NAMEccrypt - encrypt and decrypt files and streams
ccrypt [mode] [options] [file...] ccencrypt [options] [file...] ccdecrypt [options] [file...] ccat [options] file...
DESCRIPTIONccrypt is a tool for encrypting and decrypting files and streams. It is based on the Rijndael block cipher, a version of which is also used in the Advanced Encryption Standard (AES, see http://www.nist.gov/aes). This cipher is believed to provide very strong cryptographic security.
The algorithm provided by ccrypt is not symmetric, i.e., one must specify whether to encrypt or decrypt. The most common way to invoke ccrypt is via the commands ccencrypt and ccdecrypt.
Encryption and decryption depends on a keyword (or key phrase) supplied by the user. By default, the user is prompted to enter a keyword from the terminal. Keywords can consist of any number of characters, and all characters are significant (although ccrypt internally hashes the key to 256 bits). Longer keywords provide better security than short ones, since they are less likely to be discovered by exhaustive search.
MODESccrypt can operate in five different modes. If more than one mode is specified, the last one specified takes precedence. The aliases ccencrypt, ccdecrypt, and ccat are provided as a convenience; they are equivalent to ccrypt -e, ccrypt -d, and ccrypt -c, respectively.
OPTIONSThe following options are supported in addition to the modes described above:
NOTES ON USAGEThe user interface of ccrypt intentionally resembles that of GNU gzip, although it is not identical. When invoked with filename arguments, ccrypt normally modifies the files in place, overwriting their old content. Unlike gzip, the output is not first written to a temporary file; instead, the data is literally overwritten. For encryption, this is usually the desired behavior, since one does not want copies of the unencrypted data to remain in hidden places in the file system. The disadvantage is that if ccrypt is interrupted in the middle of writing to a file, the file will end up in a corrupted, partially encrypted state. However, in such cases it is possible to recover most of the data; see RECOVERING DATA FROM CORRUPTED FILES below. If you want to force ccrypt to use temporary files, use the --tmpfiles option.
When ccrypt receives an interrupt signal (Ctrl-C) while updating a file in place, it does not exit immediately, but rather delays the exit until after it finishes writing to the current file. This is to prevent files from being partially overwritten and thus corrupted. If you want to force ccrypt to exit immediately, just press Ctrl-C twice quickly.
The encryption algorithm used by ccrypt uses a random seed that is different each time. As a result, encrypting the same file twice will never yield the same result. The advantage of this method is that similarities in plaintext do not lead to similarities in ciphertext; there is no way of telling whether the content of two encrypted files is similar or not.
Because of the use of a random seed, decrypting and re-encrypting a file with the same key will not lead to an identical file. It is primarily for this reason that ccrypt refuses to decrypt files with a non-matching key; if this were allowed, there would be no way afterwards to restore the original file, and the data would be irretrievably lost.
When overwriting files, special care is taken with hard links and symbolic links. Each physical file (i.e., each inode) is processed at most once, no matter how many paths to it are encountered on the command line or in subdirectories traversed recursively. For each file that has multiple hard links, a warning is printed, to alert the user that not all paths to the file might have been properly renamed. Symbolic links are ignored except in cat mode, or unless the -l or -R option is given.
Unlike gzip, ccrypt does not complain about files that have improper suffixes. It is legal to doubly encrypt a file. It is also legal to decrypt a file that does not have the .cpt suffix, provided the file contains valid data for the given decryption key. Use the --strictsuffix option if you want to prevent ccrypt from encrypting files that already have a .cpt suffix.
Regarding encryption and compression: encrypted data is statistically indistinguishable from random data, and thus it cannot be compressed. But of course it is possible to compress the data first, then encrypt it. Suggested file suffixes are .gz.cpt or .gzc.
RECOVERING DATA FROM CORRUPTED FILESEncrypted data might be corrupted for a number of reasons. For instance, a file might have been partially encrypted or decrypted if ccrypt was interrupted while processing the file. Or data might be corrupted by a software or hardware error, or during transmission over a network. The encryption algorithm used by ccrypt is designed to allow recovery from errors. In general, only a few bytes of data will be lost near where the error occurred.
Data encrypted by ccrypt can be thought of as a sequence of 32-byte blocks. To decrypt a particular block, ccrypt only needs to know the decryption key, the data of the block itself, and the data of the block immediately preceding it. ccrypt cannot tell whether a block is corrupted or not, except the very first block, which is special. Thus, if the encrypted data has been altered in the middle or near the end of a file, ccrypt can be run to decrypt it as usual, and most of the data will be decrypted correctly, except near where the corruption occurred.
The very first block of encrypted data is special, because it does not actually correspond to any plaintext data; this block holds the random seed generated at encryption time. ccrypt also uses the very first block to decide whether the given keyword matches the data or not. If the first block has been corrupted, ccrypt will likely decide that the keyword does not match; in such cases, the -m option can be used to force ccrypt to decrypt the data anyway.
If a file contains some encrypted and some unencrypted data, or data encrypted with two different keys, one should decrypt the entire file with each applicable key, and then piece together the meaningful parts manually.
Finally, decryption will only produce meaningful results if the data is aligned correctly along block boundaries. If the block boundary information has been lost, one has to try all 32 possibilities.
DESCRIPTION OF THE CIPHERBlock ciphers operate on data segments of a fixed length. For instance, the Rijndael block cipher used in ccrypt has a block length of 32 bytes or 256 bits. Thus, this cipher encrypts 32 bytes at a time.
Stream ciphers operate on data streams of any length. There are several standard modes for operating a block cipher as a stream cipher. One such standard is Cipher Feedback (CFB), defined in NIST Special Publication 800-38A and ANSI X3.106-1983. ccrypt implements a stream cipher by operating the Rijndael block cipher in CFB mode.
Let P[i] and C[i] be the ith block of the plaintext and ciphertext, respectively. CFB mode specifies that
Here ^ denotes the bitwise exclusive or function, and E(k,x) denotes the encryption of the block x under the key k using the block cipher. Thus, each block of the ciphertext is calculated from the corresponding block of plaintext and the previous block of ciphertext. Note that in fact, each byte of P[i] can be calculated from the corresponding byte of C[i], so that the stream cipher can be applied to one byte at a time. In particular, the stream length need not be a multiple of the block size.
Assuming that blocks are numbered starting from 0, a special "initial" ciphertext block C[-1] is needed to provide the base case for the above formula. This value C[-1] is called the initialization vector or seed. The seed is chosen at encryption time and written as the first block of the encrypted stream. It is important that the seed is unpredictable; in particular, the same seed should never by used more than once. Otherwise, the two resulting ciphertext blocks C could be related by a simple xor to obtain information about the corresponding plaintext blocks P. If unpredictable seeds are used, CFB is provably as secure as the underlying block cipher.
In ccrypt, the seed is constructed as follows: first, a nonce is contructed by hashing a combination of the host name, current time, process id, and an internal counter into a 28-byte value, using a cryptographic hash function. The nonce is combined with a fixed four-byte "magic number", and the resulting 32-byte value is encrypted by one round of the Rijndael block cipher with the given key. This encrypted block is used as the seed and appended to the beginning of the ciphertext. The use of the magic number allows ccrypt to detect non-matching keys before decryption.
SECURITYccrypt is believed to provide very strong cryptographic security, equivalent to that of the Rijndael cipher with 256-bit block size and 256-bit key size. Another version of the Rijndael cipher (with a smaller block size) is used in the U.S. government's Advanced Encryption Standard (AES, see http://www.nist.gov/aes). Therefore, this cipher is very well studied and subject to intensive public scrutiny. This scrutiny has a positive effect on the cipher's security. In particular, if an exploitable weakness in this cipher were ever discovered, this would become widely publicized.
In practical terms, the security of ccrypt means that, without knowledge of the encryption key, it is effectively impossible to obtain any information about the plaintext from a given ciphertext. This is true even if a large number of plaintext-ciphertext pairs are already known for the same key. Moreover, because ccrypt uses a key size of 256 bits, an exhaustive search of the key space is not feasible, at least as long as sufficiently long and hard-to-guess keys are actually used in practice. No cipher is secure if users choose insecure keywords.
On the other hand, ccrypt does not attempt to provide data integrity, i.e., it will not attempt to detect whether the ciphertext was modified after encryption. In particular, encrypted data can be truncated, leaving the corresponding decrypted data also truncated, but otherwise consistent. If one needs to ensure data integrity as well as secrecy, this can be achieved by other methods. The recommended method is to prepend a cryptographic hash (for instance, an SHA-1 hash) to the data before encryption.
ccrypt does not claim to provide any particular safeguards against information leaking via the local operating system. While reasonable precautions are taken, there is no guarantee that keywords and plaintexts have been physically erased after encryption in completed; parts of such data might still exist in memory or on disk. ccrypt does not currently use privileged memory pages.
When encrypting files, ccrypt by default accesses them in read-write mode. This normally causes the original file to be physically overwritten, but on some file systems, this might not be the case.
Note that the use of the -K option is unsafe in a multiuser environment, because the command line of a process is visible to other users running the ps command. The use of the -E option is potentially unsafe for the same reason, although recent versions of ps don't tend to display environment information to other users. The use of the -T option is unsafe for encryption because the original plaintext will remain in unused sectors of the file system.
EMACS PACKAGEThere is an emacs package for reading and writing encrypted files. (Note that this package currently only works with emacs, not with xemacs.) This package hooks into the low-level file I/O functions of emacs, prompting the user for a password where appropriate. It is implemented in much the same way as support for compressed files. If you have both the ps-ccrypt and jka-compr packages installed, emacs can open encrypted files and compressed files; however, it does not currently work for files that are encrypted and compressed.
To use the package, simply load ps-ccrypt, then edit as usual. When you open a file with the ".cpt" extension, emacs will prompt you for a password for the file. It will remember the password for the buffer, and when you save the file later, it will be automatically encrypted again (provided you save it with a ".cpt" extension). Except for the password prompt, the operation of the package should be transparent to the user. The command M-x ccrypt-set-buffer-password can be used to change the current password of a buffer.
The simplest way to use this package is to include the lines
in your .emacs file, where path is the directory that holds the file ps-ccrypt.el.
Limitations of the emacs package: there is no guarantee that unencrypted information cannot leak to the file system; in fact, the package sometimes writes unencrypted data to temporary files. However, auto-saved files are normally treated correctly (i.e., encrypted). For details, see the comments in the file ps-ccrypt.el.
EXIT STATUSThe exit status is 0 on successful completion, and non-zero otherwise. An exit status of 1 means illegal command line, 2 is out of memory or another system error, 3 is a fatal i/o error, 4 is a non-matching key or wrong file format, 6 is interrupt, 7 is mistyped key in --timid mode, 8 is a non-fatal i/o error, and 9 means that no key was obtained because the user failed to enter it, or because the specified keyfile or environment variable could not be read. An exit status of 10 means that the file specified by the --keyref option could not be read, or did not match the requested encryption key.
Fatal i/o errors are those that occur while processing a file that is already open. Such errors cause ccrypt to abort its operation immediately with an exit status of 3. Non-fatal i/o errors are those that occur while handling files that are not already open; typically, such errors are caused by files that are missing, not readable, or can't be created. When encountering a non-fatal i/o error, ccrypt simply continues to process the next available input file. The exit status of 8 is delayed until after all the files have been processed.
Non-matching keys and wrong file formats are also considered non-fatal errors, and cause ccrypt to continue with processing the next available input file. In this case, an exit status of 4 is given after all the files have been processed. If there is a conflict between exit status 4 and 8, then 8 is returned.
The former exit status 5 ("wrong file format") has been eliminated, and is now covered under exit status 4 ("non-matching key or wrong file format"). Note that ccrypt does not really have a "file format" in the proper sense of the word; any file of length at least 32 bytes is potentially a valid encrypted file.
LIMITATIONSLike all encryption programs that depend on a user-supplied key, the encryption is only as strong as the key you provide. You must assume that adversaries have the ability to try billions of different keys per second, or more. So if you use a key that is too short, or a key that is long but easy to guess, you should assume that it can and will be broken.
While ccrypt can handle keywords of arbitrary length, some operating systems limit the length of an input line to 1024 characters.
The renaming of files (adding or removing the .cpt suffix) can go wrong if a filename is repeated on the command line. In this case, the file is only encrypted/decrypted once, but the suffix may be added or removed several times. This is because ccrypt thinks it encountered different hardlinks for the same file.
The --strictsuffix option can behave in unexpected ways if one file has several hardlinks, some of which have the suffix and some of which don't. In this case, the inode will be encrypted/decrypted, but the suffix will be changed only for those filenames that allow it. Similarly, if a file cannot be renamed because a file of the given name already exists, the file may still be encrypted/decrypted if it has another hardlink.
AUTHORPeter Selinger <selinger at users.sourceforge.net>
COPYRIGHTCopyright (C) 2000-2018 Peter Selinger
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
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