diff --git a/src/key_exchange/tripledh.rs b/src/key_exchange/tripledh.rs
index 5194596..f1df66f 100644
--- a/src/key_exchange/tripledh.rs
+++ b/src/key_exchange/tripledh.rs
@@ -468,7 +468,7 @@ fn hkdf_expand_label_extracted<D: Hash>(
     let mut okm = vec![0u8; length];
 
     let mut hkdf_label: Vec<u8> = Vec::new();
-    hkdf_label.extend_from_slice(&length.to_be_bytes()[6..]);
+    hkdf_label.extend_from_slice(&length.to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
 
     let mut opaque_label: Vec<u8> = Vec::new();
     opaque_label.extend_from_slice(&STR_OPAQUE);
diff --git a/src/serialization/mod.rs b/src/serialization/mod.rs
index 261e589..58b71f8 100644
--- a/src/serialization/mod.rs
+++ b/src/serialization/mod.rs
@@ -82,7 +82,8 @@ impl<T: CipherSuite> From<&LoginThirdMessage<T>> for ProtocolMessageType {
 
 pub(crate) fn serialize(input: &[u8], max_bytes: usize) -> Vec<u8> {
     let mut output: Vec<u8> = Vec::new();
-    output.extend_from_slice(&input.len().to_be_bytes()[8 - max_bytes..]);
+    output
+        .extend_from_slice(&input.len().to_be_bytes()[std::mem::size_of::<usize>() - max_bytes..]);
     output.extend_from_slice(&input[..]);
     output
 }
@@ -96,7 +97,15 @@ pub(crate) fn tokenize(input: Vec<u8>, size_bytes: usize) -> Result<(Vec<u8>, Ve
     for i in 0..size_bytes {
         size_array[8 - size_bytes + i] = input[i];
     }
-    let size = usize::from_be_bytes(size_array);
+
+    let big_size = u64::from_be_bytes(size_array);
+
+    // TODO:: check RFC compliance in refusing this
+    if big_size >= u32::MAX as u64 {
+        return Err(PakeError::SerializationError);
+    }
+
+    let size = big_size as usize;
 
     if size_bytes + size > input.len() {
         return Err(PakeError::SerializationError);
diff --git a/src/serialization/tests.rs b/src/serialization/tests.rs
index 8e82593..0023912 100644
--- a/src/serialization/tests.rs
+++ b/src/serialization/tests.rs
@@ -118,8 +118,8 @@ fn register_first_message_roundtrip() {
 
     let mut input = Vec::new();
     input.extend_from_slice(&[ProtocolMessageType::RegistrationRequest as u8 + 1]);
-    input.extend_from_slice(&total_length.to_be_bytes()[8 - 3..]);
-    input.extend_from_slice(&alpha_length.to_be_bytes()[8 - 2..]);
+    input.extend_from_slice(&total_length.to_be_bytes()[std::mem::size_of::<usize>() - 3..]);
+    input.extend_from_slice(&alpha_length.to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
     input.extend_from_slice(pt_bytes.as_slice());
 
     let r1 = RegisterFirstMessage::<RistrettoPoint>::deserialize(input.as_slice()).unwrap();
@@ -142,10 +142,10 @@ fn register_second_message_roundtrip() {
 
     let mut input = Vec::new();
     input.extend_from_slice(&[ProtocolMessageType::RegistrationResponse as u8 + 1]);
-    input.extend_from_slice(&total_length.to_be_bytes()[8 - 3..]);
-    input.extend_from_slice(&beta_length.to_be_bytes()[8 - 2..]);
+    input.extend_from_slice(&total_length.to_be_bytes()[std::mem::size_of::<usize>() - 3..]);
+    input.extend_from_slice(&beta_length.to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
     input.extend_from_slice(beta_bytes.as_slice());
-    input.extend_from_slice(&pubkey_length.to_be_bytes()[8 - 2..]);
+    input.extend_from_slice(&pubkey_length.to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
     input.extend_from_slice(&pubkey_bytes.as_slice());
     input.extend_from_slice(&credential_types);
 
@@ -175,9 +175,9 @@ fn register_third_message_roundtrip() {
 
     let mut input = Vec::new();
     input.extend_from_slice(&[ProtocolMessageType::RegistrationUpload as u8 + 1]);
-    input.extend_from_slice(&total_length.to_be_bytes()[8 - 3..]);
+    input.extend_from_slice(&total_length.to_be_bytes()[std::mem::size_of::<usize>() - 3..]);
     input.extend_from_slice(&envelope_bytes);
-    input.extend_from_slice(&pubkey_length.to_be_bytes()[8 - 2..]);
+    input.extend_from_slice(&pubkey_length.to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
     input.extend_from_slice(&pubkey_bytes[..]);
 
     let r3 = RegisterThirdMessage::<X25519KeyPair, sha2::Sha256>::deserialize(&input[..]).unwrap();
@@ -202,8 +202,10 @@ fn login_first_message_roundtrip() {
 
     let mut input = Vec::new();
     input.extend_from_slice(&[ProtocolMessageType::CredentialRequest as u8 + 1]);
-    input.extend_from_slice(&total_length_without_ke1m.to_be_bytes()[8 - 3..]);
-    input.extend_from_slice(&alpha_length.to_be_bytes()[8 - 2..]);
+    input.extend_from_slice(
+        &total_length_without_ke1m.to_be_bytes()[std::mem::size_of::<usize>() - 3..],
+    );
+    input.extend_from_slice(&alpha_length.to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
     input.extend_from_slice(&alpha_bytes);
     input.extend_from_slice(&ke1m[..]);
 
@@ -242,8 +244,10 @@ fn login_second_message_roundtrip() {
 
     let mut input = Vec::new();
     input.extend_from_slice(&[ProtocolMessageType::CredentialResponse as u8 + 1]);
-    input.extend_from_slice(&total_length_without_ke2m.to_be_bytes()[8 - 3..]);
-    input.extend_from_slice(&pt_bytes.len().to_be_bytes()[8 - 2..]);
+    input.extend_from_slice(
+        &total_length_without_ke2m.to_be_bytes()[std::mem::size_of::<usize>() - 3..],
+    );
+    input.extend_from_slice(&pt_bytes.len().to_be_bytes()[std::mem::size_of::<usize>() - 2..]);
     input.extend_from_slice(pt_bytes.as_slice());
     input.extend_from_slice(&envelope.to_bytes());
     input.extend_from_slice(&ke2m[..]);