Perl pack() function and "B" format character to Php - php

Question

What does the "B" do in this pack statement from Perl code?
$hce_hash=pack('B*', $hce_hash);
Is there an equivalent function in PHP?

Answer 1

PHP’s pack doesn’t support a format of B*, but it does support H*. In Perl, you could emulate it with
sub pack_Bstar {
my($bits) = #_;
my $Hstar;
my $nybble = 0;
for (my $i = 0; $i < length $bits; ++$i) {
$nybble *= 2;
$nybble += int substr($bits, $i, 1);
if ($i % 4 == 3) {
$Hstar .= sprintf "%x", $nybble;
$nybble = 0;
}
}
my $pad = 4 - length($bits) % 4;
if ($pad != 4) {
$nybble = ($nybble << $pad);
$Hstar .= sprintf "%x", $nybble;
}
pack "H*", $Hstar;
}
The code above is not idiomatic Perl, but translation to PHP should be straightforward.
The H* format wants a hex string with high nybble (4 bits) first. The code above chews off four bits at a time to compute each nybble value. For example, for a bit string of 1011, tracing the algorithm gives
nybble = 0
nybble = 2 * 0 + 1 = 1
nybble = 2 * 1 + 0 = 2
nybble = 2 * 2 + 1 = 5
nybble = 2 * 5 + 1 = 11
10112 is indeed 1110, which is b16. If the last nybble is incomplete (between one and three bits), we left-shift the bit the appropriate number of places. This has the effect of zero-padding on the right.
Tests:
my #tests = (
["01001010011101010111001101110100" => "Just"],
["0110000101101110011011110111010001101000011001010111001" => "another"],
["01010000010010000101000000101111010100000110010101110010011011" => "PHP/Perl"],
["01101000011000010110001101101011011001010111001000101100" => "hacker,"],
);
for (#tests) {
my($input,$expect) = #$_;
my $got = pack_Bstar $input;
print "$input: ", ($got eq $expect ? "PASS" : "FAIL"), " ($got)\n";
}
Output:
01001010011101010111001101110100: PASS (Just)
0110000101101110011011110111010001101000011001010111001: PASS (another)
01010000010010000101000000101111010100000110010101110010011011: PASS (PHP/Perl)
01101000011000010110001101101011011001010111001000101100: PASS (hacker,)

Answer 2

pack 'B*', $s returns the bytes represented by the string of 0 and 1 characters that form up the string in $s. The value of $s is right-padded with zeros to a length divisible by 8 if necessary.
For example,
pack 'B*', '0100101000110101'
results in
chr(0b01001010) . chr(0b00110101);

Answer 3

As others have noted, PHP's pack() does not support the B template, which in Perl's pack() turns a bitstring, represented as a literal string of 0 and 1 characters, into a packed byte string with 8 bits per byte.
However, since PHP's pack() does support the H template, which does the same except for hexadecimal digits instead of bits, we can emulate Perl's B template in PHP by first using base_convert() to turn the bits into hex digits and then packing those:
function pack_B( $bits, $len = false ) {
// truncate input to desired length, if given:
if ( $len === false ) $len = strlen( $bits );
else $bits = substr( $bits, 0, $len );
// pad input with zeros to next multiple of 4 above $len:
$hexlen = (int)( ($len + 3) / 4 );
$bits = str_pad( $bits, 4*$hexlen, "0" );
// split input into chunks of 4 bits, convert each to hex and pack them:
$nibbles = str_split( $bits, 4 );
foreach ( $nibbles as $i => $nibble ) {
$nibbles[$i] = base_convert( $nibble, 2, 16 );
}
return pack( "H*", implode( "", $nibbles ) );
}
(The reason we can't just feed the whole input string to base_convert() is that it stores its intermediate result as a PHP float, and thus doesn't produce correct results for numbers too large to be accurately represented by a float. Doing it one hex digit at a time works fine, however.)