How can * be a safe hashed password?

phpass is a widely used hashing 'framework'. While evaluating phpass' HashPassword I came across this odd method fragment.

 function HashPassword($password)
    {
        // <snip> trying to generate a hash…

        # Returning '*' on error is safe here, but would _not_ be safe
        # in a crypt(3)-like function used _both_ for generating new
        # hashes and for validating passwords against existing hashes.
        return '*';
    }

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Answer: we agree this class assumes that we test our hash for equality on * as a means of validating. This is why I will wrap this class, because it's interface is not great. I expect false in case of failure.

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This is the complete phpsalt class:

# Portable PHP password hashing framework.
#
# Version 0.2 / genuine.
#
# Written by Solar Designer <solar at op开发者_StackOverflow中文版enwall.com> in 2004-2006 and placed in
# the public domain.
#
#
#
class PasswordHash {
    var $itoa64;
    var $iteration_count_log2;
    var $portable_hashes;
    var $random_state;

    function PasswordHash($iteration_count_log2, $portable_hashes)
    {
        $this->itoa64 = './0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';

        if ($iteration_count_log2 < 4 || $iteration_count_log2 > 31)
            $iteration_count_log2 = 8;
        $this->iteration_count_log2 = $iteration_count_log2;

        $this->portable_hashes = $portable_hashes;

        $this->random_state = microtime() . getmypid();
    }

    function get_random_bytes($count)
    {
        $output = '';
        if (is_readable('/dev/urandom') &&
            ($fh = @fopen('/dev/urandom', 'rb'))) {
            $output = fread($fh, $count);
            fclose($fh);
        }

        if (strlen($output) < $count) {
            $output = '';
            for ($i = 0; $i < $count; $i += 16) {
                $this->random_state =
                    md5(microtime() . $this->random_state);
                $output .=
                    pack('H*', md5($this->random_state));
            }
            $output = substr($output, 0, $count);
        }

        return $output;
    }

    function encode64($input, $count)
    {
        $output = '';
        $i = 0;
        do {
            $value = ord($input[$i++]);
            $output .= $this->itoa64[$value & 0x3f];
            if ($i < $count)
                $value |= ord($input[$i]) << 8;
            $output .= $this->itoa64[($value >> 6) & 0x3f];
            if ($i++ >= $count)
                break;
            if ($i < $count)
                $value |= ord($input[$i]) << 16;
            $output .= $this->itoa64[($value >> 12) & 0x3f];
            if ($i++ >= $count)
                break;
            $output .= $this->itoa64[($value >> 18) & 0x3f];
        } while ($i < $count);

        return $output;
    }

    function gensalt_private($input)
    {
        $output = '$P$';
        $output .= $this->itoa64[min($this->iteration_count_log2 +
            ((PHP_VERSION >= '5') ? 5 : 3), 30)];
        $output .= $this->encode64($input, 6);

        return $output;
    }

    function crypt_private($password, $setting)
    {
        $output = '*0';
        if (substr($setting, 0, 2) == $output)
            $output = '*1';

        if (substr($setting, 0, 3) != '$P$')
            return $output;

        $count_log2 = strpos($this->itoa64, $setting[3]);
        if ($count_log2 < 7 || $count_log2 > 30)
            return $output;

        $count = 1 << $count_log2;

        $salt = substr($setting, 4, 8);
        if (strlen($salt) != 8)
            return $output;

        # We're kind of forced to use MD5 here since it's the only
        # cryptographic primitive available in all versions of PHP
        # currently in use.  To implement our own low-level crypto
        # in PHP would result in much worse performance and
        # consequently in lower iteration counts and hashes that are
        # quicker to crack (by non-PHP code).
        if (PHP_VERSION >= '5') {
            $hash = md5($salt . $password, TRUE);
            do {
                $hash = md5($hash . $password, TRUE);
            } while (--$count);
        } else {
            $hash = pack('H*', md5($salt . $password));
            do {
                $hash = pack('H*', md5($hash . $password));
            } while (--$count);
        }

        $output = substr($setting, 0, 12);
        $output .= $this->encode64($hash, 16);

        return $output;
    }

    function gensalt_extended($input)
    {
        $count_log2 = min($this->iteration_count_log2 + 8, 24);
        # This should be odd to not reveal weak DES keys, and the
        # maximum valid value is (2**24 - 1) which is odd anyway.
        $count = (1 << $count_log2) - 1;

        $output = '_';
        $output .= $this->itoa64[$count & 0x3f];
        $output .= $this->itoa64[($count >> 6) & 0x3f];
        $output .= $this->itoa64[($count >> 12) & 0x3f];
        $output .= $this->itoa64[($count >> 18) & 0x3f];

        $output .= $this->encode64($input, 3);

        return $output;
    }

    function gensalt_blowfish($input)
    {
        # This one needs to use a different order of characters and a
        # different encoding scheme from the one in encode64() above.
        # We care because the last character in our encoded string will
        # only represent 2 bits.  While two known implementations of
        # bcrypt will happily accept and correct a salt string which
        # has the 4 unused bits set to non-zero, we do not want to take
        # chances and we also do not want to waste an additional byte
        # of entropy.
        $itoa64 = './ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

        $output = '$2a$';
        $output .= chr(ord('0') + $this->iteration_count_log2 / 10);
        $output .= chr(ord('0') + $this->iteration_count_log2 % 10);
        $output .= '$';

        $i = 0;
        do {
            $c1 = ord($input[$i++]);
            $output .= $itoa64[$c1 >> 2];
            $c1 = ($c1 & 0x03) << 4;
            if ($i >= 16) {
                $output .= $itoa64[$c1];
                break;
            }

            $c2 = ord($input[$i++]);
            $c1 |= $c2 >> 4;
            $output .= $itoa64[$c1];
            $c1 = ($c2 & 0x0f) << 2;

            $c2 = ord($input[$i++]);
            $c1 |= $c2 >> 6;
            $output .= $itoa64[$c1];
            $output .= $itoa64[$c2 & 0x3f];
        } while (1);

        return $output;
    }

    function HashPassword($password)
    {
        $random = '';

        if (CRYPT_BLOWFISH == 1 && !$this->portable_hashes) {
            $random = $this->get_random_bytes(16);
            $hash =
                crypt($password, $this->gensalt_blowfish($random));
            if (strlen($hash) == 60)
                return $hash;
        }

        if (CRYPT_EXT_DES == 1 && !$this->portable_hashes) {
            if (strlen($random) < 3)
                $random = $this->get_random_bytes(3);
            $hash =
                crypt($password, $this->gensalt_extended($random));
            if (strlen($hash) == 20)
                return $hash;
        }

        if (strlen($random) < 6)
            $random = $this->get_random_bytes(6);
        $hash =
            $this->crypt_private($password,
            $this->gensalt_private($random));
        if (strlen($hash) == 34)
            return $hash;

        # Returning '*' on error is safe here, but would _not_ be safe
        # in a crypt(3)-like function used _both_ for generating new
        # hashes and for validating passwords against existing hashes.
        return '*';
    }

    function CheckPassword($password, $stored_hash)
    {
        $hash = $this->crypt_private($password, $stored_hash);
        if ($hash[0] == '*')
            $hash = crypt($password, $stored_hash);

        return $hash == $stored_hash;
    }
}

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I'm not sure exactly what you're asking. The comment directly says that it returns * on an error, so it's not "a safe hashed password", it indicates that an error occurred while trying to generate the hash. Strange choice for a return value, but it is what it is.

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The reason that * can be returned on error is that * is not a possible hash value of any password. Therefore, returning that value will make a value that is obviously not a real hash value, and can't possibly match against another hash.