NullReferenceException in BouncyCastleSecureMimeContext

[alxbauer]

In very rare cases i get an NullReferenceException in BouncyCastleSecureMimeContext while decoding S/Mime messages.

The problem is caused by malformed SMIMECapabilities in the signature.
These contain the RC2-CBC algorithm, but without the required parameter.
This causes the NullReferenceException to occur in the following section of code.

Line 846

MimeKit/MimeKit/Cryptography/BouncyCastleSecureMimeContext.cs

Lines 834 to 855 in e9b1905

	if (identifier.Algorithm.Id == CmsEnvelopedGenerator.RC2Cbc) {
	if (identifier.Parameters is DerSequence) {
	var param = (DerSequence) identifier.Parameters;
	var version = (DerInteger) param[0];
	int bits = version.Value.IntValue;
	switch (bits) {
	case 58: algorithm = EncryptionAlgorithm.RC2128; return true;
	case 120: algorithm = EncryptionAlgorithm.RC264; return true;
	case 160: algorithm = EncryptionAlgorithm.RC240; return true;
	}
	} else {
	var param = (DerInteger) identifier.Parameters;
	int bits = param.Value.IntValue;
	switch (bits) {
	case 128: algorithm = EncryptionAlgorithm.RC2128; return true;
	case 64: algorithm = EncryptionAlgorithm.RC264; return true;
	case 40: algorithm = EncryptionAlgorithm.RC240; return true;
	}
	}
	}

I think the simplest solution would be to use a else if condition to prevent the NullReferenceException:

if (identifier.algorithm.id == cmsenvelopedgenerator.rc2cbc) {
	if (identifier.parameters is dersequence) {
		var param = (dersequence) identifier.parameters;
		var version = (derinteger) param[0];
		int bits = version.value.intvalue;

		switch (bits) {
		case 58: algorithm = encryptionalgorithm.rc2128; return true;
		case 120: algorithm = encryptionalgorithm.rc264; return true;
		case 160: algorithm = encryptionalgorithm.rc240; return true;
		}
	} else if (identifier.parameters is derinteger) {
		var param = (derinteger) identifier.parameters;
		int bits = param.value.intvalue;

		switch (bits) {
		case 128: algorithm = encryptionalgorithm.rc2128; return true;
		case 64: algorithm = encryptionalgorithm.rc264; return true;
		case 40: algorithm = encryptionalgorithm.rc240; return true;
		}
	}
}

br,
Alex