Mcrypt
is a simple crypting program, a replacement for the old unix
crypt(1).
When encrypting or decrypting a file, a new file is created with the
extension
.nc
and mode 0600. The new file keeps the modification date of the original.
The original file may be deleted by specifying the -u
parameter.
If no files are specified, the standard input is encrypted to the standard
output.
Mcrypt
uses all the symmetric algorithms included in libmcrypt.
HINTS
By default,
mcrypt
, when one of these algorithms is specified, prompts something like:
Enter passphrase: ...
You should then enter a passphrase long enough (512 characters is the
maximum length). That passphrase is transformed using the specified (or the default) key
generation algorithm, and then is used as the key, which is fed to the
algorithm.
Algorithm Vulnerability:
Most algorithms today are designed to resist in specific attacks.
None of them is proved not to be vulnerable to some kind of attack not
as yet known.
Compression:
By compressing your data before encryption you gain
both in efficiency (faster encryption) and safety of your data (language redundancy
is removed). A drawback is that most compression programs will add specific
headers in the compressed file, thus making known plaintext attacks easier.
Compression after encryption is useless and may result to compressed files with longer size than the original.
Error Recovery:
There is some error recovery in mcrypt.
If bytes are removed or lost from the file or stream in ECB, CBC and OFB modes,
are impossible to recover, although CFB mode will recover. If some
bytes are altered then a full block of plaintext is affected in ECB mode,
two blocks in CBC and CFB modes, but only the corresponding byte in OFB mode.
Mcrypt uses a 32 bit CRC to check for errors in the encrypted files.
Extra security:
For the very paranoid, if mcrypt is executed with superuser priviledges it
ensures that no important data (keys etc.) are written to disk, as swap etc.
Keep in mind that mcrypt was not designed to be a setuid program, so you
shouldn't make it one.
Do not rely on the fact that an algorithm has a large key size, try to use
long passphrases and try to make them unpredictable.
All the block algorithms above support these modes of encryption:
ECB:
The Electronic CodeBook mode. It is the simplest mode to use with a
block cipher. Encrypts each block independently.
CBC:
The Cipher Block Chaining mode. It is better than ECB since the plaintext
is XOR'ed with the previous ciphertext. A random block is placed as the
first block so the same block or messages always encrypt to something
different. (This is the default mode)
CFB:
The Cipher-Feedback Mode (in 8bit). This is a self-synchronizing
stream cipher implemented from a block cipher.
OFB:
The Output-Feedback Mode (in 8bit). This is a synchronous
stream cipher implemented from a block cipher. It is intended for use
in noisy lines, because corrupted ciphertext blocks do not corrupt the
plaintext blocks that follow. Insecure (because used in 8bit mode) so I recommend against
using it. Added just for completeness.
nOFB:
The Output-Feedback Mode (in nbit). n Is the size of the block of the
algorithm. This is a synchronous stream cipher implemented from a block
cipher. It is intended for use in noisy lines, because corrupted ciphertext
blocks do not corrupt the plaintext blocks that follow.
Encrypted files can be restored to their original form using
mcrypt -d
or
mdecrypt
mdecrypt
takes a list of files on its command line and creates a new file for each
file whose name ends with .nc by removing the ".nc" or by adding ".dc"
to the end of the file name if .nc is not in the encrypted file's name.
OPTIONS
-F --force
Force output on standard output or input from stdin if that is a terminal.
By default mcrypt will not output encrypted data to terminal, nor read
encrypted data from it.
-z --gzip
Use gzip (if it exists in your system) to compress files before encryption.
If specified at decryption time it will decompress these files.
-p --bzip2
Use bzip2 (if it exists in your system) to compress files before encryption.
If specified at decryption time it will decompress these files.
-d --decrypt
Decrypt.
--help
Display a help screen and quit.
-v --version
Version. Display the version number and quit.
-L --license
Display the
mcrypt's
license and quit.
-o --keymode MODE
MODE may be one of the keymodes listed by the --keymodeslist parameter.
It actually is the convertion to the key before it is fed to the algorithm.
It is recommended to leave it as is, if you do not know what it is.
-h --hash HASH_ALGORITHM
HASH_ALGORITHM may be one of the algorithms listed by the --hashlist parameter.
This is the digest that will be appended to the file to be encrypted,
in order to detect file corruption. The default is the CRC32 checksum.
-s --keysize SIZE
SIZE is the algorithm's key size in bytes (not the size of the passphrase). It
defaults to the maximum key supported by the algorithm. The maximum key sizes of
the algorithms may be obtained by the --list parameter. It is safe not to
touch this.
-b --bare
No important information like the algorithm, mode, the bit mode and
the crc32 of the original file are written in the encrypted file.
The security lies on the algorithm not on obscurity so this is NOT the
default. This flag must also be specified when decrypting a bare encrypted
file.
When the bare flag is specified decryption and encryption are faster. This
may be usefull when using mcrypt to encrypt a link or something like that.
--flush
Flushes the output (ciphertext or plaintext) immediately. Usefull if mcrypt
is used with pipes.
--nodelete
When this option is specified mcrypt does not delete the output file, even
if decryption failed. This is usefull if you want to decrypt a corrupted
file.
-q --quiet
Suppress some not critical warnings.
-u --unlink
Unlink (delete) the input file if the whole process of encryption/decryption
succeeds. This is not the default in order to use an external program
to remove sensitive data.
--list
Lists all the algorithms current supported.
--keymodeslist
Lists all the key modes current supported.
--hashlist
Lists all the hash algorithms current supported.
-r --random
Use /dev/(s)random instead of /dev/urandom. This may need some key input
or mouse move to proceed.
If your system does not support /dev/random or /dev/urandom, it does nothing,
and you count on your libc for random data (actually pseudo-random).
-k --key KEY1 KEY2 ...
Enter the keyword(s) via the command line. The KEY(s) is/are then used as
keyword instead of prompting for them. Keep in mind that someone
may see the command you are executing and so your keyword(s).
-c --config FILE
Use the specified configuration file. The default is .mcryptrc
in your home directory. The format of the configuration
file is the same as the parameters. An example file is:
algorithm safer+
mode cbc
key a_very_secret_one
-f --keyfile FILE
Enter the keyword(s) via a file. One keyword is read per line.
The first keyword read is used for the first file, the second for
the second file etc. If the keywords are less than the files then
the last keyword is used for the remaining. A limitation is that you cannot
use the NULL (\0) and the Newline (\n) character in the key.
A solution to this problem is to specify the keyword in hex mode.
-m --mode MODE
Mode of encryption and decryption. These modes are currently supported:
ECB, CFB, OFB, nOFB, CBC and STREAM. CBC is the default. Unless the bare flag
is specified there is no need to specify these modes for decryption.
For stream algorithms (like WAKE) mode should be STREAM.
-a --algorithm ALGORITHM
The algorithm used to encrypt and decrypt. Unless the bare flag is specified
there is no need to specify these for decryption.
The algorithms currently supported are shown with the --list parameter.
EXAMPLES
For mcrypt to be compatible with the solaris
des(1),
the following parameters are needed: "mcrypt -a des --keymode pkdes --bare --noiv filename".
For mcrypt to be compatible with the unix
crypt(1),
the following parameters are needed: "mcrypt -a enigma --keymode scrypt --bare filename".
To encrypt a file using a stream algorithm (eg. Arcfour), the following parameters
are needed: "mcrypt -a arcfour --mode stream filename".
ENVIRONMENT
Mcrypt uses the following environment variables:
MCRYPT_KEY:
to specify the key
MCRYPT_ALGO:
to specify the algorithm
MCRYPT_MODE:
to specify the algorithm's mode
MCRYPT_KEY_MODE:
to specify the key mode
You can use these instead of using the command line (which is insecure),
but note that only one key should be used in MCRYPT_KEY.
