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My information security teacher gave me homework - hack smime.p7m. He said that the key is simple and bruteforce will not take much time, but I can not find a utility to do this. Maybe you know these?

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    Do you need a utility? It sounds like the key is something obvious that you could guess. – schroeder Feb 8 '18 at 13:35
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I would take a look at this tool (I haven't tried personally though):

https://github.com/SaschaWessel/rc2-40-cbc

rc2-40-cbc
A brute force tool for 40 Bit RC2 encrypted email messages.

In S/MIME messages [1] the plaintext is encrypted with a symmetric key (e.g. 3DES, RC2) and this symmetric key is encrypted with an asymmetric public key (e.g. RSA). To decrypt the message, only the symmetric key is required and this key can be brute forced for small key sizes. This tool implements this for 40 Bit RC2 keys.

[1] S/MIME Version 2 Message Specification: http://tools.ietf.org/html/rfc2311

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For S/MIME you can do better: https://www.schneier.com/academic/smime/download.html Note that the tricks used don't work for all protocols. For example SSL does a MD5 to a 128-bit key that makes this not work.

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