CRYPT(3POSIX)

💖 CRYPT(3POSIX) 💖

okay bestie, gather round, we're doing crypt from the POSIX Programmer's Manual. yes it's called CRYPT, yes it's in section 3POSIX, and yes it's giving main character energy for a function that's basically "make string look like nonsense on purpose". let's GO 🦋

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🌸 PROLOG 🌸

okay so real talk first, because this part matters:

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

so like… this is the POSIX spec version, not necessarily your Linux version. lowkey important. check your local man page, bestie. 💅

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✨ NAME ✨

crypt — string encoding function (CRYPT)

it encodes strings. that's the vibe. no cap.

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🎀 SYNOPSIS 🎀

#include <unistd.h>

char *crypt(const char *key, const char *salt);

two args, one pointer back. iconic signature honestly.

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💜 DESCRIPTION 💜

The crypt() function is a string encoding function. The algorithm is implementation-defined.

read that again. implementation-defined. so whatever it does to your string is between your implementation and the algorithm. not us. 🫶

The key argument points to a string to be encoded.

The salt argument shall be a string of at least two bytes in length, not including the null character, chosen from the set:

a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9 . /

that's lowercase, uppercase, digits, dot and slash. 64 characters, the whole little alphabet. 🌈

The first two bytes of this string may be used to perturb the encoding algorithm. so the salt's first two chars are the flavor packet, basically. 🍓

The return value of crypt() points to static data that is overwritten by each call.

🚨 bestie. STATIC DATA. it gets overwritten every single call. do not hold onto that pointer and act surprised later. copy it if you need it. i'm serious. 😭

The crypt() function need not be thread-safe.

translation: it is not promising you thread safety. if you're threading, that's a you problem, and the static buffer is right there waiting to betray you. 💀

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⭐ RETURN VALUE ⭐

Upon successful completion, crypt() shall return a pointer to the encoded string. The first two bytes of the returned value shall be those of the salt argument.

otherwise? it shall return a null pointer and set errno to indicate the error. so like, always check. always. 🙈

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🔥 ERRORS 🔥

The crypt() function shall fail if:

• ENOSYS — The functionality is not supported on this implementation.

that's the only one listed, and that's the whole list. slay.

The following sections are informative.

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💖 EXAMPLES 💖

🌸 Encoding Passwords 🌸

The following example finds a user database entry matching a particular user name and changes the current password to a new password.

The crypt() function generates an encoded version of each password. The first call to crypt() produces an encoded version of the old password; that encoded password is then compared to the password stored in the user database. The second call to crypt() encodes the new password before it is stored.

The putpwent() function, used in the following example, is not part of POSIX.1‐2008. (noted, bestie, don't come for me.)

#include <unistd.h>
#include <pwd.h>
#include <string.h>
#include <stdio.h>
...
int valid_change;
int pfd;  /* Integer for file descriptor returned by open(). */
FILE *fpfd;  /* File pointer for use in putpwent(). */
struct passwd *p;
char user[100];
char oldpasswd[100];
char newpasswd[100];
char savepasswd[100];
...
valid_change = 0;
while ((p = getpwent()) != NULL) {
    /* Change entry if found. */
    if (strcmp(p->pw_name, user) == 0) {
        if (strcmp(p->pw_passwd, crypt(oldpasswd, p->pw_passwd)) == 0) {
            strcpy(savepasswd, crypt(newpasswd, user));
            p->pw_passwd = savepasswd;
            valid_change = 1;
        }
        else {
            fprintf(stderr, "Old password is not valid\n");
        }
    }
    /* Put passwd entry into ptmp. */
    putpwent(p, fpfd);
}

see how it salts with the existing stored password to check the old one, then salts with user for the new one? that's the whole trick. 🧚

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🦋 APPLICATION USAGE 🦋

The values returned by this function need not be portable among XSI-conformant systems.

Several implementations offer extensions via characters outside of the set specified for the salt argument for specifying alternative algorithms; while not portable, these extensions may offer better security.

The use of crypt() for anything other than password hashing is not recommended.

💅 so like: don't use it as a general-purpose hash. it's a password hasher. that's the assignment.

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👑 RATIONALE 👑

None.

literally none. the rationale said "no thoughts, head empty". 😭

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🌈 FUTURE DIRECTIONS 🌈

None.

same energy. nothing coming. 🎀

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💜 SEE ALSO 💜

• encrypt(), setkey()
• The Base Definitions volume of POSIX.1‐2017, <unistd.h>

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🫶 COPYRIGHT 🫶

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group.

In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, bestie!! brought to you by the IEEE/The Open Group, 2017, straight out of the POSIX Programmer's Manual 💖 now go check your return value before you touch that static buffer, ily, bye ✨