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633 bytes added ,  11:49, 5 February 2019
Undo revision 10673 by CelesteBlue (talk)
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# 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 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.
 
# 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.
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# 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.
 
# Transmit to PC or PS3.
   −
=== Secrets ===
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=== 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.
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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:
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* 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>
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* 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 ====
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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
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#define CMA_PASSPHRASE_LEN 25
 +
 
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static uint8_t passphrase[CMA_PASSPHRASE_LEN] = "Sri Jayewardenepura Kotte";
   −
Buffer of an example of PSN Account Id:
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{
<source>
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    ...
01 23 45 67 89 AB CD EF 53 72 69 20 4A 61 79 65 77
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    memcpy(seed, aid, NP_ACCOUNT_ID_LEN);
61 72 64 65 6E 65 70 75 72 61 20 4B 6F 74 74 65
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    memcpy(seed + NP_ACCOUNT_ID_LEN, passphrase, CMA_PASSPHRASE_LEN);
</source>
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    ...
 +
   
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    int seed_len = (NP_ACCOUNT_ID_LEN + CMA_PASSPHRASE_LEN);
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    sha256((uint8_t*)seed, seed_len, session_key, 0);
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    aes_ecb_decrypt(session_key, CMA_SESSION_KEY_LEN);
   −
SHA256 of this buffer consists in the AES256-CBC session key:
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    return session_key;
<source>
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}
02EAAB5A00EC9D4207E8B1F53F8A2F3F91F1A73AAFDD2A81CCFEE3E83E5B101A
   
</source>
 
</source>
   −
To generate the PSVIMG AES 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 ).
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==== 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.
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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  00 00 00 00 00 00 00 00 53 72 69 20 4A 61 79 65  ........Sri Jaye
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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:
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SHA-256 hash of this 32 byte buffer:
 
<source>
 
<source>
186F29050C0D0D99038D86EFA9B6AD332E59564B7FFCA97985C09D64BD4BC442
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02EAAB5A00EC9D4207E8B1F53F8A2F3F91F1A73AAFDD2A81CCFEE3E83E5B101A
 
</source>
 
</source>
   −
The encrypted above hash produces the following result:
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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  65 DE EF DE 3A 14 10 B5 6F 60 D0 AB B9 D9 9D FA eÞïÞ:..µo`Ы¹Ù.ú
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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>
 
</source>
 +
 +
This 32 byte (256 bits) sized buffer is the PSVIMG AES256-CBC key.
       
[[Category:Formats]]
 
[[Category:Formats]]
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