1CRYPT(3) Linux Programmer's Manual CRYPT(3)
2
6 crypt, crypt_r - password and data encryption
7
9 #define _XOPEN_SOURCE /* See feature_test_macros(7) */
10 #include <unistd.h>
11 char *crypt(const char *key, const char *salt);
13 #define _GNU_SOURCE /* See feature_test_macros(7) */
15 #include <crypt.h>
16 char *crypt_r(const char *key, const char *salt,
18 struct crypt_data *data);
19 Link with -lcrypt.
21
23 crypt() is the password encryption function. It is based on the Data
24 Encryption Standard algorithm with variations intended (among other
25 things) to discourage use of hardware implementations of a key search.
26 key is a user's typed password.
28 salt is a two-character string chosen from the set [a–zA–Z0–9./]. This
30 string is used to perturb the algorithm in one of 4096 different ways.
31 By taking the lowest 7 bits of each of the first eight characters of
33 the key, a 56-bit key is obtained. This 56-bit key is used to encrypt
34 repeatedly a constant string (usually a string consisting of all
35 zeros). The returned value points to the encrypted password, a series
36 of 13 printable ASCII characters (the first two characters represent
37 the salt itself). The return value points to static data whose content
38 is overwritten by each call.
39 Warning: The key space consists of 2**56 equal 7.2e16 possible values.
41 Exhaustive searches of this key space are possible using massively par‐
42 allel computers. Software, such as crack(1), is available which will
43 search the portion of this key space that is generally used by humans
44 for passwords. Hence, password selection should, at minimum, avoid
45 common words and names. The use of a passwd(1) program that checks for
46 crackable passwords during the selection process is recommended.
47 The DES algorithm itself has a few quirks which make the use of the
49 crypt() interface a very poor choice for anything other than password
50 authentication. If you are planning on using the crypt() interface for
51 a cryptography project, don't do it: get a good book on encryption and
52 one of the widely available DES libraries.
53 crypt_r() is a reentrant version of crypt(). The structure pointed to
55 by data is used to store result data and bookkeeping information.
56 Other than allocating it, the only thing that the caller should do with
57 this structure is to set data->initialized to zero before the first
58 call to crypt_r().
59
61 On success, a pointer to the encrypted password is returned. On error,
62 NULL is returned.
63
65 ENOSYS The crypt() function was not implemented, probably because of
66 U.S.A. export restrictions.
67
69 Multithreading (see pthreads(7))
70 The crypt() function is not thread-safe.
71 The crypt_r() function is thread-safe.
73
75 crypt(): SVr4, 4.3BSD, POSIX.1-2001. crypt_r() is a GNU extension.
76
78 Glibc notes
79 The glibc2 version of this function supports additional encryption
80 algorithms.
81 If salt is a character string starting with the characters "$id$" fol‐
83 lowed by a string terminated by "$":
84 $id$salt$encrypted
86 then instead of using the DES machine, id identifies the encryption
88 method used and this then determines how the rest of the password
89 string is interpreted. The following values of id are supported:
90 ID | Method
92 ─────────────────────────────────────────────────────────
93 1 | MD5
94 2a | Blowfish (not in mainline glibc; added in some
95 | Linux distributions)
96 5 | SHA-256 (since glibc 2.7)
97 6 | SHA-512 (since glibc 2.7)
98 So $5$salt$encrypted is an SHA-256 encoded password and
100 $6$salt$encrypted is an SHA-512 encoded one.
101 "salt" stands for the up to 16 characters following "$id$" in the salt.
103 The encrypted part of the password string is the actual computed pass‐
104 word. The size of this string is fixed:
105 MD5 | 22 characters
107 SHA-256 | 43 characters
108 SHA-512 | 86 characters
109 The characters in "salt" and "encrypted" are drawn from the set
111 [a–zA–Z0–9./]. In the MD5 and SHA implementations the entire key is
112 significant (instead of only the first 8 bytes in DES).
113
115 login(1), passwd(1), encrypt(3), getpass(3), passwd(5)
116
118 This page is part of release 3.53 of the Linux man-pages project. A
119 description of the project, and information about reporting bugs, can
120 be found at http://www.kernel.org/doc/man-pages/.
121 2013-06-21 CRYPT(3)