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BNS Subdomains

BNS names are strongly owned because the owner of its private key can generate valid transactions that update its zone file hash and owner. However, this comes at the cost of requiring a name owner to pay for the underlying transaction in the blockchain. Moreover, this approach limits the rate of BNS name registrations and operations to the underlying blockchain's transaction bandwidth.

BNS overcomes this with subdomains. A BNS subdomain is a type of BNS name whose state and owner are stored outside of the blockchain, but whose existence and operation history are anchored to the blockchain. Like their on-chain counterparts, subdomains are globally unique, strongly owned, and human-readable. BNS gives them their own name state and public keys. Unlike on-chain names, subdomains can be created and managed cheaply, because they are broadcast to the BNS network in batches. A single blockchain transaction can send up to 120 subdomain operations.

This is achieved by storing subdomain records in the BNS name zone files. An on-chain name owner broadcasts subdomain operations by encoding them as TXT records within a DNS zone file. To broadcast the zone file, the name owner sets the new zone file hash with a NAME_UPDATE transaction and replicates the zone file. This, in turn, replicates all subdomain operations it contains, and anchors the set of subdomain operations to an on-chain transaction. The BNS node's consensus rules ensure that only valid subdomain operations from valid NAME_UPDATE transactions will ever be stored.

For example, the name verified.podcast once wrote the zone file hash 247121450ca0e9af45e85a82e61cd525cd7ba023, which is the hash of the following zone file:

$TTL 3600
1yeardaily TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAxeWVhcmRhaWx5CiRUVEwgMzYwMApfaHR0cC5fdGNwIFVSSSAxMCAxICJodHRwczovL3BoLmRvdHBvZGNhc3QuY28vMXllYXJkYWlseS9oZWFkLmpzb24iCg=="
2dopequeens TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAyZG9wZXF1ZWVucwokVFRMIDM2MDAKX2h0dHAuX3RjcCBVUkkgMTAgMSAiaHR0cHM6Ly9waC5kb3Rwb2RjYXN0LmNvLzJkb3BlcXVlZW5zL2hlYWQuanNvbiIK"
10happier TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAxMGhhcHBpZXIKJFRUTCAzNjAwCl9odHRwLl90Y3AgVVJJIDEwIDEgImh0dHBzOi8vcGguZG90cG9kY2FzdC5jby8xMGhhcHBpZXIvaGVhZC5qc29uIgo="
31thoughts TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAzMXRob3VnaHRzCiRUVEwgMzYwMApfaHR0cC5fdGNwIFVSSSAxMCAxICJodHRwczovL3BoLmRvdHBvZGNhc3QuY28vMzF0aG91Z2h0cy9oZWFkLmpzb24iCg=="
359 TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAzNTkKJFRUTCAzNjAwCl9odHRwLl90Y3AgVVJJIDEwIDEgImh0dHBzOi8vcGguZG90cG9kY2FzdC5jby8zNTkvaGVhZC5qc29uIgo="
30for30 TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAzMGZvcjMwCiRUVEwgMzYwMApfaHR0cC5fdGNwIFVSSSAxMCAxICJodHRwczovL3BoLmRvdHBvZGNhc3QuY28vMzBmb3IzMC9oZWFkLmpzb24iCg=="
onea TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiBvbmVhCiRUVEwgMzYwMApfaHR0cC5fdGNwIFVSSSAxMCAxICJodHRwczovL3BoLmRvdHBvZGNhc3QuY28vb25lYS9oZWFkLmpzb24iCg=="
10minuteteacher TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAxMG1pbnV0ZXRlYWNoZXIKJFRUTCAzNjAwCl9odHRwLl90Y3AgVVJJIDEwIDEgImh0dHBzOi8vcGguZG90cG9kY2FzdC5jby8xMG1pbnV0ZXRlYWNoZXIvaGVhZC5qc29uIgo="
36questionsthepodcastmusical TXT "owner=1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH" "seqn=0" "parts=1" "zf0=JE9SSUdJTiAzNnF1ZXN0aW9uc3RoZXBvZGNhc3RtdXNpY2FsCiRUVEwgMzYwMApfaHR0cC5fdGNwIFVSSSAxMCAxICJodHRwczovL3BoLmRvdHBvZGNhc3QuY28vMzZxdWVzdGlvbnN0aGVwb2RjYXN0bXVzaWNhbC9oZWFkLmpzb24iCg=="
_http._tcp URI 10 1 "https://dotpodcast.co/"

Each TXT record in this zone file encodes a subdomain-creation. For example, 1yeardaily.verified.podcast resolves to:

{
  "address": "1MwPD6dH4fE3gQ9mCov81L1DEQWT7E85qH",
  "blockchain": "bitcoin",
  "last_txid": "d87a22ebab3455b7399bfef8a41791935f94bc97aee55967edd5a87f22cce339",
  "status": "registered_subdomain",
  "zonefile_hash": "e7acc97fd42c48ed94fd4d41f674eddbee5557e3",
  "zonefile_txt": "$ORIGIN 1yeardaily\n$TTL 3600\n_http._tcp URI 10 1 \"https://ph.dotpodcast.co/1yeardaily/head.json\"\n"
}

This information was extracted from the 1yeardaily TXT resource record in the zone file for verified.podcast.

Subdomain Lifecycle​

Note that 1yeardaily.verified.podcast has a different public key hash (address) than verified.podcast. A BNS node will only process a subsequent subdomain operation on 1yeardaily.verified.podcast if it includes a signature from this address's private key. verified.podcast cannot generate updates; only the owner of 1yeardaily.verified.podcast can do so.

The lifecycle of a subdomain and its operations is shown in Figure 2.

   subdomain                  subdomain                  subdomain
   creation                   update                     transfer
+----------------+         +----------------+         +----------------+
| cicero         |         | cicero         |         | cicero         |
| owner="1Et..." | signed  | owner="1Et..." | signed  | owner="1cJ..." |
| zf0="7e4..."   |<--------| zf0="111..."   |<--------| zf0="111..."   |<---- ...
| seqn=0         |         | seqn=1         |         | seqn=2         |
|                |         | sig="xxxx"     |         | sig="xxxx"     |
+----------------+         +----------------+         +----------------+
        |                          |                          |
        |        off-chain         |                          |
~ ~ ~ ~ | ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~|~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ | ~ ~ ~ ~ ~ ~ ~ ...
        |         on-chain         |                          |
        V                          V (zone file hash    )     V
+----------------+         +----------------+         +----------------+
| res_publica.id |         |    jude.id     |         | res_publica.id |
|  NAME_UPDATE   |<--------|  NAME_UPDATE   |<--------|  NAME_UPDATE   |<---- ...
+----------------+         +----------------+         +----------------+
   blockchain                 blockchain                 blockchain
   block                      block                      block


Figure 2:  Subdomain lifetime with respect to on-chain name operations .A new
subdomain operation will only be accepted if it has a later "sequence=" number,
and a valid signature in "sig=" over the transaction body .The "sig=" field
includes both the public key and signature, and the public key must hash to
the previous subdomain operation's "addr=" field.

The subdomain-creation and subdomain-transfer transactions for
"cicero.res_publica.id" are broadcast by the owner of "res_publica.id."
However, any on-chain name ("jude.id" in this case) can broadcast a subdomain
update for "cicero.res_publica.id."

Subdomain operations are ordered by sequence number, starting at 0. Each new subdomain operation must include:

  • The next sequence number
  • The public key that hashes to the previous subdomain transaction's address
  • A signature from the corresponding private key over the entire subdomain operation.

If two correctly signed but conflicting subdomain operations are discovered (that is, they have the same sequence number), the one that occurs earlier in the blockchain's history is accepted. Invalid subdomain operations are ignored.

Combined, this ensures that a BNS node with all of the zone files with a given subdomain's operations will be able to determine the valid sequence of state-transitions it has undergone, and determine the current zone file and public key hash for the subdomain.

Subdomain Creation and Management​

Unlike an on-chain name, a subdomain owner needs an on-chain name owner's help to broadcast their subdomain operations. In particular:

  • A subdomain-creation transaction can only be processed by the owner of the on-chain name that shares its suffix. For example, only the owner of res_publica.id can broadcast subdomain-creation transactions for subdomain names ending in .res_publica.id.
  • A subdomain-transfer transaction can only be broadcast by the owner of the on-chain name that created it. For example, the owner of cicero.res_publica.id needs the owner of res_publica.id to broadcast a subdomain-transfer transaction to change cicero.res_publica.id's public key.
  • In order to send a subdomain-creation or subdomain-transfer, all of an on-chain name owner's zone files must be present in the Atlas network. This lets the BNS node prove the absence of any conflicting subdomain-creation and subdomain-transfer operations when processing new zone files.
  • A subdomain update transaction can be broadcast by any on-chain name owner, but the subdomain owner needs to find one who will cooperate. For example, the owner of verified.podcast can broadcast a subdomain-update transaction created by the owner of cicero.res_publica.id.

That said, to create a subdomain, the subdomain owner generates a subdomain-creation operation for their desired name and gives it to the on-chain name owner.

Once created, a subdomain owner can use any on-chain name owner to broadcast a subdomain-update operation. To do so, they generate and sign the requisite subdomain operation and give it to an on-chain name owner, who then packages it with other subdomain operations into a DNS zone file and broadcasts it to the network.

If the subdomain owner wants to change the address of their subdomain, they need to sign a subdomain-transfer operation and give it to the on-chain name owner who created the subdomain. They then package it into a zone file and broadcast it.

Subdomain Registrars​

Because subdomain names are cheap, developers may be inclined to run subdomain registrars on behalf of their applications. For example, the name personal.id is used to register usernames without requiring them to spend any Bitcoin.

We supply a reference implementation of a BNS Subdomain Registrar to help developers broadcast subdomain operations. Users would still own their subdomain names; the registrar simply gives developers a convenient way for them to register and manage them in the context of a particular application.

BNS and DID Standards

BNS names are compliant with the emerging Decentralized Identity Foundation protocol specification for decentralized identifiers (DIDs).

Each name in BNS has an associated DID. The DID format for BNS is:

    did:stack:v0:{address}-{index}

Where:

  • {address} is an on-chain public key hash (for example a Bitcoin address).
  • {index} refers to the nth name this address created.

For example, the DID for personal.id is did:stack:v0:1dARRtzHPAFRNE7Yup2Md9w18XEQAtLiV-0, because the name personal.id was the first-ever name created by 1dARRtzHPAFRNE7Yup2Md9w18XEQAtLiV.

As another example, the DID for jude.id is did:stack:v0:16EMaNw3pkn3v6f2BgnSSs53zAKH4Q8YJg-1. Here, the address 16EMaNw3pkn3v6f2BgnSSs53zAKH4Q8YJg had created one earlier name in history prior to this one (which happens to be abcdefgh123456.id).

The purpose of a DID is to provide an eternal identifier for a public key. The public key may change, but the DID will not.

Stacks Blockchain implements a DID method of its own in order to be compatible with other systems that use DIDs for public key resolution. In order for a DID to be resolvable, all of the following must be true for a name:

  • The name must exist
  • The name's zone file hash must be the hash of a well-formed DNS zone file
  • The DNS zone file must be present in the Stacks node's data.
  • The DNS zone file must contain a URI resource record that points to a signed JSON Web Token
  • The public key that signed the JSON Web Token (and is included with it) must hash to the address that owns the name

Not all names will have DIDs that resolve to public keys. However, names created by standard tooling will have DIDs that do.

A RESTful API is under development.

DID Encoding for Subdomains​

Every name and subdomain in BNS has a DID. The encoding is slightly different for subdomains, so the software can determine which code-path to take.

  • For on-chain BNS names, the {address} is the same as the Bitcoin address that owns the name. Currently, both version byte 0 and version byte 5 addresses are supported (that is, addresses starting with 1 or 3, meaning p2pkh and p2sh addresses).

  • For off-chain BNS subdomains, the {address} has version byte 63 for subdomains owned by a single private key, and version byte 50 for subdomains owned by a m-of-n set of private keys. That is, subdomain DID addresses start with S or M, respectively.

The {index} field for a subdomain's DID is distinct from the {index} field for a BNS name's DID, even if the same created both names and subdomains. For example, the name abcdefgh123456.id has the DID did:stack:v0:16EMaNw3pkn3v6f2BgnSSs53zAKH4Q8YJg-0, because it was the first name created by 16EMaNw3pkn3v6f2BgnSSs53zAKH4Q8YJg. However, 16EMaNw3pkn3v6f2BgnSSs53zAKH4Q8YJg also created jude.statism.id as its first subdomain name. The DID for jude.statism.id is did:stack:v0:SSXMcDiCZ7yFSQSUj7mWzmDcdwYhq97p2i-0. Note that the address SSXMcDiCZ7yFSQSUj7mWzmDcdwYhq97p2i encodes the same public key hash as the address 16EMaNw3pkn3v6f2BgnSSs53zAKH4Q8YJg (the only difference between these two strings is that the first is base58check-encoded with version byte 0, and the second is encoded with version byte 63).