diff --git a/fstar/api/MLS.fst b/fstar/api/MLS.fst
index a6fb07c..b7aea19 100644
--- a/fstar/api/MLS.fst
+++ b/fstar/api/MLS.fst
@@ -696,7 +696,7 @@ let process_welcome_message w (sign_pk, sign_sk) lookup =
   let? interim_transcript_hash = MLS.TreeDEM.Message.Transcript.compute_interim_transcript_hash #bytes group_info.tbs.confirmation_tag group_info.tbs.group_context.confirmed_transcript_hash in
   let? tree_hash = MLS.TreeSync.API.compute_tree_hash treesync_state in
   let? group_context = compute_group_context group_info.tbs.group_context.group_id group_info.tbs.group_context.epoch tree_hash group_info.tbs.group_context.confirmed_transcript_hash in
-  let? epoch_secret = MLS.TreeKEM.KeySchedule.secret_joiner_to_epoch (secrets.joiner_secret <: bytes) [] (serialize _ group_context) in
+  let? epoch_secret = MLS.TreeKEM.KeySchedule.secret_joiner_to_epoch (secrets.joiner_secret <: bytes) [] group_context in
   let? (treekem_state, encryption_secret) = MLS.TreeKEM.API.welcome treekem leaf_decryption_key opt_path_secret_and_inviter_ind leaf_index epoch_secret in
 
   let? tree_hash = MLS.TreeSync.API.compute_tree_hash treesync_state in
diff --git a/fstar/test/MLS.Test.FromExt.KeySchedule.fst b/fstar/test/MLS.Test.FromExt.KeySchedule.fst
index c86e627..c5821b7 100644
--- a/fstar/test/MLS.Test.FromExt.KeySchedule.fst
+++ b/fstar/test/MLS.Test.FromExt.KeySchedule.fst
@@ -17,7 +17,7 @@ let gen_epoch_output #cb group_id last_init_secret epoch inp =
   let commit_secret = hex_string_to_bytes inp.commit_secret in
   let psk_secret = hex_string_to_bytes inp.psk_secret in
 
-  let group_context = (ps_prefix_to_ps_whole ps_group_context_nt).serialize (gen_group_context (ciphersuite #bytes) (hex_string_to_bytes group_id) epoch (hex_string_to_bytes inp.tree_hash) (hex_string_to_bytes inp.confirmed_transcript_hash)) in
+  let group_context = gen_group_context (ciphersuite #bytes) (hex_string_to_bytes group_id) epoch (hex_string_to_bytes inp.tree_hash) (hex_string_to_bytes inp.confirmed_transcript_hash) in
   let last_init_secret = hex_string_to_bytes last_init_secret in
   let joiner_secret = extract_result (secret_init_to_joiner last_init_secret (Some commit_secret) group_context) in
   let welcome_secret = extract_result (secret_joiner_to_welcome_internal #bytes joiner_secret psk_secret) in
@@ -36,11 +36,11 @@ let gen_epoch_output #cb group_id last_init_secret epoch inp =
     if not (string_is_ascii inp.exporter_label && String.strlen inp.exporter_label < pow2 30 - 8) then
       failwith "gen_epoch_output: exporter label is not ascii"
     else
-      extract_result (mls_exporter exporter_secret inp.exporter_label (hex_string_to_bytes inp.exporter_context) (UInt32.v inp.exporter_length))
+      extract_result (mls_exporter (exporter_secret <: bytes) inp.exporter_label (hex_string_to_bytes inp.exporter_context) (UInt32.v inp.exporter_length))
   in
 
   {
-    group_context = bytes_to_hex_string group_context;
+    group_context = bytes_to_hex_string (serialize _ group_context);
     joiner_secret = bytes_to_hex_string joiner_secret;
     welcome_secret = bytes_to_hex_string welcome_secret;
     init_secret = bytes_to_hex_string init_secret;
diff --git a/fstar/test/MLS.Test.FromExt.Welcome.fst b/fstar/test/MLS.Test.FromExt.Welcome.fst
index 5899af3..939c32b 100644
--- a/fstar/test/MLS.Test.FromExt.Welcome.fst
+++ b/fstar/test/MLS.Test.FromExt.Welcome.fst
@@ -54,7 +54,7 @@ let test_welcome_one t =
       failwith "test_welcome_one: bad signature"
     );
 
-    let group_context = (ps_prefix_to_ps_whole ps_group_context_nt).serialize (gen_group_context (ciphersuite #bytes) group_info.tbs.group_context.group_id group_info.tbs.group_context.epoch group_info.tbs.group_context.tree_hash group_info.tbs.group_context.confirmed_transcript_hash) in
+    let group_context = gen_group_context (ciphersuite #bytes) group_info.tbs.group_context.group_id group_info.tbs.group_context.epoch group_info.tbs.group_context.tree_hash group_info.tbs.group_context.confirmed_transcript_hash in
     let joiner_secret = group_secrets.joiner_secret in
     let epoch_secret = extract_result (secret_joiner_to_epoch joiner_secret [] group_context) in
     let confirmation_key = extract_result (secret_epoch_to_confirmation #bytes epoch_secret) in
diff --git a/fstar/treekem/code/MLS.TreeKEM.API.KeySchedule.fst b/fstar/treekem/code/MLS.TreeKEM.API.KeySchedule.fst
index 1378055..e68b2e0 100644
--- a/fstar/treekem/code/MLS.TreeKEM.API.KeySchedule.fst
+++ b/fstar/treekem/code/MLS.TreeKEM.API.KeySchedule.fst
@@ -48,8 +48,8 @@ val commit:
   group_context_nt bytes ->
   result (treekem_keyschedule_state bytes & bytes & secrets_for_welcome bytes)
 let commit #bytes #cb st opt_commit_secret psks new_group_context =
-  let? joiner_secret: bytes = secret_init_to_joiner st.init_secret opt_commit_secret (serialize _ new_group_context) in
-  let? epoch_secret: bytes = secret_joiner_to_epoch joiner_secret psks (serialize _ new_group_context) in
+  let? joiner_secret: bytes = secret_init_to_joiner st.init_secret opt_commit_secret new_group_context in
+  let? epoch_secret: bytes = secret_joiner_to_epoch joiner_secret psks new_group_context in
   let? welcome_secret: bytes = secret_joiner_to_welcome joiner_secret psks in
   let? (new_st, encryption_secret) = create epoch_secret in
   return (new_st, encryption_secret, {
diff --git a/fstar/treekem/code/MLS.TreeKEM.KeySchedule.fst b/fstar/treekem/code/MLS.TreeKEM.KeySchedule.fst
index eb19925..b9e0725 100644
--- a/fstar/treekem/code/MLS.TreeKEM.KeySchedule.fst
+++ b/fstar/treekem/code/MLS.TreeKEM.KeySchedule.fst
@@ -2,13 +2,14 @@ module MLS.TreeKEM.KeySchedule
 
 open Comparse
 open MLS.Crypto
+open MLS.NetworkTypes
 open MLS.TreeKEM.NetworkTypes
 open MLS.TreeKEM.PSK
 open MLS.Result
 
 val secret_init_to_joiner:
   #bytes:Type0 -> {|crypto_bytes bytes|} ->
-  bytes -> option bytes -> bytes ->
+  bytes -> option bytes -> group_context_nt bytes ->
   result (lbytes bytes (kdf_length #bytes))
 let secret_init_to_joiner #bytes #cb init_secret opt_commit_secret group_context =
   let commit_secret =
@@ -17,7 +18,7 @@ let secret_init_to_joiner #bytes #cb init_secret opt_commit_secret group_context
     | None -> zero_vector #bytes
   in
   let? prk = kdf_extract init_secret commit_secret in
-  expand_with_label #bytes prk "joiner" group_context (kdf_length #bytes)
+  expand_with_label #bytes prk "joiner" (serialize _ group_context) (kdf_length #bytes)
 
 // this version is tested in the test vectors
 val secret_joiner_to_welcome_internal:
@@ -39,15 +40,15 @@ let secret_joiner_to_welcome #bytes #cb joiner_secret psks =
 // this version is tested in the test vectors
 val secret_joiner_to_epoch_internal:
   #bytes:Type0 -> {|crypto_bytes bytes|} ->
-  bytes -> bytes -> bytes ->
+  bytes -> bytes -> group_context_nt bytes ->
   result (lbytes bytes (kdf_length #bytes))
 let secret_joiner_to_epoch_internal #bytes #cb joiner_secret psk_secret group_context =
   let? prk = kdf_extract joiner_secret psk_secret in
-  expand_with_label #bytes prk "epoch" group_context (kdf_length #bytes)
+  expand_with_label #bytes prk "epoch" (serialize _ group_context) (kdf_length #bytes)
 
 val secret_joiner_to_epoch:
   #bytes:Type0 -> {|crypto_bytes bytes|} ->
-  bytes -> list (pre_shared_key_id_nt bytes & bytes) -> bytes ->
+  bytes -> list (pre_shared_key_id_nt bytes & bytes) -> group_context_nt bytes ->
   result (lbytes bytes (kdf_length #bytes))
 let secret_joiner_to_epoch #bytes #cb joiner_secret psks group_context =
   let? psk_secret = compute_psk_secret psks in