25-10-2018 | Remy van Elst | Text only version of this article
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.
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.
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
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!
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)
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
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.
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.
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.