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This is a major rewrite of the description of 'crypt', 'getentropy', and 'getrandom'. A few highlights of the content changes: - Throughout the manual, public headers, and user-visible messages, I replaced the term "password" with "passphrase", the term "password database" with "user database", and the term "encrypt(ion)" with "(one-way) hashing" whenever it was applied to passphrases. I didn't bother making this change in internal code or tests. The use of the term "password" in ruserpass.c survives, because that refers to a keyword in netrc files, but it is adjusted to make this clearer. There is a note in crypt.texi explaining that they were traditionally called passwords but single words are not good enough anymore, and a note in users.texi explaining that actual passphrase hashes are found in a "shadow" database nowadays. - There is a new short introduction to the "Cryptographic Functions" section, explaining how we do not intend to be a general-purpose cryptography library, and cautioning that there _are_, or have been, legal restrictions on the use of cryptography in many countries, without getting into any kind of detail that we can't promise to keep up to date. - I added more detail about what a "one-way function" is, and why they are used to obscure passphrases for storage. I removed the paragraph saying that systems not connected to a network need no user authentication, because that's a pretty rare situation nowadays. (It still says "sometimes it is necessary" to authenticate the user, though.) - I added documentation for all of the hash functions that glibc actually supports, but not for the additional hash functions supported by libxcrypt. If we're going to keep this manual section around after the transition is more advanced, it would probably make sense to add them then. - There is much more detailed discussion of how to generate a salt, and the failure behavior for crypt is documented. (Returning an invalid hash on failure is what libxcrypt does; Solar Designer's notes say that this was done "for compatibility with old programs that assume crypt can never fail".) - As far as I can tell, the header 'crypt.h' is entirely a GNU invention, and never existed on any other Unix lineage. The function 'crypt', however, was in Issue 1 of the SVID and is now in the XSI component of POSIX. I tried to make all of the @standards annotations consistent with this, but I'm not sure I got them perfectly right. - The genpass.c example has been improved to use getentropy instead of the current time to generate the salt, and to use a SHA-256 hash instead of MD5. It uses more random bytes than is strictly necessary because I didn't want to complicate the code with proper base64 encoding. - The testpass.c example has three hardwired hashes now, to demonstrate that different one-way functions produce different hashes for the same input. It also demonstrates how DES hashing only pays attention to the first eight characters of the input. - There is new text explaining in more detail how a CSPRNG differs from a regular random number generator, and how getentropy/getrandom are not exactly a CSPRNG. I tried not to make specific falsifiable claims here. I also tried to make the blocking/cancellation/error behavior of both getentropy and getrandom clearer.
60 lines
1.6 KiB
C
60 lines
1.6 KiB
C
/* Encrypting Passwords
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Copyright (C) 1991-2018 Free Software Foundation, Inc.
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License
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as published by the Free Software Foundation; either version 2
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of the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, if not, see <http://www.gnu.org/licenses/>.
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*/
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#include <stdio.h>
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#include <unistd.h>
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#include <crypt.h>
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int
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main(void)
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{
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unsigned char ubytes[16];
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char salt[20];
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const char *const saltchars =
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"./0123456789ABCDEFGHIJKLMNOPQRST"
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"UVWXYZabcdefghijklmnopqrstuvwxyz";
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char *hash;
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int i;
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/* Retrieve 16 unpredictable bytes from the operating system. */
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if (getentropy (ubytes, sizeof ubytes))
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{
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perror ("getentropy");
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return 1;
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}
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/* Use them to fill in the salt string. */
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salt[0] = '$';
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salt[1] = '5'; /* SHA-256 */
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salt[2] = '$';
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for (i = 0; i < 16; i++)
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salt[3+i] = saltchars[ubytes[i] & 0x3f];
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salt[3+i] = '\0';
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/* Read in the user's passphrase and hash it. */
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hash = crypt (getpass ("Enter new passphrase: "), salt);
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if (!hash || hash[0] == '*')
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{
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perror ("crypt");
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return 1;
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}
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/* Print the results. */
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puts (hash);
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return 0;
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}
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