user1084509 user1084509 - 3 months ago 84
C# Question

CryptographicException in RSA.ImportParameters() - Bad Data in special 1024 keys

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

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.

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