What is a .aff file

Question

I am working on a project that requires me to load a firmware file (.aff) to an embedded device using a proprietary protocol. Google shows that aff stands for augmented file format but I am not sure what that means in this context.

Answer 1

Presuming the format is "augmented firmware file", if you google for is, you wil get this page. This file seems to be loading an aff file and uploading it to a controller.

You can see that the file starts with a css_header struct (128 bytes if I am not mistaken), then contains stuff like modulus, signature and other, meaning the actual firmware data starts somewhere at a rather distant offset (908 bytes from the file start):

struct css_header {
    u32 module_type;
    u32 header_len;
    u32 header_version;
    u32 module_id;
    u32 module_vendor;
    u32 date;               /* BCD yyyymmdd */
    u32 size;               /* in DWORDs */
    u32 key_size;           /* in DWORDs */
    u32 modulus_size;       /* in DWORDs */
    u32 exponent_size;      /* in DWORDs */
    u32 reserved[22];
};

#define KEY_SIZE      256
#define MU_SIZE         8
#define EXPONENT_SIZE   4

/* the file itself */
struct augmented_firmware_file {
    struct css_header css_header;
    u8 modulus[KEY_SIZE];
    u8 exponent[EXPONENT_SIZE];
    u8 signature[KEY_SIZE];
    u8 r2[KEY_SIZE];
    u8 mu[MU_SIZE];
    u8 firmware[];     // <--- 908 B from struct start
};

I would recommend opening the aff file using a hex editor (like HxD mentioned in the comment above) and then search for the bytes that you captured using the serial sniffer to see where they are in this file. If the data above is correct, you should find the actual firmware data offseted inside the file.

Answer 2

The key information that I was missing was that the file I was working with was in fact an SRecord. Once I understood the format of an SRecord I was able to parse out the correct data from the record and successfully load the firmware image.