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incrypt.1
incrypt - command line implementation of crypt(3)
incrypt [ -h hash ] [ -S salt ] passphrase salt
incrypt is a command line impementation of crypt(3) password encryption function. The crypt functions irreversibly “hash” phrase for storage in the system password database (shadow(5)) using a cryptographic “hashing method.” The result of this operation is called a “hashed passphrase” or just a “hash.” Hashing methods are described in crypt(5).
hash controls which hashing method to use, and also supplies various parameters to the chosen method, most importantly a random “salt” which en‐ sures that no two stored hashes are the same, even if the phrase strings are the same.
If salt is a character string starting with the characters "$id$" followed by a string terminated by "$":
$id$salt$encrypted
then instead of using the DES machine, id identifies the encryption method used and this then determines how the rest of the password string is interpreted. The following values of id are supported:
ID | Method 1 | MD5 2a | Blowfish (not in mainline glibc; added in some | Linux distributions) 5 | SHA-256 (since glibc 2.7) 6 | SHA-512 (since glibc 2.7)
So
"salt" stands for the up to 16 characters following "$id$" in the salt. The encrypted part of the password string is the actual computed password. The size of this string is fixed:
| MD5 | 22 characters | |
| SHA-256 | 43 characters | |
| SHA-512 | 86 characters |
The characters in "salt" and "encrypted" are drawn from the set [a-zA-Z0-9./]. In the MD5 and SHA implementations the entire key is significant (instead of only the first 8 bytes in DES).
Warning: The key space consists of 2**56 equal 7.2e16 possible values. Exhaustive searches of this key space are possible using massively parallel computers. Software, such as crack(1), is available which will search the portion of this key space that is generally used by humans for passwords. Hence, password selection should, at minimum, avoid common words and names. The use of a passwd(1) program that checks for crackable passwords during the selection process is recommended.
-h hash
Specifiy hash which is one of DES, MD5, SHA-256, SHA-512. Here ID is
the id in /etc/shadow. See incrypt(1).
| HASH ID Description | |
|---|---|
| DES - DES encryption (shouldn't be used) | |
| MD5 1 MD5 encryption (shouldn't be used) | |
| SHA-256 5 SHA256 encryption | |
| SHA-512 6 SHA512 encryption |
-S salt
Specify a base64 encoded salt to be used when generating SCRAM password.
If not specified, this will be generated using libsodium/gsasl. Here
base64 implies characters [0-9], [a-z], [A-Z] and the two
characters [./].
The original hashing method from Unix V7, based on the DES block cipher. Because DES is cheap on modern hardware, because there are only 4096 possible salts and 2**56 possible hashes, and because it truncates passphrases to 8 characters, it is feasible to discover any passphrase hashed with this method. It should only be used if you absolutely have to generate hashes that will work on an old operating system that supports nothing else.
It is based on the Data Encryption Standard algorithm with variations intended (among other things) to discourage use of hardware implementations of a key search.
passphrase is a user's typed password.
salt is a two-character string chosen from the set [a-zA-Z0-9./]. This string is used to perturb the algorithm in one of 4096 different ways.
By taking the lowest 7 bits of each of the first eight characters of the key, a 56-bit key is obtained. This 56-bit key is used to encrypt repeatedly a constant string (usually a string consisting of all zeros). The returned value points to the encrypted password, a series of 13 printable ASCII characters (the first two characters represent the salt itself). The return value points to static data whose content is overwritten by each call.
On success, the encrypted password is printed on stdout. On error, non zero status is returned.
crypt(3)