diff --git a/tor-llcrypto/src/pk/rsa.rs b/tor-llcrypto/src/pk/rsa.rs
index 3cd51017d..0ee3ce0b9 100644
--- a/tor-llcrypto/src/pk/rsa.rs
+++ b/tor-llcrypto/src/pk/rsa.rs
@@ -8,6 +8,11 @@
 //! signatures, and encoding and decoding keys from DER.
 //!
 //! Currently missing is signing and RSA-OEAP.
+//!
+//! # Limitations:
+//!
+//! XXXX This module should expose RustCrypto trait-based wrappers,
+//! but the rsa crate didn't support them as of initial writing.
 use arrayref::array_ref;
 use std::fmt;
 use subtle::*;
diff --git a/tor-llcrypto/src/util.rs b/tor-llcrypto/src/util.rs
index 9cd98dc6a..58ac17dab 100644
--- a/tor-llcrypto/src/util.rs
+++ b/tor-llcrypto/src/util.rs
@@ -61,3 +61,29 @@ impl<'a> Asn1<'a> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    // A cert generated by chutney
+    const CHUTNEY_CERT: &[u8] = include_bytes!("../testdata/tor.der");
+    // current (Sep 2020) certificate for www.torproject.org
+    const TPO_CERT: &[u8] = include_bytes!("../testdata/tpo.der");
+
+    #[test]
+    fn extract_rsa() {
+        use super::*;
+        use hex_literal::hex;
+        let pk = x509_extract_rsa_subject_kludge(CHUTNEY_CERT).unwrap();
+        assert_eq!(pk.bits(), 2048);
+        assert!(pk.exponent_is(65537));
+        assert_eq!(pk.to_der(), &hex!(
+            "3082010a028201010097a08af777515b03d99702a7e25382438ac93c1ba89601ece7b9ce1a1c5667ba6b97ef6c489c9e269da5f42b70ce09d6cf8c91b77bed2c66885be394f5de1c2e0a7e6464f5c9bc988dd5f3fd495af77041b64c7546930d99f3a5183186a5f188baa8f7ad579083d3bff0ce6508d1961d4f5da26fd84d1a693f903c47b1cfb3f6910843eafbcebafff58e74e6e737e4517514746880cb1b5b4fbf75243ca713c0fbd50176595e8d6e4b9bffb5434479ccbe6ed5cf1e22982826b60123eec3064e84f657da88312dcc5258320d52cfa3dda9fe372db38ebb2448e0411a26d3a95a35de9d617f8bf9ff2d9a099bf8123763229a3cd0ccd5c3c812c36c710bc0d3510203010001")[..]);
+
+        let pk = x509_extract_rsa_subject_kludge(TPO_CERT).unwrap();
+        assert_eq!(pk.bits(), 4096);
+        assert!(pk.exponent_is(65537));
+        assert_eq!(pk.to_der(), &hex!("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")[..]);
+
+        assert!(x509_extract_rsa_subject_kludge("hello world".as_bytes()).is_none());
+    }
+}
diff --git a/tor-llcrypto/testdata/tor.der b/tor-llcrypto/testdata/tor.der
new file mode 100644
index 000000000..5a391a446
Binary files /dev/null and b/tor-llcrypto/testdata/tor.der differ
diff --git a/tor-llcrypto/testdata/tpo.der b/tor-llcrypto/testdata/tpo.der
new file mode 100644
index 000000000..5ce216707
Binary files /dev/null and b/tor-llcrypto/testdata/tpo.der differ