$iv = pack("h*", $iv);
$fixed_key = pack("h*", $fixed_key);
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Is that correct? How does openssl interpret iv and key strings? Note that "With format "h", the first character of the pair determines the least-significant nybble of the output character"

$ perl -E 'say ord pack "h*", "01"'
16  # that is, 0x10

$ perl -E 'say ord pack "H*", "01"'
1
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