RSA Digital Signature
Hello, I'm doing some lab work and I can't figure out how to get a sane hash image. In general, the essence of the problem is 1. Calculating the hash image h = h (T), where T is the original message, h(T) is the hash function (for MD5, the hash image length is 128 bits).
For example, after compressing a string into a number, we got a small number 7. Moreover, no algorithm is specified for this, and even an example of a string. In general, then you will need to raise this number to a power to calculate digital signature. And here I do not understand how to get such a number. I honestly know only two methods:
- By hashing with MD5. The result is given in a matrix form, which does not suit me at all. I used the System.Security class.Cryptography.MD5.
- the usual GetHashCode method on the string. But there are values of the form -8893475.
They didn't give me any information at all, and I basically couldn't Google it, because I don't really know what to Google. I understand that maybe there is not enough information for the post. And the question may be stupid, but I will be grateful if someone answers.
In general, the question itself is how to actually get a certain number for this algorithm (calculate the hash sum or how to call it correctly)?
1 answers
The calculation of the RSA digital signature consists of 3 parts:
- Calculating the hash from the message
- Next, we align the length of the hash so that it is equal to the length of the RSA key (for example, we stupidly trim or somehow supplement it with padding algorithms
- To the resulting aligned padded hash, apply
h^d mod n
Now more specifically, in relation to your case: you have an md5 hash - 026f8e459c8f89ef75fa7a78265a0025 - this is its 16-bit representation, you need it first trim according to the length of your key and convert to a digit:
number = BigInteger.Parse(
"026f8e459c8f89ef75fa7a78265a0025", //не обрезан
NumberStyles.HexNumber);
Next, you have a number that you can work with properly.