LMS Signature Verification . . . . . . . . . . . . . 24 6. Hierarchical Signatures . . . . . . . . . . . . . . . . . . . 26 6.1. Key Generation . . . . . . . . . . . . . . . . . . . . . 29 6.2. Signature Generation . . . . . . . . . . . . . . . . . . 30 6.3. Signature Verification . . . . . . . . . . . . . . . . . 32 6.4. Parameter Set Recommendations . . . . . . . . . . . . . . 32 7. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.1. Security String . . . . . . . . . . . . . . . . . . . . . 35

8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 9. Security Considerations . . . . . . . . . . . . . . . . . . . 38 9.1. Hash Formats . . . . . . . . . . . . . . . . . . . . . . 39 9.2. Stateful Signature Algorithm . . . . . . . . . . . . . . 40 9.3. Security of LM-OTS Checksum . . . . . . . . . . . . . . . 41 10. Comparison with Other Work . . . . . . . . . . . . . . . . . 42 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 11.1. Normative References . . . . . . . . . . . . . . . . . . 43 11.2. Informative References . . . . . . . . . . . . . . . . . 43 Appendix A. Pseudorandom Key Generation . . . . . . . . . . . . 45 Appendix B. LM-OTS Parameter Options . . . . . . . . . . . . . . 45 Appendix C. An Iterative Algorithm for Computing an LMS Public Key . . . . . . . . . . . . . . . . . . . . . . . . 47 Appendix D. Method for Deriving Authentication Path for a Signature . . . . . . . . . . . . . . . . . . . . . 48 Appendix E. Example Implementation . . . . . . . . . . . . . . . 49 Appendix F. Test Cases . . . . . . . . . . . . . . . . . . . . . 49 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 60 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 61 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 moderately slow key generation (in comparison with RSA and ECDSA (Elliptic Curve Digital Signature Algorithm)). Private keys can be made very small by appropriate key generation, for example, as described in Appendix A. 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 will become insecure if more than one is signed with the same public/

private key pair. 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 the Merkle scheme, a binary tree of height h is used to hold 2^h OTS key pairs. Each interior node of the tree holds a value that is the hash of the values of its two child nodes. The public key of the tree is the value of the root node (a recursive hash of the OTS public keys), while the private key of the tree is the collection of all the OTS private keys, together with the index of the next OTS private key to sign the next message with. In this note, we describe the Leighton-Micali Signature (LMS) system (a variant of the Merkle scheme) with the Hierarchical Signature System (HSS) built on top of it that allows it to efficiently scale to larger numbers of signatures. In order to support signing a large number of messages on resource-constrained systems, the Merkle tree can be subdivided into a number of smaller trees. Only the bottommost tree is used to sign messages, while trees above that are used to sign the public keys of their children. For example, in the simplest case with two levels with both levels consisting of height h trees, the root tree is used to sign 2^h trees with 2^h OTS key pairs, and each second-level tree has 2^h OTS key pairs, for a total of 2^(2h) bottom-level key pairs, and so can sign 2^(2h) messages. The advantage of this scheme is that only the active trees need to be instantiated, which saves both time (for key generation) and space (for key storage). On the other hand, using a multilevel signature scheme increases the size of the signature as well as the signature verification time. This note is structured as follows. Notes on post-quantum cryptography are discussed in Section 1.1. Intellectual property issues are discussed in Section 1.2. The notation used within this note is defined in Section 3, and the public formats are described in Section 3.3. The Leighton-Micali One-Time Signature (LM-OTS) system is described in Section 4, and the LMS and HSS N-time signature systems are described in Sections 5 and 6, respectively. Sufficient detail is provided to ensure interoperability. The rationale for the design decisions is given in Section 7. The IANA registry for these signature systems is described in Section 8. Security considerations are presented in Section 9. Comparison with another hash-based signature algorithm (eXtended Merkle Signature Scheme (XMSS)) is in Section 10. This document represents the rough consensus of the CFRG.

1.1. CFRG Note on Post-Quantum Cryptography All post-quantum algorithms documented by the Crypto Forum Research Group (CFRG) are today considered ready for experimentation and further engineering development (e.g., to establish the impact of performance and sizes on IETF protocols). However, at the time of writing, we do not have significant deployment experience with such algorithms. Many of these algorithms come with specific restrictions, e.g., change of classical interface or less cryptanalysis of proposed parameters than established schemes. The CFRG has consensus that all documents describing post-quantum technologies include the above paragraph and a clear additional warning about any specific restrictions, especially as those might affect use or deployment of the specific scheme. That guidance may be changed over time via document updates. Additionally, for LMS: CFRG consensus is that we are confident in the cryptographic security of the signature schemes described in this document against quantum computers, given the current state of the research community's knowledge about quantum algorithms. Indeed, we are confident that the security of a significant part of the Internet could be made dependent on the signature schemes defined in this document, if developers take care of the following. In contrast to traditional signature schemes, the signature schemes described in this document are stateful, meaning the secret key changes over time. If a secret key state is used twice, no cryptographic security guarantees remain. In consequence, it becomes feasible to forge a signature on a new message. This is a new property that most developers will not be familiar with and requires careful handling of secret keys. Developers should not use the schemes described here except in systems that prevent the reuse of secret key states. Note that the fact that the schemes described in this document are stateful also implies that classical APIs for digital signatures cannot be used without modification. The API MUST be able to handle a dynamic secret key state; that is, the API MUST allow the signature-generation algorithm to update the secret key state.

1.2. Intellectual Property This document is based on U.S. Patent 5,432,852, which was issued over twenty years ago and is thus expired. 1.2.1. 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 [RFC8179] and at <https://datatracker.ietf.org/ipr/about>. 1.3. Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 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; it returns an indication of whether or not the signature-and-message pair is valid.

A message/signature pair is 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 is 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 leading "0x". For example, 0xe534f0 is a byte string with a length of three. An array of byte strings is an ordered set, indexed starting at zero, in which all strings have the same length. Unsigned integers are converted into byte strings by representing them in network byte order. To make the number of bytes in the representation explicit, we define the functions u8str(X), u16str(X), and u32str(X), which take a nonnegative integer X as input and return one-, two-, and four-byte strings, respectively. We also make use of the function strTou32(S), which takes a four-byte string S as input and returns a nonnegative integer; the identity u32str(strTou32(S)) = S holds for any 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 (with a and b being restricted to integers in this case) + : a + b denotes the sum of a and b - : a - b denotes the difference of a and b AND : a AND b denotes the bitwise AND of the two nonnegative integers a and b (represented in binary notation)

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 by i bit positions. Similarly, B << i denotes the logical left-shift operation. If S and T are byte strings, then S || T denotes the concatenation of S and T. If S and T are equal-length byte strings, then S AND T denotes the bitwise logical and operation. The i-th element in an array A is denoted as A[i]. 3.1.2. Functions If r is a nonnegative real number, then we define the following functions: ceil(r) : returns the smallest integer greater than or equal to r floor(r) : returns the largest integer less than or equal to 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 the index starts at 0 at the left. Hence, byte(S, 0) is the leftmost byte of S, byte(S, 1) is the second byte from the left, and (assuming S is n bytes long) byte(S, n-1) is the rightmost byte of S. 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 MUST raise an error. 3.2. 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 External Data Representation (XDR) enumerations (see Section 3.3). 3.3. Notation and Formats The signature and public key formats are formally defined in XDR to provide an unambiguous, machine-readable definition [RFC4506]. The private key format is not included as it is not needed for interoperability and an implementation MAY use any private key format. However, for clarity, we include an example of private key data in Test Case 2 of Appendix F. 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 lmots_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 lmots_signature switch (lmots_algorithm_type type) { case lmots_sha256_n32_w1: lmots_signature_n32_p265 sig_n32_p265; case lmots_sha256_n32_w2: lmots_signature_n32_p133 sig_n32_p133; case lmots_sha256_n32_w4: lmots_signature_n32_p67 sig_n32_p67; case lmots_sha256_n32_w8: lmots_signature_n32_p34 sig_n32_p34; default: void; /* error condition */ };

/* hash-based signatures (hbs) */ enum lms_algorithm_type { lms_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 (lms_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; lmots_signature lmots_sig; lms_path nodes; }; struct lms_key_n32 { lmots_algorithm_type ots_alg_type; opaque I[16]; opaque K[32]; }; union lms_public_key switch (lms_algorithm_type type) { case lms_sha256_n32_h5: case lms_sha256_n32_h10: case lms_sha256_n32_h15: case lms_sha256_n32_h20: case lms_sha256_n32_h25: lms_key_n32 z_n32;

default: void; /* error condition */ }; /* hierarchical signature system (hss) */ struct hss_public_key { unsigned int L; lms_public_key pub; }; struct signed_public_key { lms_signature sig; lms_public_key pub; }; struct hss_signature { signed_public_key signed_keys<7>; lms_signature sig_of_message; }; 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 parameters I and q. The parameter I is a 16-byte string that indicates which Merkle tree this LM-OTS is used with. The parameter q is a 32-bit integer that indicates the leaf of the Merkle tree where the OTS public key appears. These parameters are used as part of the security string, as listed in Section 7.1. When the LM-OTS signature system is used outside of an N-time signature system, the value I MAY be used to differentiate this one-time signature from others; however, the value q MUST 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; that is, the number of bits from the hash or checksum that is used with a single Winternitz chain. 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. This is a function of n and w; the values for the defined parameter sets are listed in Table 1; it can also be computed by the algorithm given in Appendix B. ls : the number of left-shift bits used in the checksum function Cksm (defined in Section 4.4). H : a second-preimage-resistant cryptographic hash function that accepts byte strings of any length and returns an n-byte string. For more background on the cryptographic security requirements for H, see Section 9. 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. Table 1 illustrates various combinations of n, w, p and ls, along with the resulting signature length. 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 and generate and verify the signature. In general, the LM-OTS signature is 4+n*(p+1) bytes long, and public key generation will take p*(2^w - 1) + 1 hash computations (and signature generation and verification will take approximately half that on average).

+---------------------+--------+----+---+-----+----+---------+ | Parameter Set Name | H | n | w | p | ls | sig_len | +---------------------+--------+----+---+-----+----+---------+ | LMOTS_SHA256_N32_W1 | SHA256 | 32 | 1 | 265 | 7 | 8516 | | | | | | | | | | LMOTS_SHA256_N32_W2 | SHA256 | 32 | 2 | 133 | 6 | 4292 | | | | | | | | | | LMOTS_SHA256_N32_W4 | SHA256 | 32 | 4 | 67 | 4 | 2180 | | | | | | | | | | LMOTS_SHA256_N32_W8 | SHA256 | 32 | 8 | 34 | 0 | 1124 | +---------------------+--------+----+---+-----+----+---------+ Table 1 Here SHA256 denotes the SHA-256 hash function defined in NIST standard [FIPS180]. 4.2. 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 values of q and I (the 16-byte identifier of the LMS public/private key pair) from the LMS tree 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. 4.3. 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, 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 where D_PBLC is the fixed two-byte value 0x8080, which is used to distinguish the last hash from every other hash in this system. The public key is the value returned by Algorithm 1. 4.4. Checksum A checksum is used to ensure that any forgery attempt that manipulates the elements of an existing signature will be detected. This checksum is needed because an attacker can freely advance any of the Winternitz chains. That is, if this checksum were not present, then an attacker who could find a hash that has every digit larger than the valid hash could replace it (and adjust the Winternitz

chains). The security property that the checksum provides is detailed in Section 9. 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) ls is the parameter that shifts the significant bits of the checksum into the positions that will actually be used by the coef function when encoding the digits of the checksum. The actual ls parameter is a function of the n and w parameters; the values for the currently defined parameter sets are shown in Table 1. It is calculated by the algorithm given in Appendix B. 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. 4.5. Signature Generation The LM-OTS signature of a message is generated by doing the following in sequence: prepending the LMS key identifier I, the LMS leaf identifier q, the value D_MESG (0x8181), and the randomizer C to the message; computing the hash; concatenating the checksum of the hash to the hash itself; 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 4.3. The signature is the string returned by Algorithm 3. Section 3.3 formally defines the structure of the string as the lmots_signature union. 4.6. 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. Set n according to the pubkey and Table 1; 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, pubtype, 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. Algorithm 4b: Computing a Public Key Candidate Kc from a Signature, Message, Signature Typecode pubtype, 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.

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