Internet Engineering Task Force D. DeVault Internet-Draft SourceHut Intended status: Informational 19 October 2020 Expires: 22 April 2021 Binary Application Record Encoding (BARE) draft-devault-bare-00 Abstract The Binary Application Record Encoding (BARE) is a data format used to represent application records for storage or transmission between programs. BARE messages are concise and have a well-defined schema, and implementations may be simple and broadly compatible. A schema language is also provided to express message schemas out-of-band. Comments Comments are solicited and should be addressed to the mailing list at ~sircmpwn/public-inbox@lists.sr.ht and/or the author(s). 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 https://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 22 April 2021. Copyright Notice Copyright (c) 2020 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 DeVault Expires 22 April 2021 [Page 1] Internet-Draft BARE October 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Specification of the BARE Message Encoding . . . . . . . . . 3 2.1. Primitive Types . . . . . . . . . . . . . . . . . . . . . 3 2.2. Aggregate Types . . . . . . . . . . . . . . . . . . . . . 5 2.3. User-Defined Types . . . . . . . . . . . . . . . . . . . 6 2.4. Invariants . . . . . . . . . . . . . . . . . . . . . . . 6 3. BARE Schema Language Specification . . . . . . . . . . . . . 7 3.1. Lexical Analysis . . . . . . . . . . . . . . . . . . . . 7 3.2. ABNF Grammar . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Semantic Elements . . . . . . . . . . . . . . . . . . . . 8 4. Application Considerations . . . . . . . . . . . . . . . . . 9 5. Future Considerations . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. Normative References . . . . . . . . . . . . . . . . . . . . 10 Appendix A. Example message schema . . . . . . . . . . . . . . . 11 Appendix B. Example Messages . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction The purpose of the BARE message encoding, like hundreds of others, is to encode application messages. The goals of such encodings vary (leading to their proliferation); BARE's goals are the following: * Concise messages * A well-defined message schema * Broad compatibility with programming environments * Simplicity of implementation This document specifies the BARE message encoding, as well as a schema language which may be used to describe the layout of a BARE message. The schema of a message must be agreed upon in advance by each party exchanging a BARE message; message structure is not encoded into the representation. The schema language is useful for this purpose, but not required. DeVault Expires 22 April 2021 [Page 2] Internet-Draft BARE October 2020 1.1. Terminology 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 RFC 2119 [RFC2119]. 2. Specification of the BARE Message Encoding A BARE message is a single value of a pre-defined type, which may be of an aggregate type enclosing multiple values. Unless otherwise specified there is no additional container or structure around the value; it is encoded plainly. A BARE message does not necessarily have a fixed length, but the schema author may make a deliberate choice to constrain themselves to types of well-defined lengths if this is desired. The names for each type are provided to establish a vocabulary for describing a BARE message schema out-of-band, by parties who plan to exchange BARE messages. The type names used here are provided for this informative purpose, but are more rigourously specified by the schema language specification in Section 3. 2.1. Primitive Types Primitive types represent exactly one value. uint An unsigned integer with a variable-length encoding. Each octet of the encoded value has the most-significant bit set, except for the last octet. The remaining bits are the integer value in 7-bit groups, least-significant first. The maximum precision of such a number is 64-bits. The maximum length of an encoded uint is therefore 10 octets. int A signed integer with a variable-length encoding. Signed integers are represented as uint using a "zig-zag" encoding: positive values x are written as 2x + 0, negative values are written as 2(^x) + 1. In other words, negative numbers are complemented and whether to complement is encoded in bit 0. The maximum precision of such a number is 64-bits. The maximum length of an encoded int is therefore 10 octets. DeVault Expires 22 April 2021 [Page 3] Internet-Draft BARE October 2020 u8, u16, u32, u64 Unsigned integers of a fixed precision, respectively 8, 16, 32, and 64 bits. They are encoded in little-endian (least significant octet first). i8, i16, i32, i64 Signed integers of a fixed precision, respectively 8, 16, 32, and 64 bits. They are encoded in little-endian (least significant octet first), with two's compliment notation. f32, f64 Floating-point numbers represented with the IEEE 754 [IEEE.754.1985] binary32 and binary64 floating point number formats. bool A boolean value, either true or false, encoded as a u8 type with a value of one or zero, respectively representing true or false. If a value other than one or zero is found in the u8 representation of the bool, the message is considered invalid, and the decoder SHOULD raise an error if it encounters such a value. enum An unsigned integer value from a set of possible values agreed upon in advance, encoded with the uint type. An enum whose uint value is not a member of the values agreed upon in advance is considered invalid, and the decoder SHOULD raise an error if it encounters such a value. Note that this makes the enum type unsuitable for representing a several enum values which have been combined with a bitwise OR operation. string A string of text. The length of the text in octets is encoded first as a uint, followed by the text data represented with the UTF-8 encoding [RFC3629]. If the data is found to contain invalid UTF-8 sequences, it is considered invalid, and the decoder SHOULD raise an error if it encounters such a value. DeVault Expires 22 April 2021 [Page 4] Internet-Draft BARE October 2020 data Arbitrary data with a fixed "length" in octets, e.g. data<16>. The data is encoded literally in the message, and MUST NOT be greater than 18,446,744,073,709,551,615 octets in length (the maximum value of a u64). data Arbitrary data of a variable length in octets. The length is encoded first as a uint, followed by the data itself encoded literally. void A type with zero length. It is not encoded into BARE messages. 2.2. Aggregate Types Aggregate types may store zero or more primitive or aggregate values. optional A value of "type" which may or may not be present, e.g. optional. Represented as either a u8 with a value of zero, indicating that the optional value is unset; or a u8 with a value of one, followed by the encoded data of the optional type. An optional value whose initial u8 is set to a number other than zero or one is considered invalid, and the decoder SHOULD raise an error if it encounters such a value. [length]type A list of "length" values of "type", e.g. [10]uint. The length is not encoded into the message. The encoded values of each member of the list are concatenated to form the encoded list. []type A variable-length list of values of "type", e.g. []string. The length of the list (in values) is encoded as a uint, followed by the encoded values of each member of the list concatenated. map[type A]type B DeVault Expires 22 April 2021 [Page 5] Internet-Draft BARE October 2020 An associative list of values of type B keyed by values of type A, e.g. map[u32]string. The encoded representation of a map begins with the number of key/value pairs as a uint, followed by the encoded key/value pairs concatenated. Each key/value pair is encoded as the encoded key concatenated with the encoded value. A message with repeated keys is considered invalid, and the decoder SHOULD raise an error if it encounters such a value. (type | type | ...) A tagged union whose value may be one of any type from a set of types, e.g. (int | uint | string). Each type in the set is assigned a numeric identifier. The value is encoded as the selected type's identifier represented with the uint encoding, followed by the encoded value of that type. A union with a tag value that does not have a corresponding type assigned is considered invalid, and the decoder SHOULD raise an error if it encounters such a value. struct A set of values of arbitrary types, concatenated in an order agreed upon in advance. Each value is referred to as a "field", and field has a name and type. 2.3. User-Defined Types A user-defined type gives a name to another type. This creates a distinct type whose representation is equivalent to the named type. An arbitrary number of user-defined types may be used for the same underlying type; each is distinct from the other. 2.4. Invariants The following invariants are specified: * Any type which is ultimately a void type (either directly or via a user-defined type) MUST NOT be used as an optional type, struct member, list member, map key, or map value. Void types may only be used as members of the set of types in a tagged union. * The lengths of fixed-length arrays and data types MUST be at least one. * Structs MUST have at least one field. DeVault Expires 22 April 2021 [Page 6] Internet-Draft BARE October 2020 * Unions MUST have at least one type, and each type MUST NOT be repeated. * Map keys MUST be of a primitive type which is not data or data. * Each named value of an enum type MUST have a unique value. 3. BARE Schema Language Specification The use of the schema language is optional. Implementations SHOULD support decoding arbitrary BARE messages without a schema document, by defining the schema in a manner which utilizes more native tools available from the programming environment. However, it may be useful to have a schema document for use with code generation, documentation, or interoperability. A domain-specific language is provided for this purpose. 3.1. Lexical Analysis During lexical analysis, "#" is used for comments; if encountered, the "#" character and any subsequent characters are discarded until a line feed (%x0A) is found. 3.2. ABNF Grammar The syntax of the schema language is provided here in Augmented Backus-Naur form [RFC5234]. However, this grammar differs from [RFC5234] in that strings are case-sensitive (e.g. "type" does not match TypE). schema = [WS] user-types [WS] user-type = "type" WS user-type-name WS non-enum-type user-type =/ "enum" WS user-type-name WS enum-type user-types = user-type / (user-types WS user-type) type = non-enum-type / enum-type non-enum-type = primitive-type / aggregate-type / user-type-name user-type-name = UPPER *(ALPHA / DIGIT) ; First letter is uppercase primitive-type = "int" / "i8" / "i16" / "i32" / "i64" primitive-type =/ "uint" / "u8" / "u16" / "u32" / "u64" primitive-type =/ "f32" / "f64" primitive-type =/ "bool" primitive-type =/ "string" DeVault Expires 22 April 2021 [Page 7] Internet-Draft BARE October 2020 primitive-type =/ "data" / ("data<" integer ">") primitive-type =/ "void" enum-type = "{" [WS] enum-values [WS] "}" enum-values = enum-value / (enum-values WS enum-value) enum-value = enum-value-name enum-value =/ (enum-value-name [WS] "=" [WS] integer) enum-value-name = UPPER *(UPPER / DIGIT / "_") aggregate-type = optional-type aggregate-type =/ array-type aggregate-type =/ map-type aggregate-type =/ union-type aggregate-type =/ struct-type optional-type = "optional<" type ">" array-type = "[" [integer] "]" type integer = 1*DIGIT map-type = "map[" type "]" type union-type = "(" union-members ")" union-members = union-member union-members =/ (union-members [WS] "|" [WS] union-member) union-member = type [[WS] "=" [WS] integer] struct-type = "{" [WS] fields [WS] "}" fields = field / (fields WS field) field = 1*ALPHA [WS] ":" [WS] type UPPER = %x41-5A ; uppercase ASCII letters ALPHA = %x41-5A / %x61-7A ; A-Z / a-z DIGIT = %x30-39 ; 0-9 WS = 1*(%x0A / %x09 / " ") ; whitespace See Appendix A for an example schema written in this language. 3.3. Semantic Elements The names of fields and user-defined types are informational: they are not represented in BARE messages. They may be used by code generation tools to inform the generation of field and type names in the native programming environment. DeVault Expires 22 April 2021 [Page 8] Internet-Draft BARE October 2020 Enum values are also informational. Values without an integer token are assigned automatically in the order that they appear, starting from zero and incrementing for each subsequent unassigned value. If a value is explicitly specified, automatic assignment continues from that value plus one for subsequent enum values. Union type members are assigned a tag in the order that they appear, starting from zero and incrementing for each subsequent type. If a tag value is explicitly specified, automatic assignment continues from that value plus one for subsequent values. 4. Application Considerations Message authors who wish to design a schema which is backwards- and forwards-compatible with future messages are encouraged to use union types for this purpose. New types may be appended to the members of a union type while retaining backwards compatibility with older message types. The choice to do this must be made from the first message version-- moving a struct into a union _does not_ produce a backwards-compatible message. The following schema provides an example: type Message (MessageV1 | MessageV2 | MessageV3) type MessageV1 ... type MessageV2 ... type MessageV3 ... An updated schema which adds a MessageV4 type would still be able to decode versions 1, 2, and 3. If a message version is later deprecated, it may be removed in a manner compatible with future versions 2 and 3 if the initial tag is specified explicitly. type Message (MessageV2 = 1 | MessageV3) 5. Future Considerations To ensure message compatibility between implementations and backwards- and forwards-compatibility of messages, constraints on vendor extensions are required. This specification is final, and new types or extensions will not be added in the future. Implementors MUST NOT define extensions to this specification. DeVault Expires 22 April 2021 [Page 9] Internet-Draft BARE October 2020 To support the encoding of novel data structures, the implementor SHOULD make use of user-defined types in combination with the data or data types. 6. IANA Considerations This memo includes no request to IANA. 7. Security Considerations Message parsers are common vectors for security vulnerabilities. BARE addresses this by making the message format as simple as possible. However, the parser MUST be prepared to handle a number of error cases when decoding untrusted messages, such as a union type with an invalid tag, or an enum with an invalid value. Such errors may also arise by mistake, for example when attempting to decode a message with the wrong schema. Support for data types of an arbitrary, message-defined length (lists, maps, strings, etc) is commonly exploited to cause the implementation to exhaust its resources while decoding a message. However, legitimate use-cases for extremely large data types (possibly larger than the system has the resources to store all at once) do exist. The decoder MUST manage its resources accordingly, and SHOULD provide the application a means of providing their own decoder implementation for values which are expected to be large. There is only one valid interpretation of a BARE message for a given schema, and different decoders and encoders should be expected to provide that interpretation. If an implementation has limitations imposed from the programming environment (such as limits on numeric precision), the implementor MUST document these limitations, and prevent conflicting interpretations from causing undesired behavior. 8. Normative References [IEEE.754.1985] Institute of Electrical and Electronics Engineers, "Standard for Binary Floating-Point Arithmetic", IEEE Standard 754, August 1985. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . DeVault Expires 22 April 2021 [Page 10] Internet-Draft BARE October 2020 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . Appendix A. Example message schema The following is an example of a schema written in the BARE schema language. DeVault Expires 22 April 2021 [Page 11] Internet-Draft BARE October 2020 type PublicKey data<128> type Time string # ISO 8601 enum Department { ACCOUNTING ADMINISTRATION CUSTOMER_SERVICE DEVELOPMENT # Reserved for the CEO JSMITH = 99 } type Customer { name: string email: string address: Address orders: []{ orderId: i64 quantity: i32 } metadata: map[string]data } type Employee { name: string email: string address: Address department: Department hireDate: Time publicKey: optional metadata: map[string]data } type TerminatedEmployee void type Person (Customer | Employee | TerminatedEmployee) type Address { address: [4]string city: string state: string country: string } DeVault Expires 22 April 2021 [Page 12] Internet-Draft BARE October 2020 Appendix B. Example Messages Some basic example messages in hexadecimal are provided for the schema specified in Appendix A. A "Person" value of type "Customer" with the following values: name James Smith email jsmith@example.org address 123 Main Street; Philadelphia; PA; United States orders (1) orderId: 4242424242; quantity: 5 metadata (unset) Encoded BARE message: 00 0b 4a 61 6d 65 73 20 53 6d 69 74 68 12 6a 73 6d 69 74 68 40 65 78 61 6d 70 6c 65 2e 6f 72 67 0b 31 32 33 20 4d 61 69 6e 20 53 74 00 00 00 0c 50 68 69 6c 61 64 65 6c 70 68 69 61 02 50 41 0d 55 6e 69 74 65 64 20 53 74 61 74 65 73 01 b2 41 de fc 00 00 00 00 05 00 00 00 00 A "Person" value of type "Employee" with the following values: name Tiffany Doe email tiffanyd@acme.corp address 123 Main Street; Philadelphia; PA; United States department ADMINISTRATION hireDate 2020-06-21T21:18:05Z publicKey (unset) metadata (unset) Encoded BARE message: DeVault Expires 22 April 2021 [Page 13] Internet-Draft BARE October 2020 01 0b 54 69 66 66 61 6e 79 20 44 6f 65 12 74 69 66 66 61 6e 79 64 40 61 63 6d 65 2e 63 6f 72 70 0b 31 32 33 20 4d 61 69 6e 20 53 74 00 00 00 0c 50 68 69 6c 61 64 65 6c 70 68 69 61 02 50 41 0d 55 6e 69 74 65 64 20 53 74 61 74 65 73 01 19 32 30 32 30 2d 30 36 2d 32 31 54 32 31 3a 31 38 3a 30 35 2b 30 30 3a 30 30 00 00 A "Person" value of type "TerminatedEmployee": Encoded BARE message: 02 Author's Address Drew DeVault SourceHut 454 E. Girard Ave #2R Philadelphia, PA 19125 United States of America Phone: +1 719 213 5473 Email: sir@cmpwn.com DeVault Expires 22 April 2021 [Page 14]