Question

When I create an RSA keypair should I be be doing

KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
kpg.initialize(2048);
KeyPair kp = kpg.genKeyPair();
PublicKey publicKey = kp.getPublic();
PrivateKey privateKey = kp.getPrivate();

save("public.key",publicKey.getEncoded())
save("private.key",privateKey.getEncoded())

OR

KeyFactory fact = KeyFactory.getInstance("RSA");
RSAPublicKeySpec pub = fact.getKeySpec(kp.getPublic(),RSAPublicKeySpec.class);
RSAPrivateKeySpec priv = fact.getKeySpec(kp.getPrivate(),RSAPrivateKeySpec.class);

saveToFile("public.key", pub.getModulus(),pub.getPublicExponent());
saveToFile("private.key", priv.getModulus(),priv.getPrivateExponent());

Which is better and what is the difference?

Answer 1

For public keys, it doesn’t make much difference. For private keys, getEncoded() returns much more information than the private key.

Here is the ASN.1 schema for RSA Private Key,

-- 
-- Representation of RSA private key with information for the CRT algorithm.
--
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 
}

Version ::= INTEGER { two-prime(0), multi(1) }
    (CONSTRAINED BY {-- version must be multi if otherPrimeInfos present --})

OtherPrimeInfos ::= SEQUENCE SIZE(1..MAX) OF OtherPrimeInfo


OtherPrimeInfo ::= SEQUENCE {
    prime             INTEGER,  -- ri
    exponent          INTEGER,  -- di
    coefficient       INTEGER   -- ti
}

Those extra parameters will speed up private key operations considerably. So you should always use getEncoded().