Parsing bug in RSACryptoServiceProvider.ImportParameters with zero-byte-prefix elimination?

Question

I had some problems where RSA keys created using the Security.Cryptography.RSAParameters were working only most of the time with RSACryptoServiceProvider.ImportParameters.

After a bunch of debugging it appears that the properties of that object want very specific byte buffer sizes. My ASN.1 parsing code has zero-byte-prefix elimination. In fact, some fields of the RSAParameters only work after zero-byte-prefix elimination and others don't work at all if zero-byte-prefix elimination has been done.

Every so often one of the parameters does have more leading zeros due to normal randomization and caused the resulting key to not work properly.

Is this something that is considered a bug?

Answer 1

Why are you messing around with those zero bytes? The correct DER encoding of a positive ASN.1 integer may involve a single leading zero byte. Simply put, if the high-order byte of the integer is 128 or larger then a leading zero byte must be prepended to the encoding. Without that zero byte you have the DER encoding of a negative integer.