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File encryption using OpenSSL

Symmetic encryption

For symmetic encryption, you can use the following:

To encrypt:

openssl aes-256-cbc -salt -a -e -in plaintext.txt -out encrypted.txt

To decrypt:

openssl aes-256-cbc -salt -a -d -in encrypted.txt -out plaintext.txt

Asymmetric encryption

For Asymmetric encryption you must first generate your private key and extract the public key.

openssl genrsa -aes256 -out private.key 8912
openssl rsa -in private.key -pubout -out public.key

To encrypt:

openssl rsautl -encrypt -pubin -inkey public.key -in plaintext.txt -out encrypted.txt

To decrypt:

openssl rsautl -decrypt -inkey private.key -in encrypted.txt -out plaintext.txt

Encrypting files

You can't directly encrypt a large file using rsautl. Instead, do the following:

  • Generate a key using openssl rand, e.g. openssl rand 32 -out keyfile.
  • Encrypt the key file using openssl rsautl.
  • Encrypt the data using openssl enc, using the generated key from step 1.
  • Package the encrypted key file with the encrypted data. The recipient will need to decrypt the key with their private key, then decrypt the data with the resulting key.

Ultimate solution for safe and high secured encode anyone file in OpenSSL and command-line:

Private key generation (encrypted private key):

openssl genrsa -aes256 -out private.pem 8912
openssl rsa -in private.pem -pubout -out public.pem

With unecrypted private key:

openssl req -x509 -nodes -days 100000 -newkey rsa:8912 -keyout private_key.pem -out certificate.pem

With encrypted private key:

openssl req -x509 -days 100000 -newkey rsa:8912 -keyout private_key.pem -out certificate.pem

With existing encrypted (unecrypted) private key:

openssl req -x509 -new -days 100000 -key private_key.pem -out certificate.pem

Encrypt a file

Encrypt binary file:

openssl smime -encrypt -binary -aes-256-cbc -in -out -outform DER yourSslCertificate.pem

Encrypt text file:

openssl smime -encrypt -aes-256-cbc -in input.txt -out output.txt -outform DER yourSslCertificate.pem

What is what:

  • smime — ssl command for S/MIME utility (smime(1)).
  • -encrypt — chosen method for file process.
  • -binary — use safe file process. Normally the input message is converted to "canonical" format as required by the S/MIME specification, this switch disable it. It is necessary for all binary files (like a images, sounds, ZIP archives).
  • -aes-256-cbc — chosen cipher AES in 256 bit for encryption (strong). If not specified 40 bit RC2 is used (very weak). (Supported ciphers).
  • -in — input file name.
  • -out — output file name.
  • -outform DER — encode output file as binary. If is not specified, file is encoded by base64 and file size will be increased by 30%.
  • yourSslCertificate.pem — file name of your certificate's. That should be in PEM format.

That command can very effectively a strongly encrypt any file regardless of its size or format.

Decrypt a file

Decrypt binary file:

openssl smime -decrypt -binary -in -inform DER -out -inkey private.key -passin pass:your_password

For text files:

openssl smime -decrypt -in encrypted_input.txt -inform DER -out -inkey private.key -passin pass:your_password

What is what:

  • -inform DER — same as -outform above.
  • -inkey private.key — file name of your private key. That should be in PEM format and can be encrypted by password.
  • -passin pass:your_password — (optional) your password for private key encrypt.


Creating a signed digest of a file:

openssl dgst -sha512 -sign private_key.pem -out digest.sha512 file.txt

Verify a signed digest:

openssl dgst -sha512 -verify public_key.pem -signature digest.sha512 file.txt


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bmoser05 commented Sep 6, 2020

^-^ i tried it but it did not work

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FilBot3 commented Jan 25, 2021

Could you go into the Encrypting Files part a bit more? I'm not following what you mean by send your keys. I understand sending your public key for people to use for you, but since rsautl doesn't do large files well, can you explain that more?

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