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  • openssl/ossl_cipher.c


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Document-class: OpenSSL::Cipher

Provides symmetric algorithms for encryption and decryption. The
algorithms that are available depend on the particular version
of OpenSSL that is installed.

=== Listing all supported algorithms

A list of supported algorithms can be obtained by

  puts OpenSSL::Cipher.ciphers

=== Instantiating a Cipher

There are several ways to create a Cipher instance. Generally, a
Cipher algorithm is categorized by its name, the key length in bits
and the cipher mode to be used. The most generic way to create a
Cipher is the following

  cipher ='<name>-<key length>-<mode>')

That is, a string consisting of the hyphenated concatenation of the
individual components name, key length and mode. Either all uppercase
or all lowercase strings may be used, for example:

 cipher ='AES-128-CBC')

For each algorithm supported, there is a class defined under the
Cipher class that goes by the name of the cipher, e.g. to obtain an
instance of AES, you could also use

  # these are equivalent
  cipher =, :CBC)
  cipher =, 'CBC')
  cipher ='128-CBC')

Finally, due to its wide-spread use, there are also extra classes
defined for the different key sizes of AES

  cipher =
  cipher =
  cipher =

=== Choosing either encryption or decryption mode

Encryption and decryption are often very similar operations for
symmetric algorithms, this is reflected by not having to choose
different classes for either operation, both can be done using the
same class. Still, after obtaining a Cipher instance, we need to
tell the instance what it is that we intend to do with it, so we
need to call either




on the Cipher instance. This should be the first call after creating
the instance, otherwise configuration that has already been set could
get lost in the process.

=== Choosing a key

Symmetric encryption requires a key that is the same for the encrypting
and for the decrypting party and after initial key establishment should
be kept as private information. There are a lot of ways to create
insecure keys, the most notable is to simply take a password as the key
without processing the password further. A simple and secure way to
create a key for a particular Cipher is

 cipher =
 key = cipher.random_key # also sets the generated key on the Cipher

If you absolutely need to use passwords as encryption keys, you
should use Password-Based Key Derivation Function 2 (PBKDF2) by
generating the key with the help of the functionality provided by
OpenSSL::PKCS5.pbkdf2_hmac_sha1 or OpenSSL::PKCS5.pbkdf2_hmac.

Although there is Cipher#pkcs5_keyivgen, its use is deprecated and
it should only be used in legacy applications because it does not use
the newer PKCS#5 v2 algorithms.

=== Choosing an IV

The cipher modes CBC, CFB, OFB and CTR all need an "initialization
vector", or short, IV. ECB mode is the only mode that does not require
an IV, but there is almost no legitimate use case for this mode
because of the fact that it does not sufficiently hide plaintext
patterns. Therefore

<b>You should never use ECB mode unless you are absolutely sure that
you absolutely need it</b>

Because of this, you will end up with a mode that explicitly requires
an IV in any case. Note that for backwards compatibility reasons,
setting an IV is not explicitly mandated by the Cipher API. If not
set, OpenSSL itself defaults to an all-zeroes IV ("\0", not the
character). Although the IV can be seen as public information, i.e.
it may be transmitted in public once generated, it should still stay
unpredictable to prevent certain kinds of attacks. Therefore, ideally

<b>Always create a secure random IV for every encryption of your

A new, random IV should be created for every encryption of data. Think
of the IV as a nonce (number used once) - it's public but random and
unpredictable. A secure random IV can be created as follows

 cipher = ...
 key = cipher.random_key
 iv = cipher.random_iv # also sets the generated IV on the Cipher

 Although the key is generally a random value, too, it is a bad choice
 as an IV. There are elaborate ways how an attacker can take advantage
 of such an IV. As a general rule of thumb, exposing the key directly
 or indirectly should be avoided at all cost and exceptions only be
 made with good reason.

=== Calling Cipher#final

ECB (which should not be used) and CBC are both block-based modes.
This means that unlike for the other streaming-based modes, they
operate on fixed-size blocks of data, and therefore they require a
"finalization" step to produce or correctly decrypt the last block of
data by appropriately handling some form of padding. Therefore it is
essential to add the output of OpenSSL::Cipher#final to your
encryption/decryption buffer or you will end up with decryption errors
or truncated data.

Although this is not really necessary for streaming-mode ciphers, it is
still recommended to apply the same pattern of adding the output of
Cipher#final there as well - it also enables you to switch between
modes more easily in the future.

=== Encrypting and decrypting some data

  data = "Very, very confidential data"

  cipher =, :CBC)
  key = cipher.random_key
  iv = cipher.random_iv

  encrypted = cipher.update(data) +
  decipher =, :CBC)
  decipher.key = key
  decipher.iv = iv

  plain = decipher.update(encrypted) +

  puts data == plain #=> true

=== Authenticated Encryption and Associated Data (AEAD)

If the OpenSSL version used supports it, an Authenticated Encryption
mode (such as GCM or CCM) should always be preferred over any
unauthenticated mode. Currently, OpenSSL supports AE only in combination
with Associated Data (AEAD) where additional associated data is included
in the encryption process to compute a tag at the end of the encryption.
This tag will also be used in the decryption process and by verifying
its validity, the authenticity of a given ciphertext is established.

This is superior to unauthenticated modes in that it allows to detect
if somebody effectively changed the ciphertext after it had been
encrypted. This prevents malicious modifications of the ciphertext that
could otherwise be exploited to modify ciphertexts in ways beneficial to
potential attackers.

If no associated data is needed for encryption and later decryption,
the OpenSSL library still requires a value to be set - "" may be used in
case none is available. An example using the GCM (Galois Counter Mode):

  cipher =, :GCM)
  key = cipher.random_key
  iv = cipher.random_iv
  cipher.auth_data = ""

  encrypted = cipher.update(data) +
  tag = cipher.auth_tag

  decipher =, :GCM)
  decipher.key = key
  decipher.iv = iv
  decipher.auth_tag = tag
  decipher.auth_data = ""

  plain = decipher.update(encrypted) +

  puts data == plain #=> true