AuraVPN/crates/aura-pki/src/ca.rs

//! The Aura certificate authority: generation, persistence and issuance.

use std::fs;
use std::path::Path;

use anyhow::Context;
use rcgen::string::Ia5String;
use rcgen::{
    BasicConstraints, CertificateParams, DnType, ExtendedKeyUsagePurpose, IsCa, Issuer, KeyPair,
    KeyUsagePurpose, SanType,
};
use time::{Duration, OffsetDateTime};

/// Default lifetime of issued leaf certificates.
const LEAF_VALIDITY_DAYS: i64 = 365;
/// Default lifetime of the CA certificate.
const CA_VALIDITY_DAYS: i64 = 3650;
/// Small backdating of `not_before` to tolerate minor clock skew.
const CLOCK_SKEW_MINUTES: i64 = 5;

/// A freshly issued certificate together with its private key, both PEM-encoded.
#[derive(Debug, Clone)]
pub struct IssuedCert {
    /// The leaf certificate in PEM format.
    pub cert_pem: String,
    /// The leaf's private key in PKCS#8 PEM format.
    pub key_pem: String,
}

/// A self-signed Aura certificate authority.
///
/// Holds the CA's self-signed certificate (PEM) and `KeyPair`. The issuing
/// [`Issuer`] is rebuilt from the cert PEM + key for each signature, which makes
/// [`AuraCa::generate`] and [`AuraCa::load`] share exactly the same signing
/// path.
pub struct AuraCa {
    cert_pem: String,
    key: KeyPair,
}

impl AuraCa {
    /// Generate a brand new self-signed CA with the given Common Name.
    pub fn generate(common_name: &str) -> anyhow::Result<AuraCa> {
        let key = KeyPair::generate().context("generating CA key pair")?;
        let params = ca_params(common_name)?;
        let cert = params
            .self_signed(&key)
            .context("self-signing the CA certificate")?;
        Ok(AuraCa {
            cert_pem: cert.pem(),
            key,
        })
    }

    /// Persist the CA certificate and private key to the given paths (PEM).
    pub fn save(&self, cert_path: &Path, key_path: &Path) -> anyhow::Result<()> {
        fs::write(cert_path, self.cert_pem.as_bytes())
            .with_context(|| format!("writing CA cert to {}", cert_path.display()))?;
        fs::write(key_path, self.key.serialize_pem().as_bytes())
            .with_context(|| format!("writing CA key to {}", key_path.display()))?;
        Ok(())
    }

    /// Load a CA previously written with [`AuraCa::save`].
    pub fn load(cert_path: &Path, key_path: &Path) -> anyhow::Result<AuraCa> {
        let cert_pem = fs::read_to_string(cert_path)
            .with_context(|| format!("reading CA cert from {}", cert_path.display()))?;
        let key_pem = fs::read_to_string(key_path)
            .with_context(|| format!("reading CA key from {}", key_path.display()))?;
        let key = KeyPair::from_pem(&key_pem).context("parsing CA key PEM")?;
        let ca = AuraCa { cert_pem, key };
        // Validate eagerly that the stored cert can be used as an issuer, so a
        // corrupt CA fails at load time rather than at first issuance.
        ca.issuer().context("validating loaded CA certificate")?;
        Ok(ca)
    }

    /// Issue a server certificate carrying `DNS:domain` as a SAN and the
    /// `serverAuth` extended key usage.
    pub fn issue_server_cert(&self, domain: &str) -> anyhow::Result<IssuedCert> {
        let leaf_key = KeyPair::generate().context("generating server leaf key")?;
        let mut params = leaf_params(domain)?;
        let dns = Ia5String::try_from(domain)
            .with_context(|| format!("'{domain}' is not a valid IA5/DNS name"))?;
        params.subject_alt_names.push(SanType::DnsName(dns));
        params
            .extended_key_usages
            .push(ExtendedKeyUsagePurpose::ServerAuth);
        self.finish_leaf(params, leaf_key)
    }

    /// Issue a client certificate with `CN = client_id` and the `clientAuth`
    /// extended key usage.
    pub fn issue_client_cert(&self, client_id: &str) -> anyhow::Result<IssuedCert> {
        let leaf_key = KeyPair::generate().context("generating client leaf key")?;
        let mut params = leaf_params(client_id)?;
        params
            .extended_key_usages
            .push(ExtendedKeyUsagePurpose::ClientAuth);
        self.finish_leaf(params, leaf_key)
    }

    /// The CA's own self-signed certificate, PEM-encoded. Needed to build an
    /// [`crate::AuraCertVerifier`].
    pub fn ca_cert_pem(&self) -> String {
        self.cert_pem.clone()
    }

    /// Rebuild an [`Issuer`] from the stored CA cert PEM + key.
    fn issuer(&self) -> anyhow::Result<Issuer<'_, &KeyPair>> {
        Issuer::from_ca_cert_pem(&self.cert_pem, &self.key)
            .context("reconstructing issuer from CA certificate")
    }

    /// Sign a prepared leaf and bundle it with its key.
    fn finish_leaf(
        &self,
        params: CertificateParams,
        leaf_key: KeyPair,
    ) -> anyhow::Result<IssuedCert> {
        let issuer = self.issuer()?;
        let cert = params
            .signed_by(&leaf_key, &issuer)
            .context("signing leaf certificate with the CA")?;
        Ok(IssuedCert {
            cert_pem: cert.pem(),
            key_pem: leaf_key.serialize_pem(),
        })
    }
}

/// Build the `CertificateParams` for a self-signed CA.
fn ca_params(common_name: &str) -> anyhow::Result<CertificateParams> {
    let mut params = CertificateParams::new(Vec::<String>::new())
        .context("constructing CA certificate params")?;
    params.is_ca = IsCa::Ca(BasicConstraints::Unconstrained);
    params
        .distinguished_name
        .push(DnType::CommonName, common_name);
    params.key_usages = vec![
        KeyUsagePurpose::KeyCertSign,
        KeyUsagePurpose::CrlSign,
        KeyUsagePurpose::DigitalSignature,
    ];
    let now = OffsetDateTime::now_utc();
    params.not_before = now - Duration::minutes(CLOCK_SKEW_MINUTES);
    params.not_after = now + Duration::days(CA_VALIDITY_DAYS);
    Ok(params)
}

/// Build the common `CertificateParams` shared by client and server leaves.
/// `cn` is placed in the subject Common Name.
fn leaf_params(cn: &str) -> anyhow::Result<CertificateParams> {
    let mut params = CertificateParams::new(Vec::<String>::new())
        .context("constructing leaf certificate params")?;
    params.is_ca = IsCa::NoCa;
    params.distinguished_name.push(DnType::CommonName, cn);
    params.key_usages = vec![
        KeyUsagePurpose::DigitalSignature,
        KeyUsagePurpose::KeyEncipherment,
    ];
    let now = OffsetDateTime::now_utc();
    params.not_before = now - Duration::minutes(CLOCK_SKEW_MINUTES);
    params.not_after = now + Duration::days(LEAF_VALIDITY_DAYS);
    Ok(params)
}