siguldry 0.5.0

// SPDX-License-Identifier: MIT
// Copyright (c) Microsoft Corporation.

//! Functions for encrypting and decrypting key passwords.
//!
//! Keys stored in the database are encrypted using a password generated by Siguldry, and PKCS #11
//! modules are protected by a PIN. Both of these secrets are then encrypted using a combination of
//! user-provided secrets ("user passwords") and keys stored in PKCS#11 modules on the server
//! (examples include a Yubikey or TPM, but anything with a PKCS#11 module will work).
//!
//! This implementation was based of the one in the Sigul project.

use std::{
    io::{Read, Write},
    process::Stdio,
};

use anyhow::Context;
use openssl::pkey::PKey;
use openssl::{
    cms::{CMSOptions, CmsContentInfo},
    hash::MessageDigest,
    stack::Stack,
    symm::Cipher,
    x509::X509,
};
use sequoia_openpgp::{
    crypto::Password,
    parse::{
        Parse,
        stream::{DecryptionHelper, DecryptorBuilder, VerificationHelper},
    },
    policy::StandardPolicy,
    serialize::stream::{Armorer, Encryptor, LiteralWriter, Message},
    types::{AEADAlgorithm, SymmetricAlgorithm},
};
use serde::{Deserialize, Serialize};

use crate::server::{config::Pkcs11Binding, db};

/// The intermediate data format for secrets.
///
/// This structure is serialized and, depending on what type of secret it is, futher encrypted.
/// Private keys which have been encrypted with a server-generated secret can be bound with
/// PKCS11 tokens before being serialized and written to the database.
///
/// The key password used to encrypt the private key can be bound with PKCS11 tokens, then encrypted
/// with the user-provided password before being written to the database.
///
/// The serialized JSON looks like:
///
/// `{"None": {"secret": "my-password"}}`
///
/// or
///
/// `{"Pkcs11WithCMS": {"fingerprint": "hexencodedsha256sum", "secret": "-----BEGIN CMS-----..." `
#[derive(Debug, Clone, Serialize, Deserialize)]
enum BoundSecret {
    /// No binding was used.
    None { secret: String },
    /// Secrets bound by asymmetric keys stored in a device accessible via PKCS#11.
    ///
    /// Secrets of this variant have been encrypted using OpenSSL's CMS interface and the
    /// results are PEM-encoded.
    ///
    /// Examples of key stores include SoftHSMv2 or any HSM that provides a PKCS#11 interface,
    /// Yubikeys via the libykcs11 library, and Trusted Platform Modules (TPMs) via the
    /// libtpm2_pkcs11 library. Creating and managing the key pairs is up to the administrator.
    Pkcs11WithCMS {
        /// The SHA256 digest of the DER-encoded X509 certificate used in this binding.
        fingerprint: String,
        /// The secret that's been encrypted by the X509 certificate identified in
        /// `key_fingerprint`. The string contains a PEM-encoded CMS structure.
        secret: String,
    },
}

impl BoundSecret {
    /// Encrypt some data using a [`Binding`] configuration.
    ///
    /// The data is typically the password used to encrypt a private key, or the private key itself.
    ///
    /// The data is encrypted with the X509 certificate in the provided binding to a CMS (RFC 5652)
    /// structure which is PEM-encoded. AES-256-GCM is used for the cipher.
    ///
    /// If there are no bindings configured, the secret is returned, wrapped in a structure suitable
    /// to be serialized and encrypted with a user-provided password.
    pub(crate) fn bind(bindings: &[Pkcs11Binding], secret: &[u8]) -> anyhow::Result<Vec<Self>> {
        let secret_str = str::from_utf8(secret)
            .map_err(|_| anyhow::anyhow!("Secrets that are bound must be encoded with UTF-8"))?;
        let mut bound_secrets = bindings
            .iter()
            .map(|binding| {
                tracing::info!(certificate=?binding.certificate, "binding secret");

                let certificate = std::fs::read_to_string(&binding.certificate)?;
                let certificate = X509::from_pem(certificate.as_bytes())?;
                let mut cert_stack = Stack::new()?;
                cert_stack.push(certificate)?;
                let encrypted = CmsContentInfo::encrypt(
                    &cert_stack,
                    secret,
                    Cipher::aes_256_gcm(),
                    CMSOptions::empty(),
                )?;
                let pem = encrypted.to_pem()?;
                let certificate = cert_stack.pop().expect("we just pushed a cert");
                let fingerprint = hex::encode_upper(certificate.digest(MessageDigest::sha256())?);
                Ok::<_, anyhow::Error>(BoundSecret::Pkcs11WithCMS {
                    fingerprint,
                    secret: String::from_utf8(pem)?,
                })
            })
            .collect::<Result<Vec<_>, _>>()?;

        if bound_secrets.is_empty() {
            tracing::trace!("binding isn't enabled for this secret");
            bound_secrets.push(BoundSecret::None {
                secret: secret_str.to_string(),
            });
        }

        tracing::debug!(
            tokens_bound = bindings.len(),
            "secret has been successfully bound"
        );

        Ok(bound_secrets)
    }

    pub(crate) fn unbind(&self, bindings: &[Pkcs11Binding]) -> anyhow::Result<Password> {
        match self {
            BoundSecret::None { secret } => return Ok(Password::from(secret.as_bytes())),
            BoundSecret::Pkcs11WithCMS {
                fingerprint,
                secret,
            } => {
                for binding in bindings.iter().filter(|binding| binding.can_unbind()) {
                    if let Ok(secret) =
                        binding_decrypt(binding.clone(), secret.clone().into_bytes())
                            .map(Password::from)
                    {
                        return Ok(secret);
                    } else {
                        tracing::debug!(
                            certificate = fingerprint,
                            key_uri = binding.private_key,
                            "Failed to unbind key password"
                        );
                    }
                }
            }
        }

        Err(anyhow::anyhow!("Unable to unbind key password"))
    }
}

/// Decrypt a bound password using a PIN-protected private key in a PKCS11 token.
fn binding_decrypt(binding: Pkcs11Binding, data: Vec<u8>) -> anyhow::Result<Vec<u8>> {
    let private_key = binding.private_key.as_ref().ok_or_else(|| {
        anyhow::anyhow!(
            "Binding configuration is missing the 'private_key' field and can't be used to decrypt"
        )
    })?;
    // In the future maybe we'll get openssl provider APIs? Alternatively, if Sequioa gets
    // PKCS#11 support, we could switch to using it in a migration.
    let mut command = std::process::Command::new("openssl");
    let mut child = command
        .args([
            "cms",
            "-decrypt",
            "-inform",
            "pem",
            "-provider",
            "pkcs11",
            "-passin",
            "stdin",
            "-inkey",
        ])
        .arg(private_key)
        .stdin(Stdio::piped())
        .stdout(Stdio::piped())
        .stderr(Stdio::piped())
        .spawn()?;
    let mut stdin = child
        .stdin
        .take()
        .ok_or_else(|| anyhow::anyhow!("openssl-cms command missing stdin"))?;

    binding
        .pin
        .ok_or_else(|| anyhow::anyhow!("Binding must include a PIN"))?
        .map(|pin| stdin.write_all(pin))?;
    stdin.write_all(b"\n")?;
    stdin.write_all(&data)?;
    drop(stdin);

    let output = child.wait_with_output()?;
    let stderr = String::from_utf8_lossy(&output.stderr);
    if !output.status.success() {
        return Err(anyhow::anyhow!(
            "Failed to decrypt data via PKCS#11 using openssl-cms (exited {:?}): {stderr}",
            output.status.code()
        ));
    }

    Ok(output.stdout)
}

/// Decrypt a key password to enable access to the key itself.
pub async fn decrypt_key_password(
    bindings: &[Pkcs11Binding],
    user_password: Password,
    data: &[u8],
) -> anyhow::Result<Password> {
    let key_bindings: Vec<BoundSecret> = symmetric_decrypt(user_password, data)
        .map(|data| serde_json::from_slice(&data))
        .context("User password is invalid")?
        .map_err(|_e| anyhow::anyhow!("JSON content in database could not be deserialized!"))?;

    key_bindings
        .iter()
        .map(|s| s.unbind(bindings).ok())
        .find(|result| result.is_some())
        .flatten()
        .ok_or_else(|| anyhow::anyhow!("Unable to unbind key password"))
}

/// Encrypt a key password for storage.
pub fn encrypt_key_password(
    bindings: &[Pkcs11Binding],
    user_password: Password,
    key_password: Password,
) -> anyhow::Result<Vec<u8>> {
    let bound_passwords = key_password.map(|password| BoundSecret::bind(bindings, password))?;

    tracing::debug!(
        tokens_bound = bindings.len(),
        "Key password has been successfully bound"
    );
    symmetric_encrypt(
        user_password,
        serde_json::to_vec(&bound_passwords)?.as_slice(),
    )
    .context("Failed to PGP-encrypt the bound password")
}

/// Bind a string with the provided [`Pkcs11Binding`] and serialize it to JSON for database storage.
///
/// If there are no bindings (e.g. an empty slice) the string is wrapped in [`BoundSecret::None`].
pub(crate) fn bind_with_pkcs11(bindings: &[Pkcs11Binding], secret: &str) -> anyhow::Result<String> {
    let bound = BoundSecret::bind(bindings, secret.as_bytes())?;
    serde_json::to_string(&bound).context("Failed to serialize bound secret")
}

/// Unbind key material to recover the encrypted PEM string.
///
/// This is the inverse of [`bind_key_material`].
pub(crate) fn unbind_with_pkcs11(
    bindings: &[Pkcs11Binding],
    bound_secret: &str,
) -> anyhow::Result<String> {
    let bound_secrets: Vec<BoundSecret> =
        serde_json::from_str(bound_secret).context("Failed to deserialize bound key material")?;
    bound_secrets
        .iter()
        .find_map(|s| s.unbind(bindings).ok())
        .ok_or_else(|| anyhow::anyhow!("Unable to unbind key material"))
        .and_then(|password| {
            password
                .map(|p| String::from_utf8(p.to_vec()))
                .map_err(|e| anyhow::anyhow!("Unbound key material is not valid UTF-8: {e}"))
        })
}

/// Decrypt a soft key's private key material.
pub(crate) async fn decrypt_private_key(
    key: &db::Key,
    encrypted_password: &[u8],
    bindings: &[Pkcs11Binding],
    user_password: Password,
) -> anyhow::Result<PKey<openssl::pkey::Private>> {
    let key_material = if let Some(material) = &key.key_material {
        material
    } else {
        return Err(anyhow::anyhow!(
            "Can't decrypt private key for a PKCS#11 token"
        ));
    };
    let key_password = decrypt_key_password(bindings, user_password, encrypted_password).await?;
    let encrypted_pem = unbind_with_pkcs11(bindings, key_material)?;
    Ok(key_password.map(|passphrase| {
        PKey::private_key_from_pem_passphrase(encrypted_pem.as_bytes(), passphrase)
    })?)
}

/// Implement a helper for unsigned, symmetrically encrypted data for Sequoia.
struct SymmetricHelper {
    password: Password,
}

// Decrypt exclusively via symmetrically encrypted session keys.
impl DecryptionHelper for SymmetricHelper {
    fn decrypt(
        &mut self,
        _pkesks: &[sequoia_openpgp::packet::PKESK],
        symmetric_session_keys: &[sequoia_openpgp::packet::SKESK],
        _sym_algo: Option<sequoia_openpgp::types::SymmetricAlgorithm>,
        decrypt: &mut dyn FnMut(
            Option<sequoia_openpgp::types::SymmetricAlgorithm>,
            &sequoia_openpgp::crypto::SessionKey,
        ) -> bool,
    ) -> sequoia_openpgp::Result<Option<sequoia_openpgp::Cert>> {
        for session_key in symmetric_session_keys {
            if session_key
                .decrypt(&self.password)
                .map(|(algorithm, session_key)| decrypt(algorithm, &session_key))
                .unwrap_or(false)
            {
                return Ok(None);
            }
        }
        Err(anyhow::anyhow!("Bad passphrase"))
    }
}

// A no-op verification helper implementation since the data is not expected to be signed.
impl VerificationHelper for SymmetricHelper {
    fn get_certs(
        &mut self,
        _ids: &[sequoia_openpgp::KeyHandle],
    ) -> sequoia_openpgp::Result<Vec<sequoia_openpgp::Cert>> {
        Ok(vec![])
    }

    fn check(
        &mut self,
        _structure: sequoia_openpgp::parse::stream::MessageStructure<'_>,
    ) -> sequoia_openpgp::Result<()> {
        Ok(())
    }
}

/// Encrypts some data with the given [`Password`] using OpenPGP.
///
/// Returns the ASCII-armored, encrypted `key_passphrase`.
fn symmetric_encrypt(password: Password, data: &[u8]) -> anyhow::Result<Vec<u8>> {
    let mut buffer = vec![];
    {
        let message = Armorer::new(Message::new(&mut buffer)).build()?;
        let encryptor = Encryptor::with_passwords(message, Some(password))
            .symmetric_algo(SymmetricAlgorithm::AES256)
            .aead_algo(AEADAlgorithm::GCM)
            .build()?;
        let mut message = LiteralWriter::new(encryptor).build()?;
        message.write_all(data)?;
        message.finalize()?;
    }

    Ok(buffer)
}

/// Decrypt data using GPG.
///
/// This is the inverse of [`symmetric_encrypt`]. Data is not expected to be signed and signatures are not checked.
fn symmetric_decrypt(password: Password, data: &[u8]) -> anyhow::Result<Vec<u8>> {
    let policy = StandardPolicy::new();
    let helper = SymmetricHelper { password };
    let mut decryptor = DecryptorBuilder::from_bytes(&data)?.with_policy(&policy, None, helper)?;
    let mut user_passphrase = vec![];
    decryptor.read_to_end(&mut user_passphrase)?;

    Ok(user_passphrase)
}

/// This module contains functions for working with Sigul-bound keys.
///
/// It should only be used when accessing a Sigul database for the purpose of
/// migrating it to Siguldry.
pub mod sigul {
    use super::*;

    /// The format, serialized as JSON, that Sigul uses to store secrets bound with PKCS11.
    #[derive(Debug, Clone, Serialize, Deserialize)]
    #[allow(dead_code)]
    struct SigulPkcs11BoundPassword {
        method: String,
        /// The encrypted password (or, if nesting is being used, the encrypted output of some other
        /// binding).
        ///
        /// Sigul used openssl-smime to encrypt the password with the X509 certificate for a private
        /// key accessible via PKCS#11.
        value: String,
        /// A string to identify the token; we don't bother using this and just try to decrypt with
        /// any tokens we have.
        token: String,
    }

    // Shell out to pgp to decrypt the key password.
    //
    // This is done rather than use Sequioa because the OpenPGP data structure written by
    // sigul is considered malformed to Sequoia.
    fn gpg_symmetric_decrypt(password: Password, data: &[u8]) -> anyhow::Result<Vec<u8>> {
        let mut child = std::process::Command::new("gpg")
            .args([
                "--batch",
                "--yes",
                "--quiet",
                "--decrypt",
                "--passphrase-fd",
                "0",
            ])
            .stdin(Stdio::piped())
            .stdout(Stdio::piped())
            .stderr(Stdio::piped())
            .spawn()
            .context("Failed to spawn pgp process")?;

        let mut stdin = child
            .stdin
            .take()
            .ok_or_else(|| anyhow::anyhow!("Failed to open pgp stdin"))?;
        password.map(|p| {
            stdin.write_all(p)?;
            stdin.write_all(b"\n")?;
            stdin.write_all(data)
        })?;
        drop(stdin);

        let output = child
            .wait_with_output()
            .context("Failed to wait for pgp process")?;

        if !output.status.success() {
            let stderr = String::from_utf8_lossy(&output.stderr);
            return Err(anyhow::anyhow!(
                "gpg decryption failed (exit code {:?}): {}",
                output.status.code(),
                stderr
            ));
        }

        Ok(output.stdout)
    }

    // This is based off the bind_passphrase/unbind_passphrase implementations in the utils.py module
    // of Sigul.
    //
    // While Sigul supports several binding methods (such at TPM 1.2), we only handle the PKCS#11 case
    // as that's what is used in Fedora's production deployment.
    pub async fn unbind_key_password(
        user_password: Password,
        encrypted_passphrase: &[u8],
        sigul_binding: &Option<Pkcs11Binding>,
    ) -> anyhow::Result<Password> {
        let bound_key_password = gpg_symmetric_decrypt(user_password, encrypted_passphrase)
            .context("Decryption via the user password failed")?;
        tracing::debug!("User password used to decrypt key access; unbinding...");
        let mut bound_key_password = String::from_utf8(bound_key_password)
            .map_err(|_| anyhow::anyhow!("Passwords are expected to be UTF-8"))?;

        let key_password = loop {
            match bound_key_password.as_str() {
                bound_object if bound_key_password.starts_with("{") => {
                    // Sigul changed its format from being a single object to a list of those objects.
                    if let Some(sigul_binding) = sigul_binding.as_ref() {
                        let pkcs11_binding: SigulPkcs11BoundPassword =
                            serde_json::from_str(bound_object)
                                .context("Key is bound with an unsupported method")?;
                        let inner = super::binding_decrypt(
                            sigul_binding.clone(),
                            pkcs11_binding.value.as_bytes().to_vec(),
                        )?;
                        bound_key_password = String::from_utf8(inner)?;
                    } else {
                        return Err(anyhow::anyhow!(
                            "Sigul key is hardware-bound but no PKCS#11 unbinding key was provided"
                        ));
                    }
                }
                bound_list if bound_key_password.starts_with("[") => {
                    if let Some(sigul_binding) = sigul_binding.as_ref() {
                        let binding_entries: Vec<serde_json::Value> =
                            serde_json::from_str(bound_list)?;
                        for binding in binding_entries {
                            if let Ok(pkcs11_binding) =
                                serde_json::from_value::<SigulPkcs11BoundPassword>(binding)
                            {
                                let inner = super::binding_decrypt(
                                    sigul_binding.clone(),
                                    pkcs11_binding.value.as_bytes().to_vec(),
                                );
                                if let Ok(inner) = inner {
                                    bound_key_password = String::from_utf8(inner)?;
                                    break;
                                } else {
                                    tracing::debug!(
                                        binding_token = pkcs11_binding.token,
                                        "Failed to decrypt the password"
                                    );
                                }
                            } else {
                                tracing::debug!("Unknown binding format found, skipping");
                            }
                        }
                    } else {
                        return Err(anyhow::anyhow!(
                            "Sigul key is hardware-bound but no PKCS#11 unbinding key was provided"
                        ));
                    }
                }
                unbound => break Ok::<_, anyhow::Error>(unbound),
            };
        }?;

        tracing::debug!("Key password successfully unbound");
        Ok(key_password.into())
    }
}

#[cfg(test)]
mod tests {

    use anyhow::Result;
    use tempfile::NamedTempFile;

    use super::*;
    use crate::server::crypto::test_utils::setup_hsm;

    // Encrypting and then decrypting should give us the key back
    #[test]
    fn encrypt_decrypt() -> Result<()> {
        let user_passphrase = Password::from("this grants a user access to the key passphrase");
        let data = "this encrypts the private key";
        let encrypted_data = symmetric_encrypt(user_passphrase.clone(), data.as_bytes())?;
        let decrypted_data = symmetric_decrypt(user_passphrase, &encrypted_data)?;
        assert_eq!(data.as_bytes(), decrypted_data);
        Ok(())
    }

    // Ensure something encrypted with sq's CLI is decrypted by our implementation
    #[test]
    fn encrypt_with_sq_decrypt() -> Result<()> {
        let user_passphrase = "this grants a user access to the key passphrase".to_string();
        let data = "this encrypts the private key";
        let mut password_file = NamedTempFile::new()?;
        let mut message = NamedTempFile::new()?;
        password_file.write_all(user_passphrase.as_bytes())?;
        message.write_all(data.as_bytes())?;

        let mut command = std::process::Command::new("sq");
        let result = command
            .arg("encrypt")
            .arg(format!(
                "--with-password-file={}",
                password_file.path().display()
            ))
            .arg("--without-signature")
            .arg(message.path())
            .output()?;

        let retrieved_key_passphrase =
            symmetric_decrypt(Password::from(user_passphrase), &result.stdout)?;
        assert_eq!(data.as_bytes(), retrieved_key_passphrase);
        Ok(())
    }

    // Ensure something encrypted with our implementation is decryptable by sq's CLI
    #[test]
    fn encrypt_decrypt_with_sq() -> Result<()> {
        let user_passphrase = "this grants a user access to the key passphrase".to_string();
        let data = "this encrypts the private key";
        let encrypted_passphrase =
            symmetric_encrypt(Password::from(user_passphrase.as_bytes()), data.as_bytes())?;
        let mut password_file = NamedTempFile::new()?;
        let mut encrypted_message = NamedTempFile::new()?;
        password_file.write_all(user_passphrase.as_bytes())?;
        encrypted_message.write_all(&encrypted_passphrase)?;

        let mut command = std::process::Command::new("sq");
        let result = command
            .arg(format!(
                "--password-file={}",
                password_file.path().display()
            ))
            .arg("decrypt")
            .arg(encrypted_message.path())
            .output()?;
        assert_eq!(data.as_bytes(), result.stdout);

        Ok(())
    }

    /// Assert encrypting and then decrypting for bindings works.
    #[tokio::test]
    async fn encrypt_decrypt_binding() -> Result<()> {
        let softhsm = setup_hsm()?;

        let bound_secrets = BoundSecret::bind(&softhsm.bindings, b"some data")?;
        let bound_password = bound_secrets.first().unwrap();

        assert!(
            matches!(
                &bound_password,
                &BoundSecret::Pkcs11WithCMS {
                    fingerprint: _,
                    secret: _
                }
            ),
            "Expected PKCS11 binding"
        );
        let decrypted_data = bound_password
            .unbind(&softhsm.bindings)?
            .map(|p| p.to_vec());
        assert_eq!(b"some data".as_slice(), decrypted_data);

        Ok(())
    }

    /// Assert the complete encryption/decryption process roundtrips as expected.
    #[tokio::test]
    async fn encrypt_decrypt_key_password() -> Result<()> {
        let softhsm = setup_hsm()?;

        let key_password = Password::from("a secret that never leaves the server");
        let user_password = Password::from("some long password clients provide");
        let blob = encrypt_key_password(
            &softhsm.bindings,
            user_password.clone(),
            key_password.clone(),
        )?;
        let roundtrip_key_password =
            decrypt_key_password(&softhsm.bindings, user_password, &blob).await?;

        assert_eq!(key_password, roundtrip_key_password);

        Ok(())
    }

    // Assert if no bindings include keys, we get an error
    #[tokio::test]
    async fn encrypt_decrypt_key_password_binding_no_key() -> Result<()> {
        let softhsm = setup_hsm()?;

        let key_password = Password::from("a secret that never leaves the server");
        let user_password = Password::from("some long password clients provide");
        let blob = encrypt_key_password(
            &softhsm.bindings,
            user_password.clone(),
            key_password.clone(),
        )?;
        let result =
            decrypt_key_password(softhsm.bindings.get(1..).unwrap(), user_password, &blob).await;
        assert!(result.is_err_and(|err| err.to_string().contains("Unable to unbind key password")));

        Ok(())
    }

    // We should get an error if the user password is incorrect
    #[tokio::test]
    async fn encrypt_decrypt_key_password_wrong_user_password() -> Result<()> {
        let softhsm = setup_hsm()?;

        let key_password = Password::from("a secret that never leaves the server");
        let user_password = Password::from("some long password clients provide");
        let blob = encrypt_key_password(&softhsm.bindings, user_password, key_password.clone())?;
        let user_password = Password::from("the wrong password");
        let result =
            decrypt_key_password(softhsm.bindings.get(1..).unwrap(), user_password, &blob).await;
        let err_str = result.map_err(|e| e.to_string()).err().unwrap();
        assert_eq!(err_str, "User password is invalid");

        Ok(())
    }

    // Assert if no bindings are configured, just the user password is sufficient.
    #[tokio::test]
    async fn encrypt_decrypt_key_password_no_bindings() -> Result<()> {
        let key_password = Password::from("a secret that never leaves the server");
        let user_password = Password::from("some long password clients provide");
        let blob = encrypt_key_password(&[], user_password.clone(), key_password.clone())?;
        let roundtrip_key_password = decrypt_key_password(&[], user_password, &blob).await?;
        assert_eq!(key_password, roundtrip_key_password);
        Ok(())
    }
}