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+# Introduction
+
+This repository contains the [KeyValuePairs extra-field specification](zip_keyvalue_extra_field_specification.md),
+to be able to encode arbitrary key-value pairs of metadata associated with a file
+within a ZIP. For example to store the Content-Type of a file.
diff --git a/zip_keyvalue_extra_field_specification.md b/zip_keyvalue_extra_field_specification.md
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+# Version
+
+- Version: 0.5.0
+- Date: 2023-Jan-06
+
+# License
+
+This specification document is (C) 2022-2023 Even Rouault and licensed under the
+[CC-BY-4.0](https://spdx.org/licenses/CC-BY-4.0.html) terms.
+
+Note: the scope of the copyrighted material does, of course, not extend onto
+any source or binary code derived from the specification.
+
+# Introduction
+
+This document specifies a third-party extra field, to be able to encode arbitrary
+key-value pairs of metadata associated with a file within a ZIP. For example
+to store the Content-Type of a file.
+
+# Specification
+
+The encoding MUST follow the requirements of "4.6 Third Party Mappings" of the
+[ZIP specification](https://pkware.cachefly.net/webdocs/APPNOTE/APPNOTE-6.3.9.TXT)
+
+In the following, ``uint16`` is a 16-bit unsigned integer, encoded in little-endian
+order (least significant byte first), and ``byte`` a 8-bit unsigned integer.
+
+| Offset | Type   |    Name         | Comment                                    |
+| ------ | ------ | --------------- | ------------------------------------------ |
+|    0   | uint16 | header_id       | Constant 0x564B (bytes 'K' 'V')            |
+|    2   | uint16 | payload_size    | Number of bytes following this field.      |
+|    4   | byte[] | signature       | Constant "KeyValuePairs" (13 bytes)        |
+|   17   | byte   | kv_count        | Number of key-value pairs.                 |
+|   18   | uint16 | kv1_key_size    | Size in bytes of the first key.            |
+|   20   | byte[] | kv1_key_text    | UTF-8 encoded value of the first key.      |
+|        | uint16 | kv1_val_size    | Size in bytes of the first value.          |
+|        | byte[] | kv1_val_text    | UTF-8 encoded value of the first value.    |
+|        |        | ...             | ....                                       |
+|        | uint16 | kvN_key_size    | Size in bytes of the last key.             |
+|        | byte[] | kvN_key_text    | UTF-8 encoded value of the last key.       |
+|        | uint16 | kvN_val_size    | Size in bytes of the last value.           |
+|        | byte[] | kvN_val_text    | UTF-8 encoded value of the last value.     |
+
+# Example
+
+Encoding of a ("Content-Type", "text/plain") key-value pair:
+
+| Offset  | Bytes                                  | Comment                                    |
+| ------- | -------------------------------------- | ------------------------------------------ |
+|    0    | 4B 56                                  | header_id                                  |
+|    2    | 28 00                                  | payload_size (40 bytes)                    |
+|    4    | 4B 65 79 56 61 6C 75 65 50 61 69 72 73 | signature ("KeyValuePairs")                |
+|   17    | 01                                     | kv_count                                   |
+|   18    | 0C 00                                  | kv1_key_size (12 bytes)                    |
+|   20    | 43 6F 6E 74 65 6E 74 2D 54 79 70 65    | kv1_key_text ("Content-Type")              |
+|   32    | 0A 00                                  | kv1_val_size (10 bytes)                    |
+|   34    | 74 65 78 74 2F 70 6C 61 69 6E          | kv1_val_text ("text/plain")                |
+
+Total: 44 bytes
+
+# Annex A: Software implementations
+
+## GDAL (C/C++ library)
+
+The [sozip](https://github.com/rouault/gdal/tree/sozip) development branch
+of [GDAL](https://gdal.org) contains an implementation of this specification.
+