Integer encoding not being handled correctly

[jakekgrog]

Hi there,

First off, I'm no ASN.1 expert so please correct me and close the issue if I'm wrong.

My understanding is that INTEGER types can be positive, negative, zero and have any magnitude.

In most languages you have the ability to specify a size and sign of a number

If the number is signed, then the high-order bit (the first bit of the most significant byte) determines whether the value is negative or positive.

To account for this, I understand ASN.1 Integers are padded with 0x00 if the high order bit is set. Hence a "2048-bit" (256-byte) number may actually use 257 bytes in ASN.1.

In pseudo code that is:

# parsing
if 0x00 == bytes[0] {
  bytes = bytes.slice(1)
}

# packing
if 0x80 & bytes[0] {
  bytes = bytes.prepend(0)
}

The prepending of 0x00 doesn't appear to be happening and I've had to manually add a 0x00 byte as the first byte if the first byte is > 0x80.

As an example:

const param = "FFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFF";

let output = new ASN1.Integer({
    valueHex: new Uint8Array([hexStringToArray(param)]).buffer,
});

console.log(Buffer.from(param.toBER(), 'hex').toString('hex'));

// The above should output '022100ffffffff00000001000000000000000000000000ffffffffffffffffffffffff'
// Note the prepended 0x00
// However that isn't happening
// Instead the output is '0220ffffffff00000001000000000000000000000000ffffffffffffffffffffffff'

As DER is a subset of BER, my understanding is BER encoding should prepend the 0x00 byte?