man3/crypt.3

   1 .\" Michael Haardt (michael@cantor.informatik.rwth.aachen.de)
   2 .\"     Sat Sep  3 22:00:30 MET DST 1994
   3 .\"
   4 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
   5 .\" This is free documentation; you can redistribute it and/or
   6 .\" modify it under the terms of the GNU General Public License as
   7 .\" published by the Free Software Foundation; either version 2 of
   8 .\" the License, or (at your option) any later version.
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  10 .\" The GNU General Public License's references to "object code"
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  12 .\" document formatting or typesetting system, including
  13 .\" intermediate and printed output.
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  16 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
  17 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  18 .\" GNU General Public License for more details.
  19 .\"
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  24 .\"
  25 .\" Sun Feb 19 21:32:25 1995, faith@cs.unc.edu edited details away
  26 .\"
  27 .\" TO DO: This manual page should go more into detail how DES is perturbed,
  28 .\" which string will be encrypted, and what determines the repetition factor.
  29 .\" Is a simple repetition using ECB used, or something more advanced?  I hope
  30 .\" the presented explanations are at least better than nothing, but by no
  31 .\" means enough.
  32 .\"
  33 .\" added _XOPEN_SOURCE, aeb, 970705
  34 .\" added GNU MD5 stuff, aeb, 011223
  35 .\"
  36 .TH CRYPT 3 2017-09-15 "" "Linux Programmer's Manual"
  37 .SH NAME
  38 crypt, crypt_r \- password and data encryption
  39 .SH SYNOPSIS
  40 .nf
  41 .BR "#define _XOPEN_SOURCE" "       /* See feature_test_macros(7) */"
  42 .B #include <unistd.h>
  43 .PP
  44 .BI "char *crypt(const char *" key ", const char *" salt );
  45
  46 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
  47 .B #include <crypt.h>
  48 .PP
  49 .BI "char *crypt_r(const char *" key ", const char *" salt ,
  50 .BI "              struct crypt_data *" data );
  51 .fi
  52 .PP
  53 Link with \fI\-lcrypt\fP.
  54 .SH DESCRIPTION
  55 .BR crypt ()
  56 is the password encryption function.
  57 It is based on the Data Encryption
  58 Standard algorithm with variations intended (among other things) to
  59 discourage use of hardware implementations of a key search.
  60 .PP
  61 .I key
  62 is a user's typed password.
  63 .PP
  64 .I salt
  65 is a two-character string chosen from the set
  66 [\fBa\-zA\-Z0\-9./\fP].
  67 This string is used to
  68 perturb the algorithm in one of 4096 different ways.
  69 .PP
  70 By taking the lowest 7 bits of each of the first eight characters of the
  71 .IR key ,
  72 a 56-bit key is obtained.
  73 This 56-bit key is used to encrypt repeatedly a
  74 constant string (usually a string consisting of all zeros).
  75 The returned
  76 value points to the encrypted password, a series of 13 printable ASCII
  77 characters (the first two characters represent the salt itself).
  78 The return value points to static data whose content is
  79 overwritten by each call.
  80 .PP
  81 Warning: the key space consists of
  82 .if t 2\s-2\u56\s0\d
  83 .if n 2**56
  84 equal 7.2e16 possible values.
  85 Exhaustive searches of this key space are
  86 possible using massively parallel computers.
  87 Software, such as
  88 .BR crack (1),
  89 is available which will search the portion of this key space that is
  90 generally used by humans for passwords.
  91 Hence, password selection should,
  92 at minimum, avoid common words and names.
  93 The use of a
  94 .BR passwd (1)
  95 program that checks for crackable passwords during the selection process is
  96 recommended.
  97 .PP
  98 The DES algorithm itself has a few quirks which make the use of the
  99 .BR crypt ()
 100 interface a very poor choice for anything other than password
 101 authentication.
 102 If you are planning on using the
 103 .BR crypt ()
 104 interface for a cryptography project, don't do it: get a good book on
 105 encryption and one of the widely available DES libraries.
 106 .PP
 107 .BR crypt_r ()
 108 is a reentrant version of
 109 .BR crypt ().
 110 The structure pointed to by
 111 .I data
 112 is used to store result data and bookkeeping information.
 113 Other than allocating it,
 114 the only thing that the caller should do with this structure is to set
 115 .I data->initialized
 116 to zero before the first call to
 117 .BR crypt_r ().
 118 .SH RETURN VALUE
 119 On success, a pointer to the encrypted password is returned.
 120 On error, NULL is returned.
 121 .SH ERRORS
 122 .TP
 123 .B EINVAL
 124 .I salt
 125 has the wrong format.
 126 .TP
 127 .B ENOSYS
 128 The
 129 .BR crypt ()
 130 function was not implemented, probably because of U.S.A. export restrictions.
 131 .\" This level of detail is not necessary in this man page. . .
 132 .\" .PP
 133 .\" When encrypting a plain text P using DES with the key K results in the
 134 .\" encrypted text C, then the complementary plain text P' being encrypted
 135 .\" using the complementary key K' will result in the complementary encrypted
 136 .\" text C'.
 137 .\" .PP
 138 .\" Weak keys are keys which stay invariant under the DES key transformation.
 139 .\" The four known weak keys 0101010101010101, fefefefefefefefe,
 140 .\" 1f1f1f1f0e0e0e0e and e0e0e0e0f1f1f1f1 must be avoided.
 141 .\" .PP
 142 .\" There are six known half weak key pairs, which keys lead to the same
 143 .\" encrypted data.  Keys which are part of such key clusters should be
 144 .\" avoided.
 145 .\" Sorry, I could not find out what they are.
 146 .\""
 147 .\" .PP
 148 .\" Heavily redundant data causes trouble with DES encryption, when used in the
 149 .\" .I codebook
 150 .\" mode that
 151 .\" .BR crypt ()
 152 .\" implements.  The
 153 .\" .BR crypt ()
 154 .\" interface should be used only for its intended purpose of password
 155 .\" verification, and should not be used as part of a data encryption tool.
 156 .\" .PP
 157 .\" The first and last three output bits of the fourth S-box can be
 158 .\" represented as function of their input bits.  Empiric studies have
 159 .\" shown that S-boxes partially compute the same output for similar input.
 160 .\" It is suspected that this may contain a back door which could allow the
 161 .\" NSA to decrypt DES encrypted data.
 162 .\" .PP
 163 .\" Making encrypted data computed using crypt() publicly available has
 164 .\" to be considered insecure for the given reasons.
 165 .TP
 166 .B EPERM
 167 .I /proc/sys/crypto/fips_enabled
 168 has a nonzero value,
 169 and an attempt was made to use a weak encryption type, such as DES.
 170 .SH ATTRIBUTES
 171 For an explanation of the terms used in this section, see
 172 .BR attributes (7).
 173 .TS
 174 allbox;
 175 lb lb lb
 176 l l l.
 177 Interface       Attribute       Value
 178 T{
 179 .BR crypt ()
 180 T}      Thread safety   MT-Unsafe race:crypt
 181 T{
 182 .BR crypt_r ()
 183 T}      Thread safety   MT-Safe
 184 .TE
 185 .SH CONFORMING TO
 186 .BR crypt ():
 187 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 188 .BR crypt_r ()
 189 is a GNU extension.
 190 .SH NOTES
 191 .SS Availability in glibc
 192 The
 193 .BR crypt (),
 194 .BR encrypt (),
 195 and
 196 .BR setkey ()
 197 functions are part of the POSIX.1-2008 XSI Options Group for Encryption
 198 and are optional.
 199 If the interfaces are not available then the symbolic constant
 200 .BR _XOPEN_CRYPT
 201 is either not defined or defined to -1, and can be checked at runtime with
 202 .BR sysconf ().
 203 This may be the case if the downstream distribution has switched from glibc
 204 crypt to libxcrypt.
 205 When recompiling applications in such distributions the
 206 user must detect if
 207 .BR _XOPEN_CRPYT
 208 is not available and include crypt.h for the function prototypes;
 209 otherwise
 210 libxcrypt is a ABI compatible drop-in replacement.
 211 .SS Features in glibc
 212 The glibc version of this function supports additional
 213 encryption algorithms.
 214 .PP
 215 If
 216 .I salt
 217 is a character string starting with the characters "$\fIid\fP$"
 218 followed by a string optionally terminated by "$",
 219 then the result has the form:
 220 .RS
 221 .PP
 222 $\fIid\fP$\fIsalt\fP$\fIencrypted\fP
 223 .PP
 224 .RE
 225 .I id
 226 identifies the encryption method used instead of DES and this
 227 then determines how the rest of the password string is interpreted.
 228 The following values of
 229 .I id
 230 are supported:
 231 .RS
 232 .TS
 233 l l.
 234 ID  | Method
 235 _
 236 1   | MD5
 237 2a  | Blowfish (not in mainline glibc; added in some
 238     | Linux distributions)
 239 .\" openSUSE has Blowfish, but AFAICS, this option is not supported
 240 .\" natively by glibc -- mtk, Jul 08
 241 .\"
 242 .\" md5 | Sun MD5
 243 .\" glibc doesn't appear to natively support Sun MD5; I don't know
 244 .\" if any distros add the support.
 245 5   | SHA-256 (since glibc 2.7)
 246 6   | SHA-512 (since glibc 2.7)
 247 .TE
 248 .RE
 249 .PP
 250 Thus, $5$\fIsalt\fP$\fIencrypted\fP and $6$\fIsalt\fP$\fIencrypted\fP
 251 contain the password encrypted with, respectively, functions
 252 based on SHA-256 and SHA-512.
 253 .PP
 254 "\fIsalt\fP" stands for the up to 16 characters
 255 following "$\fIid\fP$" in the salt.
 256 The "\fIencrypted\fP"
 257 part of the password string is the actual computed password.
 258 The size of this string is fixed:
 259 .TS
 260 l l.
 261 MD5     | 22 characters
 262 SHA-256 | 43 characters
 263 SHA-512 | 86 characters
 264 .TE
 265 .sp 1
 266 The characters in "\fIsalt\fP" and "\fIencrypted\fP" are drawn from the set
 267 [\fBa\-zA\-Z0\-9./\fP].
 268 In the MD5 and SHA implementations the entire
 269 .I key
 270 is significant (instead of only the first
 271 8 bytes in DES).
 272 .PP
 273 Since glibc 2.7,
 274 .\" glibc commit 9425cb9eea6a62fc21d99aafe8a60f752b934b05
 275 the SHA-256 and SHA-512 implementations support a user-supplied number of
 276 hashing rounds, defaulting to 5000.
 277 If the "$\fIid\fP$" characters in the salt are
 278 followed by "rounds=\fIxxx\fP$", where \fIxxx\fP is an integer, then the
 279 result has the form
 280 .RS
 281 .PP
 282 $\fIid\fP$\fIrounds=yyy\fP$\fIsalt\fP$\fIencrypted\fP
 283 .PP
 284 .RE
 285 where \fIyyy\fP is the number of hashing rounds actually used.
 286 The number of rounds actually used is 1000 if
 287 .I xxx
 288 is less than
 289 1000, 999999999 if
 290 .I xxx
 291 is greater than 999999999, and
 292 is equal to
 293 .I xxx
 294 otherwise.
 295 .SH SEE ALSO
 296 .BR login (1),
 297 .BR passwd (1),
 298 .BR encrypt (3),
 299 .BR getpass (3),
 300 .BR passwd (5)