PSVIMG

From Vita Development Wiki

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].

Contents

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 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. This key is set through kprx_auth_sm using service 0x50001. It is then used in conjunction with encdec_w_portability to decrypt/encrypt PSVIMG blocks.
  7. Transmit to PC or PS3.

PSVIMG 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 the 0x19 byte PSVIMG secret passphrase.
  • Calculating a SHA-256 hash of buffer above.
  • Decrypting the 32 byte output of the calculated SHA-256 hash with AES128-ECB (128 bit key stored in kprx_auth_sm).
  • This decrypted output is the derived PSVIMG AES256-CBC key.

When you backup with CMA, the files are stored in 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\ with '0123456789abcdef' being your account id.

Pseudocode

#define NP_ACCOUNT_ID_LEN 8
#define CMA_PASSPHRASE_LEN 25

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

{
     ...
     memcpy(seed, aid, NP_ACCOUNT_ID_LEN);
     memcpy(seed + NP_ACCOUNT_ID_LEN, passphrase, CMA_PASSPHRASE_LEN);
     ...
     
     int seed_len = (NP_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

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.