RSA_private_decrypt returns -1

Question

I am trying to encrypt plain text using RSA algorithm. I have a problem with my code.

RSA        *_RSA ;
unsigned char text[2560] = "A";
unsigned char sectext[2560];
unsigned char decrypttext[2560];
int i = 0;

_RSA = RSA_generate_key ( 1024, 65537, NULL, NULL );
i = RSA_public_encrypt ( 1, text,    sectext,     _RSA, RSA_PKCS1_OAEP_PADDING );
i = RSA_private_decrypt( 1, sectext, decrypttext, _RSA, RSA_PKCS1_OAEP_PADDING);
RSA_free ( _RSA );

The return value RSA_public_encryptis 128, which is the size of the ciphertext. RSA_private_decryptreturns -1, which is an error. If I try to display the recovered text, I get nothing.

Why RSA_private_decryptreturns -1?

Answer 1

You should not pass 1 to RSA_private_decrypt, but the length of the block you are trying to decrypt, i.e. return value RSA_public_encrypt= 128. You do not know the length of the plaintext when decrypting!

This complete sample program results in:

128 from encrypt. 
1 from decrypt: 'A'

Source:

#include <openssl/rsa.h>
#include <openssl/engine.h>
#include <stdio.h>

int main(int argc, char **argv)
{
    RSA *myRSA;
    unsigned char cleartext[2560] = "A";
    unsigned char encrypted[2560] = { 0 };
    unsigned char decrypted[2560] = { 0 };
    int resultEncrypt = 0;
    int resultDecrypt = 0;

    myRSA = RSA_generate_key ( 1024, 65537, NULL, NULL );
    resultEncrypt = RSA_public_encrypt ( 1 /* strlen(cleartext) */, cleartext, encrypted, myRSA, RSA_PKCS1_OAEP_PADDING );
    printf("%d from encrypt.\n", resultEncrypt);
    resultDecrypt = RSA_private_decrypt( 128 /* resultEncrypt */, encrypted, decrypted, myRSA, RSA_PKCS1_OAEP_PADDING);
    printf("%d from decrypt: '%s'\n", resultDecrypt, decrypted);
    RSA_free ( myRSA );

    return 0;
}

Answer 2

ERR_error_string(), .