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 = OpenSSL::Cipher.new('<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 = OpenSSL::Cipher.new('aes-128-cbc')

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

cipher.encrypt

or

cipher.decrypt

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 = OpenSSL::Cipher.new('aes-256-cfb')
cipher.encrypt
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

You should never use ECB mode unless you are absolutely sure that you absolutely need it

Because of this, you will end up with a mode that explicitly requires an IV in any case. 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

Always create a secure random IV for every encryption of your Cipher

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 = ...
cipher.encrypt
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 = OpenSSL::Cipher.new('aes-128-cbc')
cipher.encrypt
key = cipher.random_key
iv = cipher.random_iv

encrypted = cipher.update(data) + cipher.final
...
decipher = OpenSSL::Cipher.new('aes-128-cbc')
decipher.decrypt
decipher.key = key
decipher.iv = iv

plain = decipher.update(encrypted) + decipher.final

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.

Associated data, also called additional authenticated data (AAD), is optionally used where there is additional information, such as headers or some metadata, that must be also authenticated but not necessarily need to be encrypted.

An example using the GCM (Galois/Counter Mode). You have 16 bytes key, 12 bytes (96 bits) nonce and the associated data auth_data. Be sure not to reuse the key and nonce pair. Reusing an nonce ruins the security guarantees of GCM mode.

key = OpenSSL::Random.random_bytes(16)
nonce = OpenSSL::Random.random_bytes(12)
auth_data = "authenticated but unencrypted data"
data = "encrypted data"

cipher = OpenSSL::Cipher.new('aes-128-gcm').encrypt
cipher.key = key
cipher.iv = nonce
cipher.auth_data = auth_data

encrypted = cipher.update(data) + cipher.final
tag = cipher.auth_tag(16)

Now you are the receiver. You know the key and have received nonce, auth_data, encrypted and tag through an untrusted network. Note that GCM accepts an arbitrary length tag between 1 and 16 bytes. You may additionally need to check that the received tag has the correct length, or you allow attackers to forge a valid single byte tag for the tampered ciphertext with a probability of 1/256.

raise "tag is truncated!" unless tag.bytesize == 16
decipher = OpenSSL::Cipher.new('aes-128-gcm').decrypt
decipher.key = key
decipher.iv = nonce
decipher.auth_tag = tag # could be called at any time before #final
decipher.auth_data = auth_data

decrypted = decipher.update(encrypted) + decipher.final

puts data == decrypted #=> true

Note that other AEAD ciphers may require additional steps, such as setting the expected tag length (auth_tag_len=) or the total data length (ccm_data_len=) in advance. Make sure to read the relevant man page for details.

Class Methods

OpenSSL::Cipher.ciphers → array[string...]

.

Returns the names of all available ciphers in an array.

Cipher.new(string) → cipher

.

The string must contain a valid cipher name like “aes-256-cbc”.

A list of cipher names is available by calling OpenSSL::Cipher.ciphers.

Instance Methods

cipher.auth_data = string

#

Sets additional authenticated data (AAD), also called associated data, for this Cipher. This method is available for AEAD ciphers.

The contents of this field should be non-sensitive data which will be added to the ciphertext to generate the authentication tag which validates the contents of the ciphertext.

This method must be called after key= and iv= have been set, but before starting actual encryption or decryption with update. In some cipher modes, auth_tag_len= and ccm_data_len= may also need to be called before this method.

See also the “AEAD Interface” section of the man page EVP_EncryptInit(3). This method internally calls EVP_CipherUpdate() with the output buffer set to NULL.

cipher.authenticated? → true | false

#

Indicates whether this Cipher instance uses an AEAD mode.

cipher.auth_tag(tag_len = 16) → String

#

Gets the generated authentication tag. This method is available for AEAD ciphers, and should be called after encryption has been finalized by calling final.

The returned tag will be tag_len bytes long. Some cipher modes require the desired length in advance using a separate call to auth_tag_len=, before starting encryption.

See also the “AEAD Interface” section of the man page EVP_EncryptInit(3). This method internally calls EVP_CIPHER_CTX_ctrl() with EVP_CTRL_AEAD_GET_TAG.

cipher.auth_tag = string

#

Sets the authentication tag to verify the integrity of the ciphertext.

The authentication tag must be set before final is called. The tag is verified during the final call.

Note that, for CCM mode and OCB mode, the expected length of the tag must be set before starting decryption by a separate call to auth_tag_len=. The content of the tag can be provided at any time before final is called.

NOTE: The caller must ensure that the String passed to this method has the desired length. Some cipher modes support variable tag lengths, and this method may accept a truncated tag without raising an exception.

See also the “AEAD Interface” section of the man page EVP_EncryptInit(3). This method internally calls EVP_CIPHER_CTX_ctrl() with EVP_CTRL_AEAD_SET_TAG.

cipher.auth_tag_len = integer

#

Sets the length of the expected authentication tag for this Cipher. This method is available for some of AEAD ciphers that require the length to be set before starting encryption or decryption, such as CCM mode or OCB mode.

For CCM mode and OCB mode, the tag length must be set before iv= is set.

See also the “AEAD Interface” section of the man page EVP_EncryptInit(3). This method internally calls EVP_CIPHER_CTX_ctrl() with EVP_CTRL_AEAD_SET_TAG and a NULL buffer.

cipher.block_size → integer

#

Returns the size in bytes of the blocks on which this Cipher operates on.

cipher.ccm_data_len = integer

#

Sets the total length of the plaintext / ciphertext message that will be processed by update in CCM mode.

Make sure to call this method after key= and iv= have been set, and before auth_data= or update are called.

This method is only available for CCM mode ciphers.

See also the “AEAD Interface” section of the man page EVP_EncryptInit(3).

cipher.decrypt → self

#

Initializes the Cipher for decryption.

Make sure to call either encrypt or decrypt before using the Cipher for any operation or setting any parameters.

Internally calls EVP_CipherInit_ex(ctx, NULL, NULL, NULL, NULL, 0).

cipher.encrypt → self

#

Initializes the Cipher for encryption.

Make sure to call either encrypt or decrypt before using the Cipher for any operation or setting any parameters.

Internally calls EVP_CipherInit_ex(ctx, NULL, NULL, NULL, NULL, 1).

cipher.final → string

#

Returns the remaining data held in the cipher object. Further calls to Cipher#update or Cipher#final are invalid. This call should always be made as the last call of an encryption or decryption operation, after having fed the entire plaintext or ciphertext to the Cipher instance.

When encrypting using an AEAD cipher, the authentication tag can be retrieved by auth_tag after final has been called.

When decrypting using an AEAD cipher, this method will verify the integrity of the ciphertext and the associated data with the authentication tag, which must be set by auth_tag= prior to calling this method. If the verification fails, CipherError will be raised.

cipher.iv = string

#

Sets the cipher IV. Please note that since you should never be using ECB mode, an IV is always explicitly required and should be set prior to encryption. The IV itself can be safely transmitted in public.

This method expects the String to have the length equal to iv_len. To use a different IV length with an AEAD cipher, iv_len= must be set prior to calling this method.

NOTE: In OpenSSL API conventions, the IV value may correspond to the “nonce” instead in some cipher modes. Refer to the OpenSSL man pages for details.

See also the man page EVP_CipherInit_ex(3).

cipher.iv_len → integer

#

Returns the expected length in bytes for an IV for this Cipher.

cipher.iv_len = integer

#

Sets the IV/nonce length for this Cipher. This method is available for AEAD ciphers that support variable IV lengths. This method can be called if a different IV length than OpenSSL’s default is desired, prior to calling iv=.

See also the “AEAD Interface” section of the man page EVP_EncryptInit(3). This method internally calls EVP_CIPHER_CTX_ctrl() with EVP_CTRL_AEAD_SET_IVLEN.

cipher.key = string

#

Sets the cipher key. To generate a key, you should either use a secure random byte string or, if the key is to be derived from a password, you should rely on PBKDF2 functionality provided by OpenSSL::PKCS5. To generate a secure random-based key, Cipher#random_key may be used.

Only call this method after calling Cipher#encrypt or Cipher#decrypt.

See also the man page EVP_CipherInit_ex(3).

cipher.key_len → integer

#

Returns the key length in bytes of the Cipher.

cipher.key_len = integer

#

Sets the key length of the cipher. If the cipher is a fixed length cipher then attempting to set the key length to any value other than the fixed value is an error.

Under normal circumstances you do not need to call this method (and probably shouldn’t).

See EVP_CIPHER_CTX_set_key_length for further information.

cipher.name → string

#

Returns the short name of the cipher which may differ slightly from the original name provided.

cipher.padding = 1 or 0

#

Enables or disables padding. By default encryption operations are padded using standard block padding and the padding is checked and removed when decrypting. If the pad parameter is zero then no padding is performed, the total amount of data encrypted or decrypted must then be a multiple of the block size or an error will occur.

See EVP_CIPHER_CTX_set_padding for further information.

cipher.pkcs5_keyivgen(pass, salt = nil, iterations = 2048, digest = "MD5") → nil

#

Generates and sets the key/IV based on a password.

WARNING: This method is deprecated and should not be used. This method corresponds to EVP_BytesToKey(), a non-standard OpenSSL extension of the legacy PKCS #5 v1.5 key derivation function. See OpenSSL::KDF for other options to derive keys from passwords.

Parameters

  • salt must be an 8 byte string if provided.

  • iterations is an integer with a default of 2048.

  • digest is a Digest object that defaults to ‘MD5’

cipher.random_iv → iv

#

Generate a random IV with OpenSSL::Random.random_bytes and sets it to the cipher, and returns it.

You must call encrypt or decrypt before calling this method.

cipher.random_key → key

#

Generate a random key with OpenSSL::Random.random_bytes and sets it to the cipher, and returns it.

You must call encrypt or decrypt before calling this method.

cipher.reset → self

#

Fully resets the internal state of the Cipher. By using this, the same Cipher instance may be used several times for encryption or decryption tasks.

Internally calls EVP_CipherInit_ex(ctx, NULL, NULL, NULL, NULL, -1).

cipher.update(data [, buffer]) → string or buffer

#

Encrypts data in a streaming fashion. Hand consecutive blocks of data to the update method in order to encrypt it. Returns the encrypted data chunk. When done, the output of Cipher#final should be additionally added to the result.

If buffer is given, the encryption/decryption result will be written to it. buffer will be resized automatically.

NOTE: When decrypting using an AEAD cipher, the integrity of the output is not verified until final has been called.