user1084509 - 4 months ago 109

C# 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

`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.