BCAT Content Container

From Nintendo Switch Brew
Jump to: navigation, search

Content downloaded by BCAT normally uses this container.

Header

Offset Size Description
0x0 0x4 Magicnum "bcat"
0x4 0x1 ?
0x5 0x1 Crypto type: 0x1 = AES-128-CTR, 0x2 = AES-192-CTR, 0x3 = AES-256-CTR. Everything else indicates plaintext.
0x6 0x1 RSA/Hash Type: {0x0,0x2} = SHA1, {0x1,0x3} = SHA256. The former value in each pair is RSA PKCS1, while the later value in each pair is RSA PSS. Value >0x3 eventually results in an error being thrown.
0x7 0x1 Secret-data index
0x8 0x8 Normally zero?
0x10 0x10 Base IV/CTR
0x20 0x100 RSA-2048 signature

The header is 0x120-bytes.

Settings config for 1-byte <"bcat", "production_mode"> is loaded, the output is used to determine which RSA modulo to use.

Key generation

The AES keydata is the output from pkcs5_pbkdf2_hmac, with SHA256: password = <passphrase string loaded from file>, salt = <below salt string>, and iteration_count=4096.

The passphrase is loaded from "<basepath>/passphrase.bin" (this data originally comes from the title's control.nacp @ 0x3100). passphrase_len=strlen(passphrase). 0x40-bytes are read from the file, with 0x0 being written to passphrase[total_read_data]. salt_len = strlen(salt).

salt is the snprintf output from "%016llx%s", where the former is the titleID, while the latter is secretdata_str. secretdata_str = secretdata_array[<value of hdr+0x7>].

Settings config for 1-byte <"bcat", "production_mode"> is loaded, when retval!=1 or output_config!=0, then secretdata_array = secretdata_array0(retail), otherwise secretdata_array = secretdata_array1(devunit). The latter only contains empty strings, while the former contains lower-case hex strings. There are 16 entries in each array.