diff --git a/src/proto/extension.rs b/src/proto/extension.rs
index 4c939df..606a5ba 100644
--- a/src/proto/extension.rs
+++ b/src/proto/extension.rs
@@ -1,6 +1,9 @@
-use ssh_encoding::{Decode, Reader};
+use ssh_encoding::{CheckedSum, Decode, Encode, Error as EncodingError, Reader, Writer};
 use ssh_key::{public::KeyData, Signature};
 
+// Reserved fields are marked with an empty string
+const RESERVED_FIELD: &str = "";
+
 /// session-bind@openssh.com extension
 ///
 /// This extension allows a ssh client to bind an agent connection to a
@@ -32,6 +35,30 @@ impl Decode for SessionBind {
     }
 }
 
+impl Encode for SessionBind {
+    fn encoded_len(&self) -> ssh_encoding::Result<usize> {
+        [
+            self.host_key.encoded_len_prefixed()?,
+            self.session_id.encoded_len()?,
+            self.signature.encoded_len_prefixed()?,
+            1u8.encoded_len()?,
+        ]
+        .checked_sum()
+    }
+
+    fn encode(&self, writer: &mut impl Writer) -> ssh_encoding::Result<()> {
+        self.host_key.encode_prefixed(writer)?;
+        self.session_id.encode(writer)?;
+        self.signature.encode_prefixed(writer)?;
+
+        if self.is_forwarding {
+            1u8.encode(writer)
+        } else {
+            0u8.encode(writer)
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct RestrictDestination {
     pub constraints: Vec<DestinationConstraint>,
@@ -49,50 +76,111 @@ impl Decode for RestrictDestination {
     }
 }
 
+impl Encode for RestrictDestination {
+    fn encoded_len(&self) -> ssh_encoding::Result<usize> {
+        self.constraints.iter().try_fold(0, |acc, e| {
+            let constraint_len = e.encoded_len()?;
+            usize::checked_add(acc, constraint_len).ok_or(EncodingError::Length)
+        })
+    }
+
+    fn encode(&self, writer: &mut impl Writer) -> ssh_encoding::Result<()> {
+        for constraint in &self.constraints {
+            constraint.encode(writer)?;
+        }
+        Ok(())
+    }
+}
+
 #[derive(Debug, Clone)]
-pub struct DestinationConstraint {
-    pub from_username: String,
-    pub from_hostname: String,
-    pub from_hostkeys: Vec<KeySpec>,
-    pub to_username: String,
-    pub to_hostname: String,
-    pub to_hostkeys: Vec<KeySpec>,
+pub struct HostTuple {
+    pub username: String,
+    pub hostname: String,
+    pub keys: Vec<KeySpec>,
 }
 
-impl Decode for DestinationConstraint {
+impl Decode for HostTuple {
     type Error = crate::proto::error::ProtoError;
 
     fn decode(reader: &mut impl Reader) -> Result<Self, Self::Error> {
-        fn read_user_host_keys(
-            reader: &mut impl Reader,
-        ) -> Result<(String, String, Vec<KeySpec>), crate::proto::error::ProtoError> {
-            let username = String::decode(reader)?;
-            let hostname = String::decode(reader)?;
-            let _reserved = String::decode(reader)?;
-
-            let mut keys = Vec::new();
-            while !reader.is_finished() {
-                keys.push(KeySpec::decode(reader)?);
-            }
-
-            Ok((username, hostname, keys))
+        let username = String::decode(reader)?;
+        let hostname = String::decode(reader)?;
+        let _reserved = String::decode(reader)?;
+
+        let mut keys = Vec::new();
+        while !reader.is_finished() {
+            keys.push(KeySpec::decode(reader)?);
         }
 
-        let (from_username, from_hostname, from_hostkeys) =
-            reader.read_prefixed(read_user_host_keys)?;
-        let (to_username, to_hostname, to_hostkeys) = reader.read_prefixed(read_user_host_keys)?;
-        let _reserved = String::decode(reader)?;
         Ok(Self {
-            from_username,
-            from_hostname,
-            from_hostkeys,
-            to_username,
-            to_hostname,
-            to_hostkeys,
+            username,
+            hostname,
+            keys,
         })
     }
 }
 
+impl Encode for HostTuple {
+    fn encoded_len(&self) -> ssh_encoding::Result<usize> {
+        let prefix = [
+            self.username.encoded_len()?,
+            self.hostname.encoded_len()?,
+            RESERVED_FIELD.encoded_len()?,
+        ]
+        .checked_sum()?;
+        self.keys.iter().try_fold(prefix, |acc, e| {
+            let key_len = e.encoded_len()?;
+            usize::checked_add(acc, key_len).ok_or(EncodingError::Length)
+        })
+    }
+
+    fn encode(&self, writer: &mut impl Writer) -> ssh_encoding::Result<()> {
+        self.username.encode(writer)?;
+        self.hostname.encode(writer)?;
+        RESERVED_FIELD.encode(writer)?;
+        for key in &self.keys {
+            key.encode(writer)?;
+        }
+        Ok(())
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct DestinationConstraint {
+    pub from: HostTuple,
+    pub to: HostTuple,
+}
+
+impl Decode for DestinationConstraint {
+    type Error = crate::proto::error::ProtoError;
+
+    fn decode(reader: &mut impl Reader) -> Result<Self, Self::Error> {
+        let from = reader.read_prefixed(HostTuple::decode)?;
+        let to = reader.read_prefixed(HostTuple::decode)?;
+        let _reserved = String::decode(reader)?;
+
+        Ok(Self { from, to })
+    }
+}
+
+impl Encode for DestinationConstraint {
+    fn encoded_len(&self) -> ssh_encoding::Result<usize> {
+        [
+            self.from.encoded_len_prefixed()?,
+            self.to.encoded_len_prefixed()?,
+            RESERVED_FIELD.encoded_len()?,
+        ]
+        .checked_sum()
+    }
+
+    fn encode(&self, writer: &mut impl Writer) -> ssh_encoding::Result<()> {
+        self.from.encode_prefixed(writer)?;
+        self.to.encode_prefixed(writer)?;
+        RESERVED_FIELD.encode(writer)?;
+        Ok(())
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct KeySpec {
     pub keyblob: KeyData,
@@ -111,6 +199,23 @@ impl Decode for KeySpec {
     }
 }
 
+impl Encode for KeySpec {
+    fn encoded_len(&self) -> ssh_encoding::Result<usize> {
+        [self.keyblob.encoded_len_prefixed()?, 1u8.encoded_len()?].checked_sum()
+    }
+
+    fn encode(&self, writer: &mut impl Writer) -> ssh_encoding::Result<()> {
+        self.keyblob.encode_prefixed(writer)?;
+        // TODO: contribute `impl Encode for bool` in ssh-encoding
+        // <https://www.rfc-editor.org/rfc/rfc4251#section-5>
+        if self.is_ca {
+            1u8.encode(writer)
+        } else {
+            0u8.encode(writer)
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use hex_literal::hex;