Digest::SHA(3p)

NAME
Digest::SHA - Perl extension for SHA-1/224/256/384/512

SYNOPSIS
In programs:

                # Functional interface

        use Digest::SHA qw(sha1 sha1_hex sha1_base64 ...);

        $digest = sha1($data);
        $digest = sha1_hex($data);
        $digest = sha1_base64($data);

        $digest = sha256($data);
        $digest = sha384_hex($data);
        $digest = sha512_base64($data);

                # Object-oriented

        use Digest::SHA;

        $sha = Digest::SHA->new($alg);

        $sha->add($data);               # feed data into stream

        $sha->addfile(*F);
        $sha->addfile($filename);

        $sha->add_bits($bits);
        $sha->add_bits($data, $nbits);

        $sha_copy = $sha->clone;        # if needed, make copy of
        $sha->dump($file);              #       current digest state,
        $sha->load($file);              #       or save it on disk

        $digest = $sha->digest;         # compute digest
        $digest = $sha->hexdigest;
        $digest = $sha->b64digest;

From the command line:

        $ shasum files

        $ shasum --help

SYNOPSIS (HMAC-SHA)
                # Functional interface only

        use Digest::SHA qw(hmac_sha1 hmac_sha1_hex ...);

        $digest = hmac_sha1($data, $key);
        $digest = hmac_sha224_hex($data, $key);
        $digest = hmac_sha256_base64($data, $key);

ABSTRACT
Digest::SHA is a complete implementation of the NIST Secure Hash Standard. It gives Perl programmers a convenient way to calculate SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256 message digests. The module can handle all types of input, including partial-byte data.

DESCRIPTION
Digest::SHA is written in C for speed. If your platform lacks a C compiler, you can install the functionally equivalent (but much slower) Digest::SHA::PurePerl module.

The programming interface is easy to use: it's the same one found in CPAN's Digest module. So, if your applications currently use Digest::MD5 and you'd prefer the stronger security of SHA, it's a simple matter to convert them.

The interface provides two ways to calculate digests: all-at-once, or in stages. To illustrate, the following short program computes the SHA-256 digest of "hello world" using each approach:

        use Digest::SHA qw(sha256_hex);

        $data = "hello world";
        @frags = split(//, $data);

        # all-at-once (Functional style)
        $digest1 = sha256_hex($data);

        # in-stages (OOP style)
        $state = Digest::SHA->new(256);
        for (@frags) { $state->add($_) }
        $digest2 = $state->hexdigest;

        print $digest1 eq $digest2 ?
                "whew!\n" : "oops!\n";

To calculate the digest of an n-bit message where n is not a multiple of 8, use the add_bits() method. For example, consider the 446-bit message consisting of the bit-string "110" repeated 148 times, followed by "11". Here's how to display its SHA-1 digest:

        use Digest::SHA;
        $bits = "110" x 148 . "11";
        $sha = Digest::SHA->new(1)->add_bits($bits);
        print $sha->hexdigest, "\n";

Note that for larger bit-strings, it's more efficient to use the two-argument version add_bits($data, $nbits), where $data is in the customary packed binary format used for Perl strings.

The module also lets you save intermediate SHA states to disk, or display them on standard output. The dump() method generates portable, human-readable text describing the current state of computation. You can subsequently retrieve the file with load() to resume where the calculation left off.

To see what a state description looks like, just run the following:

        use Digest::SHA;
        Digest::SHA->new->add("Shaw" x 1962)->dump;

As an added convenience, the Digest::SHA module offers routines to calculate keyed hashes using the HMAC-SHA-1/224/256/384/512 algorithms. These services exist in functional form only, and mimic the style and behavior of the sha(), sha_hex(), and sha_base64() functions.

        # Test vector from draft-ietf-ipsec-ciph-sha-256-01.txt

        use Digest::SHA qw(hmac_sha256_hex);
        print hmac_sha256_hex("Hi There", chr(0x0b) x 32), "\n";

UNICODE AND SIDE EFFECTS
Perl supports Unicode strings as of version 5.6. Such strings may contain wide characters, namely, characters whose ordinal values are greater than 255. This can cause problems for digest algorithms such as SHA that are specified to operate on sequences of bytes.

The rule by which Digest::SHA handles a Unicode string is easy to state, but potentially confusing to grasp: the string is interpreted as a sequence of byte values, where each byte value is equal to the ordinal value (viz. code point) of its corresponding Unicode character. That way, the Unicode string 'abc' has exactly the same digest value as the ordinary string 'abc'.

Since a wide character does not fit into a byte, the Digest::SHA routines croak if they encounter one. Whereas if a Unicode string contains no wide characters, the module accepts it quite happily. The following code illustrates the two cases:

        $str1 = pack('U*', (0..255));
        print sha1_hex($str1);          # ok

        $str2 = pack('U*', (0..256));
        print sha1_hex($str2);          # croaks

Be aware that the digest routines silently convert UTF-8 input into its equivalent byte sequence in the native encoding (cf. utf8::downgrade). This side effect influences only the way Perl stores the data internally, but otherwise leaves the actual value of the data intact.

NIST STATEMENT ON SHA-1
NIST acknowledges that the work of Prof. Xiaoyun Wang constitutes a practical collision attack on SHA-1. Therefore, NIST encourages the rapid adoption of the SHA-2 hash functions (e.g. SHA-256) for applications requiring strong collision resistance, such as digital signatures.

ref. <http://csrc.nist.gov/groups/ST/hash/statement.html>

PADDING OF BASE64 DIGESTS
By convention, CPAN Digest modules do not pad their Base64 output. Problems can occur when feeding such digests to other software that expects properly padded Base64 encodings.

For the time being, any necessary padding must be done by the user. Fortunately, this is a simple operation: if the length of a Base64-encoded digest isn't a multiple of 4, simply append "=" characters to the end of the digest until it is:

        while (length($b64_digest) % 4) {
                $b64_digest .= '=';
        }

To illustrate, sha256_base64("abc") is computed to be

        ungWv48Bz+pBQUDeXa4iI7ADYaOWF3qctBD/YfIAFa0

which has a length of 43. So, the properly padded version is

        ungWv48Bz+pBQUDeXa4iI7ADYaOWF3qctBD/YfIAFa0=

EXPORT
None by default.

EXPORTABLE FUNCTIONS
Provided your C compiler supports a 64-bit type (e.g. the long long of C99, or __int64 used by Microsoft C/C++), all of these functions will be available for use. Otherwise, you won't be able to perform the SHA-384 and SHA-512 transforms, both of which require 64-bit operations.

Functional style

sha1($data, ...)
sha224($data, ...)
sha256($data, ...)
sha384($data, ...)
sha512($data, ...)
sha512224($data, ...)
sha512256($data, ...)
    Logically joins the arguments into a single string, and returns its SHA-1/224/256/384/512 digest encoded as a binary string.

sha1_hex($data, ...)
sha224_hex($data, ...)
sha256_hex($data, ...)
sha384_hex($data, ...)
sha512_hex($data, ...)
sha512224_hex($data, ...)
sha512256_hex($data, ...)
    Logically joins the arguments into a single string, and returns its SHA-1/224/256/384/512 digest encoded as a hexadecimal string.

sha1_base64($data, ...)
sha224_base64($data, ...)
sha256_base64($data, ...)
sha384_base64($data, ...)
sha512_base64($data, ...)
sha512224_base64($data, ...)
sha512256_base64($data, ...)
    Logically joins the arguments into a single string, and returns its SHA-1/224/256/384/512 digest encoded as a Base64 string.

    It's important to note that the resulting string does not contain the padding characters typical of Base64 encodings. This omission is deliberate, and is done to maintain compatibility with the family of CPAN Digest modules. See "PADDING OF BASE64 DIGESTS" for details.

OOP style

new($alg)
    Returns a new Digest::SHA object. Allowed values for $alg are 1, 224, 256, 384, 512, 512224, or 512256. It's also possible to use common string representations of the algorithm (e.g. "sha256", "SHA-384"). If the argument is missing, SHA-1 will be used by default.

    Invoking new as an instance method will not create a new object; instead, it will simply reset the object to the initial state associated with $alg. If the argument is missing, the object will continue using the same algorithm that was selected at creation.

reset($alg)
    This method has exactly the same effect as new($alg). In fact, reset is just an alias for new.

hashsize
    Returns the number of digest bits for this object. The values are 160, 224, 256, 384, 512, 224, and 256 for SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224 and SHA-512/256, respectively.

algorithm
    Returns the digest algorithm for this object. The values are 1, 224, 256, 384, 512, 512224, and 512256 for SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256, respectively.

clone
    Returns a duplicate copy of the object.

add($data, ...)
    Logically joins the arguments into a single string, and uses it to update the current digest state. In other words, the following statements have the same effect:

            $sha->add("a"); $sha->add("b"); $sha->add("c");
            $sha->add("a")->add("b")->add("c");
            $sha->add("a", "b", "c");
            $sha->add("abc");

    The return value is the updated object itself.

add_bits($data, $nbits)
add_bits($bits)
    Updates the current digest state by appending bits to it. The return value is the updated object itself.

    The first form causes the most-significant $nbits of $data to be appended to the stream. The $data argument is in the customary binary format used for Perl strings.

    The second form takes an ASCII string of "0" and "1" characters as its argument. It's equivalent to

            $sha->add_bits(pack("B*", $bits), length($bits));

    So, the following two statements do the same thing:

            $sha->add_bits("111100001010");
            $sha->add_bits("\xF0\xA0", 12);

addfile(*FILE)
    Reads from FILE until EOF, and appends that data to the current state. The return value is the updated object itself.

addfile($filename [, $mode])
    Reads the contents of $filename, and appends that data to the current state. The return value is the updated object itself.

    By default, $filename is simply opened and read; no special modes or I/O disciplines are used. To change this, set the optional $mode argument to one of the following values:

            "b"     read file in binary mode

            "p"     use portable mode

            "0"     use BITS mode

    The "p" mode ensures that the digest value of $filename will be the same when computed on different operating systems. It accomplishes this by internally translating all newlines in text files to UNIX format before calculating the digest. Binary files are read in raw mode with no translation whatsoever.

    The BITS mode ("0") interprets the contents of $filename as a logical stream of bits, where each ASCII '0' or '1' character represents a 0 or 1 bit, respectively. All other characters are ignored. This provides a convenient way to calculate the digest values of partial-byte data by using files, rather than having to write programs using the add_bits method.

dump($filename)
    Provides persistent storage of intermediate SHA states by writing a portable, human-readable representation of the current state to $filename. If the argument is missing, or equal to the empty string, the state information will be written to STDOUT.

load($filename)
    Returns a Digest::SHA object representing the intermediate SHA state that was previously dumped to $filename. If called as a class method, a new object is created; if called as an instance method, the object is reset to the state contained in $filename. If the argument is missing, or equal to the empty string, the state information will be read from STDIN.

digest
    Returns the digest encoded as a binary string.

    Note that the digest method is a read-once operation. Once it has been performed, the Digest::SHA object is automatically reset in preparation for calculating another digest value. Call $sha->clone->digest if it's necessary to preserve the original digest state.

hexdigest
    Returns the digest encoded as a hexadecimal string.

    Like digest, this method is a read-once operation. Call $sha->clone->hexdigest if it's necessary to preserve the original digest state.

    This method is inherited if Digest::base is installed on your system. Otherwise, a functionally equivalent substitute is used.

b64digest
    Returns the digest encoded as a Base64 string.

    Like digest, this method is a read-once operation. Call $sha->clone->b64digest if it's necessary to preserve the original digest state.

    This method is inherited if Digest::base is installed on your system. Otherwise, a functionally equivalent substitute is used.

    It's important to note that the resulting string does not contain the padding characters typical of Base64 encodings. This omission is deliberate, and is done to maintain compatibility with the family of CPAN Digest modules. See "PADDING OF BASE64 DIGESTS" for details.

HMAC-SHA-1/224/256/384/512

hmac_sha1($data, $key)
hmac_sha224($data, $key)
hmac_sha256($data, $key)
hmac_sha384($data, $key)
hmac_sha512($data, $key)
hmac_sha512224($data, $key)
hmac_sha512256($data, $key)
    Returns the HMAC-SHA-1/224/256/384/512 digest of $data/$key, with the result encoded as a binary string. Multiple $data arguments are allowed, provided that $key is the last argument in the list.

hmac_sha1_hex($data, $key)
hmac_sha224_hex($data, $key)
hmac_sha256_hex($data, $key)
hmac_sha384_hex($data, $key)
hmac_sha512_hex($data, $key)
hmac_sha512224_hex($data, $key)
hmac_sha512256_hex($data, $key)
    Returns the HMAC-SHA-1/224/256/384/512 digest of $data/$key, with the result encoded as a hexadecimal string. Multiple $data arguments are allowed, provided that $key is the last argument in the list.

hmac_sha1_base64($data, $key)
hmac_sha224_base64($data, $key)
hmac_sha256_base64($data, $key)
hmac_sha384_base64($data, $key)
hmac_sha512_base64($data, $key)
hmac_sha512224_base64($data, $key)
hmac_sha512256_base64($data, $key)
    Returns the HMAC-SHA-1/224/256/384/512 digest of $data/$key, with the result encoded as a Base64 string. Multiple $data arguments are allowed, provided that $key is the last argument in the list.

    It's important to note that the resulting string does not contain the padding characters typical of Base64 encodings. This omission is deliberate, and is done to maintain compatibility with the family of CPAN Digest modules. See "PADDING OF BASE64 DIGESTS" for details.

SEE ALSO
Digest, Digest::SHA::PurePerl

The Secure Hash Standard (Draft FIPS PUB 180-4) can be found at:

<http://csrc.nist.gov/publications/drafts/fips180-4/Draft-FIPS180-4_Feb2011.pdf>

The Keyed-Hash Message Authentication Code (HMAC):

<http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf>

AUTHOR
        Mark Shelor     <mshelor@cpan.org>

ACKNOWLEDGMENTS
The author is particularly grateful to

        Gisle Aas
        Sean Burke
        Chris Carey
        Alexandr Ciornii
        Jim Doble
        Thomas Drugeon
        Julius Duque
        Jeffrey Friedl
        Robert Gilmour
        Brian Gladman
        Adam Kennedy
        Andy Lester
        Alex Muntada
        Steve Peters
        Chris Skiscim
        Martin Thurn
        Gunnar Wolf
        Adam Woodbury

"who by trained skill rescued life from such great billows and such thick darkness and moored it in so perfect a calm and in so brilliant a light" - Lucretius

COPYRIGHT AND LICENSE
Copyright (C) 2003-2013 Mark Shelor

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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