searching Tags (TLV) with C function - c

Question

I' m working on Mastercard Paypass transactions, I Have sent a READ RECORD command and got the result:
70 81 AB 57 11 54 13 33 00 89 60 10 83 D2 51 22
20 01 23 40 91 72 5A 08 54 13 33 00 89 60 10 83
5F 24 03 25 12 31 5F 25 03 04 01 01 5F 28 02 00
56 5F 34 01 01 8C 21 9F 02 06 9F 03 06 9F 1A 02
95 05 5F 2A 02 9A 03 9C 01 9F 37 04 9F 35 01 9F
45 02 9F 4C 08 9F 34 03 8D 0C 91 0A 8A 02 95 05
9F 37 04 9F 4C 08 8E 0E 00 00 00 00 00 00 00 00
42 03 5E 03 1F 03 9F 07 02 FF 00 9F 08 02 00 02
9F 0D 05 00 00 00 00 00 9F 0E 05 00 08 00 60 00
9F 0F 05 00 00 00 00 00 9F 42 02 09 78 9F 4A 01
82 9F 14 01 00 9F 23 01 00 9F 13 02 00 00
This response contains TLV data objects (without spaces). I have converted the response as described in the following:
// Read Record 1 with SFI2
//---------------------------------SEND READ RECORD-------------------
inCtlsSendVAPDU(0x2C,0x03,(unsigned char *)"\x00\xB2\x01\x14\x00",5);
clrscr();
inRet = inCTLSRecv2(Response, 269);
LOG_HEX_PRINTF("Essai EMV4 Read record 1 EMV Paypass:",Response,inRet);
if(Response[14]==0x70)
{
sprintf(Response_PPSE,"%02X%02X",Response[12],Response[13]);//To retrieve length of received data
t1=hexToInt(Response_PPSE);// Convert length to integer
t11=t1-2;
i=14;
k=0;
//--------------------------- Converting data to be used later---------
while(i<t11+14)// 14 to escape the header+ command+ status+ length
{
sprintf(READ1+(2*k),"%02X",Response[i]);
i++;
k++;
}
Now I should check if this Response contains the Mandatory Tags:
5A - Application Primary Account Number (PAN)
5F24 - Application Expiration Date
8C - Card Risk Management Data Object List 1 (CDOL1)
8D - Card Risk Management Data Object List 2 (CDOL2)
So I tried the following to check for the 5A tag (Application Primary Account Number (PAN)):
i=0;
t11=2*t11;
while(i<=t11)
{
strncpy(Response_PPSE,READ1+i,2);
if(strncmp(Response_PPSE,"\x05\x0A")==0)
{
write_at("true",4,1,1);// Just to test on the terminal display
goto end;
}
else
i=i+2;
}
goto end;
I don't know why nothing is displayed on the terminal, The if block is not executed!
I tried to print the 5A tag manually by:
strncpy(Response_PPSE,READ1+44,2);
write_at(Response_PPSE,strlen(Response_PPSE),1,1);
And it display the right value!!
Can someone helps to resolve this issue?

Answer 1

You don't find that tag because you are not searching for the string "5A" but for the string "\x05\x0A" (ENQ character + line feed character). Moreover, I wonder if the above code actually compiles as you did not specify the mandatory length argument to strncmp(). You could try something like
if(strncmp(Response_PPSE,"5A", 2)==0)
instead.
However, you should understand that you are scanning the whole response data for the value 5A. Therefore, finding this value could also mean that it was part of some other TLV tag's data field, length field or even part of a multi-byte tag field. It would therefore make sense to implement (or use an existing) TLV parser for the BER (Basic Encoding Rules) format.

Answer 2

It's not a good approach to search a specific byte in a raw byte-stream data using strings functions at first place.
The generic TLV parser is a very easy algorithm and you will do it in 30 minutes or so.
In general a pseudo-code for TLV parser that look for a specific Tag would be something like this:
index = 0
while (byte[i] != 0x5A or EOF)
{
length = DecodeLength(byte[i+1])
i += length + 2 // + 1 for L (length) byte itself, it might be encoded with
// 2 bytes so the function DecodeLength can return the number
// of bytes lenght has been encoded
// +1 for T (tag) byte
}
if(EOF) return tag_not_found
return byte[i + 2], length // pointer to data for Tag '5A'and length of data