dvPublicKeyFromCertificate function

RSAPublicKey dvPublicKeyFromCertificate(String base64Der)

Extracts the RSA public key from a base64 DER X.509 certificate.

Implementation

RSAPublicKey dvPublicKeyFromCertificate(String base64Der) {
  final Uint8List der =
      base64.decode(base64Der.replaceAll(RegExp(r'\s|-----[^-]+-----'), ''));
  final ASN1Parser parser = ASN1Parser(der);
  final ASN1Sequence certificate = parser.nextObject() as ASN1Sequence;
  final ASN1Sequence tbs = certificate.elements!.first as ASN1Sequence;

  // SubjectPublicKeyInfo is the first element that is itself a sequence
  // containing an algorithm identifier and a bit string. Located by shape
  // rather than by index, because the optional version field shifts every
  // index after it.
  for (final ASN1Object element in tbs.elements!) {
    if (element is! ASN1Sequence) continue;
    final List<ASN1Object>? parts = element.elements;
    if (parts == null || parts.length != 2) continue;
    if (parts[0] is! ASN1Sequence || parts[1] is! ASN1BitString) continue;

    final ASN1BitString bits = parts[1] as ASN1BitString;
    final Uint8List keyBytes = Uint8List.fromList(bits.stringValues!);
    final ASN1Sequence key =
        ASN1Parser(keyBytes).nextObject() as ASN1Sequence;
    return RSAPublicKey(
      (key.elements![0] as ASN1Integer).integer!,
      (key.elements![1] as ASN1Integer).integer!,
    );
  }
  throw const FormatException(
    'No SubjectPublicKeyInfo found in the certificate.',
  );
}