Offset Size Description Example 0x0 0x20 MAGIC ("keystone") and some constant bytes 6b 65 79 73 74 6f 6e 65 02 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x20 0x20 HMAC-SHA256 (32 bytes) of the bytes of the passcode using keystone_passcode_secret as key - 0x40 0x20 HMAC-SHA256 (32 bytes) of the previous two sections using keystone_ks_secret as key -

Code: public static byte [] GenerateKeystoneFile (string passcode) { // 1. The first 32 bytes are constant byte[] keystone = { 0x6B, 0x65, 0x79, 0x73, 0x74, 0x6F, 0x6E, 0x65, 0x02, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 }; // 2. Convert the 32 characters of the passcode to a byte array byte[] passcodeInHEX = Encoding.ASCII.GetBytes(passcode); // 3. Calculate the fingerprint of the passcode HMACSHA256 hmac = new HMACSHA256(); hmac.Key = keystone_passcode_secret; byte[] fingerprint = hmac.ComputeHash(passcodeInHEX); // 4. Concat the 32 bytes from point 1 and the 32 bytes from point 3 keystone = keystone.Concat(fingerprint).ToArray(); // 5. Calculate the SHA256Hmac of the 64 bytes from point 4 hmac.Key = keystone_ks_secret; byte[] sha256hmac = hmac.ComputeHash(keystone); // 6. Concat the constant bytes from point 1, the fingerprint from point 3 and the hmac from point 5 keystone = keystone.Concat(sha256hmac).ToArray(); return keystone; }

Code: The first 32 are constant: 00000000 6b 65 79 73 74 6f 6e 65 02 00 01 00 00 00 00 00 |keystone........| 00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| Fingerprint of the passcode: 00000020 29 4a 5e d0 6d b1 70 61 8f 2e ed 8c 42 4b 9d 82 |)J^.m.pa....BK..| 00000030 88 79 c0 80 cc 66 fb c4 86 4f 69 e9 74 de b8 56 |.y...f...Oi.t..V| SHA256Hmac of the first 64 bytes of the file: 00000040 fa 0d 0c 2e bd 6a 00 80 63 71 3d e8 81 0d 7e 10 |.....j..cq=...~.| 00000050 b7 32 14 3b 91 cd 2e 4f ea 2d 20 53 10 6e b7 5d |.2.;...O.- S.n.]|

Since the PS4 EBOOT / ELF FW Patcher Barthen Method and recent Free PS4 SaveData Decrypting Tools in development, CarlosCorcoles (aka) let us know on Twitter that he's added some information about keystone files to the PS4 developer wiki today!To quote from the PSDevWiki page:This file is generated on app package generation based on the passcode provided. It is then included in every savegame created by the app. It is used to prevent apps from mounting savedata of other apps, as you need to know at least the fingerprint to do it.The passcode is a 32 character string used on package generation to create the keystone file.Size is always 96 (0x60) bytes for both PS4 and PSVita.Sample keystone file created when provided a passcode consisting of all zeros "00000000000000000000000000000000":Documented UserModules KeySet Changes on DevWiki for those interested!