#![deny(missing_docs)]

//! # HTTP Signature Normaliztion
//! _An HTTP Signatures library that leaves the signing to you_
//!
//! - [crates.io](https://crates.io/crates/http-signature-normalization)
//! - [docs.rs](https://docs.rs/http-signature-normalization)
//! - [Hit me up on Mastodon](https://asonix.dog/@asonix)
//!
//! Http Signature Normalization is a minimal-dependency crate for producing HTTP Signatures with user-provided signing and verification. The API is simple; there's a series of steps for creation and verification with types that ensure reasonable usage.
//!
//! ```rust
//! use http_signature_normalization::Config;
//! use std::{collections::BTreeMap, time::Duration};
//!
//! fn main() -> Result<(), Box<dyn std::error::Error>> {
//!     let config = Config::default().set_expiration(Duration::from_secs(5));
//!
//!     let headers = BTreeMap::new();
//!
//!     let signature_header_value = config
//!         .begin_sign("GET", "/foo?bar=baz", headers)?
//!         .sign("my-key-id".to_owned(), |signing_string| {
//!             // sign the string here
//!             Ok(signing_string.to_owned()) as Result<_, Box<dyn std::error::Error>>
//!         })?
//!         .signature_header();
//!
//!     let mut headers = BTreeMap::new();
//!     headers.insert("Signature".to_owned(), signature_header_value);
//!
//!     let verified = config
//!         .begin_verify("GET", "/foo?bar=baz", headers)?
//!         .verify(|sig, signing_string| {
//!             // Verify the signature here
//!             sig == signing_string
//!         });
//!
//!     assert!(verified);
//!     Ok(())
//! }
//! ```

use std::{
    collections::{BTreeMap, HashSet},
    num::ParseIntError,
    time::{Duration, SystemTime, UNIX_EPOCH},
};

pub mod create;
pub mod verify;

use self::{
    create::Unsigned,
    verify::{ParseSignatureError, ParsedHeader, Unverified, ValidateError},
};

const REQUEST_TARGET: &str = "(request-target)";
const CREATED: &str = "(created)";
const EXPIRES: &str = "(expires)";

const KEY_ID_FIELD: &str = "keyId";
const ALGORITHM_FIELD: &str = "algorithm";
const ALGORITHM_VALUE: &str = "hs2019";
const CREATED_FIELD: &str = "created";
const EXPIRES_FIELD: &str = "expires";
const HEADERS_FIELD: &str = "headers";
const SIGNATURE_FIELD: &str = "signature";

#[derive(Clone, Debug)]
/// Configuration for signing and verifying signatures
///
/// By default, the config is set up to create and verify signatures that expire after 10
/// seconds, and use the `(created)` and `(expires)` fields that were introduced in draft 11
pub struct Config {
    expires_after: Duration,
    use_created_field: bool,
    required_headers: Vec<String>,
}

#[derive(Debug)]
/// Error preparing a header for validation
///
/// This could be due to a missing header, and unparsable header, or an expired header
pub enum PrepareVerifyError {
    /// Error validating the header
    Validate(ValidateError),

    /// Error parsing the header
    Parse(ParseSignatureError),

    /// Missing required headers
    Required(RequiredError),
}

impl std::fmt::Display for PrepareVerifyError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Validate(ref e) => std::fmt::Display::fmt(e, f),
            Self::Parse(ref e) => std::fmt::Display::fmt(e, f),
            Self::Required(ref e) => std::fmt::Display::fmt(e, f),
        }
    }
}

impl std::error::Error for PrepareVerifyError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::Validate(ref e) => Some(e),
            Self::Parse(ref e) => Some(e),
            Self::Required(ref e) => Some(e),
        }
    }
}

impl From<ValidateError> for PrepareVerifyError {
    fn from(e: ValidateError) -> Self {
        Self::Validate(e)
    }
}
impl From<ParseSignatureError> for PrepareVerifyError {
    fn from(e: ParseSignatureError) -> Self {
        Self::Parse(e)
    }
}
impl From<RequiredError> for PrepareVerifyError {
    fn from(e: RequiredError) -> Self {
        Self::Required(e)
    }
}

#[derive(Debug)]
/// Failed to build a signing string due to missing required headers
pub struct RequiredError(HashSet<String>);

impl std::fmt::Display for RequiredError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "Missing required headers {:?}", self.0)
    }
}

impl std::error::Error for RequiredError {}

impl RequiredError {
    /// Retrieve the missing headers from the error
    pub fn headers(&self) -> &HashSet<String> {
        &self.0
    }

    /// Take the headers from the error
    pub fn take_headers(&mut self) -> HashSet<String> {
        std::mem::take(&mut self.0)
    }
}

impl Config {
    /// Create a new Config with a default expiration of 10 seconds
    pub const fn new() -> Self {
        Config {
            expires_after: Duration::from_secs(10),
            use_created_field: true,
            required_headers: Vec::new(),
        }
    }

    /// Enable mastodon compatibility
    ///
    /// This is the same as disabling the use of `(created)` and `(expires)` signature fields,
    /// requiring the Date header, and requiring the Host header
    pub fn mastodon_compat(self) -> Self {
        self.dont_use_created_field().require_header("host")
    }

    /// Require the Digest header be set
    ///
    /// This is useful for POST, PUT, and PATCH requests, but doesn't make sense for GET or DELETE.
    pub fn require_digest(self) -> Self {
        self.require_header("Digest")
    }

    /// Opt out of using the (created) and (expires) fields introduced in draft 11
    ///
    /// Note that by not requiring the created field, the Date header becomes required. This is to
    /// prevent replay attacks.
    pub fn dont_use_created_field(mut self) -> Self {
        self.use_created_field = false;
        self.require_header("date")
    }

    /// Set the expiration to a custom duration
    pub const fn set_expiration(mut self, expires_after: Duration) -> Self {
        self.expires_after = expires_after;
        self
    }

    /// Mark a header as required
    pub fn require_header(mut self, header: &str) -> Self {
        self.required_headers.push(header.to_lowercase());
        self
    }

    /// Perform the neccessary operations to produce an [`Unsigned`] type, which can be used to
    /// sign the header
    pub fn begin_sign(
        &self,
        method: &str,
        path_and_query: &str,
        headers: BTreeMap<String, String>,
    ) -> Result<Unsigned, RequiredError> {
        let mut headers = headers
            .into_iter()
            .map(|(k, v)| (k.to_lowercase(), v))
            .collect();

        let sig_headers = self.build_headers_list(&headers);

        let (created, expires) = if self.use_created_field {
            let created = SystemTime::now();
            let expires = created + self.expires_after;

            (Some(created), Some(expires))
        } else {
            (None, None)
        };

        let signing_string = build_signing_string(
            method,
            path_and_query,
            created,
            expires,
            &sig_headers,
            &mut headers,
            self.required_headers.iter().cloned().collect(),
        )?;

        Ok(Unsigned {
            signing_string,
            sig_headers,
            created,
            expires,
        })
    }

    /// Perform the neccessary operations to produce and [`Unverified`] type, which can be used to
    /// verify the header
    pub fn begin_verify(
        &self,
        method: &str,
        path_and_query: &str,
        headers: BTreeMap<String, String>,
    ) -> Result<Unverified, PrepareVerifyError> {
        let mut headers: BTreeMap<String, String> = headers
            .into_iter()
            .map(|(k, v)| (k.to_lowercase(), v))
            .collect();

        let header = headers
            .remove("authorization")
            .or_else(|| headers.remove("signature"))
            .ok_or(ValidateError::Missing)?;

        let parsed_header: ParsedHeader = header.parse()?;
        let unvalidated = parsed_header.into_unvalidated(
            method,
            path_and_query,
            &mut headers,
            self.required_headers.iter().cloned().collect(),
        )?;

        Ok(unvalidated.validate(self.expires_after)?)
    }

    fn build_headers_list(&self, btm: &BTreeMap<String, String>) -> Vec<String> {
        let http_header_keys: Vec<String> = btm.keys().cloned().collect();

        let mut sig_headers = if self.use_created_field {
            vec![
                REQUEST_TARGET.to_owned(),
                CREATED.to_owned(),
                EXPIRES.to_owned(),
            ]
        } else {
            vec![REQUEST_TARGET.to_owned()]
        };

        sig_headers.extend(http_header_keys);

        sig_headers
    }
}

fn build_signing_string(
    method: &str,
    path_and_query: &str,
    created: Option<SystemTime>,
    expires: Option<SystemTime>,
    sig_headers: &[String],
    btm: &mut BTreeMap<String, String>,
    mut required_headers: HashSet<String>,
) -> Result<String, RequiredError> {
    let request_target = format!("{} {}", method.to_string().to_lowercase(), path_and_query);

    btm.insert(REQUEST_TARGET.to_owned(), request_target);
    if let Some(created) = created {
        btm.insert(CREATED.to_owned(), unix_timestamp(created).to_string());
    }
    if let Some(expires) = expires {
        btm.insert(EXPIRES.to_owned(), unix_timestamp(expires).to_string());
    }

    let signing_string = sig_headers
        .iter()
        .filter_map(|h| {
            let opt = btm.remove(h).map(|v| format!("{}: {}", h, v));
            if opt.is_some() {
                required_headers.remove(h);
            }
            opt
        })
        .collect::<Vec<_>>()
        .join("\n");

    if !required_headers.is_empty() {
        return Err(RequiredError(required_headers));
    }

    Ok(signing_string)
}

impl Default for Config {
    fn default() -> Self {
        Self::new()
    }
}

fn unix_timestamp(time: SystemTime) -> u64 {
    time.duration_since(UNIX_EPOCH)
        .expect("UNIX_EPOCH is never in the future")
        .as_secs()
}

fn parse_unix_timestamp(s: &str) -> Result<SystemTime, ParseIntError> {
    let u: u64 = s.parse()?;
    Ok(UNIX_EPOCH + Duration::from_secs(u))
}

#[cfg(test)]
mod tests {
    use super::Config;
    use std::collections::BTreeMap;

    fn prepare_headers() -> BTreeMap<String, String> {
        let mut headers = BTreeMap::new();
        headers.insert(
            "Content-Type".to_owned(),
            "application/activity+json".to_owned(),
        );
        headers
    }

    #[test]
    fn required_header() {
        let headers = prepare_headers();
        let config = Config::default().require_header("date");

        let res = config.begin_sign("GET", "/foo?bar=baz", headers);

        assert!(res.is_err())
    }

    #[test]
    fn round_trip_authorization() {
        let headers = prepare_headers();
        let config = Config::default().require_header("content-type");

        let authorization_header = config
            .begin_sign("GET", "/foo?bar=baz", headers)
            .unwrap()
            .sign("hi".to_owned(), |s| {
                Ok(s.to_owned()) as Result<_, std::io::Error>
            })
            .unwrap()
            .authorization_header();

        let mut headers = prepare_headers();
        headers.insert("Authorization".to_owned(), authorization_header);

        let verified = config
            .begin_verify("GET", "/foo?bar=baz", headers)
            .unwrap()
            .verify(|sig, signing_string| sig == signing_string);

        assert!(verified);
    }

    #[test]
    fn round_trip_signature() {
        let headers = prepare_headers();
        let config = Config::default();

        let signature_header = config
            .begin_sign("GET", "/foo?bar=baz", headers)
            .unwrap()
            .sign("hi".to_owned(), |s| {
                Ok(s.to_owned()) as Result<_, std::io::Error>
            })
            .unwrap()
            .signature_header();

        let mut headers = prepare_headers();
        headers.insert("Signature".to_owned(), signature_header);

        let verified = config
            .begin_verify("GET", "/foo?bar=baz", headers)
            .unwrap()
            .verify(|sig, signing_string| sig == signing_string);

        assert!(verified);
    }
}