How to convert an ECDSA signature between DER and raw in Kotlin

1import java.math.BigInteger
2 
3/** DER SEQUENCE { INTEGER r, INTEGER s } to fixed-width r‖s; size: 32 P-256, 48 P-384, 66 P-521. */
4fun ByteArray.derToRaw(size: Int): ByteArray {
5    var pos = 0
6    fun next(): Int = this[pos++].toInt() and 0xff // Byte is signed in Kotlin
7    require(next() == 0x30) { "not a DER SEQUENCE" }
8    var length = next()
9    if (length == 0x81) length = next() // long form: P-521 only
10    require(length == this.size - pos) { "bad SEQUENCE length" }
11    fun integer(): ByteArray {
12        require(next() == 0x02) { "expected INTEGER" }
13        val len = next()
14        val value = copyOfRange(pos, pos + len).dropWhile { it == 0.toByte() }.toByteArray()
15        pos += len
16        require(value.size <= size) { "integer does not fit the curve" }
17        return ByteArray(size - value.size) + value // left-pad with zeros
18    }
19    val raw = integer() + integer()
20    require(pos == this.size) { "trailing bytes" }
21    return raw
22}
23 
24/** Fixed-width r‖s to DER; BigInteger.toByteArray() is already a minimal DER INTEGER body. */
25fun ByteArray.rawToDer(): ByteArray {
26    val half = size / 2
27    fun integer(bytes: ByteArray): ByteArray {
28        val value = BigInteger(1, bytes).toByteArray()
29        return byteArrayOf(0x02, value.size.toByte()) + value
30    }
31    val body = integer(copyOfRange(0, half)) + integer(copyOfRange(half, size))
32    val length = if (body.size > 127) byteArrayOf(0x81.toByte(), body.size.toByte()) else byteArrayOf(body.size.toByte())
33    return byteArrayOf(0x30) + length + body
34}
35 

1fun main() {
2    val der = "30450221009f4118b059a34a2f344c39a1d085d93bab560402938281dc1cb2f3dfb8eee58602200bd54b34a72042551c3316e10f461a1d69036ba4f0d72d95d42465340b56def8".hexToByteArray()
3    val raw = der.derToRaw(32)
4    println("raw: ${raw.toHexString()}")
5    println("round trip: ${raw.rawToDer().contentEquals(der)}") // not ==, which compares references
6}
7