feat(notsecrets): SSH RSA identity and recipient (OAEP-SHA256)

crates/notsecrets/src/identities/mod.rs

@@ -9,6 +9,9 @@ pub use ssh_ed25519::SshEd25519Identity;
 pub mod scrypt;
 pub use scrypt::ScryptIdentity;
 
+pub mod ssh_rsa;
+pub use ssh_rsa::SshRsaIdentity;
+
 /// The symmetric file encryption key — 16 random bytes.
 #[derive(Clone, Debug)]
 pub struct FileKey([u8; 16]);

crates/notsecrets/src/identities/ssh_rsa.rs

@@ -0,0 +1,104 @@
+use crate::error::AgeError;
+use crate::identities::{FileKey, Identity, Stanza};
+use base64::{Engine, engine::general_purpose::STANDARD_NO_PAD};
+use rsa::{Oaep, RsaPrivateKey, RsaPublicKey, traits::PublicKeyParts};
+use sha2::{Digest, Sha256};
+
+const TAG: &str = "ssh-rsa";
+
+pub struct SshRsaIdentity {
+    private_key: RsaPrivateKey,
+}
+
+impl SshRsaIdentity {
+    pub fn from_private_key(private_key: RsaPrivateKey) -> Self {
+        Self { private_key }
+    }
+
+    fn fingerprint(&self) -> [u8; 4] {
+        rsa_pubkey_fingerprint(&RsaPublicKey::from(&self.private_key))
+    }
+}
+
+impl Identity for SshRsaIdentity {
+    fn unwrap_file_key(&self, stanza: &Stanza) -> Option<Result<FileKey, AgeError>> {
+        if stanza.tag != TAG {
+            return None;
+        }
+        if stanza.args.len() != 1 {
+            return Some(Err(AgeError::ParseError(
+                "ssh-rsa stanza must have 1 arg".to_string(),
+            )));
+        }
+        let fp = match STANDARD_NO_PAD.decode(&stanza.args[0]) {
+            Ok(b) => b,
+            Err(e) => return Some(Err(AgeError::ParseError(format!("fingerprint base64: {e}")))),
+        };
+        if fp.as_slice() != self.fingerprint() {
+            return None;
+        }
+        Some(unwrap(stanza, &self.private_key))
+    }
+}
+
+fn unwrap(stanza: &Stanza, private_key: &RsaPrivateKey) -> Result<FileKey, AgeError> {
+    let padding = Oaep::new::<Sha256>();
+    let file_key_bytes = private_key
+        .decrypt(padding, &stanza.body)
+        .map_err(|_| AgeError::CryptoError("RSA-OAEP decrypt failed".to_string()))?;
+    FileKey::try_from(file_key_bytes.as_slice())
+}
+
+pub(crate) fn rsa_pubkey_fingerprint(public_key: &RsaPublicKey) -> [u8; 4] {
+    let n_bytes = public_key.n().to_bytes_be();
+    let e_bytes = public_key.e().to_bytes_be();
+    let mut hasher = Sha256::new();
+    hasher.update(&n_bytes);
+    hasher.update(&e_bytes);
+    let hash = hasher.finalize();
+    hash[..4].try_into().expect("SHA-256 is 32 bytes")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::recipients::ssh_rsa::SshRsaRecipient;
+    use crate::recipients::Recipient;
+    use rand::rngs::OsRng;
+
+    fn test_rsa_keypair() -> (RsaPrivateKey, RsaPublicKey) {
+        let private_key = RsaPrivateKey::new(&mut OsRng, 2048).expect("RSA keygen failed");
+        let public_key = RsaPublicKey::from(&private_key);
+        (private_key, public_key)
+    }
+
+    #[test]
+    fn ssh_rsa_wrap_unwrap_roundtrip() {
+        let (private_key, public_key) = test_rsa_keypair();
+        let recipient = SshRsaRecipient::from_public_key(public_key);
+        let identity = SshRsaIdentity::from_private_key(private_key);
+
+        let file_key = FileKey::new([0x55u8; 16]);
+        let stanza = recipient.wrap_file_key(&file_key).expect("wrap should succeed");
+        assert_eq!(stanza.tag, "ssh-rsa");
+        assert_eq!(stanza.args.len(), 1);
+
+        let unwrapped = identity
+            .unwrap_file_key(&stanza)
+            .expect("identity should match")
+            .expect("unwrap should succeed");
+        assert_eq!(unwrapped.as_bytes(), file_key.as_bytes());
+    }
+
+    #[test]
+    fn ssh_rsa_wrong_fingerprint_returns_none() {
+        let (private_key1, _) = test_rsa_keypair();
+        let (_, public_key2) = test_rsa_keypair();
+        let recipient = SshRsaRecipient::from_public_key(public_key2);
+        let identity = SshRsaIdentity::from_private_key(private_key1);
+
+        let file_key = FileKey::new([0x55u8; 16]);
+        let stanza = recipient.wrap_file_key(&file_key).unwrap();
+        assert!(identity.unwrap_file_key(&stanza).is_none());
+    }
+}