Trying to understand Java RSA key size
The key generator was initilized with a size of 1024, so why the printed sizes are 635 and 162?
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
public class TEST {
public static KeyPair generateKeyPair() throws NoSuchAlgorithmException, NoSuchProviderException {
KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA", "BC");
keyPairGenerator.initialize(1024);
return keyPairGenerator.generateKeyPair();
}
public static void main(String[] args) throws Exception {
KeyPair keyPair = generateKeyPair();
RSAPrivateKey privateKey = (RSAPrivateKey) ke开发者_Python百科yPair.getPrivate();
RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();
System.out.println("Size = " + privateKey.getEncoded().length);
System.out.println("Size = " + publicKey.getEncoded().length);
}
}
RSA keys are made of Modulus and Exponent. The key size refers to the bits in modulus. So even without any encoding overhead, you will need more than 128 bytes to store 1024-bit keys.
getEncoded() returns ASN.1 DER encoded objects. The private key even contains CRT parameters so it's very large.
To get key size, do something like this,
System.out.println("Key size = " + publicKey.getModulus().bitLength());
Here are the relevant ASN.1 objects,
RSAPrivateKey ::= SEQUENCE {
version Version,
modulus INTEGER, -- n
publicExponent INTEGER, -- e
privateExponent INTEGER, -- d
prime1 INTEGER, -- p
prime2 INTEGER, -- q
exponent1 INTEGER, -- d mod (p-1)
exponent2 INTEGER, -- d mod (q-1)
coefficient INTEGER, -- (inverse of q) mod p
otherPrimeInfos OtherPrimeInfos OPTIONAL
}
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER -- e
}
First hint: 1024 bits = 128 bytes
Second hint: privateKey.getEncoded()
returns an encoded
representation (i.e. not raw).
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