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+# meta.dat
+
+Dat uses a simple metadata file called `meta.dat`. The purpose of this file is to store the fingerprints of the files in a Dat repository. If you create a `meta.dat` file for a set of files, you can host it on a static HTTP server along with the files and Dat clients will be able to download and verify your files, even if you aren't running a Dat server!
+
+# File format
+
+```
+<Header><Entries Index...><Entries...>
+```
+
+The format is a header followed by an index of many entries. Entry order is based on the indexing determined by the [Flat In-Order Tree](hyperdrive.md#flat-in-order-trees) algorithm we use in Dat. After the entry index, a concatinated list of entries follows.
+
+### Header format
+
+```
+<varint header-length><header protobuf>
+```
+
+The header protobuf has this schema:
+
+``` proto
+message Header {
+  required bytes datLink = 1;
+  required uint64 entries = 2;
+  optional bool isSigned = 3;
+  optional string hashType = 4 [default = "sha256"];
+  optional uint32 hashLength = 5 [default = 32];
+  optional string signatureType = 6 [default = "ed25519"];
+  optional uint32 signatureLength = 7 [default = 64];
+}
+```
+
+### Entry index format
+
+For non-signed entries:
+
+```
+<8-byte-chunk-end><chunk-hash>
+```
+
+The 8-byte-chunk-end is an unsigned big endian 64 bit integer that should be the absolute position in the file for the **end of the chunk**.
+
+For signed entries in live feeds (only applies to even numbered nodes e.g. leaf nodes):
+
+```
+<8-byte-chunk-end><chunk-signature><chunk-hash>
+```
+
+For any odd nodes, in either a live or a non-live feed, the non-signed entry format will be used.
+
+## Example
+
+Given a tree like this you might want to look up in a `meta.dat` file the metadata for a specific node:
+
+```
+0─┐  
+  1─┐
+2─┘ │
+    3
+4─┐ │
+  5─┘
+6─┘
+```
+
+If you wanted to look up the metadata for 3, you could read the third (or any!) entry from meta.dat:
+
+First you have to read the varint at the beginning of the file so you know how big the header is:
+
+``` js
+var varint = require('varint') // https://github.com/chrisdickinson/varint
+var headerLength = varint.decode(firstChunkOfFile)
+```
+
+Now you can read the header from the file
+
+``` js
+var headerOffset = varint.encodingLength(headerLength)
+var headerEndOffset = headerOffset + headerLength
+var headerBytes = firstChunkOfFile.slice(headerOffset, headerEndOffset)
+```
+
+To decode the header use the protobuf schema. We can use the [protocol-buffers](https://github.com/mafintosh/protocol-buffers) module to do that.
+
+``` js
+var messages = require('protocol-buffers')(fs.readFileSync('meta.dat.proto'))
+var header = messages.Header.decode(headerBytes)
+```
+
+Now we have all the configuration required to calculate an entry offset.
+
+``` js
+var entryNumber = 42
+var entryOffset = headerEndOffset + entryNumber * (8 + header.hashLength)
+```
+
+If you have a signed feed, you have to take into account the extra space required for the signatures in the even nodes.
+
+``` js
+var entryOffset = headerLength + entryNumber * (8 + header.hashLength)
+                  + Math.floor(entryNumber / 2) * header.signatureLength
+```