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PhabricatorHash.php
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Thu, Nov 14, 06:24

PhabricatorHash.php
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<?php
final class PhabricatorHash extends Phobject {
const INDEX_DIGEST_LENGTH = 12;
const ANCHOR_DIGEST_LENGTH = 12;
/**
* Digest a string using HMAC+SHA1.
*
* Because a SHA1 collision is now known, this method should be considered
* weak. Callers should prefer @{method:digestWithNamedKey}.
*
* @param string Input string.
* @return string 32-byte hexadecimal SHA1+HMAC hash.
*/
public static function weakDigest($string, $key = null) {
if ($key === null) {
$key = PhabricatorEnv::getEnvConfig('security.hmac-key');
}
if (!$key) {
throw new Exception(
pht(
"Set a '%s' in your Phabricator configuration!",
'security.hmac-key'));
}
return hash_hmac('sha1', $string, $key);
}
/**
* Digest a string for use in, e.g., a MySQL index. This produces a short
* (12-byte), case-sensitive alphanumeric string with 72 bits of entropy,
* which is generally safe in most contexts (notably, URLs).
*
* This method emphasizes compactness, and should not be used for security
* related hashing (for general purpose hashing, see @{method:digest}).
*
* @param string Input string.
* @return string 12-byte, case-sensitive, mostly-alphanumeric hash of
* the string.
*/
public static function digestForIndex($string) {
$hash = sha1($string, $raw_output = true);
static $map;
if ($map === null) {
$map = '0123456789'.
'abcdefghij'.
'klmnopqrst'.
'uvwxyzABCD'.
'EFGHIJKLMN'.
'OPQRSTUVWX'.
'YZ._';
}
$result = '';
for ($ii = 0; $ii < self::INDEX_DIGEST_LENGTH; $ii++) {
$result .= $map[(ord($hash[$ii]) & 0x3F)];
}
return $result;
}
/**
* Digest a string for use in HTML page anchors. This is similar to
* @{method:digestForIndex} but produces purely alphanumeric output.
*
* This tries to be mostly compatible with the index digest to limit how
* much stuff we're breaking by switching to it. For additional discussion,
* see T13045.
*
* @param string Input string.
* @return string 12-byte, case-sensitive, purely-alphanumeric hash of
* the string.
*/
public static function digestForAnchor($string) {
$hash = sha1($string, $raw_output = true);
static $map;
if ($map === null) {
$map = '0123456789'.
'abcdefghij'.
'klmnopqrst'.
'uvwxyzABCD'.
'EFGHIJKLMN'.
'OPQRSTUVWX'.
'YZ';
}
$result = '';
$accum = 0;
$map_size = strlen($map);
for ($ii = 0; $ii < self::ANCHOR_DIGEST_LENGTH; $ii++) {
$byte = ord($hash[$ii]);
$low_bits = ($byte & 0x3F);
$accum = ($accum + $byte) % $map_size;
if ($low_bits < $map_size) {
// If an index digest would produce any alphanumeric character, just
// use that character. This means that these digests are the same as
// digests created with "digestForIndex()" in all positions where the
// output character is some character other than "." or "_".
$result .= $map[$low_bits];
} else {
// If an index digest would produce a non-alphumeric character ("." or
// "_"), pick an alphanumeric character instead. We accumulate an
// index into the alphanumeric character list to try to preserve
// entropy here. We could use this strategy for all bytes instead,
// but then these digests would differ from digests created with
// "digestForIndex()" in all positions, instead of just a small number
// of positions.
$result .= $map[$accum];
}
}
return $result;
}
public static function digestToRange($string, $min, $max) {
if ($min > $max) {
throw new Exception(pht('Maximum must be larger than minimum.'));
}
if ($min == $max) {
return $min;
}
$hash = sha1($string, $raw_output = true);
// Make sure this ends up positive, even on 32-bit machines.
$value = head(unpack('L', $hash)) & 0x7FFFFFFF;
return $min + ($value % (1 + $max - $min));
}
/**
* Shorten a string to a maximum byte length in a collision-resistant way
* while retaining some degree of human-readability.
*
* This function converts an input string into a prefix plus a hash. For
* example, a very long string beginning with "crabapplepie..." might be
* digested to something like "crabapp-N1wM1Nz3U84k".
*
* This allows the maximum length of identifiers to be fixed while
* maintaining a high degree of collision resistance and a moderate degree
* of human readability.
*
* @param string The string to shorten.
* @param int Maximum length of the result.
* @return string String shortened in a collision-resistant way.
*/
public static function digestToLength($string, $length) {
// We need at least two more characters than the hash length to fit in a
// a 1-character prefix and a separator.
$min_length = self::INDEX_DIGEST_LENGTH + 2;
if ($length < $min_length) {
throw new Exception(
pht(
'Length parameter in %s must be at least %s, '.
'but %s was provided.',
'digestToLength()',
new PhutilNumber($min_length),
new PhutilNumber($length)));
}
// We could conceivably return the string unmodified if it's shorter than
// the specified length. Instead, always hash it. This makes the output of
// the method more recognizable and consistent (no surprising new behavior
// once you hit a string longer than `$length`) and prevents an attacker
// who can control the inputs from intentionally using the hashed form
// of a string to cause a collision.
$hash = self::digestForIndex($string);
$prefix = substr($string, 0, ($length - ($min_length - 1)));
return $prefix.'-'.$hash;
}
public static function digestWithNamedKey($message, $key_name) {
$key_bytes = self::getNamedHMACKey($key_name);
return self::digestHMACSHA256($message, $key_bytes);
}
public static function digestHMACSHA256($message, $key) {
if (!is_string($message)) {
throw new Exception(
pht('HMAC-SHA256 can only digest strings.'));
}
if (!is_string($key)) {
throw new Exception(
pht('HMAC-SHA256 keys must be strings.'));
}
if (!strlen($key)) {
throw new Exception(
pht('HMAC-SHA256 requires a nonempty key.'));
}
$result = hash_hmac('sha256', $message, $key, $raw_output = false);
// Although "hash_hmac()" is documented as returning `false` when it fails,
// it can also return `null` if you pass an object as the "$message".
if ($result === false || $result === null) {
throw new Exception(
pht('Unable to compute HMAC-SHA256 digest of message.'));
}
return $result;
}
/* -( HMAC Key Management )------------------------------------------------ */
private static function getNamedHMACKey($hmac_name) {
$cache = PhabricatorCaches::getImmutableCache();
$cache_key = "hmac.key({$hmac_name})";
$hmac_key = $cache->getKey($cache_key);
if (!strlen($hmac_key)) {
$hmac_key = self::readHMACKey($hmac_name);
if ($hmac_key === null) {
$hmac_key = self::newHMACKey($hmac_name);
self::writeHMACKey($hmac_name, $hmac_key);
}
$cache->setKey($cache_key, $hmac_key);
}
// The "hex2bin()" function doesn't exist until PHP 5.4.0 so just
// implement it inline.
$result = '';
for ($ii = 0; $ii < strlen($hmac_key); $ii += 2) {
$result .= pack('H*', substr($hmac_key, $ii, 2));
}
return $result;
}
private static function newHMACKey($hmac_name) {
$hmac_key = Filesystem::readRandomBytes(64);
return bin2hex($hmac_key);
}
private static function writeHMACKey($hmac_name, $hmac_key) {
$unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
id(new PhabricatorAuthHMACKey())
->setKeyName($hmac_name)
->setKeyValue($hmac_key)
->save();
unset($unguarded);
}
private static function readHMACKey($hmac_name) {
$table = new PhabricatorAuthHMACKey();
$conn = $table->establishConnection('r');
$row = queryfx_one(
$conn,
'SELECT keyValue FROM %T WHERE keyName = %s',
$table->getTableName(),
$hmac_name);
if (!$row) {
return null;
}
return $row['keyValue'];
}
}

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