The library library(sha) provides Secure Hash
Algorihms approved by FIPS (Federal Information Processing
Standard). Quoting
Wikipedia: “The
SHA (Secure Hash Algorithm) hash functions refer to five FIPS-approved
algorithms for computing a condensed digital representation (known as a
message digest) that is, to a high degree of probability, unique for a
given input data sequence (the message). These algorithms are called‘secure’because
(in the words of the standard), “for a given algorithm, it is
computationally infeasible 1) to find a message that corresponds to a
given message digest, or 2) to find two different messages that produce
the same message digest. Any change to a message will, with a very high
probability, result in a different message digest.”
The current library supports all 5 approved algorithms, both
computing the hash-key from data and the hash Message Authentication
Code (HMAC).
A general comprehensive hash interface is provided by library(crypto),
part of the
ssl package.
Input is text, represented as an atom, packed string object or
code-list. Note that these functions operate on byte-sequences and
therefore are not meaningful on Unicode text. The result is returned as
a list of byte-values. This is the most general format that is
comfortable supported by standard Prolog and can easily be transformed
in other formats. Commonly used text formats are ASCII created by
encoding each byte as two hexadecimal digits and ASCII created using
base64 encoding. Representation as a large integer can be
desirable for computational processing.
- sha_hash(+Data,
-Hash, +Options)
- Hash is the SHA hash of Data. Data is either an atom, packed
string or list of character codes. Hash is unified with a
list of bytes (integers in the range 0..255) representing the hash. See
hash_atom/2
to convert this into the more commonly seen hexadecimal representation.
The conversion is controlled by Options:
- algorithm(+Algorithm)
- One of
sha1 (default), sha224, sha256,
sha384 or sha512
- encoding(+Encoding)
- This option defines the mapping from Prolog (Unicode) text to bytes on
which the SHA algorithm is performed. It has two values. The defualt is
utf8,
which implies that Unicode text is encoded as UTF-8 bytes. This option
can deal with any atom. The alternative is
octet, which implies that the text is considered as a
sequence of bytes. This is suitable for e.g., atoms that represent
binary data. An error is raised if the text contains code-points outside
the range 0..255.
- hmac_sha(+Key,
+Data, -HMAC, +Options)
- Quoting Wikipedia:
“A keyed-hash message authentication code, or HMAC, is a type
of message authentication code (MAC) calculated using a cryptographic
hash function in combination with a secret key. As with any MAC, it may
be used to simultaneously verify both the data integrity and the
authenticity of a message. Any iterative cryptographic hash function,
such as MD5 or SHA-1, may be used in the calculation of an HMAC; the
resulting MAC algorithm is termed HMAC-MD5 or HMAC-SHA-1 accordingly.
The cryptographic strength of the HMAC depends upon the cryptographic
strength of the underlying hash function, on the size and quality of the
key and the size of the hash output length in bits.”
Key and Data are either an atom, packed string
or list of character codes. HMAC is unified with a list of
integers representing the authentication code. Options is the
same as for
sha_hash/3,
but currently only sha1 and sha256 are
supported.
See also crypto_data_hash/3
from library(crypto) library provided by the SSL package.
- hash_atom(+Hash,
-HexAtom)
- True when HexAtom is the commonly used hexadecimal encoding
of the hash code. E.g.,
?- sha_hash('SWI-Prolog', Hash, []),
hash_atom(Hash, Hex).
Hash = [61, 128, 252, 38, 121, 69, 229, 85, 199|...],
Hex = '3d80fc267945e555c730403bd0ab0716e2a68c68'.
The underlying SHA-2 library is an unmodified copy created by Dr
Brian Gladman, Worcester, UK. It is distributed under the license
conditions below.
The free distribution and use of this software in both source and
binary form is allowed (with or without changes) provided that:
- distributions of this source code include the above copyright
notice, this list of conditions and the following disclaimer;
- distributions in binary form include the above copyright notice,
this list of conditions and the following disclaimer in the
documentation and/or other associated materials;
- the copyright holder's name is not used to endorse products built
using this software without specific written permission.
ALTERNATIVELY, provided that this notice is retained in full, this
product may be distributed under the terms of the GNU General Public
License (GPL), in which case the provisions of the GPL apply INSTEAD OF
those given above.
