Voting

The Note You're Voting On

16 years ago
Sometimes you want the digest in both readable notation (such as hexadecimal) and raw binary. At other times you want the digest in a notation other than hexadecimal.

The following getDigestNotation() function takes a binary string and returns it in base 2, 4, 8, 16, 32, or 64 notation. It works with sha1(), md5(), hash(), or anything else that can output a raw binary string.

It works similar to the session.hash_bits_per_character php.ini configuration option.

You can specify which characters to use for each position, or use the default, which matches session.hash_bits_per_character (0-9, a-z, A-Z, "-", ","). The practical range of bits to use per character ($bitsPerCharacter) is 1 to 6; you may use more, but you will have to provide your own base character string ($chars) that is at least pow(2, $bitsPerCharacter) characters long. So even with 7 bits per character you need to specify a value for $chars that is 128 characters long, which exceeds the number of printable ASCII characters.

The output's radix relates to the value of $bitsPerCharacter as follows:
1: base-2 (binary)
2: base-4
3: base-8 (octal)
4: base-16 (hexadecimal)
5: base-32
6: base-64

<?php
$raw = sha1(uniqid(mt_rand(), TRUE), TRUE);

echo getDigestNotation($raw, 6);

function getDigestNotation($rawDigest, $bitsPerCharacter, $chars = NULL)
{
    if ($chars === NULL || strlen($chars) < 2) {
        $chars '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-,';
    }

    if ($bitsPerCharacter < 1) {
        // $bitsPerCharacter must be at least 1
        $bitsPerCharacter = 1;

    } elseif (strlen($chars) < pow(2, $bitsPerCharacter)) {
        // Character length of $chars is too small for $bitsPerCharacter
        // Set $bitsPerCharacter to greatest value allowed by length of $chars
        $bitsPerCharacter = 1;

        do {
            $bitsPerCharacter++;
        } while (strlen($chars) > pow(2, $bitsPerCharacter));
    }

    $bytes = unpack('C*', $rawDigest);
    $byteCount = count($bytes);

    $out = '';
    $byte = array_shift($bytes);
    $bitsRead = 0;

    for ($i = 0; $i < $byteCount * 8 / $bitsPerCharacter; $i++) {

        if ($bitsRead + $bitsPerCharacter > 8) {
            // Not enough bits remain in this byte for the current character
            // Get remaining bits and get next byte
            $oldBits = $byte - ($byte >> 8 - $bitsRead << 8 - $bitsRead);

            if (count($bytes) == 0) {
                // Last bits; match final character and exit loop
                $out .= $chars[$oldBits];
                break;
            }

            $oldBitCount = 8 - $bitsRead;
            $byte = array_shift($bytes);
            $bitsRead = 0;

        } else {
            $oldBitCount = 0;
        }

        // Read only the needed bits from this byte
        $bits = $byte >> 8 - ($bitsRead + ($bitsPerCharacter - $oldBitCount));
        $bits = $bits - ($bits >> $bitsPerCharacter - $oldBitCount << $bitsPerCharacter - $oldBitCount);
        $bitsRead += $bitsPerCharacter - $oldBitCount;

        if ($oldBitCount > 0) {
            // Bits come from seperate bytes, add $oldBits to $bits
            $bits = ($oldBits << $bitsPerCharacter - $oldBitCount) | $bits;
        }

        $out .= $chars[$bits];
    }

    return $out;
}
?>

Lastly, depending on the digest length, there may be fewer bits remaining for the last character than $bitsPerCharacter, so the last character will be smaller. The same thing happens with PHP's session ID generator, when 5 or 6 is used for session.hash_bits_per_character.
<< Back to user notes page