Ask for a signer name and key, and hold saved Markdown whole to sign it

README.md

@@ -17,6 +17,9 @@ It registers one provider, `signature`, for the `signed-webdav` scheme.
 | Server address | The folder's address, for example `https://dav.example.com/files/` |
 | Username | Optional account name for HTTP basic authentication |
 | Password | Optional password, stored as a secret |
+| Signer name | The ENS name that signs, for example `laptop.alice.eth` |
+| Signing key | The private key of the address the name resolves to, stored as a secret |
+| Sepolia RPC address | The endpoint used to check signers against ENS, by default `https://ethereum-sepolia-rpc.publicnode.com` |
 
 Listing, reading, writing, creating directories, renaming and deleting behave
 as in the WebDAV plugin: reads and writes stream in pieces of at most 1 MiB,
@@ -24,6 +27,15 @@ conditional writes use `If-Match` and `If-None-Match`, change polling is not
 supported, a rename never replaces an existing target, and a folder with
 contents is refused rather than removed whole.
 
+## Signing
+
+Saving a `.md` file holds it whole, up to 8 MiB, and writes it once finished,
+with `signer` and `signature` set in its frontmatter; every other file streams
+straight through. Reading passes every file through unchanged. The signing
+itself lives behind one small interface that the md-ens-signature library
+fills in; until then a saved file is written exactly as it arrives. The RPC
+address is for verifying signers, which a later version adds to reads.
+
 ## Building
 
 ```sh

manifest.json

@@ -33,6 +33,28 @@
           "label": "Password",
           "kind": "password",
           "required": false
+        },
+        {
+          "key": "ensName",
+          "label": "Signer name",
+          "description": "The ENS name that signs saved Markdown, for example laptop.alice.eth",
+          "kind": "text",
+          "required": true
+        },
+        {
+          "key": "privateKey",
+          "label": "Signing key",
+          "description": "The private key of the address this name resolves to",
+          "kind": "password",
+          "required": true
+        },
+        {
+          "key": "rpcUrl",
+          "label": "Sepolia RPC address",
+          "description": "The Ethereum Sepolia endpoint used to check signers against ENS",
+          "kind": "text",
+          "required": false,
+          "default": "https://ethereum-sepolia-rpc.publicnode.com"
         }
       ]
     }

src/lib.rs

@@ -3,6 +3,7 @@
 
 mod dav;
 mod sdk;
+mod sign;
 
 use sdk::host;
 use sdk::{
@@ -10,6 +11,7 @@ use sdk::{
   GuestReader, GuestWriter, ListPage, Locator, Reader, Setting, WriteCondition,
   Writer,
 };
+use sign::Signer;
 use std::cell::{Cell, RefCell};
 
 /// The PROPFIND body. Asking for named properties rather than allprop keeps
@@ -21,10 +23,13 @@ const PROPS: &str = concat!(
   "</d:prop></d:propfind>",
 );
 const MAX_CHUNK: u32 = 1024 * 1024;
+/// The largest Markdown file this plugin will hold whole to sign.
+const MAX_MARKDOWN: usize = 8 * 1024 * 1024;
 
 struct Session {
   root: String,
   authorization: Option<String>,
+  signer: Signer,
 }
 
 thread_local! {
@@ -179,11 +184,27 @@ impl GuestReader for DavReader {
   }
 }
 
+/// Whether a path names a Markdown file, the only kind this plugin signs.
+fn is_markdown(path: &str) -> bool {
+  path.to_ascii_lowercase().ends_with(".md")
+}
+
 /// Writes one file through a PUT whose body is pushed in pieces.
 struct DavWriter {
   call: host::Exchange,
   done: RefCell<bool>,
   written: Cell<u64>,
+  /// A Markdown file waits here whole until `finish`, because its signature
+  /// covers the entire body and lands in the frontmatter at the front.
+  markdown: Option<RefCell<Vec<u8>>>,
+}
+
+impl DavWriter {
+  fn send(&self, data: &[u8]) -> Result<(), ErrorCode> {
+    self.call.push(data).map_err(transport)?;
+    self.written.set(self.written.get() + data.len() as u64);
+    Ok(())
+  }
 }
 
 impl GuestWriter for DavWriter {
@@ -191,8 +212,14 @@ impl GuestWriter for DavWriter {
     if *self.done.borrow() {
       return Err(ErrorCode::InvalidInput);
     }
-    self.call.push(&data).map_err(transport)?;
+    let Some(markdown) = &self.markdown else {
-    self.written.set(self.written.get() + data.len() as u64);
+      return self.send(&data);
+    };
+    let mut markdown = markdown.borrow_mut();
+    if markdown.len() + data.len() > MAX_MARKDOWN {
+      return Err(ErrorCode::Unsupported);
+    }
+    markdown.extend_from_slice(&data);
     Ok(())
   }
   /// One call is enough: the request body is already complete, so the response
@@ -203,6 +230,12 @@ impl GuestWriter for DavWriter {
       return Err(ErrorCode::InvalidInput);
     }
     *self.done.borrow_mut() = true;
+    if let Some(markdown) = &self.markdown {
+      let signed = session(|session| session.signer.sign(&markdown.borrow()))?;
+      for piece in signed.chunks(MAX_CHUNK as usize) {
+        self.send(piece)?;
+      }
+    }
     let head = self.call.respond().map_err(transport)?;
     dav::outcome(head.status)?;
     let revision = head
@@ -242,9 +275,14 @@ impl Guest for Webdav {
       }
       _ => None,
     };
+    let signer = Signer::new(
+      &lookup("ensName").ok_or(ErrorCode::InvalidInput)?,
+      &lookup("privateKey").ok_or(ErrorCode::InvalidInput)?,
+    )?;
     SESSION.set(Some(Session {
       root,
       authorization,
+      signer,
     }));
     Ok(Capabilities {
       write: true,
@@ -358,6 +396,7 @@ impl Guest for Webdav {
       call,
       done: RefCell::new(false),
       written: Cell::new(0),
+      markdown: is_markdown(&path).then(|| RefCell::new(Vec::new())),
     }))
   }
 

src/sign.rs

@@ -0,0 +1,42 @@
+//! Signing a saved Markdown file, the only place this plugin touches its
+//! bytes. Everything the md-ens-signature library does for the plugin goes
+//! through the interface here, so adopting the library replaces this file and
+//! nothing else.
+
+use crate::sdk::ErrorCode;
+
+/// One connection's signing identity: an ENS name and the private key bound
+/// to it.
+///
+/// Until the library lands, a signer accepts any identity and leaves every
+/// file exactly as it was saved.
+pub struct Signer;
+
+impl Signer {
+  /// Takes the configured name and key, failing with `InvalidInput` when the
+  /// key cannot sign, so a bad key refuses the connection rather than every
+  /// later save.
+  pub fn new(_name: &str, _private_key: &str) -> Result<Self, ErrorCode> {
+    Ok(Self)
+  }
+
+  /// Returns the whole file with its `signer` and `signature` frontmatter set
+  /// for this identity, other frontmatter and the body untouched.
+  pub fn sign(&self, markdown: &[u8]) -> Result<Vec<u8>, ErrorCode> {
+    Ok(markdown.to_vec())
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::Signer;
+  use crate::sdk::ErrorCode;
+
+  #[test]
+  fn a_file_keeps_its_body() -> Result<(), ErrorCode> {
+    let signer = Signer::new("laptop.alice.eth", "0x01")?;
+    let signed = signer.sign(b"# Notes\n\nHello\n")?;
+    assert!(signed.ends_with(b"# Notes\n\nHello\n"));
+    Ok(())
+  }
+}