What is the internal format of .Xauthority file?

Well, the subject.

I have searched a lot, but unfortunately, found nothing. Is there some document describing this format? Or the structure need to be extracted out from the xauth source files?

Answer

Probably not exactly what you are looking for but putting in an answer just for the formatting.

The .Xauthority is an array of structures:

typedef struct xauth {
    unsigned short   family;
    unsigned short   address_length;
    char            *address;
    unsigned short   number_length;
    char            *number;
    unsigned short   name_length;
    char            *name;
    unsigned short   data_length;
    char        *data;
} Xauth;

You would probably still need to be able to decode each entry — if nothing else by slogging through the source:Xauth.h

For example:

$ od -xc --endian=big .Xauthority | more
0000000    0100    0007    6d61    7869    6d75    7300    0130    0012
        001      a   m   a   x   i   m   u   s   001   0   022
0000020    4d49    542d    4d41    4749    432d    434f    4f4b    4945
          M   I   T   -   M   A   G   I   C   -   C   O   O   K   I   E
0000040    2d31    0010    c0ac    9e9c    ee82    ef59    f406    b7f9
          -   1   020 300 254 236 234 356 202 357   Y 364 006 267 371
0000060    b745    254e    0100    0007    6d61    7869    6d75    7300
        267   E   %   N 001      a   m   a   x   i   m   u   s  

The first short is 0x100 indicating the family

The next short is 0x0007 indicating the length of the address

The next 7 bytes are the address: maximus

The next short is 0001, the length of the seat number

The next byte is 30, ascii 0, the seat number

The next short is 0x0012, decimal 18, the length of the name

The next 18 bytes are the name: MIT-MAGIC-COOKIE-1

The next short is 0x0010, decimal 16, the length of the data

And the next 16 bytes are the data: 0xc0ac thru 0x254e.

Then it starts over.