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My knownledge about security is limited so I need some help here.

The Question

I want to Decrypt a .P7M file using its private key (.pem). I would prefer to use OpenSSL for this.

Tried so far

OpenSSL smime -decrypt -in "Secret.p7m" -inkey "Key.pem" > Result.txt

Error

unable to load signing key file 7548:error:0906D06C:PEM routines:PEM_read_bio:no start line:.\crypto\pem\pem_lib .c:701:Expecting: ANY PRIVATE KEY error in smime**The P7M File**

Content-Type: application/pkcs7-mime; name="smime.p7m"; smime-type=enveloped-data
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7m"
Content-Description: S/MIME Encrypted Message

BLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLABLA

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2
  • "Cannot seem to get it to work". What command did you use, and what was the result? Any error messages? Commented Jun 4, 2015 at 8:07
  • 1
    Edited post with code sample i used Commented Jun 4, 2015 at 8:22

3 Answers 3

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2

Your decryption command is correct.

It seems there's something wrong with your key file. Does it really start with -----BEGIN RSA PRIVATE KEY----- and end with -----END RSA PRIVATE KEY----- (mind the exact number of dashes)? Or is it perhaps DER encoded which requires you to add -keyform DER your decryption command line?

BTW: You can check the integrity of the key itself with openssl rsa -in Key.pem

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  • Checking the integrity is fine. The output gives me -----BEGIN RSA PRIVATE KEY---- BLABLABALABALBALABALBA -----END RSA PRIVATE KEY----- Commented Jun 4, 2015 at 10:34
1

I have run into this same issue and Lupo's answer is on the right track. You can try converting the key using a command like:

openssl pkcs12 -in <key file.p7m> -out <key_file.pem> -nodes
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1

A *.p7m file is a PKCS#7/CAdES container. If it wraps a PDF, you can extract the embedded PDF without verifying the certificate chain:

Windows/macOS/Linux (OpenSSL): openssl smime -verify -noverify -inform DER -in "document.pdf.p7m" -out "document.pdf"

If the container is valid and the payload is indeed a PDF, the output will be a normal PDF.

Online solution: Disclosure: I’m affiliated with CoolUtils; here’s an online tool that unwraps *.p7m with an embedded PDF in one step: https://www.coolutils.com/online/P7M-to-PDF

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