Manpages - crypt.3

The

and

functions irreversibly

for storage in the system password database

using a cryptographic

The result of this operation is called a

or just a

Hashing methods are described in

controls which hashing method to use, and also supplies various parameters to the chosen method, most importantly a random

which ensures that no two stored hashes are the same, even if the

strings are the same.

The

argument to

is a structure of type

It has at least these fields:

struct crypt_data { char output[CRYPT_OUTPUT_SIZE]; char setting[CRYPT_OUTPUT_SIZE]; char phrase[CRYPT_MAX_PASSPHRASE_SIZE]; char initialized; };

Upon a successful return from

the hashed passphrase will be stored in

Applications are encouraged, but not required, to use the

and

fields to store the strings that they will pass as

and

This will make it easier to erase all sensitive data after it is no longer needed.

The

field must be set to zero before the first time a

object is first used in a call to

We recommend zeroing the entire object, not just

and not just the documented fields, before the first use. (Of course, do this before storing anything in

and

The

argument to

should also point to a

object, and

should be the size of that object, cast to

When used with

the entire

object (except for the

and

fields) must be zeroed before its first use; this is not just a recommendation, as it is for

Otherwise, the fields of the object have the same uses that they do for

On the first call to

should be the address of a

variable set to NULL, and

should be the address of an

variable set to zero.

will allocate and initialize a

object, using

and write its address and size into the variables pointed to by

and

These can be reused in subsequent calls. After the application is done hashing passphrases, it should deallocate the

object using

Upon successful completion,

and

return a pointer to a string which encodes both the hashed passphrase, and the settings that were used to encode it. This string is directly usable as

in other calls to

and

and as

in calls to

and

It will be entirely printable ASCII, and will not contain whitespace or the characters

See

for more detail on the format of hashed passphrases.

places its result in a static storage area, which will be overwritten by subsequent calls to

It is not safe to call

from multiple threads simultaneously.

and

place their result in the

field of their

argument. It is safe to call them from multiple threads simultaneously, as long as a separate

object is used for each thread.

Upon error,

and

write an

hashed passphrase to the

field of their

argument, and

writes an invalid hash to its static storage area. This string will be shorter than 13 characters, will begin with a

and will not compare equal to

Upon error,

and

return a null pointer.

and

may also return a null pointer, or they may return a pointer to the invalid hash, depending on how libcrypt was configured. (The option to return the invalid hash is for compatibility with old applications that assume that

cannot return a null pointer. See

below.)

All four functions set

when they fail.

is invalid, or requests a hashing method that is not supported.

is too long (more than

characters; some hashing methods may have lower limits).

only:

is too small for the hashing method requested by

Failed to allocate internal scratch memory.

only: failed to allocate memory for

Hashing passphrases is not supported at all on this installation, or the hashing method requested by

is not supported. These error codes are not used by this version of libcrypt, but may be encountered on other systems.

is included in POSIX, but

and

are not part of any standard.

POSIX does not specify any hashing methods, and does not require hashed passphrases to be portable between systems. In practice, hashed passphrases are portable as long as both systems support the hashing method that was used. However, the set of supported hashing methods varies considerably from system to system.

The behavior of

on errors isn’t well standardized. Some implementations simply can’t fail (except by crashing the program), others return a null pointer or a fixed string. Most implementations don’t set

but some do. POSIX specifies returning a null pointer and setting

but it defines only one possible error,

in the case where

is not supported at all. Some older applications are not prepared to handle null pointers returned by

The behavior described above for this implementation, setting

and returning an invalid hashed passphrase different from

is chosen to make these applications fail closed when an error occurs.

Due to historical restrictions on the export of cryptographic software from the USA,

is an optional POSIX component. Applications should therefore be prepared for

not to be available, or to always fail (setting

at runtime.

POSIX specifies that

is declared in

but only if the macro

is defined and has a value greater than or equal to zero. Since libcrypt does not provide

it declares

and

instead.

On a minority of systems (notably recent versions of Solaris),

uses a thread-specific static storage buffer, which makes it safe to call from multiple threads simultaneously, but does not prevent each call within a thread from overwriting the results of the previous one.

Some implementations of

upon error, return an invalid hash that is stored in a read-only location or only initialized once, which means that it is only safe to erase the buffer pointed to by the

return value if an error did not occur.

may be quite large (32kB in this implementation of libcrypt; over 128kB in some other implementations). This is large enough that it may be unwise to allocate it on the stack.

Some recently designed hashing methods need even more scratch memory, but the

interface makes it impossible to change the size of

without breaking binary compatibility. The

interface could accommodate larger allocations for specific hashing methods, but the caller of

has no way of knowing how much memory to allocate.

does the allocation itself, but can only make a single call to

For an explanation of the terms used in this section, see

Interface	Attribute	Value
	Thread safety	MT-Unsafe race:crypt
	Thread safety	MT-Safe

A rotor-based

function appeared in

The

DES-based

first appeared in

originates with the GNU C Library. There’s also a

function on HP-UX and MKS Toolkit, but the prototypes and semantics differ.

and

originate with the Openwall project.