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If you are just discovering it, I recommend reading [this narrative description](https://www.robinsloan.com/lab/specifying-spring-83/). Don't miss my notes on [a summer spent operating the protocol with other people](https://www.robinsloan.com/lab/specifying-spring-83/#summer), either.\n\nThis is speculative software intended to invite consideration and imagination; it doesn't have any \"users\", only co-investigators.\n\nThis spec is now a historical artifact, but/and I hope it might prompt you to think about new ways of relating online.\n\nMost recent draft:\n\n* [Protocol specification](draft-20220629.md)\n\nPrevious drafts:\n\n* [20220619](draft-20220616.md)\n* [20220609](draf-20220609.md)\n\nHere are the implementations I know about currently:\n\n* [The Kingswood Palimpsest](https://cyborg.rodeo/palimpsest/), a client\n* [rdmurphy/spring-board-element](https://github.com/rdmurphy/spring-board-element), a web component\n* [rpj/spring83](https://github.com/rpj/spring83), in JavaScript\n* [royragsdale/s83](https://github.com/royragsdale/s83), in Go\n* [motevets/springboard](https://github.com/motevets/springboard), in Go (running [here](https://spring83.kindrobot.ca))\n* [michael-lazar/lets-dance](https://github.com/michael-lazar/lets-dance), in Python (with some great [notes](https://github.com/michael-lazar/lets-dance/blob/main/Notes.md))\n* [pteichman/ahoy](https://github.com/pteichman/ahoy), in Go\n* [cellu_cc/so83-gpu](https://gitlab.com/cellu_cc/so83-gpu), on GitLab, in OpenCL 🤯\n* [JohnB/spring83](https://github.com/JohnB/spring83), a client\n\nIf you've implemented a client, server, or utility, at any level of completeness, and you would like me to list it here, let me know.\n\nThis work is offered under a Creative Commons Attribution-ShareAlike license." }, { "path": "draft-20220609.md", "content": "# Spring '83\n\n*Note: this is a first draft specification published in June 2022. You can find a narrative description of the protocol, and my contact information, [in this newsletter](https://www.robinsloan.com/lab/specifying-spring-83/).*\n\n*Note also: this draft has been superceded by the [June 16 version](https://github.com/robinsloan/spring-83-spec/blob/main/draft-20220616.md), which will be superceded in turn. But we are winding our way towards something final-ish...*\n\n## Introduction\n\nSpring '83 is a protocol that allows users to follow publishers on the internet -- who might be people, computer programs, or anything else -- in a way that's simple, expressive, and predictable.\n\nThe basic unit of the protocol is the board, which is an HTML fragment, limited to 2217 bytes, unable to execute JavaScript or load external resources, but otherwise unrestricted.\n\nEach publisher maintains just one board. There is no concept of a history; think instead of a whiteboard that is amended or erased.\n\nSpring '83 aspires to be:\n\n**Simple.** This means the protocol is easy to understand and implement, even for a non-expert programmer.\n\n**Expressive.** This means protocol embraces the richness, flexibility, and chaos of modern HTML and CSS. It does not formalize interactions and relationships into database schemas.\n\n(It also means the protocol doesn't provide any mechanism for replies, likes, favorites, or, indeed, feedback of any kind. Publishers are encouraged to use the full flexibility of HTML to develop their own approaches, inviting readers to respond via email, join a live chat, send a postcard... whatever!)\n\n**Predictable.** This means boards holds their place, maintaing a steady presence. It means also that clients only receive the boards they request, when they request them; there is no mechanism by which a server can \"push\" an unsolicited board.\n\nIn addition, Spring '83 is\n\n**Federated.** This means it spans a network of servers operated by different people. The protocol is not, however, \"trustless\"; in Spring '83, connection requires conversation.\n\nAlthough the transmission portion of the protocol operates over HTTP/1.1, its approach to HTML storage is very different. Rather than store different boards on different servers, Spring '83 stores ALL the boards on EVERY server. The network converges to a consistent global state -- eventually; maybe -- using a simple gossip algorithm.\n\nSpring '83 draws inspiration from many existing protocols and technologies; you can read about these in Discussion 1.\n\n## Implementation\n\nThe 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.\n\nhttps://www.ietf.org/rfc/rfc2119.txt\n\n## Terminology\n\n*board*: a fragment of HTML -- not necessarily a valid HTML5 document -- not more than 2217 bytes long, encoded in UTF-8.\n\n*publisher*: the entity responsible for specifying a board's content.\n\n*key*: a public key on the Ed25519 curve, formatted as 64 hex characters.\n\n*signature*: a public key signature, formatted as 128 hex characters.\n\n*client*: an application, web or standalone, that publishes, retrieves, and displays boards.\n\n*server*: an application, reachable on the public internet using a domain name, that receives, stores, and provides boards.\n\n*peer*: another server.\n\n*difficulty factor*: a server's current requirement for new keys\n\n*realm*: a set of peers, listed together in a YAML document reachable on the public internet. The default *realm* is defined at TKTK.\n\n## Publishers and keys\n\nPublishers (who might be people, computer programs, or anything else) are identified and authorized by keypairs on the Ed25519 curve. The public part of the keypair, formatted as 64 hex characters, is the publisher's identifier, used to request their board from any available server. The secret part of the keypair allows the publisher alone to edit their board.\n\nEach keypair corresponds to exactly one board. Again, there is no concept of a history; think instead of a whiteboard that is amended or erased.\n\nBecause a publisher is identified by their public key, boards are easy to verify. When a client requests a board for a particular *key*, the server might send an incorrect or modified response -- appending an advertisement, perhaps -- but the client will detect the invalid signature, drop the board, and mark the server as untrustworthy.\n\nSee Discussion 2 for a consideration of the benefits (substantial) and drawbacks (likewise) of keypair identity schemes.\n\n### Format requirements\n\nKeys are generated randomly, but they must fit a particular format; this functions as a simple Hashcash variation, providing a rudimentary form of abuse mitigation. Concocting a compatible key isn't instantaneous; it takes between a few seconds and a few minutes on modern hardware.\n\nTo be used in Spring '83, a key's final six hex characters must match this regex:\n\n```\n/ed[0-9]{4}$/\n```\n\nFurthermore, the final four characters, interpreted as a decimal number, must fall in the range 2022 .. 2099.\n\nThat number has teeth. Keys are only valid in two calendar years: the year specified in their final four digits, and the year previous. For example, the key\n\n```\n1c6ffef2825b294274478bad8c80a7a610d38245a9fded18cd004c4a67ed2023\n```\n\nis valid between 2022-01-01T00:00:00:00Z and 2023-12-31T23:59:59Z.\n\nIn this way, a key's final four characters represent its \"expiration date\".\n\nThis requirement makes key rotation mandatory over the long term. Clients may implement features that make the process more convenient, but the recurring \"stress test\" on the publisher-follower link is a feature, not a bug. The goal is to keep Spring '83 relationships \"live\" and engaged, rather than weigh publishers down with zombie followers.\n\n(TKTK is this too much? I like it, but...)\n\n## Boards in the client\n\nMuch of the burden of the protocol falls on the client: to make requests on the user's behalf, store keypairs securely, and display boards safely.\n\nThe client must:\n\n* limit incoming boards to 2217 bytes\n* validate each board's cryptographic signature\n* situate each board inside its own Shadow DOM\n\nThe client must not:\n\n* execute any JavaScript included in the board\n* load any images, media, or fonts linked by the board\n\nThese two requirements should be satisfied with a Content Security Policy.\n\nThe client should:\n\n* allow users to manage a collection of followed keys\n* display each board in a region with an aspect ratio of either 1:sqrt(2) or sqrt(2):1\n* make available to boards the Spring '83 suite of CSS variables, listed in TKTK\n* open links in new windows or tabs\n\nOther than the byte-size threshold and a timestamp requirement discussed below, Spring '83 places no limitations on the HTML content of a board. Knowing that the client will not execute JavaScript, publishers will probably not want to spend their precious bytes on inert code... but they are free to do so.\n\nBoards might be:\n\n* mini home pages, updated regularly with new work\n* lists of links to web pages recently read, perhaps with brief notes\n* daily logs, wiped clean each morning\n* yawlps to the universe\n\nBeyond the standards described above, Spring '83 doesn't specify how the client should display boards. For example, the client may:\n\n* allow users to explore overflowing boards by scrolling\n* apply post-processing effects to boards (e.g., a limited color palette)\n* organize boards according to an abstruse algorithm\n* handle links to Spring '83 keys in a helpful way\n\nPublishers should expect their boards to be placed on a 2D canvas alongside many others.\n\n## Boards on the server\n\nSpring '83 servers are, in operation, very similar to \"plain old web\" servers, with a few additional behaviors.\n\nThe server must\n\n* maintain a persistent store of boards\n* enforce a TTL, dropping boards over 28 days old\n* share newly-received boards with peers\n\nThe transmission portion of Spring '83 operates over HTTP/1.1, using TLS, so it can be implemented easily using existing tools.\n\nA Spring '83 server must respond at the following HTTP endpoints:\n\n```\nPUT /\nGET /\nGET /\n```\n\nA server that provides boards to browsers will also need an appropriate OPTIONS endpoint to satisfy CORS, but that's not part of this specification.\n\nIt's a pretty small API surface! We'll go through the endpoints one by one.\n\n### Publishing boards: PUT /``\n\nTo publish a board, a client must send a request of this form:\n\n```\nPUT / HTTP/1.1\nContent-Type: text/html;charset=utf-8\nSpring-Version: 83\nIf-Unmodified-Since: \nAuthorization: Spring-83 Signature=\n\n\n```\n\nUpon receipt, servers should validate the non-cryptographic part of the PUT request immediately, and, if necessary, respond with an error code.\n\n#### Verifying boards\n\n##### Size\n\nIf the board is larger than 2217 bytes, the server must reject the request with 413 Payload Too Large.\n\n##### Timestamp\n\nThe client must include the publishing timestamp in the If-Unmodified-Since header. If this value is older or equal to the timestamp of the server's version of the board, the server must reject the request with 409 Conflict.\n\nThe If-Unmodified-Since header is transmitted as a convenience, to allow the server to \"fail fast\" if the request is out of date. However, because it's not cryptographically signed, the server can't rely on it entirely.\n\nThe client must also include a last-modified meta tag in the board's HTML:\n\n```\n\">\n```\n\nIt's important that the timestamp is transmitted this way, because it needs to be signed along with the rest of the board's HTML. The client may add the meta tag automatically, just before signing and transmitting the board.\n\nThe server must reject the PUT request, returning 400 Bad Request, if\n\n* the board is transmitted without a last-modified meta tag; or\n* it is transmitted with more than one last-modified meta tag; or\n* its last-modified meta tag isn't parsable as an HTTP-format date and time; or\n* its last-modified meta tag is set to a date in the future.\n\nThese criteria are EXTREMELY important. The last-modified meta tag modulates a board's change over time; if it gets screwed up, the publisher could lose the ability to update their board.\n\n##### Cryptographic validity\n\nThe server must verify that `` is a valid signature for the entire request body, exactly as transmitted. If the board isn't signed properly, the server should reject it, returning 401 Unauthorized.\n\n##### Cryptographic difficulty\n\nIf the current four-digit year is YYYY, and the previous four-digit year is YYYX, the server must only accept PUTs for keys that end with the four digits YYYY or YYYX, preceded in turn by the two hex digits \"ed\". This is the years-of-use requirement.\n\nThe server must reject other keys with 400 Bad Request.\n\nAdditionally, if the server doesn't have any board stored for ``, then it must apply another check. The key, interpreted as a 256-bit number, must be less than a threshold defined by the server's difficulty factor:\n\n```\nMAX_SIG = (2**256 - 1)\nkey_threshold = MAX_SIG * ( 1.0 - difficulty_factor)\n```\n\nThis check is not applied to keys for which the server already has a board stored. You can read more about the difficulty factor later in this document.\n\n##### Final considerations\n\nSpring '83 specifies a test keypair:\n\n```\npublic: fad415fbaa0339c4fd372d8287e50f67905321ccfd9c43fa4c20ac40afed1983\nsecret: a7e4d1c8be858d683ab9cb15574bd0bc3a87e6c846cdaf848da498909cb574f7\n```\n\nServers must not accept PUTs for this key, returning 401 Unauthorized. (See the section detailing GET /``, below, for more guidance.)\n\nThe server may also use a denylist to block certain keys, rejecting all PUTs for those keys.\n\nIf all the preceding checks are met, the server must store the board and share it with peers.\n\n### Sharing boards: peer to peer PUT /``\n\nA realm is a set of Spring '83 servers, specified by a YAML document reachable on the public internet. (The format of this document is TKTK.) There is no automatic or \"trustless\" way to join a realm; as with BGP, the foundational routing protocol of the internet, you gotta talk to somebody! \n\nWithin a realm, Spring '83 servers communicate directly with each other, sharing new boards using a simple gossip algorithm. Their aim is to converge on a shared state of the realm, each server's copy exactly the same. In the froth of real activity, that will never actually happen -- but we can get pretty close.\n\nAfter receiving and verifying a new board from a client, the server must share it with N peers selected randomly from the realm. N is\n\n```\nmin(round(total_peer_count * 0.5), 5)\n```\n\nNew boards should be transmitted to peers asynchronously. The server must wait at least five minutes before sharing, but it may wait longer. In this way, the server acts as a buffer, absorbing and \"compacting\" rapid PUTs.\n\nTo share a board with a peer, the server must transmit a PUT request similar to the one described above. Note the addition of the `Prefer: respond-async` header:\n\n```\nPUT / HTTP/1.1\nContent-Type: text/html;charset=utf-8\nSpring-Version: 83\nPrefer: respond-async\nIf-Unmodified-Since: \nAuthorization: Spring-83 Signature=\n\n\n```\n\nWhen the server receives a PUT with the `respond-async` preference, it should respond immediately with 202 Accepted and place the board into a queue for asynchronous validation. (If the server doesn't have a task queue, it may respond synchronously.)\n\nIf a peer is not reachable or responds with an error code, the server must wait for a minimum timeout of 5 minutes before attempting to contact that peer again. If the peer is still not reachable, the server must apply jittered exponential backoff, calculating each new timeout in this way:\n\n```\nnew_timeout = current_timeout + current_timeout * random(0.0 .. 1.0)\n```\n\nThe server should cap this timeout at some maximum; 7 days is recommended.\n\n#### Forgetting boards\n\nThe server must store boards with a TTL. One week is recommended; the maximum TTL is 28 days.\n\nThe server must provide identical responses to requests for\n\n* ``, for which it once stored a board, now deleted, and\n* ``, for which it never stored any board.\n\nFinally, the server must not enumerate keys or boards for any requester; the server must only respond to requests for specific keys.\n\n## Difficulty factor: GET /\n\nDisk space is a concern for a protocol that allows anyone on the internet with a trivially-concocted cryptographic key to store a chunk of data, \"for free\", on a network of servers. Accordingly, a Spring '83 realm is limited to 10 million boards, for a maximum possible disk size of 22.17 gigabytes. (Ambitious servers might keep the whole thing in RAM!)\n\nThis limit is enforced through a difficulty factor that draws inspiration from the mining difficulty factor used in many cryptocurrencies.\n\nSpring '83's difficulty factor ranges from 0.0 to 1.0, calculated in this way:\n\n```\ndifficulty_factor = ( num_boards_stored / 10_000_000 )**4\n```\n\n(TKTK is that the right exponent? Explain it, I suppose.)\n\nIn normal operation, the server's difficulty factor is very close to 0.0, and boards for new keys are freely accepted. As the difficulty factor rises, the concoction of a suitable key becomes more time-consuming for prospective publishers. This has the effect of both (1) slowing the introduction of new keys, and (2) dissuading some publishers entirely.\n\nWhen the server's database maxes out at 10 million boards, the difficulty factor is 1.0, and the server does not accept any boards for keys it doesn't already know.\n\nLet's consider the example of a server that is already storing 8.5 million boards.\n\n```\ndifficulty_factor = ( 8_500_000 / 10_000_000 )**4 = 0.52\n```\n\nUsing that difficulty factor, we can calculate the key threshold:\n\n```\nMAX_KEY = (2**256 - 1)\nkey_threshold = MAX_KEY * ( 1.0 - 0.52 ) = \n```\n\nThe server must reject PUT requests for new keys that are not less than ``.\n\nA client should determine a server's current difficulty factor using a request of this form:\n\n```\nGET / HTTP/1.1\nSpring-Version: 83\n```\n\nThe server's response must take the form:\n\n```\nHTTP/1.1 200 OK\nContent-Length: \nContent-Type: text/html;charset=utf-8\nSpring-Version: 83\nSpring-Difficulty: \n\n\n```\n\nUsing that information, the client can choose either to spend the time required to concoct a suitable key... or give up, and perhaps try another time.\n\n### Retrieving boards: GET /``\n\nServers are not obligated to provide this endpoint for all requesters; they might provide it only to users of a particular client, or only to users who have paid for access, or according to any other scheme. Such schemes are beyond the scope of this specification, which describes the behavior of a publicly-available server.\n\nTo retrieve the board for ``, a client must choose one or more servers to which it has access and transmit a request of this form:\n\n```\nGET / HTTP/1.1\nSpring-Version: 83\nIf-Modified-Since: \n```\n\nIf the server has a board for `` but it is not newer than the timestamp specified in If-Modified-Since, it must respond with 304 Not Modified.\n\nIf the client omits If-Modified-Since, the server should return whatever board it has for ``, if it has one.\n\nThe server's response must take the form:\n\n```\nHTTP/1.1 200 OK\nContent-Length: \nContent-Type: text/html;charset=utf-8\nSpring-Version: 83\nAuthorization: Spring-83 Signature=\n\n\n```\n\nThe client must verify that the `` is valid for `` and `` before processing or displaying the board. If the signature is not valid, the client must drop the response and remove the server from its list of trustworthy peers.\n\n(TKTK clients should assist servers with out-of-date boards.)\n\nSpring '83 specifies a test keypair:\n\n```\npublic: fad415fbaa0339c4fd372d8287e50f67905321ccfd9c43fa4c20ac40afed1983\nsecret: a7e4d1c8be858d683ab9cb15574bd0bc3a87e6c846cdaf848da498909cb574f7\n```\n\nServers should respond to GETs for this key with an ever-changing board, generated internally, with a timestamp set to the time of the request. This board is provided to help client developers understand and troubleshoot their applications. \n\n## Error code quick reference\n\n* 400: Board was submitted with impromper meta timestamp tags.\n* 401: Board was submitted without a valid signature.\n* 404: No board for this key found on this server.\n* 403: Board was submitted for a key that does not meet the difficulty factor.\n* 409: Board was submitted with a timestamp older than the server's timestamp for this key.\n* 513: Board is larger than 2217 bytes.\n\n## Discussions\n\n### 1: Inspirations\n\nThis protocol draws inspiration\n\n* from Secure Scuttlebutt: the power of cryptographic keypairs as identities\n* from Hashcash: the notion of guarding server resources with client puzzles\n* from Ethereum: the gonzo strategy of storing the whole universe in one place\n* from ZeroTier: the spirit of \"decentralize until it hurts, then centralize until it works\".\n\nand, most of all,\n\n* from [the Quote of the Day Protocol](https://datatracker.ietf.org/doc/html/rfc865), defined by Jon Postel in May 1983: the vision of simplicity\n\nFor those not familiar: QOTD operates over both TCP and UDP; the server responds to a TCP connection or a UDP datagram with a single brief message.\n\nIn fact, I very badly wanted Spring '83 to operate over UDP -- responding to a request with, in some cases, a single datagram packet: beautiful -- but the architecture of the modern internet makes that much more difficult today than it was in 1983, and, anyway, there's a universe of capable tooling for HTTP. \n\nSo, not without regret, Spring '83 goes with the flow.\n\n### 2: The agony and ecstasy of public key cryptography\n\nThere are a lot of reasons NOT to use cryptographic keypairs as identities, not least of which: the certainty that a user will eventually lose their secret key.\n\nBut the profound magic trick of the signature: that it allows a piece of content to flow around the internet, handed from peer to peer, impossible to tamper with... it's too good to pass up.\n\nAnd the way public key cryptography allows anyone to \"join\" the system, without registering anywhere, simply by generating an appropriate keypair -- again, it's a trick so good it seems like it shouldn't work.\n\nThe trapdoor function at the heart of public key cryptography is mirrored in the trapdoor of its user experience: once a secret key is lost or compromised, the identity is done. Game over. The system offers no customer support, because the system is math, and all the angels are busy assisting other callers.\n\nIt's one of the harshest if/thens in all of computing, and a steep price to pay for the magic trick. Spring '83, given its other priorities and constraints, is willing to pay that price -- but only barely.\n\nThe compromise is mandatory key rotation, enforced with the years-of-use requirement. This policy suggests, \"you're going to lose your private key eventually... why not lose it now?\" and uses that loss as a mechanism to strengthen, rather than weaken, the network.\n\nBeyond that, there is plenty of space for Spring '83 clients to offer hosted (\"custodial\") private keys, abstracting the keypair behind a more traditional username/password." }, { "path": "draft-20220616.md", "content": "# Spring '83\n\n> *Discussion: This is an updated draft specification published in June 2022. You can find a narrative description of the protocol, and my contact information, [in this newsletter](https://www.robinsloan.com/lab/specifying-spring-83/).*\n\n## Introduction\n\nSpring '83 is a protocol that allows users to follow publishers on the internet -- who might be people, computer programs, or anything else -- in a way that's simple, expressive, and predictable.\n\nThe basic unit of the protocol is the board, which is an HTML fragment, limited to [2217 bytes](http://info.cern.ch/hypertext/WWW/TheProject.html), unable to execute JavaScript or load external resources, but otherwise unrestricted.\n\nEach publisher maintains just one board. There is no concept of a history; think instead of a whiteboard that is amended or erased.\n\nSpring '83 aspires to be:\n\n**Simple.** This means the protocol is easy to understand and implement, even for a non-expert programmer.\n\n**Expressive.** This means the protocol embraces the richness, flexibility, and chaos of modern HTML and CSS. It does not formalize interactions and relationships into database schemas.\n\n(It also means the protocol doesn't provide any mechanism for replies, likes, favorites, or, indeed, feedback of any kind. Publishers are encouraged to use the flexibility of HTML to develop their own approaches, inviting readers to respond via email, join a live chat, send a postcard... whatever!)\n\n**Predictable.** This means boards holds their place, maintaining a steady presence. It means also that clients only receive the boards they request, when they request them; there is no mechanism by which a server can \"push\" an unsolicited board.\n\nIn addition, Spring '83 is\n\n**Federated.** This means boards live on different servers operated by different people. It means also that groups of peers share boards with one another, a strategy intended to support exploration and analysis of the full \"board flow\".\n\nSpring '83 draws inspiration from many existing protocols and technologies; you can read about these in Discussion 1.\n\n## Implementation\n\nThe 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.\n\nhttps://www.ietf.org/rfc/rfc2119.txt\n\n## Terminology\n\n*board*: a fragment of HTML, not necessarily a valid HTML5 document, not more than 2217 bytes long, encoded in UTF-8.\n\n*publisher*: the entity responsible for specifying a board's content.\n\n*key*: a public key on the Ed25519 curve, formatted as 64 hex characters.\n\n*signature*: a public key signature, formatted as 128 hex characters.\n\n*client*: an application, web or standalone, that publishes, retrieves, and displays boards.\n\n*server*: an application, reachable on the public internet, that accepts requests from clients to publish and retrieve boards.\n\n*listener*: an application, reachable on the public internet, that receives boards not from clients but from servers, subscribing to the full \"board flow\" through a realm. (The listener's possible reasons for doing this are described later.)\n\n*peer*: a server or listener.\n\n*realm*: a group of peers who have agreed to circulate the full \"board flow\" using a simple gossip algorithm.\n\n*difficulty factor*: a server's current requirement for new boards.\n\n## Publishers, keys, and servers\n\nPublishers -- who might be people, computer programs, or anything else -- are identified and authorized by keypairs on the Ed25519 curve. The public part of the keypair, formatted as 64 hex characters, is the publisher's identifier, used to request their board from the server. The secret part of the keypair allows the publisher alone to edit their board.\n\nSee Discussion 2 for a consideration of the benefits (substantial) and drawbacks (likewise) of keypair identity schemes.\n\nEach keypair corresponds to exactly one board. Again, there is no concept of a history; think instead of a whiteboard that is amended or erased.\n\nEvery publisher has a home server to which they transmit their board, and from which clients generally retrieve it. Publishers can always migrate to a new home server, taking their key with them. (This process is described in the section on clients, below.)\n\nBecause a publisher is identified by their key, boards are easy to verify. When a client requests a board for a particular key, the server might send an incorrect or modified response -- inserting an advertisement, perhaps -- but the client will detect the invalid signature, drop the board, and mark the server as untrustworthy.\n\nKeys are globally unique, and they act as \"coordination-free\" identifiers, similar to UUIDs. Publishers don't need to register with any central authority. Instead, they need only to generate a keypair on the Ed25519 curve that conforms to a format requirement and then select, or start, a home server.\n\n> *Aside:* The use of keys for identification and authorization is, like many things in this protocol, motivated partially by a desire to keep implementation easy and stateless for server programmers and operators. Who wants to manage a complete login system? And send password reset emails? Not me!\n\nA server is identified by a domain name and, optionally, a path. The transmission protocol is an HTTP API over TLS; therefore, the full URL for a board is\n\n```\nhttps:////\n```\n\nFor example, a board currently hosted on the server `bogbody.biz` would be identified as\n\n```\nhttps://bogbody.biz/ca93846ae61903a862d44727c16fed4b80c0522cab5e5b8b54763068b83e0623\n```\n\n> Discussion: A client may allow users to follow Spring '83 publishers alongside RSS feeds and other resources. In these situations, the client should distinguish Spring '83 URLs with a regex that recognizes conforming keys, or simply by making a request and noting the presence of a Spring-Version header.\n> The ambiguity of Spring '83 URLs is somewhat intentional; they are just HTML fragments, and users can always preview them in a web browser.\n\n### Generating conforming keys\n\nThe protocol imposes a format requirement on keys. The content of Ed25519 keypairs is mostly (but not totally) random, so conforming keys can only be generated by trial and error.\n\nThe format requirement accomplishes two things at once:\n\n1. It presents a \"client puzzle\" which requires a one-time investment of compute resources to \"solve\". Like [Hashcash](http://www.hashcash.org/), this provides a rudimentary form of abuse mitigation: a malicious publisher cannot generate conforming keys instantly and endlessly.\n\n2. It \"bakes\" some useful metadata into the key itself.\n\nA conforming key's final seven hex characters must be `83e` followed by four characters that, interpreted as MMYY, express a valid month and year in the range 01/00 .. 12/99. Formally, the key must match this regex:\n\n```\n/83e(0[1-9]|1[0-2])(\\d\\d)$/\n```\n\nAgain, a conforming keypair can only be generated by trial and error. On a single thread in an Apple M1 chip, this is accomplished in tens of minutes, not seconds or less.\n\n> *Aside:* I am not totally sure it's impossible to \"program\" the content of an Ed25519 public key, particularly if you abandon security considerations. My impression is that [it can't be done](https://crypto.stackexchange.com/questions/3596/is-it-possible-to-pick-your-ed25519-public-key), but I could be wrong. If you have any insight into this, let me know. This format requirement could always be applied instead to the SHA-256 hash of the key.\n\nThe date \"encoded\" in those final four characters has teeth: the key is only valid in the two years preceding it, and expires at the end of the last day of the month specified. (This is analogous to a [credit card expiration date](https://en.wikipedia.org/wiki/ISO/IEC_7813#Physical_characteristics).)\n\nFor example, the key\n\n```\nca93846ae61903a862d44727c16fed4b80c0522cab5e5b8b54763068b83e0623\n```\n\nhas an encoded expiration date of 0623, or 06/2023. This key is valid between 2021-06-01T00:00:00:00Z and 2023-07-01T00:00:00Z.\n\nThis expiration policy makes key rotation mandatory over the long term. Clients may implement features that make the process more convenient, even automatic, but the recurring \"stress test\" on the publisher-follower link is a feature, not a bug. The goal is to keep Spring '83 relationships \"live\" and engaged, with fresh opt-ins every two years at most.\n\nThe policy suggests, \"you're going to lose your secret key eventually... why not lose it now?\" and uses that loss as a mechanism to strengthen, rather than weaken, the network.\n\n### Discovering keys\n\nThe process of discovering a particular publisher's key, or discovering keys to follow generally, is not part of this specification.\n\nHowever, it is presumed that a home page or profile page might contain a `` element analogous to the kind used to specify RSS feeds. A client scanning a web page for an associated board should look for `` elements with the `type` attribute set to `text/board+html`.\n\n```\n\n```\n\n## Boards in the client\n\nMuch of the burden of the protocol falls on the client: to store a user's keypair securely, allow them to manage a collection of followed keys, and display boards safely.\n\nThe client must:\n\n* limit incoming boards to 2217 bytes\n* verify each board's cryptographic signature\n* situate each board inside its own Shadow DOM\n\nThe client must not:\n\n* execute any JavaScript included in the board\n* load any images, media, or fonts linked by the board\n\nThese two requirements should be satisfied with a [Content Security Policy](https://content-security-policy.com/). The design of this policy is not part of this specification, but here's an example that works for a simple web-based client:\n\n```\ndefault-src 'none';\nstyle-src 'self' 'unsafe-inline';\nfont-src 'self';\nscript-src 'self';\nform-action *;\nconnect-src *;\n```\n\nIt's important to allow `unsafe-inline` CSS so boards can style themselves!\n\n> *Aside:* Yes, HTML forms are allowed -- unless someone explains to me why this is a horrible idea 😝\n\n> *Aside:* The prohibition against images and other external resources is a matter of privacy and safety. Privacy, because it prevents the use of tracking pixels and other \"transponders\". Safety, because it lowers the stakes for malicious and illegal content. However, it is totally possible to imagine a future version of this protocol, Spring '84 or beyond, in which\n> * a particular domain could be added to the CSP, providing a shared image catalog, analogous to the funky clip art choices offered to purchasers of classified ads. \"You can use any pic you want, as long as it's on [bukk.it](https://bukk.it/)!\"\n> * or, images and other external resources could simply be permitted.\n\nThe client should:\n\n* display each board in a region with an aspect ratio of either 1:sqrt(2) or sqrt(2):1\n* make available to boards the Spring '83 suite of CSS variables, listed in TKTK\n* open links in new windows or tabs\n\nSpring '83 places no limitations on the HTML content of a board. Knowing that the client will not execute JavaScript, publishers will probably not want to spend their precious bytes on inert code... but they are free to do so.\n\nBoards might be:\n\n* mini home pages, updated regularly with new work\n* lists of links to web pages recently read, perhaps with brief notes\n* daily logs, wiped clean each morning\n* yawlps to the universe\n\nBeyond the standards described above, Spring '83 doesn't specify how the client should display boards. For example, the client may:\n\n* allow users to explore overflowing boards by scrolling\n* organize boards according to an abstruse algorithm\n* handle links to boards in a helpful way, even \"transcluding\" them (!)\n\nPublishers should expect their boards to be placed on a 2D canvas alongside many others.\n\n### Boards with special client instructions\n\nClients should scan for the `` element:\n\n```\n\">\n```\n\nThis element is used by publishers to migrate from key to key and server to server. When the client finds a `` element, it should retrieve the board at the URL, verify it normally, and update its record of the publisher's home server and/or key accordingly.\n\nThe board should contain only one one `` element. If it contains more than one, the client must honor the first and ignore the rest.\n\nThe client may also scan for arbitrary data stored in `data-spring-*` attributes throughout the board. These attributes and their uses will be defined by publishers and client developers.\n\n## Boards on the server\n\nSpring '83 servers are, in operation, very similar to \"plain old web\" servers, with a few additional behaviors.\n\nThe server must\n\n* maintain a persistent store of boards\n* enforce a TTL, dropping boards over 22 days old\n* share newly-received boards with listeners\n\nThe transmission portion of Spring '83 operates over HTTP, using TLS, so it can be implemented easily using existing tools.\n\nA Spring '83 server must respond at the following HTTP endpoints:\n\n```\nOPTIONS\nPUT /\nGET /\nGET /\n```\n\nThat's a pretty small API surface! We'll go through the endpoints one by one.\n\n### Serving clients: OPTIONS\n\nMany clients will, by dint of being web apps, depend on CORS to retrieve boards from non-origin servers. Servers must support preflight OPTIONS requests to all endpoints, replying with:\n\n```\nHTTP/1.1 204 No Content\nAccess-Control-Allow-Methods: GET, PUT, OPTIONS\nAccess-Control-Allow-Origin: *\nAccess-Control-Allow-Headers: Content-Type, If-Modified-Since, Spring-Signature, Spring-Version\nAccess-Control-Expose-Headers: Content-Type, Last-Modified, Spring-Difficulty, Spring-Signature, Spring-Version\n```\n\n### Publishing boards: PUT /``\n\nTo publish a board, a client must send a request of this form:\n\n```\nPUT / HTTP/1.1\nContent-Type: text/html;charset=utf-8\nSpring-Version: 83\nSpring-Signature: \n\n\n```\n\nUpon receipt, servers should check the non-cryptographic part of the PUT request immediately, and, if necessary, respond with an error code.\n\n#### Checking boards\n\n##### Size\n\nIf the board is larger than 2217 bytes, the server must reject the PUT request with 413 Payload Too Large.\n\n##### Timestamp\n\nThe timestamp is transmitted as part of the board HTML. The client must include a `