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//! Re-exporting RSA implementations.
//!
//! This module can currently handle public keys and signature
//! verification used in the Tor directory protocol and
//! similar places.
//! Currently, that means validating PKCSv1 signatures, and encoding
//! and decoding RSA public keys from DER.
//! # Limitations:
//! Currently missing are support for signing and RSA-OEAP. In Tor,
//! RSA signing is only needed for relays and authorities, and
//! RSA-OAEP padding is only needed for the (obsolete) TAP protocol.
//! This module should expose RustCrypto trait-based wrappers,
//! but the [`rsa`] crate didn't support them as of initial writing.
use rsa::pkcs1::{DecodeRsaPrivateKey, DecodeRsaPublicKey};
use std::fmt;
use subtle::{Choice, ConstantTimeEq};
#[cfg(feature = "memquota-memcost")]
use {derive_deftly::Deftly, tor_memquota::derive_deftly_template_HasMemoryCost};
use crate::util::{ct::CtByteArray, rng::RngCompat};
pub use rsa::Error;
/// How many bytes are in an "RSA ID"? (This is a legacy tor
/// concept, and refers to identifying a relay by a SHA1 digest
/// of its RSA public identity key.)
pub const RSA_ID_LEN: usize = 20;
/// An identifier for an RSA key, based on SHA1 and DER.
///
/// These are used (for legacy purposes) all over the Tor protocol.
/// This object is an "identity" in the sense that it identifies (up to) one RSA
/// key. It may also represent the identity for a particular entity, such as a
/// relay or a directory authority.
/// Note that for modern purposes, you should almost always identify a relay by
/// its [`Ed25519Identity`](crate::pk::ed25519::Ed25519Identity) instead of by
/// this kind of identity key.
#[derive(Clone, Copy, Hash, Ord, PartialOrd, Eq, PartialEq)]
#[cfg_attr(
feature = "memquota-memcost",
derive(Deftly),
derive_deftly(HasMemoryCost)
)]
pub struct RsaIdentity {
/// SHA1 digest of a DER encoded public key.
id: CtByteArray<RSA_ID_LEN>,
}
impl ConstantTimeEq for RsaIdentity {
fn ct_eq(&self, other: &Self) -> Choice {
self.id.ct_eq(&other.id)
impl fmt::Display for RsaIdentity {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "${}", hex::encode(&self.id.as_ref()[..]))
impl fmt::Debug for RsaIdentity {
write!(f, "RsaIdentity {{ {} }}", self)
impl safelog::Redactable for RsaIdentity {
/// Warning: This displays 16 bits of the RSA identity, which is
/// enough to narrow down a public relay by a great deal.
fn display_redacted(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "${}…", hex::encode(&self.id.as_ref()[..1]))
fn debug_redacted(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "RsaIdentity {{ {} }}", self.redacted())
impl serde::Serialize for RsaIdentity {
fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
where
S: serde::Serializer,
{
if serializer.is_human_readable() {
serializer.serialize_str(&hex::encode(&self.id.as_ref()[..]))
} else {
serializer.serialize_bytes(&self.id.as_ref()[..])
impl<'de> serde::Deserialize<'de> for RsaIdentity {
fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
D: serde::Deserializer<'de>,
if deserializer.is_human_readable() {
/// Deserialization helper
struct RsaIdentityVisitor;
impl<'de> serde::de::Visitor<'de> for RsaIdentityVisitor {
type Value = RsaIdentity;
fn expecting(&self, fmt: &mut std::fmt::Formatter<'_>) -> fmt::Result {
fmt.write_str("hex-encoded RSA identity")
fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
E: serde::de::Error,
RsaIdentity::from_hex(s)
.ok_or_else(|| E::custom("wrong encoding for RSA identity"))
deserializer.deserialize_str(RsaIdentityVisitor)
fmt.write_str("RSA identity")
fn visit_bytes<E>(self, bytes: &[u8]) -> Result<Self::Value, E>
RsaIdentity::from_bytes(bytes)
.ok_or_else(|| E::custom("wrong length for RSA identity"))
deserializer.deserialize_bytes(RsaIdentityVisitor)
impl RsaIdentity {
/// Expose an RsaIdentity as a slice of bytes.
pub fn as_bytes(&self) -> &[u8] {
&self.id.as_ref()[..]
/// Construct an RsaIdentity from a slice of bytes.
/// Returns None if the input is not of the correct length.
/// ```
/// use tor_llcrypto::pk::rsa::RsaIdentity;
/// let bytes = b"xyzzyxyzzyxyzzyxyzzy";
/// let id = RsaIdentity::from_bytes(bytes);
/// assert_eq!(id.unwrap().as_bytes(), bytes);
/// let truncated = b"xyzzy";
/// let id = RsaIdentity::from_bytes(truncated);
/// assert_eq!(id, None);
pub fn from_bytes(bytes: &[u8]) -> Option<Self> {
Some(RsaIdentity {
id: CtByteArray::from(<[u8; RSA_ID_LEN]>::try_from(bytes).ok()?),
})
/// Decode an `RsaIdentity` from a hexadecimal string.
/// The string must have no spaces, or any extra characters.
pub fn from_hex(s: &str) -> Option<Self> {
let mut array = [0_u8; 20];
match hex::decode_to_slice(s, &mut array) {
Err(_) => None,
Ok(()) => Some(RsaIdentity::from(array)),
/// Return true if this `RsaIdentity` is composed entirely of zero-valued
/// bytes.
/// Such all-zero values should not be used internally, since they are not
/// the ID of any valid key. Instead, they are used in some places in the
/// Tor protocols.
pub fn is_zero(&self) -> bool {
// We do a constant-time comparison to avoid side-channels.
self.id.ct_eq(&[0; RSA_ID_LEN].into()).into()
impl From<[u8; 20]> for RsaIdentity {
fn from(id: [u8; 20]) -> RsaIdentity {
RsaIdentity { id: id.into() }
/// An RSA public key.
/// This implementation is a simple wrapper so that we can define new
/// methods and traits on the type.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct PublicKey(rsa::RsaPublicKey);
/// An RSA private key.
/// This is not so useful at present, since Arti currently only has
/// client support, and Tor clients never actually need RSA private
/// keys.
pub struct KeyPair(rsa::RsaPrivateKey);
impl KeyPair {
/// Generate a new random RSA keypair.
/// This is hardcoded to generate a 1024-bit keypair, since this only exists to support the RSA
/// keys that we require for backwards compatibility (which are all 1024 bit), and we don't
/// anticipate adding anything new that uses RSA in the future.
pub fn generate<R: rand_core::RngCore + rand_core::CryptoRng>(
csprng: &mut R,
) -> Result<Self, tor_error::Bug> {
// It's maybe a bit strange to return tor_error::Bug here, but I think it makes sense: The
// only way this call can fail is if we choose invalid values for the size and exponent,
// and those values are fixed. Ideally the `rsa` crate would give us a way to check that at
// compile time and thus have a infallible call, but they don't. I don't think it's
// reasonable to bubble up the underlying rsa::Error type, because I don't expect a caller
// to be able to figure out what to do with a error that we expect to never happen.
// Returning tor_error::Bug clearly indicates that the caller won't be able to do anything
// in particular about this error, without hiding a panic anywhere unexpected.
Ok(Self(
rsa::RsaPrivateKey::new(&mut RngCompat::new(csprng), 1024).map_err(|_| {
tor_error::internal!("Generating RSA key failed, despite fixed exponent and size")
})?,
))
/// Return the public component of this key.
pub fn to_public_key(&self) -> PublicKey {
PublicKey(self.0.to_public_key())
/// Construct a PrivateKey from DER pkcs1 encoding.
pub fn from_der(der: &[u8]) -> Option<Self> {
Some(KeyPair(rsa::RsaPrivateKey::from_pkcs1_der(der).ok()?))
/// Return a reference to the underlying key type.
pub fn as_key(&self) -> &rsa::RsaPrivateKey {
&self.0
/// Sign a message using this keypair.
/// This uses PKCS#1 v1.5 padding and takes a raw bytes, rather than doing the hashing
/// internally. This is because we use PKCS padding without specifying the hash OID, which is a
/// slightly unusual setup that is understandably not supported by the rsa crate.
pub fn sign(&self, message: &[u8]) -> Result<Vec<u8>, rsa::Error> {
self.0.sign(rsa::Pkcs1v15Sign::new_unprefixed(), message)
impl PublicKey {
/// Return true iff the exponent for this key is the same
/// number as 'e'.
pub fn exponent_is(&self, e: u32) -> bool {
use rsa::traits::PublicKeyParts;
*self.0.e() == rsa::BigUint::new(vec![e])
/// Return the number of bits in the modulus for this key.
pub fn bits(&self) -> usize {
self.0.n().bits()
/// Try to check a signature (as used in Tor.) The signed hash
/// should be in 'hashed', and the alleged signature in 'sig'.
/// Tor uses RSA-PKCSv1 signatures, with hash algorithm OIDs
/// omitted.
pub fn verify(&self, hashed: &[u8], sig: &[u8]) -> Result<(), signature::Error> {
let padding = rsa::pkcs1v15::Pkcs1v15Sign::new_unprefixed();
self.0
.verify(padding, hashed, sig)
.map_err(|_| signature::Error::new())
/// Decode an alleged DER byte string into a PublicKey.
/// Return None if the DER string does not have a valid PublicKey.
/// (This function expects an RsaPublicKey, as used by Tor. It
/// does not expect or accept a PublicKeyInfo.)
Some(PublicKey(rsa::RsaPublicKey::from_pkcs1_der(der).ok()?))
/// Encode this public key into the DER format as used by Tor.
/// The result is an RsaPublicKey, not a PublicKeyInfo.
pub fn to_der(&self) -> Vec<u8> {
use der_parser::ber::BerObject;
let mut n = self.0.n().to_bytes_be();
// prepend 0 if high bit is 1 to ensure correct signed encoding
if n[0] & 0b10000000 != 0 {
n.insert(0, 0_u8);
let n = BerObject::from_int_slice(&n);
let mut e = self.0.e().to_bytes_be();
if e[0] & 0b10000000 != 0 {
e.insert(0, 0_u8);
let e = BerObject::from_int_slice(&e);
let asn1 = BerObject::from_seq(vec![n, e]);
asn1.to_vec().expect("RSA key not encodable as DER")
/// Compute the RsaIdentity for this public key.
pub fn to_rsa_identity(&self) -> RsaIdentity {
use crate::d::Sha1;
use digest::Digest;
let id: [u8; RSA_ID_LEN] = Sha1::digest(self.to_der()).into();
pub fn as_key(&self) -> &rsa::RsaPublicKey {
impl<'a> From<&'a KeyPair> for PublicKey {
fn from(value: &'a KeyPair) -> Self {
PublicKey(value.to_public_key().0)
impl From<rsa::RsaPrivateKey> for KeyPair {
fn from(value: rsa::RsaPrivateKey) -> Self {
Self(value)
impl From<rsa::RsaPublicKey> for PublicKey {
fn from(value: rsa::RsaPublicKey) -> Self {
/// An RSA signature plus all the information needed to validate it.
pub struct ValidatableRsaSignature {
/// The key that allegedly signed this signature
key: PublicKey,
/// The signature in question
sig: Vec<u8>,
/// The value we expect to find that the signature is a signature of.
expected_hash: Vec<u8>,
impl ValidatableRsaSignature {
/// Construct a new ValidatableRsaSignature.
pub fn new(key: &PublicKey, sig: &[u8], expected_hash: &[u8]) -> Self {
ValidatableRsaSignature {
key: key.clone(),
sig: sig.into(),
expected_hash: expected_hash.into(),
impl super::ValidatableSignature for ValidatableRsaSignature {
fn is_valid(&self) -> bool {
self.key
.verify(&self.expected_hash[..], &self.sig[..])
.is_ok()