1CRYPT_RN(3) Library functions CRYPT_RN(3)
3
7 crypt, crypt_r, crypt_rn, crypt_ra - passphrase hashing
8
10 #include <crypt.h>
11 char *crypt(const char *phrase, const char *setting);
13 char *crypt_r(const char *phrase, const char *setting, struct
14 crypt_data *data);
15 char *crypt_rn(const char *phrase, const char *setting, void *data, int
16 size);
17 char *crypt_ra(const char *phrase, const char *setting, void **data,
18 int *size);
19 Link with -lcrypt.
21
23 The crypt, crypt_r, crypt_rn, and crypt_ra functions irreversibly
24 “hash” phrase for storage in the system password database (shadow(5))
25 using a cryptographic “hashing method.” The result of this operation
26 is called a “hashed passphrase” or just a “hash.” Hashing methods are
27 described in crypt(5).
28 setting controls which hashing method to use, and also supplies various
30 parameters to the chosen method, most importantly a random “salt” which
31 ensures that no two stored hashes are the same, even if the phrase
32 strings are the same. The hashing methods are explained below.
33 The crypt_data structure passed to crypt_r has at least these fields:
35 struct crypt_data {
37 char output[CRYPT_OUTPUT_SIZE];
38 char setting[CRYPT_OUTPUT_SIZE];
39 char phrase[CRYPT_MAX_PASSPHRASE_SIZE];
40 char initialized;
41 };
42 Upon a successful return from crypt_r, the hashed passphrase will be
44 stored in output. Applications are encouraged, but not required, to
45 use the setting and phrase fields to store the strings that they will
46 pass as phrase and setting to crypt_r. This will make it easier to
47 erase all sensitive data after it is no longer needed.
48 The initialized field must be set to zero before the first time a
50 crypt_data object is first used in a call to crypt_r. We recommend
51 zeroing the entire crypt_data object, not just initialized and not just
52 the documented fields, before the first use. (Of course, do this
53 before storing anything in setting and phrase.)
54 The data argument to crypt_rn should also point to a crypt_data object,
56 and size should be the size of that object, cast to int. When used
57 with crypt_rn, the entire crypt_data object must be zeroed before its
58 first use; this is not just a recommendation, as it is for crypt_r.
59 (setting and phrase are still allowed to be used.) Otherwise, the
60 fields of the object have the same uses that they do for crypt_r.
61 On the first call to crypt_ra, data should be the address of a void *
63 variable set to NULL, and size should be the address of an int variable
64 set to zero. crypt_ra will allocate and initialize a crypt_data
65 object, using malloc(3), and write its address and size into *data and
66 *size. These can be reused in subsequent calls. After the application
67 is done hashing passphrases, it should deallocate *data using free(3).
68
70 Upon successful completion, crypt, crypt_r, crypt_rn, and crypt_ra
71 return a pointer to a string which encodes both the hashed passphrase,
72 and the settings that were used to encode it. This string is directly
73 usable as setting with other calls to crypt, crypt_r, crypt_rn, and
74 crypt_ra, and as prefix with calls to crypt_gensalt, crypt_gensalt_rn,
75 and crypt_gensalt_ra. It will be entirely printable ASCII, and will
76 not contain whitespace or the characters ‘:’, ‘;’, ‘*’, ‘!’, or ‘\’.
77 See crypt(5) for more detail on the format of hashed passphrases.
78 crypt places its result in a static storage area, which will be over‐
80 written by subsequent calls to crypt. It is not safe to call crypt
81 from multiple threads simultaneously.
82 crypt_r, crypt_rn, and crypt_ra place their result in the output field
84 of the crypt_data object that they are supplied with; it is safe to
85 call them from multiple threads simultaneously, as long as a separate
86 crypt_data object is used for each thread.
87 Upon error, crypt_r, crypt_rn, and crypt_ra write an invalid hashed
89 passphrase to the output field of their crypt_data object, and crypt
90 writes an invalid hash to its static storage area. This string will be
91 shorter than 13 characters, will begin with a ‘*’, and will not compare
92 equal to setting.
93 Upon error, crypt_rn and crypt_ra return a null pointer. crypt_r and
95 crypt may also return a null pointer, or they may return a pointer to
96 the invalid hash, depending on how libcrypt was configured. (The
97 option to return the invalid hash is for compatibility with old appli‐
98 cations that assume that crypt cannot return a null pointer. See
99 PORTABILITY NOTES below.)
100 All four functions set errno when they fail.
102
104 EINVAL setting is invalid, or requests a hashing method that is not
105 supported.
106 ERANGE crypt_rn only: size is too small for the hashing method
108 requested by setting.
109 ENOMEM Failed to allocate internal scratch memory.
111 crypt_ra only: failed to allocate memory for *data.
112 ENOSYS or EOPNOTSUPP
114 Hashing passphrases is not supported at all on this installa‐
115 tion, or the hashing method requested by setting is not sup‐
116 ported. These error codes are not used by this version of
117 libcrypt, but may be encountered on other systems.
118
120 crypt is included in POSIX, but crypt_r, crypt_rn, and crypt_ra are not
121 part of any standard.
122 POSIX does not specify any hashing methods, and does not require hashed
124 passphrases to be portable between systems. In practice, hashed
125 passphrases are portable as long as both systems support the hashing
126 method that was used. However, the set of supported hashing methods
127 varies considerably from system to system.
128 The behavior of crypt on errors isn't well standardized. Some imple‐
130 mentations simply can't fail (except by crashing the program), others
131 return a null pointer or a fixed string. Most implementations don't
132 set errno, but some do. POSIX specifies returning a null pointer and
133 setting errno, but it defines only one possible error, ENOSYS, in the
134 case where crypt is not supported at all. Many existing applications
135 are not prepared to handle null pointers returned by crypt. The behav‐
136 ior described above for this implementation, setting errno and return‐
137 ing an invalid hashed passphrase different from setting, is chosen to
138 make these applications fail closed when an error occurs.
139 Due to historical restrictions on the export of cryptographic software
141 from the USA, crypt is an optional POSIX component. Applications
142 should therefore be prepared for crypt not to be available, or to
143 always fail (setting errno to ENOSYS) at runtime.
144 POSIX specifies that crypt is declared in unistd.h, but only if the
146 macro _XOPEN_CRYPT is defined and has a value greater than or equal to
147 zero. Since libcrypt does not provide unistd.h, it declares crypt,
148 crypt_r, crypt_rn, and crypt_ra in crypt.h instead.
149 On a minority of systems (notably recent versions of Solaris), crypt
151 uses a thread-specific static storage buffer, which makes it safe to
152 call from multiple threads simultaneously, but does not prevent each
153 call within a thread from overwriting the results of the previous one.
154
156 Some implementations of crypt, upon error, return an invalid hash that
157 is stored in a read-only location or only initialized once, which means
158 that it is only safe to erase the buffer pointed to by the crypt return
159 value if an error did not occur.
160 struct crypt_data may be quite large (32kB in this implementation of
162 libcrypt; over 128kB in some other implementations). This is large
163 enough that it may be unwise to allocate it on the stack.
164 Some recently designed hashing methods need even more scratch memory,
166 but the crypt_r interface makes it impossible to change the size of
167 crypt_data without breaking binary compatibility. The crypt_rn inter‐
168 face could accommodate larger allocations for specific hashing methods,
169 but the caller of crypt_rn has no way of knowing how much memory to
170 allocate. crypt_ra does the allocation itself, but can only make a
171 single call to malloc(3).
172
174 For an explanation of the terms used in this section, see
175 attributes(7).
176 ┌───────────────────┬───────────────┬──────────────────────┐
178 │Interface │ Attribute │ Value │
179 ├───────────────────┼───────────────┼──────────────────────┤
180 │crypt │ Thread safety │ MT-Unsafe race:crypt │
181 ├───────────────────┼───────────────┼──────────────────────┤
182 │crypt_r, crypt_rn, │ Thread safety │ MT-Safe │
183 │crypt_ra │ │ │
184 └───────────────────┴───────────────┴──────────────────────┘
185
187 A rotor-based crypt function appeared in Version 6 AT&T UNIX. The
188 "traditional" DES-based crypt first appeared in Version 7 AT&T UNIX.
189 crypt_r originates with the GNU C Library. There's also a crypt_r
191 function on HP-UX and MKS Toolkit, but the prototypes and semantics
192 differ.
193 crypt_rn and crypt_ra originate with the Openwall project.
195
197 crypt_gensalt(3), getpass(3), getpwent(3), shadow(3), login(1),
198 passwd(1), crypt(5), passwd(5), shadow(5), pam(8)
199Openwall Project October 11, 2017 CRYPT_RN(3)