Network Working Group D. McGrew Internet-Draft Cisco Systems, Inc. Intended status: Informational P. Patnala Expires: June 22, 2009 December 19, 2008 Threshold Secret Sharing draft-mcgrew-tss-01.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on June 22, 2009. Copyright Notice Copyright (c) 2008 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 (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. McGrew & Patnala Expires June 22, 2009 [Page 1]

Internet-Draft Threshold Secret Sharing December 2008 Abstract Threshold secret sharing (TSS) provides a way to generate N shares from a value, so that any M of those shares can be used to reconstruct the original value, but any M-1 shares provide no information about that value. This method can provide shared access control on key material and other secrets that must be strongly protected. This note defines a threshold secret sharing method based on polynomial interpolation in GF(256) and a format for the storage and transmission of shares. It also provides usage guidance, describes how to test an implementation, and supplies test cases. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions Used In This Document . . . . . . . . . . . . 3 2. Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Create Shares . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Reconstruct Secret . . . . . . . . . . . . . . . . . . . . 4 3. Polynomial Interpolation over GF(256) . . . . . . . . . . . . 5 3.1. Field Representation . . . . . . . . . . . . . . . . . . . 5 3.2. Share Generation . . . . . . . . . . . . . . . . . . . . . 7 3.3. Secret Reconstruction . . . . . . . . . . . . . . . . . . 8 4. Robust Threshold Secret Sharing . . . . . . . . . . . . . . . 10 4.1. RTSS Data Format . . . . . . . . . . . . . . . . . . . . . 10 5. Error Correction and Data Recovery . . . . . . . . . . . . . . 12 5.1. Data Recovery . . . . . . . . . . . . . . . . . . . . . . 12 5.2. Error Correction . . . . . . . . . . . . . . . . . . . . . 12 5.3. A Repetition Code . . . . . . . . . . . . . . . . . . . . 14 6. Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7. Design and Rationale . . . . . . . . . . . . . . . . . . . . . 17 8. Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 9. Test Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10. Security Considerations . . . . . . . . . . . . . . . . . . . 20 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 13.1. Normative References . . . . . . . . . . . . . . . . . . . 23 13.2. Informative References . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 McGrew & Patnala Expires June 22, 2009 [Page 2]

Internet-Draft Threshold Secret Sharing December 2008 1. Introduction Threshold secret sharing (TSS) provides a way to generate N shares from a value, so that any M of those shares can be used to reconstruct the original value, but any M-1 shares provide no information about that value. This method does not rely on any assumptions about the complexity of solving a particular computational problem (such as factoring); it is information- theoretically secure. Each share is slightly longer than the original secret. In the context of secret sharing, the word "share" means a part of something, and "sharing" means the act of breaking up into parts. Readers may be confused if they think of "sharing" as meaning "giving to or posessing with others". TSS is especially useful whenever there is a need to ensure the availability of a secret, yet there is a simultaneous need to reduce the risk of compromise of the secret. By dividing the secret into multiple shares, and distributing each share to a different trusted entity, TSS reduces that risk while providing for the availability of the secret. At the time that the secret is divided into shares, the threshold defining a number of shares that are needed to reconstruct the secret is set. 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]. McGrew & Patnala Expires June 22, 2009 [Page 3]

Internet-Draft Threshold Secret Sharing December 2008 2. Operations A threshold secret sharing system provides two operations: one that creates a set of shares given a secret, and one that reconstructs the secret, given a set of shares. This section defines the inputs and ouputs of these operations. The following sections describe the details of TSS based on a polynomial interpolation in GF(256). 2.1. Create Shares This operation takes an octet string S, whose length is L octets, and a threshold parameter M, and generates a set of N shares, any M of which can be used to reconstruct the secret. The secret S is treated as an unstructured sequence of octets. It is not expected to be null-terminated. The number of octets in the secret may be anywhere from zero up to TSS_MAX_SECRET_LENGTH. The threshold parameter M is the number of shares that will be needed to reconstruct the secret. This value may be any number between one and 255. The number of shares N that will be generated MUST be between the threshold value M and 255, inclusive. The upper limit is particular to the TSS algorithm specified in this document. If the operation can not be completed successfully, then an error code should be returned. 2.2. Reconstruct Secret The reconstruct operation reconstructs the secret from a set of shares. The number of shares N must be provided as a parameter. The only other parameter is the list of shares themselves. The shares should be treated as unstructured octet strings. If the operation could be completed, then the secret value will be returned. If the operation can not be completed successfully, then an error code should be returned. McGrew & Patnala Expires June 22, 2009 [Page 4]

Internet-Draft Threshold Secret Sharing December 2008 3. Polynomial Interpolation over GF(256) A finite field is a set of elements with associated addition, multiplication, subtraction, and division operations. Each of those operations acts on elements in the field, and returns an element in the field. This specification uses the field GF(256), and each element is represented as a single octet. There are many possible ways to represent a finite field; below we define the field arithmetic operations as having inputs and outputs that are octets. This fixes a particular representation, without explicitly defining it, and it avoids the issue of the bit-representation of octets. 3.1. Field Representation The elements of the field GF(256) are represented as octets. In the following, each octet is represented as a hexadecimal number with a leading "0x", as in ANSI/ISO C. The representation of the finite field that we use is defined in terms of the addition, subtraction, multiplication, and division operations. We define these operations as taking two octets as input and returning a single octet as output. These operations are defined in terms of two tables, the EXP table (Figure 1) and the LOG table (Figure 2), which define the exponential function and the logarithmic function, respectively. The ith elements of these tables are denoted as EXP[i] and LOG[i]. LOG takes a non-zero field element as input, and returns an integer, and EXP takes an integer and returns a field element. The addition operation returns the bitwise exclusive-or of its operands. The subtraction operation is identical, because the field has characteristic two. The multiplication operation takes two elements X and Y as input and proceeds as follows. If either X or Y is equal to 0x00, then the operation returns 0x00. Otherwise, the value EXP[ (LOG[X] + LOG[Y]) modulo 255] is returned. The division operation takes a dividend X and a divisor Y as input and computes X divided by Y as follows. If X is equal to 0x00, then the operation returns 0x00. If Y is equal to 0x00, then the input is invalid, and an error condition occurs. Otherwise, the value EXP[ (LOG[X] - LOG[Y]) modulo 255] is returned. The operation of raising a field element X to a power i, where i is a positive integer, is denoted as X^i, and it consists of multiplying X by itself i times. McGrew & Patnala Expires June 22, 2009 [Page 5]

Internet-Draft Threshold Secret Sharing December 2008 0x01, 0x03, 0x05, 0x0f, 0x11, 0x33, 0x55, 0xff, 0x1a, 0x2e, 0x72, 0x96, 0xa1, 0xf8, 0x13, 0x35, 0x5f, 0xe1, 0x38, 0x48, 0xd8, 0x73, 0x95, 0xa4, 0xf7, 0x02, 0x06, 0x0a, 0x1e, 0x22, 0x66, 0xaa, 0xe5, 0x34, 0x5c, 0xe4, 0x37, 0x59, 0xeb, 0x26, 0x6a, 0xbe, 0xd9, 0x70, 0x90, 0xab, 0xe6, 0x31, 0x53, 0xf5, 0x04, 0x0c, 0x14, 0x3c, 0x44, 0xcc, 0x4f, 0xd1, 0x68, 0xb8, 0xd3, 0x6e, 0xb2, 0xcd, 0x4c, 0xd4, 0x67, 0xa9, 0xe0, 0x3b, 0x4d, 0xd7, 0x62, 0xa6, 0xf1, 0x08, 0x18, 0x28, 0x78, 0x88, 0x83, 0x9e, 0xb9, 0xd0, 0x6b, 0xbd, 0xdc, 0x7f, 0x81, 0x98, 0xb3, 0xce, 0x49, 0xdb, 0x76, 0x9a, 0xb5, 0xc4, 0x57, 0xf9, 0x10, 0x30, 0x50, 0xf0, 0x0b, 0x1d, 0x27, 0x69, 0xbb, 0xd6, 0x61, 0xa3, 0xfe, 0x19, 0x2b, 0x7d, 0x87, 0x92, 0xad, 0xec, 0x2f, 0x71, 0x93, 0xae, 0xe9, 0x20, 0x60, 0xa0, 0xfb, 0x16, 0x3a, 0x4e, 0xd2, 0x6d, 0xb7, 0xc2, 0x5d, 0xe7, 0x32, 0x56, 0xfa, 0x15, 0x3f, 0x41, 0xc3, 0x5e, 0xe2, 0x3d, 0x47, 0xc9, 0x40, 0xc0, 0x5b, 0xed, 0x2c, 0x74, 0x9c, 0xbf, 0xda, 0x75, 0x9f, 0xba, 0xd5, 0x64, 0xac, 0xef, 0x2a, 0x7e, 0x82, 0x9d, 0xbc, 0xdf, 0x7a, 0x8e, 0x89, 0x80, 0x9b, 0xb6, 0xc1, 0x58, 0xe8, 0x23, 0x65, 0xaf, 0xea, 0x25, 0x6f, 0xb1, 0xc8, 0x43, 0xc5, 0x54, 0xfc, 0x1f, 0x21, 0x63, 0xa5, 0xf4, 0x07, 0x09, 0x1b, 0x2d, 0x77, 0x99, 0xb0, 0xcb, 0x46, 0xca, 0x45, 0xcf, 0x4a, 0xde, 0x79, 0x8b, 0x86, 0x91, 0xa8, 0xe3, 0x3e, 0x42, 0xc6, 0x51, 0xf3, 0x0e, 0x12, 0x36, 0x5a, 0xee, 0x29, 0x7b, 0x8d, 0x8c, 0x8f, 0x8a, 0x85, 0x94, 0xa7, 0xf2, 0x0d, 0x17, 0x39, 0x4b, 0xdd, 0x7c, 0x84, 0x97, 0xa2, 0xfd, 0x1c, 0x24, 0x6c, 0xb4, 0xc7, 0x52, 0xf6, 0x00 Figure 1: The EXP table. The elements are to be read from top to bottom and left to right. For example, EXP[0] is 0x01, EXP[8] is 0x1a, and so on. Note that the EXP[255] entry is present only as a placeholder, and is not actually used in any computation. McGrew & Patnala Expires June 22, 2009 [Page 6]

Internet-Draft Threshold Secret Sharing December 2008 0x00, 0x00, 0x19, 0x01, 0x32, 0x02, 0x1a, 0xc6, 0x4b, 0xc7, 0x1b, 0x68, 0x33, 0xee, 0xdf, 0x03, 0x64, 0x04, 0xe0, 0x0e, 0x34, 0x8d, 0x81, 0xef, 0x4c, 0x71, 0x08, 0xc8, 0xf8, 0x69, 0x1c, 0xc1, 0x7d, 0xc2, 0x1d, 0xb5, 0xf9, 0xb9, 0x27, 0x6a, 0x4d, 0xe4, 0xa6, 0x72, 0x9a, 0xc9, 0x09, 0x78, 0x65, 0x2f, 0x8a, 0x05, 0x21, 0x0f, 0xe1, 0x24, 0x12, 0xf0, 0x82, 0x45, 0x35, 0x93, 0xda, 0x8e, 0x96, 0x8f, 0xdb, 0xbd, 0x36, 0xd0, 0xce, 0x94, 0x13, 0x5c, 0xd2, 0xf1, 0x40, 0x46, 0x83, 0x38, 0x66, 0xdd, 0xfd, 0x30, 0xbf, 0x06, 0x8b, 0x62, 0xb3, 0x25, 0xe2, 0x98, 0x22, 0x88, 0x91, 0x10, 0x7e, 0x6e, 0x48, 0xc3, 0xa3, 0xb6, 0x1e, 0x42, 0x3a, 0x6b, 0x28, 0x54, 0xfa, 0x85, 0x3d, 0xba, 0x2b, 0x79, 0x0a, 0x15, 0x9b, 0x9f, 0x5e, 0xca, 0x4e, 0xd4, 0xac, 0xe5, 0xf3, 0x73, 0xa7, 0x57, 0xaf, 0x58, 0xa8, 0x50, 0xf4, 0xea, 0xd6, 0x74, 0x4f, 0xae, 0xe9, 0xd5, 0xe7, 0xe6, 0xad, 0xe8, 0x2c, 0xd7, 0x75, 0x7a, 0xeb, 0x16, 0x0b, 0xf5, 0x59, 0xcb, 0x5f, 0xb0, 0x9c, 0xa9, 0x51, 0xa0, 0x7f, 0x0c, 0xf6, 0x6f, 0x17, 0xc4, 0x49, 0xec, 0xd8, 0x43, 0x1f, 0x2d, 0xa4, 0x76, 0x7b, 0xb7, 0xcc, 0xbb, 0x3e, 0x5a, 0xfb, 0x60, 0xb1, 0x86, 0x3b, 0x52, 0xa1, 0x6c, 0xaa, 0x55, 0x29, 0x9d, 0x97, 0xb2, 0x87, 0x90, 0x61, 0xbe, 0xdc, 0xfc, 0xbc, 0x95, 0xcf, 0xcd, 0x37, 0x3f, 0x5b, 0xd1, 0x53, 0x39, 0x84, 0x3c, 0x41, 0xa2, 0x6d, 0x47, 0x14, 0x2a, 0x9e, 0x5d, 0x56, 0xf2, 0xd3, 0xab, 0x44, 0x11, 0x92, 0xd9, 0x23, 0x20, 0x2e, 0x89, 0xb4, 0x7c, 0xb8, 0x26, 0x77, 0x99, 0xe3, 0xa5, 0x67, 0x4a, 0xed, 0xde, 0xc5, 0x31, 0xfe, 0x18, 0x0d, 0x63, 0x8c, 0x80, 0xc0, 0xf7, 0x70, 0x07 Figure 2: The LOG table. The elements are to be read from top to bottom and left to right. For example, LOG[1] is 0x00, LOG[8] is 0x4b, and so on. Note that the LOG[0] entry is present only as a placeholder, and is not actually used in any computation. 3.2. Share Generation We first define how to share a single octet. The function f takes as input a single octet X that is not equal to 0x00, and an array A of D+1 octets, and returns a single octet. It is defined as f(X, A) = SUM A[i] * X^i i=0,D McGrew & Patnala Expires June 22, 2009 [Page 7]

Internet-Draft Threshold Secret Sharing December 2008 Because the summation takes place over GF(256), each addition uses the exclusive-or operation, and not integer addition. Note that the successive values of X^i used in the computation of the function f can be computed by multiplying a value by X once for each term in the summation. To create N shares from a secret, with a threshold of M, the following procedure, or any equivalent method, is used: For each share, a distinct x-value is generated. Each x-value is an octet other than the all-zero octet. All of the x-values used during a share generation process MUST be distinct. Each share is initialized to the x-value associated with that share. For each octet of the secret, the following steps are performed. An array A of M+1 octets is created, in which the array element A[0] contains the octet of the secret, and the array elements A[1], ..., A[M] contain octets that are selected independently and uniformly at random. For each share, the value of f(X,A) is computed, where X is the x-value of the share, and the resulting octet is appended to the share. After the procedure is done, each share contains one more octet than does the secret. The share format can be illustrated as +---------+---------+---------+---------+---------+ | X | f(X,A) | f(X,B) | f(X,C) | ... | +---------+---------+---------+---------+---------+ where X is the x-value of the share, and A, B, and C are arrays of M+1 octets; A[0] is equal to the first octet of the secret, B[0] is equal to the second octet of the secret, and so on. 3.3. Secret Reconstruction We denote the ith Lagrange function as L_i. This function takes as input a single octet X, and an array U of M octets, and is defined as X - U[j] L_i(X, U) = PRODUCT ------------- j=0,M, j!=i U[j] - U[i] Here the product runs over all of the values of j from 0 to M, excluding the value i. We denote the interpolation function as I. This function takes as McGrew & Patnala Expires June 22, 2009 [Page 8]

Internet-Draft Threshold Secret Sharing December 2008 input a single octet Z, two arrays U and V, each consisting of M octets, and returns a single octet; it is defined as I(X, U, V) = SUM L_j(X, U) * V[j]. j=0,M To reconstruct a secret from M shares, the following procedure, or any equivalent method, is used: If the shares are not equal length, then the input is inconsistent. An error should be reported, and processing must halt. The output string is initialized to the empty (zero-length) octet string. The octet array U is formed by setting U[i] equal to the first octet of the ith share. (Note that the ordering of the shares is arbitrary, but must be consistent throughout this algorithm.) The initial octet is stripped from each share. If any two elements of the array U have the same value, then an error condition has occurred; this fact should be reported, then the procedure must halt. For each octet of the shares, the following steps are performed. An array V of M+1 octets is created, in which the array element V[i] contains the octet from the ith share. The value of I(0x00, U, V) is computed, then appended to the output string. The output string is returned. After the procedure is done, the string that is returned contains one fewer octet than do the shares. McGrew & Patnala Expires June 22, 2009 [Page 9]

Internet-Draft Threshold Secret Sharing December 2008 4. Robust Threshold Secret Sharing A robust TSS system, or RTSS, is one that provides security even when one or more of the shares that are provided to the reconstruction algorithm may be crafted by a malicious adversary. In addition, an RTSS system will detect unintentional corruption of the shares. We provide robustness by adding a pre-processing step to the TSS share generation step, and a post-processing step to the TSS secret reconstruction step. The pre-processing consists of taking the secret S, then appending a hash H(S) to it. The post-processing step consists of verifying that the reconstructed secret has the form S || H(S), where the symbol || denotes the concatenation operation. An RTSS system can perform an additional operation that verifies the validity of a set of shares. This operation has the same inputs as the Reconstruct operation. Its output consists of an indication whether or not the secret could be reconstructed, but the secret itself is not returned. This operation may be useful in a situation in where the availability of a secret must be verified, for example, as part of an audit. 4.1. RTSS Data Format We use a data format with the following fields, in order: Identifier. This field contains a variable number of octets. It identifies the secret with which a share is associated. When a secret is reconstructed, each of the shares used as input should have the same value identifier. Flags. This field contains a single octet. Number of Shares. This field contains a single octet. Threshold. This field contains a single octet. Digest Algorithm. This field contains a single octet. Secret Length. This field is four octets long. Share Index. This field is a single octet in length. It contains the finite field element that is used as the "X" coordinate for the share. McGrew & Patnala Expires June 22, 2009 [Page 10]

Internet-Draft Threshold Secret Sharing December 2008 Share Data. This field has a length that is a variable number of octets. It contains the actual share data. This format is illustrated in Figure 3. +--------------------------------+ | Identifier | | (variable number of octets) | +--------------------------------+ | Flags | | (1 octets) | +--------------------------------+ ........ +--------------------------------+ | | ~ Share Data ~ | (variable number of octets) | | | +--------------------------------+ Figure 3: Share Format. McGrew & Patnala Expires June 22, 2009 [Page 11]

Internet-Draft Threshold Secret Sharing December 2008 5. Error Correction and Data Recovery TSS and RTSS are suitable for the protection of long-term key material. In such applications, it is highly desirable to provide protection against the accidental corruption of the shares. This section defines data formats that can be used to protect shares. These formats are optional extensions to the basic TSS and RTSS systems. 5.1. Data Recovery To protect against the corruption of the filesystem that is holding the shares, a "magic number" can be used as the initial part of the share data format. A magic number is a constant data string that is chosen arbitrarily, but which is unlikely to appear in other contexts, and thus can be used to recognize a data format when it appears in an arbitrary data stream. The use of a magic number in the data format for a share greatly simplifies the task of finding a share after a filesystem has been corrupted. The 8-octet magic number f628f91b52023d11 (hexadecimal) SHOULD be used. The number was selected randomly from a uniform distribution. 5.2. Error Correction To protect against data corruption in the underlying media, an error- correcting code (ECC) can be used. An ECC system consists of an encoding function, which maps the data to a codeword, and a decoding function, which maps a (possibly corrupted) codeword to the data. The simplest such code is a repetition code, in which multiple copies of the data are stored. In this specification, all ECCs must be systematic, that is, the data must appear as the initial bytes of the codeword. This property allows an implementation of the ECC to avoid the implementation of the full decoding algorithm. We use a data format that incorporates the following fields, in order: Encoding Type. This field is four octets long. It contains an unsigned integer in network byte order that denotes the type of the encoding, i.e. the algorithm that was used during the encoding process. Data Length. This field is four octets long. It contains an unsigned integer in network byte order that denotes the number of octets in the Data field. McGrew & Patnala Expires June 22, 2009 [Page 12]

Internet-Draft Threshold Secret Sharing December 2008 Redundancy Length. This field is four octets long. It contains an unsigned integer in network byte order that denotes the number of octets in the Redundancy field. Data. This field has a length that is a variable number of octets, which is indicated by the Data Length field. It contains the data that is intended to be conveyed by the code. If no data corruption has occurred, then this field will contain the data that was originally encoded. Redundancy. This field has a length that is a variable number of octets, which is indicated by the Reduncancy Length field. It contains information that can be used to check whether or not there are any errors in the Data field, and to correct some errors that may have occurred. This format is illustrated in Figure 4. +--------------------------------+ | Encoding Type | | (4 octets) | +--------------------------------+ | Data Length | | (4 octets) | +--------------------------------+ | Redundancy Length | | (4 octets) | +--------------------------------+ | | ~ Data ~ | (variable number of octets) | | | +--------------------------------+ | | ~ Redundancy ~ | (variable number of octets) | | | +--------------------------------+ Figure 4: Error Correction Format. If a code has a free parameter, the value of that parameter MUST be inferable from the values of the Data Length and Redundancy Length fields. McGrew & Patnala Expires June 22, 2009 [Page 13]

Internet-Draft Threshold Secret Sharing December 2008 5.3. A Repetition Code This section defines a format for a repetition code, which is a particular error correcting code that is conceptually simple and easy to implement. The value of the Encoding Type field is equal to 0000001 (hexadecimal). The Redundancy field contains R copies of the Data field, where R is an even number. The Redundancy Length is equal to the Data Length times R. The value of R MAY be equal to zero, in which case no error detection or correction is possible (but implementation is simple). The value of R SHOULD be at least two. For example, if the data that is encoded is equal to 68656c6c6f (hexadecimal), then the ECF data with R=2 would be <- ET -><- DL -><- RL -><- Data -><--- Redundancy ---> 00000001000000050000000a68656c6c6f68656c6c6f68656c6c6f To check the Data field for errors, that field should be compared with each of its copies in the redundancy field. The Repetition Code can be decoded by using majority-logic decoding. Considering both the Data and Redundancy fields, there are R+1 (possibly corrupted) copies of the original data, where R+1 is an odd number. The decoding process independently considers each octet of the Data field, and the corresponding octets of the copies that appear in the Redundancy field. That is, the ith octet of the Data, plus octets i, L+i, 2L+i, ... , RL+i, are analyzed independent from all other octets, where L is the value of the Data Length field. The following algorithm is applied to these octets. The binary representation of each octet is considered. For each bit in that representation, if more of the copies have a "1" in that position than have a "0" in that position, then that position is decoded to the value "1"; otherwise, it is decoded to "0". This process is repeated for all of the bit position. After all of the bits in the octet have been decoded, the value of the ith octet in the output of the decoding algorithm is computed, using the same binary representation as before. For example, if the data that was encoded in the previous example was corrupted to the value <- ET -><- DL -><- RL -><- Data -><--- Redundancy ---> 00000001000000050000000a68656c6c2f68656c6cef68656c6c6f ** ** ** McGrew & Patnala Expires June 22, 2009 [Page 14]

Internet-Draft Threshold Secret Sharing December 2008 then decoding would proceed as follows. The fifth octet of the Data field is equal to 2f, while the fifth and tenth octets of the Redundancy field are equal to ef and 6f, respectively. Using a bit representation with the most significant bit on the left, the octets and the "majority" octet are as follows: hex binary octet from Data 2f 00101111 octet from first copy ef 11101111 octet from second copy 6f 01101111 ----------------------------------- majority 6f 01101111 Thus the fifth octet in the output of the decoding algorithm will be 6f. McGrew & Patnala Expires June 22, 2009 [Page 15]

Internet-Draft Threshold Secret Sharing December 2008 6. Format This section summarizes the order of processing for when secret sharing is performed using the facilities for robustness (RTSS), error correction (ECC), and data recovery (Magic Number), and clarifies the relationships between data formats. This processing can be viewed as a layered model, as illustrated in Figure 5. (Note that we have not adhered to a strictly layered model, for the sake of simplicity, since the format defined by RTSS is used after the shares are generated.) When RTSS is used, it is applied to the secret before the sharing operation (and is removed from the secret after the reconstruction operaation). The RTSS data format MUST be used. When ECC is used, it is applied to the RTSS data after the sharing operation, so that the ECC Data field contains the entire RTSS Data Format. When a Magic Number is used, it is added after the ECC formatting is done, and it is prepended to the Error Correction Format. Secret Secret | ^ v | +------------------+ +------------------+ | Append Hash | | Verify Hash | +------------------+ +------------------+ | | +------------------+ +------------------+ | Generate Shares | |Reconstruct Secret| +------------------+ +------------------+ | | +------------------+ +------------------+ | ECC Encoding | | ECC Decoding | +------------------+ +------------------+ | | +------------------+ +------------------+ | Add Magic Number | |Strip Magic Number| +------------------+ +------------------+ | ^ v | Shares ----------------> Shares Figure 5: The combined processing model. McGrew & Patnala Expires June 22, 2009 [Page 16]

Internet-Draft Threshold Secret Sharing December 2008 7. Design and Rationale In this implementation, the secret and the shares are octet strings. Each octet is treated as an element of the finite field GF(256 ). The share-generation algorithm is applied to each octet of the secret independently. Similarly, the octets are treated independently during the reconstruction of the secrets from the shares. Shamir's original description treats the secret as a large integer modulo a large prime number [shamir]. The advantages of using a vector over GF(2^8) are that the computations are more efficient and the encoding is simpler. Multiplication and inversion over GF(2^8) can be done with two table lookups and two exors, using two fixed tables of 256 bytes each. One limitation of the GF(2^8) approach is that the number of shares that can be generated cannot be greater than 255; this limitation is unlikely to be important in practice, since fewer than ten shares are typically used. The reconstruction of the secret is done using Lagrange interpolation polynomials. This method is simple and easily tested. For large thresholds, this method is less efficient than an optimal method would be. However, performance is still good, and it is expected that the reconstruction of the secret will not be a performance- critical operation. McGrew & Patnala Expires June 22, 2009 [Page 17]

Internet-Draft Threshold Secret Sharing December 2008 8. Testing As with every crypto algorithm, it is essential to test an implementation of TSS or RTSS for correctness. This section provides guidance for such testing. Test cases are provided for known-answer tests. The Share Index field can never be equal to zero. This property SHOULD be tested. There is a simple consistency test that can be run on an implementation that uses the Lagrange form of the interpolation polynomial. Each function L_i(X,U) as defined above has the property that L_i(X,X[j]) = / unity (0x01) when i is equal to j, and \ zero (0x00) otherwise. The random source must be tested to ensure that it has high min- entropy. McGrew & Patnala Expires June 22, 2009 [Page 18]

Internet-Draft Threshold Secret Sharing December 2008 9. Test Cases This section provides test cases that can be used to validate an implementation. All values are in hexadecimal. algorithm - The algorithm used in the test case. secret - The secret value to be split into shares. threshold - The number of shares required to reconstruct a secret. share index - A share index. Each test case has multiple share values, and each share is associated with a share index. share - A share value. algorithm = TSS secret = 7465737400 threshold = 2 share index = 1 share = B9FA07E185 share index = 2 share = F5409B4511 McGrew & Patnala Expires June 22, 2009 [Page 19]

Internet-Draft Threshold Secret Sharing December 2008 10. Security Considerations It is crucial for security that the source of randomness used in the share generation process by cryptographically strong; it MUST be suitable for generating cryptographic keys. [RFC4086] provides guidance on the selection and implementation of random sources. A TSS implementation SHOULD be tested as described in Section 8. The confidentiality of the shares generated by TSS should be protected, since the exposure of too many shares will undermine the security of the system. Note that, in this regard, share values are more comparable to secret keys than to ciphertext. McGrew & Patnala Expires June 22, 2009 [Page 20]

Internet-Draft Threshold Secret Sharing December 2008 11. IANA Considerations This document has no actions for IANA. McGrew & Patnala Expires June 22, 2009 [Page 21]

Internet-Draft Threshold Secret Sharing December 2008 12. Acknowledgements Thanks to Brian Weis and Alfred Hines for constructive feedback. McGrew & Patnala Expires June 22, 2009 [Page 22]

Internet-Draft Threshold Secret Sharing December 2008 13. References 13.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. 13.2. Informative References [shamir] Shamir, A., "How to share a secret", Communications of the ACM (22): 612-613, 1979. McGrew & Patnala Expires June 22, 2009 [Page 23]

Internet-Draft Threshold Secret Sharing December 2008 Authors' Addresses David A. McGrew Cisco Systems, Inc. 510 McCarthy Blvd. Milpitas, CA 95035 US Phone: (408) 525 8651 Email: mcgrew@cisco.com URI: http://www.mindspring.com/~dmcgrew/dam.htm Praveen Patnala Email: praveenpatnala@yahoo.com McGrew & Patnala Expires June 22, 2009 [Page 24]