From 70eb09dc099a13f60d9980a122ecb01730ce5c70 Mon Sep 17 00:00:00 2001
From: Guido Casati <hello@guidocasati.com>
Date: Mon, 26 May 2025 16:28:45 +0200
Subject: [PATCH] Add NH derivation function (A.10 3GPP TS 33.501)

---
 openair3/SECU/key_nas_deriver.c | 24 ++++++++++++++++++++++++
 openair3/SECU/key_nas_deriver.h |  2 ++
 2 files changed, 26 insertions(+)

diff --git a/openair3/SECU/key_nas_deriver.c b/openair3/SECU/key_nas_deriver.c
index 3c0b05dc50..a7cf27a1ff 100644
--- a/openair3/SECU/key_nas_deriver.c
+++ b/openair3/SECU/key_nas_deriver.c
@@ -293,3 +293,27 @@ void derive_skgNB(const uint8_t *keNB, const uint16_t sk_counter, uint8_t *skgNB
   byte_array_t data = {.buf = s, .len = 5};
   kdf(keNB, data, 32, skgNB);
 }
+
+/** @brief NH derivation function (A.10 3GPP TS 33.501)
+ *  @param k_amf: input key
+ *  @param sync_input: pointer to the newly derived KgNB for the initial NH derivation,
+ *                     and the previous NH for all subsequent derivations
+ *  @param nh: pointer to the derivated NH output */
+void nr_derive_nh(const uint8_t k_amf[SECURITY_KEY_LEN], const uint8_t *sync_input, uint8_t *nh)
+{
+  uint8_t s[SECURITY_KEY_LEN + 3] = {0};
+  // FC
+  s[0] = FC_NH;
+  // P0 = SYNC-input
+  memcpy(&s[1], sync_input, SECURITY_KEY_LEN);
+  // L0 = length of SYNC-input (i.e. 0x00 0x20)
+  s[SECURITY_KEY_LEN + 1] = 0x00;
+  s[SECURITY_KEY_LEN + 2] = 0x20;
+
+  byte_array_t data = {.buf = s, .len = sizeof(s)};
+  kdf(k_amf, data, SECURITY_KEY_LEN, nh);
+
+  log_hex_buffer("KDF input (SYNC string)", s, SECURITY_KEY_LEN + 3);
+  log_hex_buffer("Derived NH", nh, SECURITY_KEY_LEN);
+
+}
diff --git a/openair3/SECU/key_nas_deriver.h b/openair3/SECU/key_nas_deriver.h
index 4cb336bce9..0ab14ad51a 100644
--- a/openair3/SECU/key_nas_deriver.h
+++ b/openair3/SECU/key_nas_deriver.h
@@ -34,4 +34,6 @@ void nr_derive_key(algorithm_type_dist_t alg_type, uint8_t alg_id, const uint8_t
 
 void nr_derive_key_ng_ran_star(uint16_t pci, uint64_t nr_arfcn_dl, const uint8_t key[32], uint8_t *key_ng_ran_star);
 
+void nr_derive_nh(const uint8_t k_amf[SECURITY_KEY_LEN], const uint8_t *sync_input, uint8_t *nh);
+
 #endif