ASN1.Spec().toDER() signed/unsigned int ERR_OUT_OF_RANGE

[chrillefkr]

ASN1.Spec doesn't seem to work correctly when converting to DER.
I'm using Node v12.16.1 on Linux.

Running ASN1.Spec(0, []).toDER() results in an error

RangeError [ERR_OUT_OF_RANGE]: The value of "value" is out of range. It must be >= -128 and <= 127. Received 160
    at writeU_Int8 (internal/buffer.js:728:11)
    at Buffer.writeInt8 (internal/buffer.js:859:10)
    at ASN1.toDER (/asn1-test/node_modules/@fidm/asn1/build/asn1.js:859:13)
    at repl:1:18
    at Script.runInThisContext (vm.js:120:20)
    at REPLServer.defaultEval (repl.js:430:29)
    at bound (domain.js:426:14)
    at REPLServer.runBound [as eval] (domain.js:439:12)
    at REPLServer.onLine (repl.js:758:10)
    at REPLServer.emit (events.js:323:22) {
  code: 'ERR_OUT_OF_RANGE'

Seems to be caused by first byte (b1) in src/asn1.ts:959 being unsigned (which I believe is JS standard behaviour), and later on is expected to a signed by buf.writeInt8.

CONTEXT_SPECIFIC = 0x80
0x80 | 0x20 == 0xa0
0xa0 unsigned is 160, while signed is -96.

Expected results is a <Buffer <a0 00>.

I resolved the issue by changing line 964 in src/asn1.ts to buf.writeUInt8(b1, 0) instead of having buf.writeInt8(b1, 0), so that we're parsing the integer as unsigned.