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SSH2: fix bug that prevented RC4 and ChaCha20 from ever being used

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This commit is contained in:
terrafrost 2024-05-19 06:37:37 -05:00
parent 787d0fdc1f
commit 3f921549f8
1 changed files with 21 additions and 1 deletions
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22
phpseclib/Net/SSH2.php
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@ -4965,10 +4965,30 @@ class SSH2
$obj->setKeyLength(preg_replace('#[^\d]#', '', $algo));
}
switch ($algo) {
// Eval engines do not exist for ChaCha20 or RC4 because they would not benefit from one.
// to benefit from an Eval engine they'd need to loop a variable amount of times, they'd
// need to do table lookups (eg. sbox subsitutions). ChaCha20 doesn't do either because
// it's a so-called ARX cipher, meaning that the only operations it does are add (A), rotate (R)
// and XOR (X). RC4 does do table lookups but being a stream cipher it works differently than
// block ciphers. with RC4 you XOR the plaintext against a keystream and the keystream changes
// as you encrypt stuff. the only table lookups are made against this keystream and thus table
// lookups are kinda unavoidable. with AES and DES, however, the table lookups that are done
// are done against substitution boxes (sboxes), which are invariant.
// OpenSSL can't be used as an engine, either, because OpenSSL doesn't support continuous buffers
// as SSH2 uses and altho you can emulate a continuous buffer with block ciphers you can't do so
// with stream ciphers. As for ChaCha20... for the ChaCha20 part OpenSSL could prob be used but
// the big slow down isn't with ChaCha20 - it's with Poly1305. SSH constructs the key for that
// differently than how OpenSSL does it (OpenSSL does it as the RFC describes, SSH doesn't).
// libsodium can't be used because it doesn't support RC4 and it doesn't construct the Poly1305
// keys in the same way that SSH does
// mcrypt could prob be used for RC4 but mcrypt hasn't been included in PHP core for yearss
case 'chacha20-poly1305@openssh.com':
case 'arcfour128':
case 'arcfour256':
if ($engine != 'Eval') {
if ($engine != 'PHP') {
continue 2;
}
break;