diff --git a/README.md b/README.md
index 356ebef..6e39784 100644
--- a/README.md
+++ b/README.md
@@ -40,4 +40,4 @@ $ source .env/bin/activate
 $ pip install -r requirements-dev.txt
 ```
 
-With that, you should be able to run `maturin develop` to build and install the extension. You can then `import biscuit_auth` in a Python shell to play around, or run `pytest` to run the Python tests.
\ No newline at end of file
+With that, you should be able to run `maturin develop` to build and install the extension. You can then `import biscuit_auth` in a Python shell to play around, or run `pytest` to run the Python tests.
diff --git a/biscuit_test.py b/biscuit_test.py
index 8d49186..f436141 100644
--- a/biscuit_test.py
+++ b/biscuit_test.py
@@ -463,3 +463,14 @@ def test(left, right):
     policy = authorizer.build_unauthenticated().authorize()
     assert policy == 0
 
+def test_append_third_party_block():
+    root_kp = KeyPair()
+    external_kp = KeyPair()
+
+    builder = BiscuitBuilder("user({user});", {'user': "123"})
+    biscuit = builder.build(root_kp.private_key)
+
+    third_party_block = BlockBuilder("external_fact({fact});", {'fact': "456"})
+    new_biscuit = biscuit.append_third_party_block(external_kp, third_party_block)
+
+    assert str(new_biscuit.block_external_key(1)) == str(external_kp.public_key)
diff --git a/src/lib.rs b/src/lib.rs
index cf049dd..bcefbd7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -83,6 +83,13 @@ impl From<PyAlgorithm> for builder::Algorithm {
     }
 }
 
+impl BiscuitBuildError {
+    /// Helper function to build `BiscuitBuildError` from other errors.
+    pub fn from_err(error: impl ToString) -> PyErr {
+        Self::new_err(error.to_string())
+    }
+}
+
 struct PyKeyProvider {
     py_value: PyObject,
 }
@@ -397,6 +404,36 @@ impl PyBiscuit {
             .map(PyBiscuit)
     }
 
+    /// Create a new `Biscuit` by appending a third party attenuation block
+    ///
+    /// :param kp: keypair used to sign the block
+    /// :type kp: KeyPair
+    /// :param block: a builder for the new block
+    /// :type block: BlockBuilder
+    /// :return: the attenuated biscuit
+    /// :rtype: Biscuit
+    pub fn append_third_party_block(
+        &self,
+        kp: &PyKeyPair,
+        block: &PyBlockBuilder,
+    ) -> PyResult<PyBiscuit> {
+        let third_party_block = self
+            .0
+            .third_party_request()
+            .and_then(|req| {
+                req.create_block(
+                    &kp.private_key().0,
+                    block.0.clone().expect("builder already consumed"),
+                )
+            })
+            .map_err(BiscuitBuildError::from_err)?;
+
+        self.0
+            .append_third_party(kp.public_key().0, third_party_block)
+            .map_err(BiscuitBuildError::from_err)
+            .map(PyBiscuit)
+    }
+
     /// The revocation ids of the token, encoded as hexadecimal strings
     #[getter]
     pub fn revocation_ids(&self) -> Vec<String> {
@@ -407,6 +444,21 @@ impl PyBiscuit {
             .collect()
     }
 
+    /// Get the external key of a block if it exists
+    ///
+    /// :param index: the block index
+    /// :type index: int
+    /// :return: the public key if it exists
+    /// :rtype: str | None
+    pub fn block_external_key(&self, index: usize) -> PyResult<Option<PyPublicKey>> {
+        let opt_key = self
+            .0
+            .block_external_key(index)
+            .map_err(|e| BiscuitBlockError::new_err(e.to_string()))?;
+
+        Ok(opt_key.map(PyPublicKey))
+    }
+
     fn __repr__(&self) -> String {
         self.0.print()
     }