Proposal for side-channel safe encoding.

sidechannel_encode.php

<?php

// THIS CODE IS EXPERIMENTAL. DO NOT USE IT.
// ALSO NOTE THERE IS NO ERROR CHECKING!

function side_channel_safe_encode($binary_string)
{
    // We only use 5 bits from every byte, so for 256 bits we need 52 bytes.
    $random = mcrypt_create_iv(52, MCRYPT_DEV_URANDOM);
    $printable_blind_key = '';
    for ($i = 0; $i < strlen($random); $i++) {
        // We want $char_num to look like 01?????0, where the ? are random bits.
        $char_num = ord($random[$i]);
        // Set the MSB and LSB to zero.
        $char_num = $char_num & 0x7E; 
        // Set the second-most significant bit to 1
        $char_num = $char_num | 0x40;
        $printable_blind_key .= chr($char_num);
    }

    $blind_key = hash("sha256", $printable_blind_key, true);

    $encrypted = mcrypt_encrypt(
        MCRYPT_RIJNDAEL_128,
        $blind_key,
        $binary_string,
        'ctr',
        str_repeat("\x00", 16) // zero IV!
    );

    // The zero IV should be ok, because with high probability, all messages are
    // encrypted with a different key.

    // We shouldn't care if bin2hex leaks $encrypted, since it's encrypted with
    // a random key.

    $encoded = $printable_blind_key . bin2hex($encrypted);

    return $encoded;
}

function side_channel_safe_decode($encoded_string)
{
    $printable_blind_key = substr($encoded_string, 0, 52);
    $blind_key = hash("sha256", $printable_blind_key, true);

    $ciphertext = substr($encoded_string, 52);

    // Likewise, we shouldn't care if pack leaks $ciphertext, since it's
    // encrypted with a random key.

    $decoded = pack("H*", $ciphertext);


    $decrypted = mcrypt_decrypt(
        MCRYPT_RIJNDAEL_128,
        $blind_key,
        $decoded,
        'ctr',
        str_repeat("\x00", 16)
    );

    return $decrypted;
}

// TODO: what about encrypting with key H(80-bit random || message) ?

$message = "Hello, world!";

echo "Message: $message\n";

$encoded1 = side_channel_safe_encode($message);
$encoded2 = side_channel_safe_encode($message);

echo "Encoded #1: $encoded1\n";
echo "Encoded #2: $encoded2\n";

$decoded1 = side_channel_safe_decode($encoded1);
$decoded2 = side_channel_safe_decode($encoded2);

echo "Decoded #1: $decoded1\n";
echo "Decoded #2: $decoded2\n";

if ($message === $decoded1 && $message === $decoded2) {
    echo "It works!\n";
} else {
    echo "It doesn't work!\n";
}

?>