<?php
declare(strict_types=1);
namespace ParagonIE\HPKE;
use ParagonIE\HPKE\Interfaces\SymmetricKeyInterface;
use SodiumException;
abstract class Context
{
public function __construct(
protected HPKE $hpke,
#[\SensitiveParameter] protected SymmetricKeyInterface $key,
#[\SensitiveParameter] protected string $baseNonce,
protected int $sequence,
#[\SensitiveParameter] protected string $exporterSecret,
) {}
/**
* @param string $exporterContext
* @param int $length
* @return string
*/
public function export(string $exporterContext, int $length): string
{
return $this->hpke->labeledExpand(
$this->exporterSecret,
'sec',
$exporterContext,
$length
);
}
/**
* @return string
*/
protected function computeNonce(): string
{
$seq_bytes = str_pad(
pack('J', $this->sequence),
$this->hpke->aead->nonceLength(),
"\0",
STR_PAD_LEFT
);
return $this->baseNonce ^ $seq_bytes;
}
/**
* @return void
*
* @throws HPKEException
*/
protected function incrementSeq(): void
{
$max = PHP_INT_MAX; // (1 << ($this->hpke->aead->nonceLength() * 8)) - 1;
if ($this->sequence >= $max) {
throw new HPKEException('Message limit reached');
}
++$this->sequence;
}
}
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