diff --git a/lib/fetch/src/lib.rs b/lib/fetch/src/lib.rs
index ebccfc8..a23d19e 100644
--- a/lib/fetch/src/lib.rs
+++ b/lib/fetch/src/lib.rs
@@ -1,4 +1,4 @@
-#![feature(iter_intersperse, yeet_expr)]
+#![feature(iter_intersperse, yeet_expr, iterator_try_collect)]
 use chrono::Utc;
 use http_body_util::BodyExt as _;
 use reqwest::Body;
diff --git a/lib/fetch/src/signatures.rs b/lib/fetch/src/signatures.rs
index d36ee86..835b265 100644
--- a/lib/fetch/src/signatures.rs
+++ b/lib/fetch/src/signatures.rs
@@ -92,18 +92,6 @@ impl Key {
     }
 }
 
-impl Key<Public> {
-    /// Encode a verification key so that it can be presented in a json.
-    pub fn serialize(self) -> Key {
-        let public_key_pem = self.inner.encode_pem();
-        Key {
-            id: self.id,
-            owner: self.owner,
-            inner: public_key_pem,
-        }
-    }
-}
-
 /// A key that can be used to generate signatures.
 #[derive(Clone)]
 pub struct Private(rsa::RsaPrivateKey);
@@ -203,6 +191,15 @@ impl VerificationKey {
             .verify(sig.signing_string.as_bytes(), &signature)
             .map_err(|e| e.to_string())
     }
+    /// Encode a verification key so that it can be presented in a json.
+    pub fn serialize(self) -> Key {
+        let public_key_pem = self.inner.encode_pem();
+        Key {
+            id: self.id,
+            owner: self.owner,
+            inner: public_key_pem,
+        }
+    }
 }
 
 /// The algorithm to sign with.
@@ -447,3 +444,324 @@ fn parse<'s, T>(req: &'s Request<T>) -> Result<Signature<'s>, String> {
         signing_string,
     })
 }
+
+mod new {
+    use base64::prelude::*;
+    use chrono::{TimeDelta, Utc};
+    use http::{HeaderName, HeaderValue, Method, Request};
+    use rsa::sha2::{Digest, Sha256};
+
+    use super::{sign_rsa_sha256, Algorithm, SigningKey, HS2019, RSA_SHA256};
+
+    /// Default for when
+    const EXPIRY_WINDOW: TimeDelta = TimeDelta::minutes(5);
+
+    /// Configuration for the behavior of the signing and verification routines.
+    ///
+    /// This struct is non-exhaustive.
+    #[derive(Clone, Copy)]
+    pub struct Options {
+        /// Whether to use the `(created)` and `(expires)`. If `false`, the `date` header is used instead.
+        ///
+        /// Defaults to `true`.
+        pub use_created: bool,
+        /// Quirk for older mastodon versions, which don't incorporate the query string into the signing
+        /// string during verification.
+        ///
+        /// Defaults to `false`.
+        pub strip_query: bool,
+        /// For how long the signature is valid.
+        ///
+        /// For signing, this only has an effect if `use_created` is also set. For verification, this is only
+        /// used if the `(expires)` pseudo-header is not present.
+        ///
+        /// Defaults to 5 minutes.
+        pub expires_after: TimeDelta,
+        /// Which signature algorithm to use.
+        ///
+        /// Defaults to [`"hs2019"`][super::HS2019].
+        pub algorithm: Algorithm,
+    }
+
+    impl Options {
+        /// Use hs2019 with the `(created)` pseudo-header.
+        pub const MODERN: Options = Options {
+            use_created: true,
+            strip_query: false,
+            expires_after: EXPIRY_WINDOW,
+            algorithm: HS2019,
+        };
+        /// Use rsa-sha256 with the `date` header.
+        pub const LEGACY: Options = Options {
+            use_created: false,
+            algorithm: RSA_SHA256,
+            ..Options::MODERN
+        };
+    }
+
+    impl Default for Options {
+        fn default() -> Self {
+            Options {
+                use_created: true,
+                strip_query: false,
+                expires_after: EXPIRY_WINDOW,
+                algorithm: HS2019,
+            }
+        }
+    }
+
+    /// Get the headers that need to be added to a request in order for the signature to be complete.
+    pub fn headers(
+        req: &Request<()>,
+        key: &SigningKey,
+        opt: Options,
+    ) -> Result<Vec<(HeaderName, HeaderValue)>, String> {
+        gen_headers(req, key, opt, Vec::new(), false)
+    }
+
+    /// Generate signature headers for a request with a body.
+    pub fn headers_with_digest<T>(
+        req: &Request<T>,
+        key: &SigningKey,
+        opt: Options,
+    ) -> Result<Vec<(HeaderName, HeaderValue)>, String>
+    where
+        T: AsRef<[u8]>,
+    {
+        let digest = {
+            let hash = <Sha256 as Digest>::digest(req.body().as_ref());
+            let encoded = BASE64_STANDARD.encode(hash);
+            format!("sha256={encoded}")
+        };
+        gen_headers(req, key, opt, vec![("digest", digest)], true)
+    }
+
+    fn gen_headers<T>(
+        req: &Request<T>,
+        key: &SigningKey,
+        opt: Options,
+        extra: Vec<(&str, String)>,
+        use_digest: bool,
+    ) -> Result<Vec<(HeaderName, HeaderValue)>, String> {
+        // We need to add a date header is one doesn't already exist.
+        const RFC_822: &str = "%a, %d %b %Y %H:%M:%S GMT";
+        let date = Utc::now().format(RFC_822).to_string();
+        let date = (!(opt.use_created || req.headers().contains_key("date")))
+            .then_some(("date", date.as_str()));
+
+        // QUIRK: some older mastodon versions don't sign the query params.
+        let path = if opt.strip_query {
+            req.uri().path()
+        } else {
+            req.uri()
+                .path_and_query()
+                .map(|s| s.as_str())
+                .unwrap_or(req.uri().path())
+        };
+
+        // Preprocess the headers into the format expected by the signing functions.
+        // NOTE: only the first value of each key is ever used.
+        let header_map: Vec<(&str, &str)> = req
+            .headers()
+            .iter()
+            // Our headers need to be turned into an association list.
+            .filter_map(|(k, v)| v.to_str().ok().map(|v| (k.as_str(), v)))
+            // The "uncommitted" headers need to be added as well
+            .chain(extra.iter().map(|(k, v)| (*k, v.as_str())))
+            // And at last, our date header too, if we're adding one.
+            .chain(date)
+            .collect();
+
+        let signature_header = make_signature_header(
+            key,
+            opt.algorithm,
+            // We only use the expiry deadline if we're using the `(created)` and `(expires)` inputs
+            // for signature generation.
+            opt.use_created.then_some(opt.expires_after),
+            use_digest,
+            &header_map,
+            req.method(),
+            path,
+        )?;
+
+        extra
+            .into_iter()
+            // Don't forget the signature :)
+            .chain([("signature", signature_header)])
+            // Turn all of the strings into header names/values.
+            .map(|(name, data)| -> Result<_, http::Error> {
+                let name = name.try_into()?;
+                let data = data.try_into()?;
+                Ok((name, data))
+            })
+            .try_collect()
+            .map_err(|e| format!("{e:?}"))
+    }
+
+    fn make_signature_header(
+        key: &SigningKey,
+        alg: Algorithm,
+        expires_after: Option<TimeDelta>,
+        use_digest: bool,
+        headers: &[(&str, &str)],
+        method: &Method,
+        path: &str,
+    ) -> Result<String, String> {
+        // List of input names that we want. Pseudo-headers are ordered before normal headers.
+        let needed = ["(request-target)"]
+            .into_iter()
+            .chain(if expires_after.is_some() {
+                vec!["(created)", "(expired)"]
+            } else {
+                vec!["date"]
+            })
+            .chain(use_digest.then_some("digest"))
+            .chain(["host"]);
+
+        let created = Utc::now();
+        let expires = created + expires_after.unwrap_or(EXPIRY_WINDOW);
+
+        let target_str = format!("{} {path}", method.as_str().to_ascii_lowercase());
+        let created_str = created.timestamp().to_string();
+        let expires_str = expires.timestamp().to_string();
+
+        // Lookup table for pseudo-headers.
+        let pseudo = [
+            ("(request-target)", target_str.as_str()),
+            ("(created)", created_str.as_str()),
+            ("(expired)", expires_str.as_str()),
+        ];
+
+        let inputs = collect_inputs(needed, &headers, &pseudo)?;
+        let signing_str = make_signing_string(&inputs);
+
+        let signature = sign_rsa_sha256(&signing_str, &key.inner)?;
+        let encoded = BASE64_STANDARD.encode(signature);
+
+        Ok(format_header(&inputs, &key.id, alg.0, &encoded))
+    }
+
+    fn collect_inputs<'a>(
+        needed: impl IntoIterator<Item = &'a str> + 'a,
+        headers: &'a [(&'a str, &'a str)],
+        pseudo: &'a [(&'a str, &'a str)],
+    ) -> Result<Vec<(&'a str, &'a str)>, String> {
+        // Quick utility function to get stuff from an association list.
+        fn get<'x>(map: &[(&str, &'x str)], key: &str) -> Option<&'x str> {
+            map.iter().find_map(|(k, v)| (*k == key).then_some(*v))
+        }
+
+        let assoc = |k| {
+            get(&headers, k)
+                .or_else(|| get(&pseudo, k))
+                .ok_or_else(|| format!("missing (pseudo)header `{k}`"))
+                .map(|v| (k, v))
+        };
+        needed.into_iter().map(assoc).try_collect()
+    }
+
+    fn format_header(inputs: &[(&str, &str)], key: &str, alg: &str, sig: &str) -> String {
+        // Quick utility function to get stuff from an association list.
+        fn get<'x>(map: &[(&str, &'x str)], key: &str) -> Option<&'x str> {
+            map.iter().find_map(|(k, v)| (*k == key).then_some(*v))
+        }
+
+        // Format all the headers in the order that we used them in the signing string.
+        let headers: String = inputs
+            .iter()
+            .map(|(k, _)| k.as_ref())
+            .intersperse(" ")
+            .collect();
+
+        // Get the time-based parameters, if they exist.
+        let created = get(inputs, "(created)").map(|v| ("created", v));
+        let expires = get(inputs, "(expires)").map(|v| ("expires", v));
+
+        // These parameters are always produced.
+        #[rustfmt::skip]
+        let table = [
+            ("keyId",     key),
+            ("algorithm", alg),
+            ("signature", sig),
+            ("headers",   &headers),
+        ];
+
+        // Now we need to format the whole shebang.
+        table
+            .into_iter()
+            // `(created)` is part of a newer draft that not everyone implements.
+            .chain(created)
+            // `(expires)` is optional per the spec
+            .chain(expires)
+            // Step 1: all the values need to be surrounded by quotes
+            .map(|(k, v)| (k, format!(r#""{v}""#)))
+            // Step 2. join each pair together
+            .map(|(k, v)| format!("{k}={v}"))
+            // Step 3. comma separate everything
+            .intersperse(", ".to_string())
+            // Step 4. fold the entire thing into one
+            .collect::<String>()
+    }
+
+    /// Partial inverse of `format_header_` (sort of, we don't have enough information to construct the inputs map from
+    /// within the function).
+    fn parse_header(header: &str) -> Result<(Vec<(&str, Option<&str>)>, &str, &str, &str), String> {
+        // Quick utility function to get stuff from an association list.
+        fn get<'x>(map: &[(&str, &'x str)], key: &str) -> Option<&'x str> {
+            map.iter().find_map(|(k, v)| (*k == key).then_some(*v))
+        }
+
+        // Parse the top-level table.
+        let table: Vec<(&str, &str)> = header
+            // Split into entries
+            .split(", ")
+            // Split entries into key-value pairs
+            .filter_map(|pair| pair.split_once('='))
+            // Undo quoting of the values
+            .filter_map(|(k, v)| v.strip_prefix('"')?.strip_suffix('"').map(|v| (k, v)))
+            .collect();
+
+        let Some(headers) = get(&table, "headers") else {
+            do yeet "Missing `headers` field";
+        };
+        let Some(key) = get(&table, "keyId") else {
+            do yeet "Missing `keyId` field";
+        };
+        let Some(algorithm) = get(&table, "algorithm") else {
+            do yeet "Missing `algorithm` field";
+        };
+        let Some(signature) = get(&table, "signature") else {
+            do yeet "Missing `signature` field"
+        };
+
+        // Construct a partial multi-map of all the inputs that are used to generate the signing string.
+        let inputs: Vec<(&str, Option<&str>)> = headers
+            // Headers and pseudo-headers are separated by spaces in the order in which they appear.
+            .split(' ')
+            // Map created and expires pseudo-headers to the ones specified in the inputs table.
+            .map(|k| match k {
+                // If these exist, the table must have them, but other than that they're optional.
+                "(created)" => get(&table, "created")
+                    .ok_or("`(created)` pseudo-header is listed, but does not exist")
+                    .map(|v| ("created", Some(v))),
+                "(expires)" => get(&table, "expires")
+                    .ok_or("`(expires)` pseudo-header is listed, but does not exist")
+                    .map(|v| ("expires", Some(v))),
+                // For anything else, we don't have the required information, and we'll need access
+                // to the entire request in order to fill in the blanks.
+                k => Ok((k, None)),
+            })
+            .try_collect()?;
+
+        Ok((inputs, key, algorithm, signature))
+    }
+
+    fn make_signing_string(data: &[(&str, &str)]) -> String {
+        data.iter()
+            // Each pair is separated by a colon and a space
+            .map(|(k, v)| format!("{k}: {v}"))
+            // Pairs must be separated by a newline
+            .intersperse("\n".to_string())
+            .collect()
+    }
+}