Encrypting Files with openssl

calendar Apr 15, 2009

Here is how to encrypt and decrypt files using public-key encryption and openssl.

I prefer this over GPG because openssl is installed everywhere (including OS X by default) and it requires no specialized subsystem (keyrings, etc.). Just two key files and the document to be encrypted.

Private Key Encryption

I use RSA encryption. There are three commands in openssl relevant to RSA:
  • genrsa - Generates an RSA private key.
  • rsa - Manages RSA private keys (includes generating the public key from it).
  • rsautl - Encrypt and decrypt files using RSA keys.
Because of the nature of the RSA algorithm, it is not possible to encrypt large files. In general, if you generate a key of n bits, then your file to encrypt may be no larger than (n − 11) bits. Thus, the most effective use of RSA encryption is to use it to encrypt a randomly generated password, and then encrypt the file with the password using symmetric key encryption.

Symmetric Key Encryption

Symmetric key encryption is an encryption algorithm that encrypts and decrypts a file using a single key or password.

In openssl, the enc command performs symmetric key encryption (among other things). To use it, you must choose (1) an algorithm, or “cipher,” and (2) a password.

The cipher I use is aes-256-cbc. The “aes” stands for Advanced Encryption Standard, which seems pretty well respected. The 256 is the key size, and the cbc stands for Cipher Block Chaining, which is less vulnerable to standard cryptanalysis than the other option, ebc.

The key is just a string of random bytes. I use a base64 encoded string of 30 bytes, which comes out to 41 characters. Since 41 characters is 328 bits, even a small RSA key will be able to encrypt it.

Procedures for Encryption and Decryption

The following steps encrypt a file:
  1. Choose a random key.
  2. Encrypt the random key using the RSA public key to produce the encrypted key.
  3. Use the (unencrypted) random key as a symmetric key encryption password, to encrypt the data, producing the encrypted data.
  4. Return the encrypted key and the encrypted data to the user.
Note that the encrypted data can only be read with the random key. The random key can only be retrieved from the encrypted key by using the RSA private key. Hence, the data can only be retrieved by one holding the RSA private key.

The steps for decryption are:

  1. Decrypt the encrypted key using the RSA private key, to retrieve the random key.
  2. Decrypt the encrypted data using the random key and symmetric key decryption.

Remembering the Commands

One of the most difficult issues is remembering the commands to do all the encryption and decryption. (The openssl commands have numerous options.) The easiest way I have thought of to do this is to write a script to do the encryption and output a combined script and data file that, when run, outputs the original data. This way, the output is something like a self-extracting executable archive: no special tools required (other than the private key, of course!) to extract the original data.

Here is the script.