libc/crypt/crypt.3

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.\" Manual page, using -mandoc macros
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.Dd $Mdocdate: May 16 2014 $
.Dt CRYPT 3
.Os
.Sh NAME
.Nm crypt ,
.Nm crypt_checkpass ,
.Nm setkey ,
.Nm encrypt ,
.Nm des_setkey ,
.Nm des_cipher ,
.Nm bcrypt_gensalt ,
.Nm bcrypt
.Nd password hashing
.Sh SYNOPSIS
.In stdlib.h
.Ft int
.Fn setkey "const char *key"
.Pp
.In unistd.h
.Ft char *
.Fn crypt "const char *key" "const char *setting"
.Ft int
.Fn crypt_checkpass "const char *password" "const char *hash"
.Ft int
.Fn encrypt "char *block" "int flag"
.Ft int
.Fn des_setkey "const char *key"
.Ft int
.Fn des_cipher "const char *in" "char *out" "int32_t salt" "int count"
.In pwd.h
.Ft char *
.Fn bcrypt_gensalt "u_int8_t log_rounds"
.Ft char *
.Fn bcrypt "const char *key" "const char *salt"
.Sh DESCRIPTION
The
.Fn crypt
function performs password hashing based on the
.Tn NBS
Data Encryption Standard (DES).
Additional code has been added to deter key search attempts and to use
stronger hashing algorithms.
.Pp
The first argument to
.Fn crypt
is a
.Dv NUL Ns -terminated
string, typically a user's typed password.
The second is in one of three forms:
if it begins with an underscore
.Pq Ql _
then an extended format is used
in interpreting both the key and the setting, as outlined below.
If it begins
with a string character
.Pq Ql $
and a number then a different algorithm is used depending on the number.
At the moment
.Ql $2
chooses Blowfish hashing; see below for more information.
.Pp
The
.Fn crypt_checkpass
function is provided to simplify checking a user's password.
If both the hash and the password are the empty string, authentication
is a success.
Otherwise, the password is hashed and compared to the provided hash.
If the hash is NULL, authentication will always fail, but a default
amount of work is performed to simulate the hashing operation.
A successful match will return 0.
A failure will return \-1 and set errno.
.Ss Extended crypt
The
.Ar key
is divided into groups of 8 characters (the last group is null-padded)
and the low-order 7 bits of each character (56 bits per group) are
used to form the DES key as follows:
the first group of 56 bits becomes the initial DES key.
For each additional group, the XOR of the encryption of the current DES
key with itself and the group bits becomes the next DES key.
.Pp
The setting is a 9-character array consisting of an underscore followed
by 4 bytes of iteration count and 4 bytes of salt.
These are encoded as printable characters, 6 bits per character,
least significant character first.
The values 0 to 63 are encoded as
.Dq \&./0-9A-Za-z .
This allows 24 bits for both
.Fa count
and
.Fa salt .
.Ss "Blowfish" crypt
The
.Tn Blowfish
version of crypt has 128 bits of
.Fa salt
in order to make building dictionaries of common passwords space consuming.
The initial state of the
.Tn Blowfish
cipher is expanded using the
.Fa salt
and the
.Fa password
repeating the process a variable number of rounds, which is encoded in
the password string.
The maximum password length is 72.
The final Blowfish password entry is created by encrypting the string
.Pp
.Dq OrpheanBeholderScryDoubt
.Pp
with the
.Tn Blowfish
state 64 times.
.Pp
The version number, the logarithm of the number of rounds and
the concatenation of salt and hashed password are separated by the
.Ql $
character.
An encoded
.Sq 8
would specify 256 rounds.
A valid Blowfish password looks like this:
.Pp
.Dq $2b$12$FPWWO2RJ3CK4FINTw0Hi8OiPKJcX653gzSS.jqltHFMxyDmmQ0Hqq .
.Pp
The whole Blowfish password string is passed as
.Fa setting
for interpretation.
.Ss "Traditional" crypt
The first 8 bytes of the key are null-padded, and the low-order 7 bits of
each character is used to form the 56-bit
.Tn DES
key.
.Pp
The setting is a 2-character array of the ASCII-encoded salt.
Thus only 12 bits of
.Fa salt
are used.
.Fa count
is set to 25.
.Ss DES Algorithm
The
.Fa salt
introduces disorder in the
.Tn DES
algorithm in one of 16777216 or 4096 possible ways
(i.e., with 24 or 12 bits: if bit
.Em i
of the
.Ar salt
is set, then bits
.Em i
and
.Em i+24
are swapped in the
.Tn DES
E-box output).
.Pp
The DES key is used to encrypt a 64-bit constant using
.Ar count
iterations of
.Tn DES .
The value returned is a
.Dv NUL Ns -terminated
string, 20 or 13 bytes (plus NUL) in length, consisting of the
.Ar setting
followed by the encoded 64-bit encryption.
.Pp
The functions
.Fn encrypt ,
.Fn setkey ,
.Fn des_setkey ,
and
.Fn des_cipher
provide access to the
.Tn DES
algorithm itself.
.Fn setkey
is passed a 64-byte array of binary values (numeric 0 or 1).
A 56-bit key is extracted from this array by dividing the
array into groups of 8, and ignoring the last bit in each group.
That bit is reserved for a byte parity check by DES, but is ignored
by these functions.
.Pp
The
.Fa block
argument to
.Fn encrypt
is also a 64-byte array of binary values.
If the value of
.Fa flag
is 0,
.Fa block
is encrypted otherwise it is decrypted.
The result is returned in the original array
.Fa block
after using the key specified by
.Fn setkey
to process it.
.Pp
The argument to
.Fn des_setkey
is a character array of length 8.
The least significant bit (the parity bit) in each character is ignored,
and the remaining bits are concatenated to form a 56-bit key.
The function
.Fn des_cipher
encrypts (or decrypts if
.Fa count
is negative) the 64-bits stored in the 8 characters at
.Fa in
using
.Xr abs 3
of
.Fa count
iterations of
.Tn DES
and stores the 64-bit result in the 8 characters at
.Fa out
(which may be the same as
.Fa in ) .
The
.Fa salt
specifies perturbations to the
.Tn DES
E-box output as described above.
.Pp
The
.Fn crypt ,
.Fn setkey ,
and
.Fn des_setkey
functions all manipulate the same key space.
.Sh RETURN VALUES
The function
.Fn crypt
returns a pointer to the encrypted value on success, and
.Dv NULL
on failure.
The functions
.Fn setkey ,
.Fn encrypt ,
.Fn des_setkey ,
and
.Fn des_cipher
return 0 on success and 1 on failure.
.Sh SEE ALSO
.Xr encrypt 1 ,
.Xr login 1 ,
.Xr passwd 1 ,
.Xr blowfish 3 ,
.Xr getpass 3 ,
.Xr md5 3 ,
.Xr passwd 5
.Sh HISTORY
A rotor-based
.Fn crypt
function appeared in
.At v3 .
The current style
.Fn crypt
first appeared in
.At v7 .
.Sh AUTHORS
.An David Burren Aq Mt davidb@werj.com.au
wrote the original DES functions.
.Sh BUGS
The
.Fn crypt
function returns a pointer to static data, and subsequent calls to
.Fn crypt
will modify the same object.
.Pp
With DES hashing, passwords containing the byte 0x80 use less key entropy
than other passwords.
This is an implementation bug, not a bug in the DES cipher.