mirror of git://sourceware.org/git/glibc.git
Remove documentation of passphrase-hashing functions.
crypt.texi itself continues to exist, since it also documents getentropy and getrandom. I deleted the paragraph at the beginning of crypt.texi about legal restrictions on cryptographic software, because after this patchset the only cryptographic code in glibc itself will be the MD5 implementation used by localedef (see first patch in this series), which is not exposed to users of the library, and the DES implementation in sunrpc/, which is also slated for removal (right?) If this paragraph should be preserved, please let me know.
This commit is contained in:
Zack Weinberg
parent
a0b80d284c
commit
0a19410103
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@ -199,7 +199,7 @@ Romain Geissler for various fixes.
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@item
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Michael Glad for the passphrase-hashing function @code{crypt} and related
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functions.
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functions (no longer part of glibc, but we still appreciate his work).
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@item
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Wolfram Gloger for contributing the memory allocation functions
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@ -1,205 +1,18 @@
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@node Cryptographic Functions, Debugging Support, System Configuration, Top
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@chapter Cryptographic Functions
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@c %MENU% Passphrase storage and strongly unpredictable bytes.
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@c %MENU% A few functions to support cryptographic applications
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@Theglibc{} includes only a few special-purpose cryptographic
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functions: one-way hash functions for passphrase storage, and access
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to a cryptographic randomness source, if one is provided by the
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operating system. Programs that need general-purpose cryptography
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should use a dedicated cryptography library, such as
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@Theglibc{} includes only one type of special-purpose cryptographic
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functions; these allow use of a source of cryptographically strong
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pseudorandom numbers, if such a source is provided by the operating
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system. Programs that need general-purpose cryptography should use
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a dedicated cryptography library, such as
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@uref{https://www.gnu.org/software/libgcrypt/,,libgcrypt}.
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Many countries place legal restrictions on the import, export,
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possession, or use of cryptographic software. We deplore these
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restrictions, but we must still warn you that @theglibc{} may be
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subject to them, even if you do not use the functions in this chapter
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yourself. The restrictions vary from place to place and are changed
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often, so we cannot give any more specific advice than this warning.
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@menu
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* Passphrase Storage:: One-way hashing for passphrases.
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* Unpredictable Bytes:: Randomness for cryptographic purposes.
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@end menu
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@node Passphrase Storage
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@section Passphrase Storage
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@cindex passphrase hashing
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@cindex one-way hashing
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@cindex hashing, passphrase
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Sometimes it is necessary to be sure that a user is authorized
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to use some service a machine provides---for instance, to log in as a
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particular user id (@pxref{Users and Groups}). One traditional way of
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doing this is for each user to choose a secret @dfn{passphrase}; then, the
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system can ask someone claiming to be a user what the user's passphrase
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is, and if the person gives the correct passphrase then the system can
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grant the appropriate privileges. (Traditionally, these were called
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``passwords,'' but nowadays a single word is too easy to guess.)
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Programs that handle passphrases must take special care not to reveal
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them to anyone, no matter what. It is not enough to keep them in a
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file that is only accessible with special privileges. The file might
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be ``leaked'' via a bug or misconfiguration, and system administrators
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shouldn't learn everyone's passphrase even if they have to edit that
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file for some reason. To avoid this, passphrases should also be
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converted into @dfn{one-way hashes}, using a @dfn{one-way function},
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before they are stored.
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A one-way function is easy to compute, but there is no known way to
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compute its inverse. This means the system can easily check
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passphrases, by hashing them and comparing the result with the stored
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hash. But an attacker who discovers someone's passphrase hash can
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only discover the passphrase it corresponds to by guessing and
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checking. The one-way functions are designed to make this process
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impractically slow, for all but the most obvious guesses. (Do not use
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a word from the dictionary as your passphrase.)
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@Theglibc{} provides an interface to four one-way functions, based on
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the SHA-2-512, SHA-2-256, MD5, and DES cryptographic primitives. New
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passphrases should be hashed with either of the SHA-based functions.
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The others are too weak for newly set passphrases, but we continue to
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support them for verifying old passphrases. The DES-based hash is
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especially weak, because it ignores all but the first eight characters
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of its input.
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@deftypefun {char *} crypt (const char *@var{phrase}, const char *@var{salt})
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@standards{X/Open, unistd.h}
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@standards{GNU, crypt.h}
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@safety{@prelim{}@mtunsafe{@mtasurace{:crypt}}@asunsafe{@asucorrupt{} @asulock{} @ascuheap{} @ascudlopen{}}@acunsafe{@aculock{} @acsmem{}}}
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@c Besides the obvious problem of returning a pointer into static
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@c storage, the DES initializer takes an internal lock with the usual
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@c set of problems for AS- and AC-Safety.
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@c The NSS implementations may leak file descriptors if cancelled.
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@c The MD5, SHA256 and SHA512 implementations will malloc on long keys,
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@c and NSS relies on dlopening, which brings about another can of worms.
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The function @code{crypt} converts a passphrase string, @var{phrase},
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into a one-way hash suitable for storage in the user database. The
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string that it returns will consist entirely of printable ASCII
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characters. It will not contain whitespace, nor any of the characters
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@samp{:}, @samp{;}, @samp{*}, @samp{!}, or @samp{\}.
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The @var{salt} parameter controls which one-way function is used, and
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it also ensures that the output of the one-way function is different
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for every user, even if they have the same passphrase. This makes it
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harder to guess passphrases from a large user database. Without salt,
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the attacker could make a guess, run @code{crypt} on it once, and
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compare the result with all the hashes. Salt forces the attacker to
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make separate calls to @code{crypt} for each user.
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To verify a passphrase, pass the previously hashed passphrase as the
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@var{salt}. To hash a new passphrase for storage, set @var{salt} to a
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string consisting of a prefix plus a sequence of randomly chosen
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characters, according to this table:
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@multitable @columnfractions .2 .1 .3
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@headitem One-way function @tab Prefix @tab Random sequence
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@item SHA-2-512
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@tab @samp{$6$}
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@tab 16 characters
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@item SHA-2-256
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@tab @samp{$5$}
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@tab 16 characters
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@item MD5
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@tab @samp{$1$}
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@tab 8 characters
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@item DES
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@tab @samp{}
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@tab 2 characters
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@end multitable
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In all cases, the random characters should be chosen from the alphabet
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@code{./0-9A-Za-z}.
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With all of the hash functions @emph{except} DES, @var{phrase} can be
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arbitrarily long, and all eight bits of each byte are significant.
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With DES, only the first eight characters of @var{phrase} affect the
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output, and the eighth bit of each byte is also ignored.
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@code{crypt} can fail. Some implementations return @code{NULL} on
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failure, and others return an @emph{invalid} hashed passphrase, which
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will begin with a @samp{*} and will not be the same as @var{salt}. In
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either case, @code{errno} will be set to indicate the problem. Some
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of the possible error codes are:
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@table @code
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@item EINVAL
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@var{salt} is invalid; neither a previously hashed passphrase, nor a
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well-formed new salt for any of the supported hash functions.
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@item EPERM
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The system configuration forbids use of the hash function selected by
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@var{salt}.
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@item ENOMEM
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Failed to allocate internal scratch storage.
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@item ENOSYS
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@itemx EOPNOTSUPP
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Hashing passphrases is not supported at all, or the hash function
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selected by @var{salt} is not supported. @Theglibc{} does not use
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these error codes, but they may be encountered on other operating
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systems.
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@end table
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@code{crypt} uses static storage for both internal scratchwork and the
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string it returns. It is not safe to call @code{crypt} from multiple
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threads simultaneously, and the string it returns will be overwritten
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by any subsequent call to @code{crypt}.
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@code{crypt} is specified in the X/Open Portability Guide and is
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present on nearly all historical Unix systems. However, the XPG does
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not specify any one-way functions.
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@code{crypt} is declared in @file{unistd.h}. @Theglibc{} also
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declares this function in @file{crypt.h}.
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@end deftypefun
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@deftypefun {char *} crypt_r (const char *@var{phrase}, const char *@var{salt}, struct crypt_data *@var{data})
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@standards{GNU, crypt.h}
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@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @asulock{} @ascuheap{} @ascudlopen{}}@acunsafe{@aculock{} @acsmem{}}}
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@tindex struct crypt_data
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@c Compared with crypt, this function fixes the @mtasurace:crypt
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@c problem, but nothing else.
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The function @code{crypt_r} is a thread-safe version of @code{crypt}.
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Instead of static storage, it uses the memory pointed to by its
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@var{data} argument for both scratchwork and the string it returns.
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It can safely be used from multiple threads, as long as different
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@var{data} objects are used in each thread. The string it returns
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will still be overwritten by another call with the same @var{data}.
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@var{data} must point to a @code{struct crypt_data} object allocated
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by the caller. All of the fields of @code{struct crypt_data} are
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private, but before one of these objects is used for the first time,
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it must be initialized to all zeroes, using @code{memset} or similar.
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After that, it can be reused for many calls to @code{crypt_r} without
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erasing it again. @code{struct crypt_data} is very large, so it is
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best to allocate it with @code{malloc} rather than as a local
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variable. @xref{Memory Allocation}.
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@code{crypt_r} is a GNU extension. It is declared in @file{crypt.h},
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as is @code{struct crypt_data}.
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@end deftypefun
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The following program shows how to use @code{crypt} the first time a
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passphrase is entered. It uses @code{getentropy} to make the salt as
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unpredictable as possible; @pxref{Unpredictable Bytes}.
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@smallexample
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@include genpass.c.texi
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@end smallexample
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The next program demonstrates how to verify a passphrase. It checks a
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hash hardcoded into the program, because looking up real users' hashed
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passphrases may require special privileges (@pxref{User Database}).
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It also shows that different one-way functions produce different
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hashes for the same passphrase.
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@smallexample
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@include testpass.c.texi
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@end smallexample
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@node Unpredictable Bytes
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@section Generating Unpredictable Bytes
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@cindex randomness source
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@ -211,27 +24,24 @@ hashes for the same passphrase.
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@cindex CSPRNG
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@cindex DRBG
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Cryptographic applications often need some random data that will be as
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difficult as possible for a hostile eavesdropper to guess. For
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instance, encryption keys should be chosen at random, and the ``salt''
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strings used by @code{crypt} (@pxref{Passphrase Storage}) should also
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be chosen at random.
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Some pseudo-random number generators do not provide unpredictable-enough
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output for cryptographic applications; @pxref{Pseudo-Random Numbers}.
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Such applications need to use a @dfn{cryptographic random number
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generator} (CRNG), also sometimes called a @dfn{cryptographically strong
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pseudo-random number generator} (CSPRNG) or @dfn{deterministic random
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bit generator} (DRBG).
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Cryptographic applications often need random data that will be as
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difficult as possible for a hostile eavesdropper to guess.
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The pseudo-random number generators provided by @theglibc{}
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(@pxref{Pseudo-Random Numbers}) are not suitable for this purpose.
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They produce output that is @emph{statistically} random, but fails to
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be @emph{unpredictable}. Cryptographic applications require a
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@dfn{cryptographic random number generator} (CRNG), also known as a
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@dfn{cryptographically strong pseudo-random number generator} (CSPRNG)
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or a @dfn{deterministic random bit generator} (DRBG).
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Currently, @theglibc{} does not provide a cryptographic random number
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generator, but it does provide functions that read random data from a
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@dfn{randomness source} supplied by the operating system. The
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randomness source is a CRNG at heart, but it also continually
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``re-seeds'' itself from physical sources of randomness, such as
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electronic noise and clock jitter. This means applications do not need
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to do anything to ensure that the random numbers it produces are
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different on each run.
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generator, but it does provide functions that read cryptographically
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strong random data from a @dfn{randomness source} supplied by the
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operating system. This randomness source is a CRNG at heart, but it
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also continually ``re-seeds'' itself from physical sources of
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randomness, such as electronic noise and clock jitter. This means
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applications do not need to do anything to ensure that the random
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numbers it produces are different on each run.
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The catch, however, is that these functions will only produce
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relatively short random strings in any one call. Often this is not a
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@ -1,59 +0,0 @@
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/* Encrypting Passwords
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Copyright (C) 1991-2023 Free Software Foundation, Inc.
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License
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as published by the Free Software Foundation; either version 2
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of the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, see <https://www.gnu.org/licenses/>.
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*/
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#include <stdio.h>
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#include <unistd.h>
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#include <crypt.h>
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int
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main(void)
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{
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unsigned char ubytes[16];
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char salt[20];
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const char *const saltchars =
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"./0123456789ABCDEFGHIJKLMNOPQRST"
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"UVWXYZabcdefghijklmnopqrstuvwxyz";
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char *hash;
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int i;
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/* Retrieve 16 unpredictable bytes from the operating system. */
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if (getentropy (ubytes, sizeof ubytes))
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{
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perror ("getentropy");
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return 1;
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}
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/* Use them to fill in the salt string. */
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salt[0] = '$';
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salt[1] = '5'; /* SHA-256 */
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salt[2] = '$';
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for (i = 0; i < 16; i++)
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salt[3+i] = saltchars[ubytes[i] & 0x3f];
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salt[3+i] = '\0';
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/* Read in the user's passphrase and hash it. */
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hash = crypt (getpass ("Enter new passphrase: "), salt);
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if (!hash || hash[0] == '*')
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{
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perror ("crypt");
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return 1;
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}
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/* Print the results. */
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puts (hash);
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return 0;
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}
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@ -1,67 +0,0 @@
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/* Verify a passphrase.
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Copyright (C) 1991-2023 Free Software Foundation, Inc.
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License
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as published by the Free Software Foundation; either version 2
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of the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, see <https://www.gnu.org/licenses/>.
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*/
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#include <stdio.h>
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#include <string.h>
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#include <unistd.h>
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#include <crypt.h>
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/* @samp{GNU's Not Unix} hashed using SHA-256, MD5, and DES. */
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static const char hash_sha[] =
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"$5$DQ2z5NHf1jNJnChB$kV3ZTR0aUaosujPhLzR84Llo3BsspNSe4/tsp7VoEn6";
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static const char hash_md5[] = "$1$A3TxDv41$rtXVTUXl2LkeSV0UU5xxs1";
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static const char hash_des[] = "FgkTuF98w5DaI";
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int
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main(void)
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{
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char *phrase;
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int status = 0;
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/* Prompt for a passphrase. */
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phrase = getpass ("Enter passphrase: ");
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/* Compare against the stored hashes. Any input that begins with
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@samp{GNU's No} will match the DES hash, but the other two will
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only match @samp{GNU's Not Unix}. */
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if (strcmp (crypt (phrase, hash_sha), hash_sha))
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{
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puts ("SHA: not ok");
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status = 1;
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}
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else
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puts ("SHA: ok");
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if (strcmp (crypt (phrase, hash_md5), hash_md5))
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{
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puts ("MD5: not ok");
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status = 1;
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}
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else
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puts ("MD5: ok");
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if (strcmp (crypt (phrase, hash_des), hash_des))
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{
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puts ("DES: not ok");
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status = 1;
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}
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else
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puts ("DES: ok");
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return status;
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}
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@ -1731,8 +1731,8 @@ most systems, but on some systems a special network server gives access
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to it.
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Historically, this database included one-way hashes of user
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passphrases (@pxref{Passphrase Storage}) as well as public information
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about each user (such as their user ID and full name). Many of the
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passphrases, as well as public information about each user
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(such as their user ID and full name). Many of the names of
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functions and data structures associated with this database, and the
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filename @file{/etc/passwd} itself, reflect this history. However,
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the information in this database is available to all users, and it is
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