crypt_gensalt (3) - Linux Manuals

crypt_gensalt: encode settings for passphrase hashing

Openwall Project


crypt_gensalt , crypt_gensalt_rn , crypt_gensalt_ra - encode settings for passphrase hashing


Lb libcrypt


In crypt.h Ft char * Fo crypt_gensalt Fa const char *prefix Fa unsigned long count Fa const char *rbytes Fa int nrbytes Fc Ft char * Fo crypt_gensalt_rn Fa const char * prefix Fa unsigned long count Fa const char *rbytes Fa int nrbytes Fa char * output Fa int output_size Fc Ft char * Fo crypt_gensalt_ra Fa const char *prefix Fa unsigned long count Fa const char *rbytes Fa int nrbytes Fc


The crypt_gensalt crypt_gensalt_rn and crypt_gensalt_ra functions compile a string for use as the Fa setting argument to crypt crypt_r crypt_rn and crypt_ra Fa prefix selects the hashing method to use. Fa count controls the CPU time cost of the hash; the valid range for Fa count and the exact meaning of ``CPU time cost'' depends on the hashing method, but larger numbers correspond to more costly hashes. Fa rbytes should point to Fa nrbytes cryptographically random bytes for use as ``salt.''

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

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


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.

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. 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 Fa output buffer, which has Fa output_size bytes of storage available. Fa output_size should be greater than or equal to CRYPT_GENSALT_OUTPUT_SIZE

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 Fa prefix .


Fa prefix is invalid or not supported by this implementation; Fa count is invalid for the requested Fa prefix ; the input Fa nrbytes is insufficient for the smallest valid salt with the requested Fa prefix .
crypt_gensalt_rn only: Fa output_size is too small to hold the compiled Fa setting string.
Failed to allocate internal scratch memory.
crypt_gensalt_ra only: failed to allocate memory for the compiled Fa setting string.
Obtaining random bytes from the operating system failed. This can only happen when Fa rbytes is a null pointer.


The following macros are defined by In crypt.h :

A null pointer can be specified as the Fa prefix argument.
A null pointer can be specified as the Fa rbytes argument.


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 Fa prefix and Fa rbytes arguments.

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


For an explanation of the terms used in this section, see attributes(7).
crypt_gensalt Thread safetyMT-Unsafe race:crypt_gensalt
crypt_gensalt_rn crypt_gensalt_ra Thread safetyMT-Safe


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