Things to keep in mind:
- The field's location within transaction data is defined by the transaction's structure. Currently (protocol v12) the transaction version is v2, and 'extra' lives between 'vout' (outputs) and 'rct_signatures' (ringct data).
- The field is interpreted as an array of bytes with little endian order (indexed 0, 1, 2). Its content is verified by neither the blockchain protocol nor the network consensus, so transaction authors can include anything they want in any order. This answer focuses on how the core implementation parses the field.
- It contains information sections, which each begin with a 'tag' byte. A tag defines how the bytes after it are interpreted. Wherever relevant, the leftmost byte (lower index) is considered most significant.
- The field's core implementation can be found here: src/cryptonote_basic/tx_extra.h
- Best practice: wallet designers should understand the rules for all known tags, in order to mitigate interference with the expectations and behavior of other wallets (especially the core wallet). In particular, non-standard tags and other information should be placed after any standard information.
- Bytes to be interpreted as 'length' or 'amount' are considered varints. Varints are up to 8 bytes long, and the subsequent byte is included only if the current byte's most significant bit is
1 (the end of the varint is either the byte with MSB not set or the 8th byte). It is an integer with up to 56 bits of information.
- The field may have unlimited elements, up to the functional limit of its transaction's maximum weight. Note: in the original CryptoNote specification the extra field began with a varint equal to the field's number of elements. Monero does not do that.
- The standard implementation interprets non-standard extra fields to the best of its ability. The parser works like this. It looks at the first byte. If it is a known tag then if the following bytes obey the tag's rules, save the tag's bytes for use elsewhere and skip ahead to the next available byte and check if it's a known tag. If an examined byte is not a known tag (specifically
0x04), then according to this it will interpret the next byte(s) as a varint, skip that number of bytes, and examine the next available byte.
Basic/standard tags (in hexidecimal format):
0x00: padding, ignore and move to the next byte; the core implementation does not add any of these to the extra field (there seems to be a 255 consecutive padding byte limit, although not clear how this affects the core parser)
0x01: public key, saves the next 32 bytes if they can be converted into an elliptic curve point; most transactions will have one of these, and it corresponds to the transaction public key; when going through an extra field, the wallet will save every valid public key it finds in order to use them to test for output ownership
0x02: extra nonce, the next byte is its length (not a varint, just an 8bit unsigned integer; may be up to 255); this is like an extra field within the extra field, and is often used by miner pools to prevent nonce overlap, and can be used to extend the proof of work nonce size to larger than 32 bits (although unlikely to be used, as a typical miner can only get through around 1/1000th or less of the available nonce values each block before resetting; see this site for current hash speeds); if the transaction type is non-miner (RCTTypeNull) the byte after its length is an 'extra nonce tag' interpreted according to special rules (see below); if the transaction type IS RCTTypeNull, there is no consistent or standard useage
0x03: merge mining tag; this doesn't seem to be used by anyone any more
0x04: additional public keys, the next byte is the number of additional keys (must be convertible to elliptic curve points, 32 bytes each), and the byte after that is the most significant byte of the first additional key; usually only one of these sets per transaction, and there should typically be either 1 or #outputs total amount of public keys per transaction (standard single transaction public key, or at-least-one-ouput-is-a-subaddress case with #outputs transaction public keys)
0xDE: mysterious minergate tag
Note: the core implementation sorts tag fields for transactions being constructed in this order [
0x00] (see function
sort_tx_extra()). It does not support other tag fields or random data, although alternative implementations can and do edit the sort function (if they use it at all) to accommodate their own tags.
Special extra nonce tags (tags within the extra nonce section, likely only relevant for non-miner/coinbase transactions):
0x00: payment ID, the next 32 bytes are a payment ID in clear text; deprecated as of core implementation v0.15, which means it will be ignored/skipped over by the main wallet
0x01: encrypted payment ID, the next 8 bytes are an encrypted payment ID (note: the core implementation adds a dummy encrypted payment ID to transactions with two outputs if one isn't specified, to improve transaction indistinguishability); only the first one of these found will be used and reported to the user if they happen to own an output in the same transaction (my understanding of best practice is encrypted payment IDs should only be included in 2-output transactions, and that the 'change'-output will ignore payment IDs)
An example from a transaction in block 2004288 (transaction hash
= [1 (transaction public key TAG),
2 (extra nonce TAG),
9 (length of extra nonce),
1 (extra nonce encrypted payment ID TAG),
P.S. Thanks to jtgrassie for his patience assisting with this topic.