Keystone

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.

Passcode

The passcode is a 32 character string used on package generation to create the keystone file.

Structure

Size is always 96 bytes

From	To	Description
00	1F	MAGIC ("keystone") and some constant bytes
20	3F	SHA-256 (32 bytes) of the bytes of the passcode using keystone_passcode_secret as key
40	5F	SHA-256 (32 bytes) of the previous two sections using keystone_ks_secret as key

Sample keystone file generation

CSharp

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;

}

Sample keystone file

Sample keystone file created when provided a passcode consisting of all zeros "00000000000000000000000000000000":

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.]|