This is a text-only version of the following page on https://raymii.org:

This is a text-only version of the following page on https://raymii.org:
---
Title : Encrypt and decrypt files to public keys via the OpenSSL Command Line
Author : Remy van Elst
Date : 25-10-2018
URL : https://raymii.org/s/tutorials/Encrypt_and_decrypt_files_to_public_keys_via_the_OpenSSL_Command_Line.html
Format : Markdown/HTML
---

This small tutorial will show you how to use the openssl command line to encrypt
and decrypt a file using a public key. We will first generate a random key,
encrypt that random key against the public key of the other person and use that
random key to encrypt the actual file with using symmetric encryption.

 Recently I removed all Google Ads from this site due to their invasive tracking, as well as Google Analytics. Please, if you found this content useful, consider a small donation using any of the options below: <a href="https://leafnode.nl">I'm developing an open source monitoring app called Leaf Node Monitoring, for windows, linux & android. Go check it out!</a> <a href="https://github.com/sponsors/RaymiiOrg/">Consider sponsoring me on Github. It means the world to me if you show your appreciation and you'll help pay the server costs.</a> <a href="https://www.digitalocean.com/?refcode=7435ae6b8212">You can also sponsor me by getting a Digital Ocean VPS. With this referral link you'll get $100 credit for 60 days. </a> 

Because of how the RSA algorithm works it is not possible to encrypt large
files. If you create a key of `n` bits, then the file you want to encrypt must
not larger than (`n` minus 11) bits. The most effective use of RSA crypto is to
encrypt a random generated password, then encrypt the file with the password
using symmetric crypto. If the file is larger then the key size the encryption
command will fail:

RSA operation error: 020:error:0406D06E:rsa routines:RSA_padding_add_PKCS1_type_2:data too large for key size:.\crypto\rsa\rsa_pk1.c:151:

We generate a random file and use that as the key to encrypt the large file with
symmetric crypto. That random file acts as the password so to say. We encrypt
the large file with the small password file as password. Then we send the
encrypted file and the encrypted key to the other party and then can decrypt the
key with their public key, the use that key to decrypt the large file.

The following commands are relevant when you work with RSA keys:

* `openssl genrsa`: Generates an RSA private keys.
* `openssl rsa`: Manage RSA private keys (includes generating a public key from it).
* `openssl rsautl`: Encrypt and decrypt files with RSA keys.

The key is just a string of random bytes. We use a base64 encoded string of 128
bytes, which is 175 characters. Since 175 characters is 1400 bits, even a small
RSA key will be able to encrypt it.

### Get the public key

Let the other party send you a certificate or their public key. If they send to
a certificate you can extract the public key using this command:

openssl rsa -in certificate.pem -out publickey.pem -outform PEM -pubout

### Generate the random password file

Use the following command to generate the random key:

openssl rand -hex 64 -out key.bin

Do this every time you encrypt a file. Use a new key every time!

**Update 25-10-2018**

The key format is HEX because the base64 format adds newlines. The `-pass`
argument later on only takes the first line of the file, so the full key is not
used. (Thanks Ken Larson for pointing this to me)

### Encrypt the file with the random key

Use the following command to encrypt the large file with the random key:

openssl enc -aes-256-cbc -salt -in largefile.pdf -out largefile.pdf.enc -pass file:./bin.key

The file size doesn't grows that much:

$ ls -larth
-rw-r--r-- 1 user group 40M Nov 9 21:14 Linux-Voice-Issue-020.pdf
-rw-r--r-- 1 user group 40M Nov 9 22:03 Linux-Voice-Issue-020.pdf.enc

It's encrypted however:

$ file Linux-Voice-Issue-020.pdf
Linux-Voice-Issue-020.pdf: PDF document, version 1.4

$ file Linux-Voice-Issue-020.pdf.enc
Linux-Voice-Issue-020.pdf.enc: data

### Encrypt the random key with the public keyfile

Use the following command to encrypt the random keyfile with the other persons
public key:

openssl rsautl -encrypt -inkey publickey.pem -pubin -in key.bin -out key.bin.enc

You can safely send the `key.bin.enc` and the `largefile.pdf.enc` to the other
party.

You might want to [sign the two files with your public key as well][2].

### Decrypt the random key with our private key file

If you want to decrypt a file encrypted with this setup, use the following
command with your privte key (beloning to the pubkey the random key was crypted
to) to decrypt the random key:

openssl rsautl -decrypt -inkey privatekey.pem -in key.bin.enc -out key.bin

This will result in the decrypted random key we encrypted the file in.

### Decrypt the large file with the random key

Once you have the random key, you can decrypt the encrypted file with the
decrypted key:

openssl enc -d -aes-256-cbc -in largefile.pdf.enc -out largefile.pdf -pass file:./bin.key

This will result in the decrypted large file.

[1]: https://www.digitalocean.com/?refcode=7435ae6b8212
[2]: https://raymii.org/s/tutorials/Sign_and_verify_text_files_to_public_keys_via_the_OpenSSL_Command_Line.html

---

License:
All the text on this website is free as in freedom unless stated otherwise.
This means you can use it in any way you want, you can copy it, change it
the way you like and republish it, as long as you release the (modified)
content under the same license to give others the same freedoms you've got
and place my name and a link to this site with the article as source.

This site uses Google Analytics for statistics and Google Adwords for
advertisements. You are tracked and Google knows everything about you.
Use an adblocker like ublock-origin if you don't want it.

All the code on this website is licensed under the GNU GPL v3 license
unless already licensed under a license which does not allows this form
of licensing or if another license is stated on that page / in that software:

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.

Just to be clear, the information on this website is for meant for educational
purposes and you use it at your own risk. I do not take responsibility if you
screw something up. Use common sense, do not 'rm -rf /' as root for example.
If you have any questions then do not hesitate to contact me.

See https://raymii.org/s/static/About.html for details.