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Diffstat (limited to 'src/osmo-bsc/osmo_bsc_bssap.c')
-rw-r--r--src/osmo-bsc/osmo_bsc_bssap.c32
1 files changed, 22 insertions, 10 deletions
diff --git a/src/osmo-bsc/osmo_bsc_bssap.c b/src/osmo-bsc/osmo_bsc_bssap.c
index 83c884c44..b14968a50 100644
--- a/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/src/osmo-bsc/osmo_bsc_bssap.c
@@ -567,6 +567,19 @@ static int bssmap_handle_paging(struct bsc_msc_data *msc,
return 0;
}
+/* select the best cipher permitted by the intersection of both masks */
+static int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask)
+{
+ uint8_t intersection = msc_mask & bsc_mask;
+ int i;
+
+ for (i = 7; i >= 0; i--) {
+ if (intersection & (1 << i))
+ return i;
+ }
+ return -1;
+}
+
/*
* GSM 08.08 ยง 3.1.9.1 and 3.2.1.21...
* release our gsm_subscriber_connection and send message
@@ -626,8 +639,8 @@ static int bssmap_handle_cipher_mode(struct gsm_subscriber_connection *conn,
int include_imeisv = 1;
const uint8_t *enc_key;
uint16_t enc_key_len;
- uint8_t enc_bits_bsc;
uint8_t enc_bits_msc;
+ int chosen_cipher;
if (!conn) {
LOGP(DMSC, LOGL_ERROR, "No lchan/msc_data in cipher mode command.\n");
@@ -668,27 +681,26 @@ static int bssmap_handle_cipher_mode(struct gsm_subscriber_connection *conn,
if (TLVP_PRESENT(&tp, GSM0808_IE_CIPHER_RESPONSE_MODE))
include_imeisv = TLVP_VAL(&tp, GSM0808_IE_CIPHER_RESPONSE_MODE)[0] & 0x1;
- /* FIXME: match up the list of permitted ciphering algorithms received from the MSC with a list
- * of ciphering algorithms configured for this BSC (the config of more than one is TODO). Finally
- * pick one of the remaining options. */
-
/* Identical to the GSM0808_IE_ENCRYPTION_INFORMATION above:
* a5_encryption == 0 --> 0x01
* a5_encryption == 1 --> 0x02
* a5_encryption == 2 --> 0x04 ... */
- enc_bits_bsc = 1 << network->a5_encryption;
enc_bits_msc = data[0];
- if (!(enc_bits_msc & enc_bits_bsc)) {
- LOGP(DMSC, LOGL_ERROR, "MSC does not permit A5/%d (permitted algorithms mask: 0x%x)\n",
- network->a5_encryption, enc_bits_msc);
+ /* The bit-mask of permitted ciphers from the MSC (sent in ASSIGNMENT COMMAND) is intersected
+ * with the vty-configured mask a the BSC. Finally, the best (highest) possible cipher is
+ * chosen. */
+ chosen_cipher = select_best_cipher(enc_bits_msc, network->a5_encryption_mask);
+ if (chosen_cipher < 0) {
+ LOGP(DMSC, LOGL_ERROR, "Reject: no overlapping A5 ciphers between BSC (0x%02x) "
+ "and MSC (0x%02x)\n", network->a5_encryption_mask, enc_bits_msc);
reject_cause = GSM0808_CAUSE_CIPHERING_ALGORITHM_NOT_SUPPORTED;
goto reject;
}
/* To complete the confusion, gsm0808_cipher_mode again expects the encryption as a number
* from 0 to 7. */
- if (gsm0808_cipher_mode(conn, network->a5_encryption, enc_key, enc_key_len,
+ if (gsm0808_cipher_mode(conn, chosen_cipher, enc_key, enc_key_len,
include_imeisv)) {
reject_cause = GSM0808_CAUSE_PROTOCOL_ERROR_BETWEEN_BSS_AND_MSC;
goto reject;