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SecureFilesystemV1_92r1.md

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rhme-2016 write-up Secure Filesystem v192.r1

Secure Filesystem v192.r1 (Crypto - 150 pts)

This is next version of Secure Filesystem with longer token 2x 192 bits. Some examples are provided:

897703036b2e18116b36353d92ac3dd978845fc99a735b8a |
dfd0f4a25b7d529e89ac030c2b681e93831e95a8186823b9 | cat.txt
897703036b2e18116b36353d92ac3dd978845fc99a735b8a |
f2bca35d472116dc6d5bebe96f1a3af249be78c63219a0dc | cat.txt:finances.csv
897703036b2e18116b36353d92ac3dd978845fc99a735b8a |
7eed666977d3861dbaefd16b2ed7dc5b639e51853ca6e7b3 | cat.txt:finances.csv:joke.txt
897703036b2e18116b36353d92ac3dd978845fc99a735b8a |
51d915246394ce976f8768cf3300087cb5b9958bbec30f9c | cat.txt:joke.txt
897703036b2e18116b36353d92ac3dd978845fc99a735b8a |
ae2a5a38b4d03f0103bce59874e41a0df19cb39b328b02fa | finances.csv
897703036b2e18116b36353d92ac3dd978845fc99a735b8a |
c66b5e48f5e600982724eca3804fb59b7b0f395a6e17e1ce | finances.csv:joke.txt
897703036b2e18116b36353d92ac3dd978845fc99a735b8a |
3a3a9b3cc5239fdf4572157296903a0237a4aaeeaa8f3d15 | joke.txt

We directly noticed the first 192 bits which is static, looks so weird to be innocent. Another point is the long processing time which feels like doing overkilling execution on a poor ATMega328p. The final hint is the title wich is v192.r1. Gathering all those hints, we found out that it might be a ECDSA over a 192 bits field. The static part is depending on a random used during ECDSA signature generation and PS3 hacks makes it famous. We are now confident!

Openssl gives us:

  secp192k1 : SECG curve over a 192 bit prime field
  prime192v1: NIST/X9.62/SECG curve over a 192 bit prime field
  prime192v2: X9.62 curve over a 192 bit prime field
  prime192v3: X9.62 curve over a 192 bit prime field
  brainpoolP192r1: RFC 5639 curve over a 192 bit prime field
  brainpoolP192t1: RFC 5639 curve over a 192 bit prime field

We have two candidates: brainpoolP192r1 and prime192v1 which has another name NIST P-192r1.

Let's try these curves in order to find the private scalar which then can be used to generate ECDSA for passwd file.