nimble/sm: Use proper key size for encryption

This patch adds proper masking of STK/LTK keys depending on encryption
key size as defined in Bluetooth Specification 4.2, Vol 3, Part H,
Section 2.3.4.

This also fixes following PTS test cases for SM:
- TP/EKS/BV-01-C
- TP/EKS/BV-02-C

X-Original-Commit: a034ac10ac6b02413e47f8d74652c99eae3763c3

nimble/host/src/ble_sm.c

@@ -314,11 +314,14 @@ ble_sm_gen_ltk(struct ble_sm_proc *proc, uint8_t *ltk)
     }
 #endif
 
-    rc = ble_hs_hci_util_rand(ltk, 16);
+    rc = ble_hs_hci_util_rand(ltk, proc->key_size);
     if (rc != 0) {
         return rc;
     }
 
+    /* Ensure proper key size */
+    memset(ltk + proc->key_size, 0, sizeof proc->ltk - proc->key_size);
+
     return 0;
 }
 
@@ -1377,6 +1380,9 @@ ble_sm_pair_cfg(struct ble_sm_proc *proc)
     if (rx_key_dist & BLE_SM_PAIR_KEY_DIST_SIGN) {
         proc->rx_key_flags |= BLE_SM_KE_F_SIGN_INFO;
     }
+
+    proc->key_size = min(proc->pair_req.max_enc_key_size,
+                         proc->pair_rsp.max_enc_key_size);
 }
 
 static void
@@ -1393,7 +1399,7 @@ ble_sm_pair_exec(struct ble_sm_proc *proc, struct ble_sm_result *res,
     cmd.io_cap = ble_hs_cfg.sm_io_cap;
     cmd.oob_data_flag = ble_hs_cfg.sm_oob_data_flag;
     cmd.authreq = ble_sm_build_authreq();
-    cmd.max_enc_key_size = 16;
+    cmd.max_enc_key_size = BLE_SM_PAIR_KEY_SZ_MAX;
 
     if (is_req) {
         cmd.init_key_dist = ble_hs_cfg.sm_our_key_dist;

nimble/host/src/ble_sm_lgcy.c

@@ -181,7 +181,10 @@ ble_sm_gen_stk(struct ble_sm_proc *proc)
         return rc;
     }
 
-    memcpy(proc->ltk, key, sizeof key);
+    memcpy(proc->ltk, key, proc->key_size);
+
+    /* Ensure proper key size */
+    memset(proc->ltk + proc->key_size, 0, sizeof key - proc->key_size);
 
     return 0;
 }

nimble/host/src/ble_sm_priv.h

@@ -257,6 +257,7 @@ struct ble_sm_proc {
     uint8_t pair_alg;
     uint8_t state;
     uint8_t rx_key_flags;
+    uint8_t key_size;
 
     struct ble_sm_pair_cmd pair_req;
     struct ble_sm_pair_cmd pair_rsp;

nimble/host/src/ble_sm_sc.c

@@ -440,6 +440,9 @@ ble_sm_sc_random_rx(struct ble_sm_proc *proc, struct ble_sm_result *res)
         return;
     }
 
+    /* Ensure proper key size */
+    memset(proc->ltk + proc->key_size, 0, sizeof proc->ltk - proc->key_size);
+
     /* Ensure the ltk gets persisted when the pairing procedure succeeds. */
     memcpy(proc->our_keys.ltk, proc->ltk, sizeof proc->our_keys.ltk);
     proc->our_keys.ltk_valid = 1;