diff --git a/include/osmocom/crypt/gprs_cipher.h b/include/osmocom/crypt/gprs_cipher.h
index 1656037..3e514ec 100644
--- a/include/osmocom/crypt/gprs_cipher.h
+++ b/include/osmocom/crypt/gprs_cipher.h
@@ -45,4 +45,10 @@
 /* Do we have an implementation for this cipher? */
 int gprs_cipher_supported(enum gprs_ciph_algo algo);
 
+/* GSM TS 04.64 / Section A.2.1 : Generation of 'input' */
+uint32_t gprs_cipher_gen_input_ui(uint32_t iov_ui, uint8_t sapi, uint32_t lfn, uint32_t oc);
+
+/* GSM TS 04.64 / Section A.2.1 : Generation of 'input' */
+uint32_t gprs_cipher_gen_input_i(uint32_t iov_i, uint32_t lfn, uint32_t oc);
+
 #endif /* _GPRS_CIPHER_H */
diff --git a/src/gprs_cipher_core.c b/src/gprs_cipher_core.c
index a72dda8..6174bd7 100644
--- a/src/gprs_cipher_core.c
+++ b/src/gprs_cipher_core.c
@@ -83,3 +83,17 @@
 
 	return 0;
 }
+
+/* GSM TS 04.64 / Section A.2.1 : Generation of 'input' */
+uint32_t gprs_cipher_gen_input_ui(uint32_t iov_ui, uint8_t sapi, uint32_t lfn, uint32_t oc)
+{
+	uint32_t sx = ((1<<27) * sapi) + (1<<31);
+
+	return (iov_ui ^ sx) + lfn + oc;
+}
+
+/* GSM TS 04.64 / Section A.2.1 : Generation of 'input' */
+uint32_t gprs_cipher_gen_input_i(uint32_t iov_i, uint32_t lfn, uint32_t oc)
+{
+	return iov_i + lfn + oc;
+}