diff --git a/src/se050_wireguard.c b/src/se050_wireguard.c
index b5ec111..42cabb3 100644
--- a/src/se050_wireguard.c
+++ b/src/se050_wireguard.c
@@ -61,54 +61,46 @@ static bool constant_time_eq(const uint8_t *a, const uint8_t *b, size_t len)
     return result == 0;
 }
 
-/* HKDF expand for WireGuard - simplified for 64 bytes output */
-static void wg_hkdf_expand(const uint8_t *prk, size_t prk_len,
-                          uint8_t *out1, uint8_t *out2)
+/* HKDF for WireGuard - always uses 32-byte PRK
+ * T(1) = HMAC(PRK, 0x01)
+ * T(2) = HMAC(PRK, T(1) || 0x02)
+ * T(3) = HMAC(PRK, T(2) || 0x03)
+ */
+static void wg_hkdf_2(const uint8_t *prk,
+                     uint8_t *out1, uint8_t *out2)
 {
-    /* WireGuard uses a simplified HKDF:
-     * T(1) = HMAC(PRK, 0x01)
-     * T(2) = HMAC(PRK, T(1) || 0x02)
-     */
-    
     /* T(1) = HMAC(PRK, 0x01) */
     uint8_t c1 = 0x01;
-    se050_hmac_blake2s(out1, prk, prk_len, &c1, 1);
+    se050_hmac_blake2s(out1, prk, 32, &c1, 1);
     
     /* T(2) = HMAC(PRK, T(1) || 0x02) */
     uint8_t t2_input[33];
     memcpy(t2_input, out1, 32);
     t2_input[32] = 0x02;
-    se050_hmac_blake2s(out2, prk, prk_len, t2_input, 33);
+    se050_hmac_blake2s(out2, prk, 32, t2_input, 33);
     
     memzero_explicit(t2_input, 33);
 }
 
-/* Compute HKDF-1 (two outputs) - WireGuard style */
-static void wg_hkdf_1(const uint8_t *prk, size_t prk_len,
-                     uint8_t *out1, uint8_t *out2)
-{
-    wg_hkdf_expand(prk, prk_len, out1, out2);
-}
-
-/* Compute HKDF-3 (three outputs) - WireGuard style */
-static void wg_hkdf_3(const uint8_t *prk, size_t prk_len,
+/* HKDF-3 (three outputs) - WireGuard style */
+static void wg_hkdf_3(const uint8_t *prk,
                      uint8_t *out1, uint8_t *out2, uint8_t *out3)
 {
     /* T(1) = HMAC(PRK, 0x01) */
     uint8_t c1 = 0x01;
-    se050_hmac_blake2s(out1, prk, prk_len, &c1, 1);
+    se050_hmac_blake2s(out1, prk, 32, &c1, 1);
     
     /* T(2) = HMAC(PRK, T(1) || 0x02) */
     uint8_t t2_input[33];
     memcpy(t2_input, out1, 32);
     t2_input[32] = 0x02;
-    se050_hmac_blake2s(out2, prk, prk_len, t2_input, 33);
+    se050_hmac_blake2s(out2, prk, 32, t2_input, 33);
     
     /* T(3) = HMAC(PRK, T(2) || 0x03) */
     uint8_t t3_input[33];
     memcpy(t3_input, out2, 32);
     t3_input[32] = 0x03;
-    se050_hmac_blake2s(out3, prk, prk_len, t3_input, 33);
+    se050_hmac_blake2s(out3, prk, 32, t3_input, 33);
     
     memzero_explicit(t2_input, 33);
     memzero_explicit(t3_input, 33);
@@ -180,10 +172,9 @@ int se050_wireguard_derive_keys(se050_wireguard_session_t *session,
     }
     
     /* Derive sending and receiving keys using HKDF
-     * WireGuard uses: HKDF(shared_secret, "WireGuard v1 zx2c4 IPsec v1")
-     * But simplified to just use shared_secret as PRK
+     * WireGuard uses simplified HKDF with 32-byte PRK
      */
-    wg_hkdf_1(shared_secret, 32, session->sending_key, session->receiving_key);
+    wg_hkdf_2(shared_secret, session->sending_key, session->receiving_key);
     
     /* Reset nonces */
     session->sending_nonce = 0;
@@ -314,7 +305,6 @@ int se050_wireguard_decrypt_packet(se050_wireguard_session_t *session,
     memcpy(nonce_buf + 4, packet + 8, 8);
     
     size_t ciphertext_len = packet_len - 16 - 16;  /* Total - header - tag */
-    *plaintext_len = ciphertext_len;
     uint8_t tag[16];
     memcpy(tag, packet + 16 + ciphertext_len, 16);
     
@@ -337,7 +327,8 @@ int se050_wireguard_decrypt_packet(se050_wireguard_session_t *session,
         return -1;
     }
     
-    /* Update plaintext length and nonce */
+    /* Update plaintext length and nonce only on success */
+    *plaintext_len = ciphertext_len;
     session->receiving_nonce = nonce;
     session->packets_received++;