PSVIMG

PSVIMG files are encrypted files generated by CMA in backing up and restoring data from the Vita. The format is documented in this tool [1].

Generating PSVIMG

When CMA is used to backup system, game, or savedata from the PSVita to a PC or PS3, the following algorithm is used:

1. Using a tar-like structure, stream all of the file data into a file.
2. If making a PSVMD file, use the deflate algorithm to compress.
3. Generate a random nonce for the first 0x10 bytes using the RndNumber syscall.
4. Generate a unique session AES256-CBC key using a secret phrase and the PSN account id.
5. Generate a SHA256 hash of the plaintext every 0x8000 bytes and insert the hash into the filestream.
6. Encrypt the stream data using sceSblDmac5EncDecKeyGen with the nonce as the header and the AES256-CBC session key.
7. Transmit to PC or PS3.

Key Derivation

The AES256-CBC session key is calculated by:

• Creating a 33 byte buffer composed of 8 byte hex binary representation of your account id followed by a secret passphrase.
• Calculating a SHA-256 hash of buffer above.
• Decrypt the 32 byte output of the calculated SHA-256 hash with AES128-ECB (128 bit key stored in kprx_sm_auth).
• This decrypted output is the derived PSVIMG AES256-CBC key.

When you backup with CMA, there should be a directory in your PC that consists of 16 character hex directory name within your backup path.

That hex directory name is a representation of your account id. ex: \Documents\PS Vita\PSAVEDATA\0123456789abcdef\ '0123456789abcdef' being your account id.

#define CMA_ACCOUNT_ID_LEN 8
#define CMA_PASSPHRASE_LEN 25

static uint8_t passphrase[CMA_PASSPHRASE_LEN] = "Sri Jayewardenepura Kotte";

{
     ...
     memcpy(seed, aid, CMA_ACCOUNT_ID_LEN);
     memcpy(seed + CMA_ACCOUNT_ID_LEN, passphrase, CMA_PASSPHRASE_LEN);
     ...
     
     int seed_len = (CMA_ACCOUNT_ID_LEN + CMA_PASSPHRASE_LEN);
     sha256((uint8_t*)seed, seed_len, session_key, 0);
     aes_ecb_decrypt(session_key, CMA_SESSION_KEY_LEN);

     return session_key;
}

Example

This is an example where we are going to use account id: 0123456789abcdef

Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

00000000  01 23 45 67 89 AB CD EF 53 72 69 20 4A 61 79 65  .#Eg‰«ÍïSri Jaye
00000010  77 61 72 64 65 6E 65 70 75 72 61 20 4B 6F 74 74  wardenepura Kott
00000020  65                                               e                                            e

SHA-256 hash of this 32 byte buffer:

02EAAB5A00EC9D4207E8B1F53F8A2F3F91F1A73AAFDD2A81CCFEE3E83E5B101A

The decrypted hash produces the following result after decrypting with AES128-ECB:

Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F

00000000  AC A8 2B 25 34 69 8F 3B 3B EB 21 20 69 E8 37 D3  ¬¨+%4i.;;ë! iè7Ó
00000010  9E 85 34 69 73 78 89 8D 0E D7 30 7A D1 30 31 D4  ž…4isx‰..×0zÑ01Ô

This 32 byte (256 bits) sized buffer is the PSVIMG AES256-CBC key.