CRYPT_GENSALT(3)

NAME
crypt_gensalt, crypt_gensalt_rn, crypt_gensalt_ra - encode settings for passphrase hashing

LIBRARY
libcrypt

SYNOPSIS
#include <crypt.h>
char *
crypt_gensalt(const char *prefix, unsigned long count, const char *rbytes, int nrbytes);
char *
crypt_gensalt_rn(const char * prefix, unsigned long count, const char *rbytes, int nrbytes, char * output, int output_size);
char *
crypt_gensalt_ra(const char *prefix, unsigned long count, const char *rbytes, int nrbytes);

DESCRIPTION
The crypt_gensalt, crypt_gensalt_rn, and crypt_gensalt_ra functions compile a string for use as the setting argument to crypt, crypt_r, crypt_rn, and crypt_ra. prefix selects the hashing method to use. count controls the processing cost of the hash; the valid range and exact meaning of count depend on the hashing method, but larger numbers correspond to more costly hashes in terms of CPU time and possibly memory usage. rbytes should point to nrbytes cryptographically random bytes for use as “salt.”

If prefix is a null pointer, the best available hashing method will be selected. (CAUTION: if prefix is an empty string, the “traditional” DES-based hashing method will be selected; this method is unacceptably weak by modern standards.) If count is 0, a low default cost will be selected. If rbytes is a null pointer, an appropriate number of random bytes will be obtained from the operating system, and nrbytes is ignored.

See crypt(5) for other strings that can be used as prefix, and valid values of count for each.

RETURN VALUES
crypt_gensalt, crypt_gensalt_rn, and crypt_gensalt_ra return a pointer to an encoded setting string. This string will be entirely printable ASCII, and will not contain whitespace or the characters ‘’:, ‘’;, ‘*’, ‘’!, or ‘\’. See crypt(5) for more detail on the format of this string. Upon error, they return a null pointer and set errno to an appropriate error code. When the functions succeed, the value of errno is unspecified and must not be relied upon.

crypt_gensalt places its result in a static storage area, which will be overwritten by subsequent calls to crypt_gensalt. It is not safe to call crypt_gensalt from multiple threads simultaneously. It's also not recommended to use the pointer returned as an argument for another call to crypt_gensalt, as some implementations, including earlier releases of libxcrypt, may overwrite the underlying static output buffer before computing the setting. However, it is safe to pass the string returned by crypt_gensalt directly to crypt without copying it; each function has its own static storage area.

crypt_gensalt_rn places its result in the supplied output buffer, which has output_size bytes of storage available. output_size should be greater than or equal to CRYPT_GENSALT_OUTPUT_SIZE. It's also not recommended to use the pointer returned as an argument for another call to crypt_gensalt_rn using the same output buffer for subsequent calls, as some implementations, including earlier releases of libxcrypt, may overwrite the supplied buffer before computing the setting.

crypt_gensalt_ra allocates memory for its result using malloc(3). It should be freed with free(3) after use.

Upon error, in addition to returning a null pointer, crypt_gensalt and crypt_gensalt_rn will write an invalid setting string to their output buffer, if there is enough space; this string will begin with a ‘*’ and will not be equal to prefix.

ERRORS

EINVAL
    prefix is invalid or not supported by this implementation; count is invalid for the requested prefix; the input nrbytes is insufficient for the smallest valid salt with the requested prefix.

ERANGE
    crypt_gensalt_rn only: output_size is too small to hold the compiled setting string.

ENOMEM
    Failed to allocate internal scratch memory.
    crypt_gensalt_ra only: failed to allocate memory for the compiled setting string.

ENOSYS, EACCES, EIO, etc.
    Obtaining random bytes from the operating system failed. This can only happen when rbytes is a null pointer.

FEATURE TEST MACROS
The following macros are defined by <crypt.h>:

CRYPT_GENSALT_IMPLEMENTS_DEFAULT_PREFIX
    A null pointer can be specified as the prefix argument.

CRYPT_GENSALT_IMPLEMENTS_AUTO_ENTROPY
    A null pointer can be specified as the rbytes argument.

PORTABILITY NOTES
The functions crypt_gensalt, crypt_gensalt_rn, and crypt_gensalt_ra are not part of any standard. They originate with the Openwall project. A function with the name crypt_gensalt also exists on Solaris 10 and newer, but its prototype and semantics differ.

The default prefix and auto entropy features are available since libxcrypt version 4.0.0. Portable software can use feature test macros to find out whether null pointers can be used for the prefix and rbytes arguments.

The set of supported hashing methods varies considerably from system to system.

ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value
 | Thread safety | MT-Unsafe race:crypt_gensalt
 | Thread safety | MT-Safe

SEE ALSO
crypt(3), getpass(3), getpwent(3), shadow(3), login(1), passwd(1), crypt(5), passwd(5), shadow(5), pam(8)