PSVIMG: Difference between revisions
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PSVIMG files are encrypted files generated by [[CMA]] in backing up and restoring data from the Vita. The format is documented in | PSVIMG files are encrypted files generated by [[CMA]] in backing up and restoring data from the PS Vita. The format is documented in the program [https://github.com/yifanlu/psvimgtools|psvimgtools]. | ||
== Generating PSVIMG == | == Generating PSVIMG == | ||
When [[CMA]] is used to backup system, game, or savedata from the | When [[CMA]] is used to backup system, game, or savedata from the PS Vita to a PC or PS3, the following algorithm is used: | ||
# Using a tar-like structure, stream all of the file data into a file. | # Using a tar-like structure, stream all of the file data into a file. | ||
# If making a PSVMD file, use the deflate algorithm to compress. | # If making a PSVMD file, use the deflate algorithm to compress. | ||
# Generate a random nonce for the first 0x10 bytes using the RndNumber syscall. | # Generate a random nonce for the first 0x10 bytes using the RndNumber syscall. | ||
# Generate a [[# | # Generate a [[#PSVIMG_Key_Derivation|unique session AES256-CBC key]] using a secret phrase and the account id. | ||
# Generate a SHA256 hash of the plaintext every 0x8000 bytes and insert the hash into the filestream. | # Generate a SHA256 hash of the plaintext every 0x8000 bytes and insert the hash into the filestream. | ||
# Encrypt the stream data using [[SceSblSsMgr#sceSblDmac5EncDecKeyGen|sceSblDmac5EncDecKeyGen]] with the nonce as the header and the AES256-CBC session key. | # Encrypt the stream data using [[SceSblSsMgr#sceSblDmac5EncDecKeyGen|sceSblDmac5EncDecKeyGen]] with the nonce as the header and the AES256-CBC session key. This key is set through [[Kprx auth sm|kprx_auth_sm]] using [[Secure_Modules_Functions#0x50001_-_sceSblAuthMgrSetDmac5Key|service 0x50001]]. It is then used in conjunction with [[Secure_Modules_Functions#encdec_w_portability_sm.self|encdec_w_portability]] to decrypt/encrypt PSVIMG blocks. | ||
# Transmit to PC or PS3. | # Transmit to PC or PS3. | ||
=== PSVIMG | === PSVIMG Key Derivation === | ||
The AES256-CBC session key is calculated by: | |||
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 ==== | |||
<source lang="C"> | |||
#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; | |||
} | |||
</source> | </source> | ||
==== Example ==== | |||
This is an example where we are going to use account id: 0123456789abcdef. | |||
<source> | <source> | ||
Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F | Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F | ||
00000000 | 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 | 00000010 77 61 72 64 65 6E 65 70 75 72 61 20 4B 6F 74 74 wardenepura Kott | ||
00000020 65 e | 00000020 65 e | ||
</source> | </source> | ||
SHA-256: | SHA-256 hash of this 32 byte buffer: | ||
<source> | <source> | ||
02EAAB5A00EC9D4207E8B1F53F8A2F3F91F1A73AAFDD2A81CCFEE3E83E5B101A | |||
</source> | </source> | ||
The | The decrypted hash produces the following result after decrypting with AES128-ECB: | ||
<source> | <source> | ||
Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F | Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F | ||
00000000 | 00000000 AC A8 2B 25 34 69 8F 3B 3B EB 21 20 69 E8 37 D3 ¬¨+%4i.;;ë! iè7Ó | ||
00000010 | 00000010 9E 85 34 69 73 78 89 8D 0E D7 30 7A D1 30 31 D4 ž…4isx‰..×0zÑ01Ô | ||
</source> | </source> | ||
This 32 byte (256 bits) sized buffer is the PSVIMG AES256-CBC key. | |||
[[Category:Formats]] | [[Category:Formats]] |
Latest revision as of 08:13, 17 July 2023
PSVIMG files are encrypted files generated by CMA in backing up and restoring data from the PS Vita. The format is documented in the program [1].
Generating PSVIMG
When CMA is used to backup system, game, or savedata from the PS Vita to a PC or PS3, the following algorithm is used:
- Using a tar-like structure, stream all of the file data into a file.
- If making a PSVMD file, use the deflate algorithm to compress.
- Generate a random nonce for the first 0x10 bytes using the RndNumber syscall.
- Generate a unique session AES256-CBC key using a secret phrase and the account id.
- Generate a SHA256 hash of the plaintext every 0x8000 bytes and insert the hash into the filestream.
- 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.
- 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.