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I create two files, my.cer and my.pem ( include certificate and private key), I encrypt a file with my.cer, I use openssl smime -encrypt command. When decrypting data, it needs my.pem.

I think 3DES is symmetric cryptography so it should just need my.cer. Please explain.

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Yes, 3DES is a symmetric cipher and you use a secret key for both encryption and decryption with it. However, that is not the same thing as the keys used in RSA or other public key cryptography – you cannot (and should not) use either of them for 3DES.

Since you are using S/MIME, you are actually using public key encryption. Any reference to 3DES is about the symmetric part of a hybric cryptosystem. That is, you use the public key encrypt/generate a secret key that encrypts the actual message. So you need the private key to decrypt it.

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  • $\begingroup$ they did that because its harder to encrypt larger files with asymmetric wasn't it? $\endgroup$ – blacklight Jun 17 '16 at 7:01
  • $\begingroup$ @blacklight, yes, that is basically the reason for hybrid cryptosystems in the case of RSA. $\endgroup$ – otus Jun 17 '16 at 8:49

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