/// <summary>
/// Reads the next value as a Integer with a specified tag, returning the contents
/// as a <see cref="BigInteger"/> over the original data.
/// </summary>
/// <param name="expectedTag">The tag to check for before reading.</param>
/// <returns>
/// The bytes of the Integer value, in signed big-endian form.
/// </returns>
/// <exception cref="CryptographicException">
/// the next value does not have the correct tag --OR--
/// the length encoding is not valid under the current encoding rules --OR--
/// the contents are not valid under the current encoding rules
/// </exception>
/// <exception cref="ArgumentException">
/// <paramref name="expectedTag"/>.<see cref="Asn1Tag.TagClass"/> is
/// <see cref="TagClass.Universal"/>, but
/// <paramref name="expectedTag"/>.<see cref="Asn1Tag.TagValue"/> is not correct for
/// the method
/// </exception>
public BigInteger ReadInteger(Asn1Tag expectedTag)
{
ReadOnlyMemory <byte> contents =
GetIntegerContents(expectedTag, UniversalTagNumber.Integer, out int headerLength);
// TODO: Split this for netcoreapp/netstandard to use the Big-Endian BigInteger parsing
byte[] tmp = CryptoPool.Rent(contents.Length);
BigInteger value;
try
{
byte fill = (contents.Span[0] & 0x80) == 0 ? (byte)0 : (byte)0xFF;
// Fill the unused portions of tmp with positive or negative padding.
new Span <byte>(tmp, contents.Length, tmp.Length - contents.Length).Fill(fill);
contents.CopyTo(tmp);
// Convert to Little-Endian.
AsnWriter.Reverse(new Span <byte>(tmp, 0, contents.Length));
value = new BigInteger(tmp);
}
finally
{
// Let CryptoPool.Return clear the whole tmp so that not even the sign bit
// is returned to the array pool.
CryptoPool.Return(tmp);
}
_data = _data.Slice(headerLength + contents.Length);
return(value);
}
