Difference between revisions of "PSVIMG"

Revision as of 11:49, 5 February 2019

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:

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.

@@ Line 8: / Line 8: @@
 # 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 [[#PSVIMG_AES256-CBC_key generation|unique session AES256-CBC key]] using a secret phrase and the PSN account id.
+# 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.
-# 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 using [[F00D_Commands#0x50001|service 0x50001]]. It is then used in conjunction with encdec_w_portability to decrypt/encrypt PSVIMG blocks.
 # Transmit to PC or PS3.
-=== PSVIMG AES256-CBC key generation ===
+=== PSVIMG Key Derivation ===
-If you look on your PC at the 16 character hex directory name included in part of the backup path, that is your PSN Account Id.
+The AES256-CBC session key is calculated by:
-The AES256-CBC session key is calculated by doing a SHA256 hash of the 8 byte hex binary representation of the PSN Account Id followed by the secret phrase:
+* Creating a 33 byte buffer composed of 8 byte hex binary representation of your account id followed by the 0x19 byte PSVIMG secret passphrase.
-<code>Sri Jayewardenepura Kotte</code> then decrypting the buffer with AES128-ECB.
+* 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.
-==== Example ====
+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";
-Buffer of an example of PSN Account Id followed by secret phrase:
+{
-<source>
+     ...
-23 45 67 89 AB CD EF 53 72 69 20 4A 61 79 65 77
+     memcpy(seed, aid, NP_ACCOUNT_ID_LEN);
-72 64 65 6E 65 70 75 72 61 20 4B 6F 74 74 65
+     memcpy(seed + NP_ACCOUNT_ID_LEN, passphrase, CMA_PASSPHRASE_LEN);
-</source>
+     ...
+     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);
-SHA256 of this buffer:
+     return session_key;
-<source>
+}
-EAAB5A00EC9D4207E8B1F53F8A2F3F91F1A73AAFDD2A81CCFEE3E83E5B101A
 </source>
-To generate the PSVIMG AES256-CBC key, the buffer is decrypted using AES128-ECB with a 128 bits key located at offset 0x82DC in the 3.60 kprx_auth_sm (Prototype units use another key located at offset 0x7294 in 1.03 kprx_auth_sm ).
+==== Example ====
-This key is set though kprx_auth_sm using [[F00D_Commands#0x50001|service 0x50001]]).
-It is then used in conjunction with encdec_w_portability to decrypt/encrypt PSVIMG blocks.
+This is an example where we are going to use account id: 0123456789abcdef.
 <source>
 Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
-00000000  00 00 00 00 00 00 00 00 53 72 69 20 4A 61 79 65  ........Sri Jaye
+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
 </source>
-SHA-256:
+SHA-256 hash of this 32 byte buffer:
 <source>
-F29050C0D0D99038D86EFA9B6AD332E59564B7FFCA97985C09D64BD4BC442
+EAAB5A00EC9D4207E8B1F53F8A2F3F91F1A73AAFDD2A81CCFEE3E83E5B101A
 </source>
-The encrypted above hash produces the following result:
+The decrypted hash produces the following result after decrypting with AES128-ECB:
 <source>
 Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
-00000000  65 DE EF DE 3A 14 10 B5 6F 60 D0 AB B9 D9 9D FA  eÞïÞ:..µo`Ð«¹Ù.ú
+00000000  AC A8 2B 25 34 69 8F 3B 3B EB 21 20 69 E8 37 D3  ¬¨+%4i.;;ë! iè7Ó
-00000010  97 45 BF 55 C9 22 E3 44 A1 13 2E EB 67 60 9C 0D  —E¿UÉ"ãD¡..ëg`œ.
+00000010  9E 85 34 69 73 78 89 8D 0E D7 30 7A D1 30 31 D4  ž…4isx‰..×0zÑ01Ô
 </source>
+This 32 byte (256 bits) sized buffer is the PSVIMG AES256-CBC key.
 [[Category:Formats]]

Difference between revisions of "PSVIMG"

Revision as of 11:49, 5 February 2019

Contents

Generating PSVIMG

PSVIMG Key Derivation

Pseudocode

Example

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