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#![warn(
    missing_debug_implementations,
    missing_docs,
    rust_2018_idioms,
    unreachable_pub
)]

//! `cashweb-auth-wrapper` is a library providing deserialization, parsing, and verification needed within the [`Authorization Wrapper Framework`].
//!
//! [`Authorization Wrapper Framework`]: https://github.com/cashweb/specifications/blob/master/authorization-wrapper/specification.mediawiki

#[allow(unreachable_pub)]
mod models;

use std::convert::TryInto;

use ring::digest::{digest, SHA256};
use secp256k1::{key::PublicKey, Error as SecpError, Message, Secp256k1, Signature};
use thiserror::Error;

pub use models::{auth_wrapper::SignatureScheme, AuthWrapper};

/// Represents an [`AuthWrapper`] post-parsing.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ParsedAuthWrapper {
    /// The public key associated with the signature.
    pub public_key: PublicKey,
    /// The signature by public key covering the payload.
    pub signature: Signature,
    /// The signature scheme used for signing.
    pub scheme: SignatureScheme,
    /// The payload covered by the signature.
    pub payload: Vec<u8>,
    /// The SHA256 digest of the payload.
    pub payload_digest: [u8; 32],
}

/// Error associated with validation and parsing of the [`AuthWrapper`].
#[derive(Debug, Clone, PartialEq, Eq, Error)]
pub enum ParseError {
    /// The public key provided was invalid.
    #[error(transparent)]
    PublicKey(SecpError),
    /// The signature provided was an invalid format.
    #[error(transparent)]
    Signature(SecpError),
    /// The signature scheme provided is unsupported.
    #[error("unsupported signature scheme")]
    UnsupportedScheme,
    /// The `payload_digest` provided was fraudulent.
    #[error("fraudulent digest")]
    FraudulentDigest,
    /// Both the digest and the payload were missing.
    #[error("digest and payload missing")]
    DigestAndPayloadMissing,
    /// The `payload_digest` was not 32 bytes long.
    #[error("unexpected length digest")]
    UnexpectedLengthDigest,
}

impl AuthWrapper {
    /// Parse the [`AuthWrapper`] to construct a [`ParsedAuthWrapper`].
    ///
    /// The involves deserialization of both public keys, calculation of the payload digest, and coercion of byte fields
    /// into fixed-length arrays.
    #[inline]
    pub fn parse(self) -> Result<ParsedAuthWrapper, ParseError> {
        // Parse public key
        let public_key = PublicKey::from_slice(&self.public_key).map_err(ParseError::PublicKey)?;

        // Parse scheme
        let scheme = SignatureScheme::from_i32(self.scheme).ok_or(ParseError::UnsupportedScheme)?;

        // Parse signature
        let signature = Signature::from_compact(&self.signature).map_err(ParseError::Signature)?;

        // Construct and validate payload digest
        let payload_digest = match self.payload_digest.len() {
            0 => {
                if self.payload.is_empty() {
                    return Err(ParseError::DigestAndPayloadMissing);
                } else {
                    let payload_digest = digest(&SHA256, &self.payload);
                    let digest_arr: [u8; 32] = payload_digest.as_ref().try_into().unwrap();
                    digest_arr
                }
            }
            32 => {
                let payload_digest = digest(&SHA256, &self.payload);
                if *payload_digest.as_ref() != self.payload_digest[..] {
                    return Err(ParseError::FraudulentDigest);
                }
                let digest_arr: [u8; 32] = self.payload_digest[..].try_into().unwrap();
                digest_arr
            }
            _ => return Err(ParseError::UnexpectedLengthDigest),
        };

        Ok(ParsedAuthWrapper {
            public_key,
            scheme,
            signature,
            payload_digest,
            payload: self.payload,
        })
    }
}

/// Error associated with verifying the signature of an [`AuthWrapper`].
#[derive(Debug, Clone, PartialEq, Eq, Error)]
pub enum VerifyError {
    /// The signature failed verification.
    #[error(transparent)]
    InvalidSignature(SecpError),
    /// The signature scheme provided is unsupported.
    #[error("unsupported signature scheme")]
    UnsupportedScheme,
}

impl ParsedAuthWrapper {
    /// Verify the signature on [`ParsedAuthWrapper`].
    #[inline]
    pub fn verify(&self) -> Result<(), VerifyError> {
        if self.scheme == SignatureScheme::Schnorr {
            // TODO: Support Schnorr
            return Err(VerifyError::UnsupportedScheme);
        }
        // Verify signature on the message
        let msg = Message::from_slice(self.payload_digest.as_ref()).unwrap(); // This is safe
        let secp = Secp256k1::verification_only();
        secp.verify(&msg, &self.signature, &self.public_key)
            .map_err(VerifyError::InvalidSignature)?;
        Ok(())
    }
}