Convert Google KMS signatures from DER-encoded format to .Net IEEE 1363 format

Question

I would like to convert signature in DER-encoded format (which I get from google KMS service) to IEEE 1363 format using C# (in order to validate the signature) How it is done in C#, .Net Core 5 ?

Answer 1

Since .NET 5, both the ASN.1/DER and the IEEE P1363 formats are supported for ECDSA signatures, see e.g. SignData() and VerifyData() (similarly SignHash() and VerifyHash()).

A specification of the format is possible with DSASignatureFormat with the values IeeeP1363FixedFieldConcatenation (IEEE P1363) and Rfc3279DerSequence (ASN.1/DER). Default is IEEE P1363.

For other versions C#/BouncyCastle can be applied.

If you want to convert the signatures directly, the easiest way is to use an ASN.1 parser (e.g. from C#/BouncyCastle).