user1084509 - 1 year ago 376
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?

`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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