CryptographicException in RSA.ImportParameters() -

Question

We have a C#/.Net 4.0 application which imports RSA Private Keys from a String in Base64 received in a WebService.

This application works perfectly for RSA-Keys in 1024 bits, but doesn't with a special kind of rsa private keys (around 1% of keys).

Here are the byte lengths:

Working-Key:

• Modulus => 128 Bytes
• Exponent => 3 Bytes
• D => 128 Bytes
• P => 64 Bytes
• Q => 64 Bytes
• DP => 64 Bytes
• DQ => 64 Bytes
• IQ => 64 Bytes

Not-Working-Key:

• Modulus => 128 Bytes
• Exponent => 3 Bytes
• D => 127 Bytes
• P => 64 Bytes
• Q => 64 Bytes
• DP => 64 Bytes
• DQ => 64 Bytes
• IQ => 64 Bytes

The difference is in the lenght of D (128 working, 127 not working). The not-working key is 1 byte shorter than the working key.

The parameters are set but when doing RSA.ImportParameters(rsaParams) it throws a CryptographicException with a "Bad Data" Message.

What should be included to solve this problem?

Answer 1

RSACryptoServiceProvider has some assumptions on the data lengths which are:

• Modulus: any even size, let's call the length n
• Exponent: (<= 4 bytes; though RSACng allows "any size"), let's call the length e
• D: n
• P: n/2
• Q: n/2
• DP: n/2
• DQ: n/2
• InverseQ: n/2

So, assuming that your second key is actually Modulus: 128 bytes (because a 64-byte P times a 64-byte Q isn't a 256 byte number), you just need to left-pad the D array with a zero to bring it up to the proper length.

byte[] newD = new byte[modulus.Length];
Buffer.BlockCopy(d, 0, newD, newD.Length - d.Length, d.Length);

.NET Core has the source code available showing that relationship. In .NET Framework it's buried inside the CLR, so not available on referencesource.