add osmo_auth_c3() (separate from gsm_milenage())
To send a Ciphering Mode Command, we may need to derive a Kc from UMTS AKA
tokens. gsm_milenage() derives Kc from 3G tokens, but also derives an SRES.
For SRES, it requires an OPC, which may need to be derived from OP first. All
we need is a Kc, so we could feed a zero OPC ... but to simplify the function
call for cases where just a Kc is required, separate the c3 function out from
gsm_milenage(), as osmo_auth_c3(). Obviously call osmo_auth_c3() from
gsm_milenage() (meaning that osmo-hlr's 55.205 derived auc tests still cover
exactly that implementation).
Prepares: If04e405426c55a81341747a9b450a69188525d5c (osmo-msc)
Related: OS#2745
Change-Id: I85a1d6ae95ad9e5ce9524ef7fc06414848afc2aa
diff --git a/src/gsm/auth_core.c b/src/gsm/auth_core.c
index 400708f..f171ed4 100644
--- a/src/gsm/auth_core.c
+++ b/src/gsm/auth_core.c
@@ -236,4 +236,17 @@
{ 0, NULL }
};
+/* Derive GSM AKA ciphering key Kc from UMTS AKA CK and IK (auth function c3 from 3GPP TS 33.103 §
+ * 4.6.1).
+ * \param[out] kc GSM AKA Kc, 8 byte target buffer.
+ * \param[in] ck UMTS AKA CK, 16 byte input buffer.
+ * \param[in] ik UMTS AKA IK, 16 byte input buffer.
+ */
+void osmo_auth_c3(uint8_t kc[], const uint8_t ck[], const uint8_t ik[])
+{
+ int i;
+ for (i = 0; i < 8; i++)
+ kc[i] = ck[i] ^ ck[i + 8] ^ ik[i] ^ ik[i + 8];
+}
+
/*! @} */