Size of RSA struct and size of public key in Openssl

Question

为什么如果我在RSA对象上调用RSA_size() ，我得到的值小于在同一个RSA对象上调用的RSA_size()的返回值（ i2d_RSAPublicKey的大小）？

Answer 1

Because i2d_RSAPublicKey gives you a PKCS#1 encoded version of the key, including public exponent and DER elements.

RSA_size() gives you just the size in bytes of the pure modulus (which is also the size of any unencoded signature or ciphertext for that key).