Re: .NET crypto - C# Discussion Boards

27-Aug-03 8:01

I am just reading thru some dot-net samples on cryptography. The samples are very good and to-the-point, man, great step forward from the old-days. There're still a few things...

QUESTION 1: A key is a byte array, okay, I have no problem with this. But is the following Key = array of sixteen HEX number? Meaning, 16x16=256bit key? Is HEX number denoted by "0x"? But shouldnt "0x10" be written as "0xA" instead?

RijndaelManaged RMCrypto = new RijndaelManaged();

byte[] Key = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16};
byte[] IV = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16};

//Create a CryptoStream, pass it the NetworkStream, and encrypt
//it with the Rijndael class.
CryptoStream CryptStream = new CryptoStream(NetStream,
RMCrypto.CreateEncryptor(Key, IV),
CryptoStreamMode.Write);

QUETSION 2: For RSA algo, public key is not an array of HEX as in case of RijndaelManaged algo. But how do I know how I should construct the public key? I understand you can check "LegalKeySizes". But say Legal key size is:
min 16 (bits)
max 32 (bits)
skip 8 (bits)
Then what? An int32 is 32 bits wide. Does it mean I can have ONE int32 in the byte array?
int32 someint = 12;
byte[] MyKey = {someint};
If so, the following key looks pretty long to me:

byte[] PublicKey = {214,46,220,83,160,73,40,39,201,155,19,202,3,11,191,178,56,
74,90,36,248,103,18,144,170,163,145,87,54,61,34,220,222,
207,137,149,173,14,92,120,206,222,158,28,40,24,30,16,175,
108,128,35,230,118,40,121,113,125,216,130,11,24,90,48,194,
240,105,44,76,34,57,249,228,125,80,38,9,136,29,117,207,139,
168,181,85,137,126,10,126,242,120,247,121,8,100,12,201,171,
38,226,193,180,190,117,177,87,143,242,213,11,44,180,113,93,
106,99,179,68,175,211,164,116,64,148,226,254,172,147};

byte[] Exponent = {1,0,1};

... some code ...

//Create a new instance of RSACryptoServiceProvider.
RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();

//Create a new instance of RSAParameters.
RSAParameters RSAKeyInfo = new RSAParameters();

//Set RSAKeyInfo to the public key values.
RSAKeyInfo.Modulus = PublicKey;
RSAKeyInfo.Exponent = Exponent;

norm

David Stone27-Aug-03 11:19

27-Aug-03 11:19

As for question one: 0x10 in Hex == 16 in Dec, 0xA in Hex == 10.

Question 2: I wrote a little app to output all the keysizes returned from RSACryptoServiceProvider.LegalKeySizes and their max, min, and skip values. It looks like this under .NET 1.1:

Max: 16384 Min: 384 Skip: 8

And those are in bits. Divide by 8 to get bytes and you have:

Max: 2048 Min: 48 Skip: 1

So it looks like you have to have a byte[] array whose length is between 47 and 2047 (because arrays are zero based). And those values can be anywhere from 0 to 255 (the possible values for a byte).

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devvvy27-Aug-03 13:18

27-Aug-03 13:18

humm... thanks.
QUESTION 1:
STEP 1: say once you have determined the legal size:
max 128
min 16
skip 16
STEP 2: Your key legal size is therefore: {16, 32, 48, 56... 128}
STEP 3: pick one size, say 56 bits
STEP 4: 56 bits = 4 x16bits = 4 HEX number = {HEX1, HEX2, HEX3, HEX4} (Is this correct?)

QUESTION 2:
Also, instead of:

byte[] MyKey = {0x10, 0x12, 0x0A, 0x3B}

Could I have generated it programmatically? Or can I load a key from a key file generated by sn.exe?

QUESTION 3:
Do I have to rely on CryptoAPI to gain access to a key in cert store - and perhaps all other tasks that has to do with cert store?

Thanks!

norm

devvvy27-Aug-03 13:35

27-Aug-03 13:35

RSA.modulus=MyPublicKey;

What about my "Private" key?

Thanks.

norm

devvvy27-Aug-03 13:38

27-Aug-03 13:38

bear with me, but one more question =)

<cryptoclass mysha1hash="MySHA1HashClass, MyAssembly
Culture='en', PublicKeyToken=a5d015c7d5a0b012,
Version=1.0.0.0">

Can be translated to:

byte [] mykey = {0x0A, 0x05, 0x0D...0x01, 0x02}

Is this correct?

norm

David Stone27-Aug-03 20:38

27-Aug-03 20:38

norm wrote:
bear with me, but one more question =)

<cryptoClass MySHA1Hash="MySHA1HashClass, MyAssembly
Culture='en', PublicKeyToken=a5d015c7d5a0b012,
Version=1.0.0.0"/>

Can be translated to:

byte [] mykey = {0x0A, 0x05, 0x0D...0x01, 0x02}

Is this correct?

Ummmm....that's a really good question. The answer is that I have no clue. I'm guessing that it's actually split up like:

{0xa5, 0xd0, 0x15...} etc, because otherwise why would that zero be there? It looks like it's just a placeholder so that the 8 byte (look, it's 16 digits) key can be translated easily.

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David Stone27-Aug-03 20:31

27-Aug-03 20:31

norm wrote:
What about my "Private" key?

That's given to the constructor of the CspParameters object:

CspParameters myCspParams = new CspParameters();
myCspParams.Flags = CspProviderFlags.UseMachineKeyStore; //Or CspProviderFlags.UseDefaultKeyStore
myCspParams.KeyNumber = myKeyNumber;

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(myCspParams);

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David Stone27-Aug-03 20:24

27-Aug-03 20:24

norm wrote:
STEP 1: say once you have determined the legal size:
max 128
min 16
skip 16
STEP 2: Your key legal size is therefore: {16, 32, 48, 56... 128}

No, you're mistaking the size for the possible values. The size is the length of that array. The possible values for System.Byte are still going to be 0 to 255. That just means that the legal key size has to be an array with between 16 bytes and 128 bytes in it.

norm wrote:
STEP 4: 56 bits = 4 x16bits = 4 HEX number = {HEX1, HEX2, HEX3, HEX4} (Is this correct?)

Again, no, you're mistaking the possible values in the array for the length of the array.

norm wrote:
QUESTION 2:
Also, instead of:

byte[] MyKey = {0x10, 0x12, 0x0A, 0x3B}

Could I have generated it programmatically? Or can I load a key from a key file generated by sn.exe?

Yes! In fact, that's what I always do. I usually do something like this:

string myPassword<br />
byte[] passBytes = System.Text.Encoding.Default.GetBytes(myPassword);

And then I take passBytes, trim it down to the legal size (actually, it always fits within the legal size for RSA), and then give that to the RSA CSP. As for pulling from an sn.exe generated file, sure. I don't see why not, especially since that's generated (I believe) for RSA.

norm wrote:
QUESTION 3:
Do I have to rely on CryptoAPI to gain access to a key in cert store - and perhaps all other tasks that has to do with cert store?

No. I don't believe so. At least that's not what they say http://www.gotdotnet.com/team/clr/cryptofaq.htm[^]:

10. Does the crypto library have a key management system different from CryptoAPI?

No, we internally use CryptoAPI key storage and you can reference keys stored in CryptoAPI key containers by using the CspParameters object. For a sample of how to store and retrieve your own asymmetric key pair in CryptoAPI key storage see the sample on www.gotdotnet.com/team/clr/about_security.aspx.

However, I couldn't find that sample. You may have to go hunting in the MSDN docs. Although, the most valuable thing I saw on that "Sample" page was this:

The Source Code for the .NET Framework Cryptography Classes[^] Cool | :cool:

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devvvy28-Aug-03 0:34

28-Aug-03 0:34

Thanks for the patience!

David Stone wrote:
STEP 1: say once you have determined the legal size:
max 128
min 16
skip 16
STEP 2: Your key legal size is therefore: {16, 32, 48, 56... 128}

>> No, I think I agree with you. {16, 32, 48, 56... 128} are just "possible" or "legal" value for key. And sure, 1 byte=8 bit=256 possibilities.

One HEX digit = 4 bits --> So, if key size=56bit, we can define key with 14 HEX digit. we can therefore write:

byte[] MyKey = {0x10, 0x12, 0x0A, 0x3B, 0x2D, 0x81, 0x7D}

Is this correct?

David Stone wrote:
string myPassword
byte[] passBytes = System.Text.Encoding.Default.GetBytes(myPassword);

That's a good trick.

norm

David Stone28-Aug-03 9:06