Add special 7-bit encoding and decoding functions for USSD coding
Handling 7-bit coding is a little different for USSD, as TS 03.38
states:
To avoid the situation where the receiving entity confuses 7 binary
zero pad bits as the @ character, the carriage return or <CR>
character shall be used for padding in this situation [...].
If <CR> is intended to be the last character and the message
(including the wanted <CR>) ends on an octet boundary, then another
<CR> must be added together with a padding bit 0. The receiving entity
will perform the carriage return function twice, but this will not
result in misoperation as the definition of <CR> [...] is identical to
the definition of <CR><CR>.
The receiving entity shall remove the final <CR> character where the
message ends on an octet boundary with <CR> as the last character.
Jacob has verified the fix with fakeBTS and the wireshark dissector.
Fixes: OW#947
Reviewed-by: Jacob Erlbeck <jerlbeck@sysmocom.de>
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index a572f50..6cd46e4 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -57,8 +57,10 @@
enum gsm_band gsm_band_parse(const char *mhz);
int gsm_7bit_decode(char *decoded, const uint8_t *user_data, uint8_t length);
+int gsm_7bit_decode_ussd(char *decoded, const uint8_t *user_data, uint8_t length);
int gsm_7bit_decode_hdr(char *decoded, const uint8_t *user_data, uint8_t length, uint8_t ud_hdr_ind);
int gsm_7bit_encode(uint8_t *result, const char *data);
+int gsm_7bit_encode_ussd(uint8_t *result, const char *data, int *octets_written);
int gsm_7bit_encode_oct(uint8_t *result, const char *data, int *octets_written);
/* the three functions below are helper functions and here for the unit test */