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Improve security & performance of Encrypt then MAC scheme. #23427

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lynn-stephenson wants to merge 5 commits into from
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Improve security & performance of Encrypt then MAC scheme. #23427

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96fd82e
Split single key into encryption & MAC key.
lynn-stephenson Oct 13, 2020
cebd718
Bump cryptographic payload version to 3.
lynn-stephenson Oct 13, 2020
79f4c68
Remove unused dependency and update docblock.
lynn-stephenson Oct 13, 2020
8ffbb4b
Remove unneeded call to hash function.
lynn-stephenson Oct 13, 2020
0ae57ce
Swap out phpseclib's HMAC function.
lynn-stephenson Oct 13, 2020
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21 changes: 8 additions & 13 deletions lib/private/Security/Crypto.php
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Original file line number Diff line number Diff line change
Expand Up @@ -31,9 +31,7 @@

use OCP\IConfig;
use OCP\Security\ICrypto;
use OCP\Security\ISecureRandom;
use phpseclib\Crypt\AES;
use phpseclib\Crypt\Hash;

/**
* Class Crypto provides a high-level encryption layer using AES-CBC. If no key has been provided
Expand All @@ -55,7 +53,6 @@ class Crypto implements ICrypto {

/**
* @param IConfig $config
* @param ISecureRandom $random
*/
public function __construct(IConfig $config) {
$this->cipher = new AES();
Expand All @@ -72,12 +69,7 @@ public function calculateHMAC(string $message, string $password = ''): string {
$password = $this->config->getSystemValue('secret');
}

// Append an "a" behind the password and hash it to prevent reusing the same password as for encryption
$password = hash('sha512', $password . 'a');

$hash = new Hash('sha512');
$hash->setKey($password);
return $hash->hash($message);
return hash_hmac('sha512', $message, $password, true);
}

/**
Expand All @@ -90,16 +82,19 @@ public function encrypt(string $plaintext, string $password = ''): string {
if ($password === '') {
$password = $this->config->getSystemValue('secret');
}
$this->cipher->setPassword($password);

$keyMaterial = hash_hkdf('sha512', $password);
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@kesselb kesselb Oct 14, 2020

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Suggested change
$keyMaterial = hash_hkdf('sha512', $password);
$keyMaterial = hash_hkdf('sha512', $password, 32);

I'm not 100% sure. The documentation says "Desired output length in bytes" 🤷‍♂️

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@lynn-stephenson lynn-stephenson Oct 14, 2020 •
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@kesselb The documentation says hash_hkdf with a $length of 0 will use the provided hash function length (which is 64 bytes in this case). Also, 32 bytes is not enough. If anything, we'll explicitly ask for 64 bytes.


$this->cipher->setPassword(substr($keyMaterial, 0, 32));

$iv = \random_bytes($this->ivLength);
$this->cipher->setIV($iv);

$ciphertext = bin2hex($this->cipher->encrypt($plaintext));
$iv = bin2hex($iv);
$hmac = bin2hex($this->calculateHMAC($ciphertext.$iv, $password));
$hmac = bin2hex($this->calculateHMAC($ciphertext.$iv, substr($keyMaterial, 32)));

return $ciphertext.'|'.$iv.'|'.$hmac.'|2';
return $ciphertext.'|'.$iv.'|'.$hmac.'|3';
}

/**
Expand Down Expand Up @@ -128,7 +123,7 @@ public function decrypt(string $authenticatedCiphertext, string $password = ''):

if ($partCount === 4) {
$version = $parts[3];
if ($version === '2') {
if ($version >= '2') {
$iv = hex2bin($iv);
}
}
Expand Down