RSA encrypt/decrypt between C and python

Question

I have server written in python and client in C . Their job is to send a secret message from server to client which is encrypted with RSA private key. I am using openssl/rsa.h library, that is I initialize a rsa object with a private key and encrypte a message with RSA_public_encrypt(length_of_message, "Secret Message", to, rsa, RSA_PKCS1_PADDING) . Then I send this encrypted message to python server and try to decrypt it with same private key using from Crypto.PublicKey import RSA library. Problem is that it does not decrypt it properly. It always outputs 128-bit length message where the secret message is randomly placed in it (e.g. '\x23\xa3x\43...Secret Message\xef\x4a'), where it should normally return just Secret Message.

"decrypt it with same private key using" - that better be a typo. — WhozCraig
– WhozCraig, Commented Mar 14, 2014 at 8:06
I am using RSA_public_encrypt(length_of_message, "Secret Message", to, rsa, RSA_PKCS1_PADDING) which encrypts public. When I generate public key from private and use it, functions gives errors. — mustafa.yavuz
– mustafa.yavuz, Commented Mar 14, 2014 at 8:32
I think it is related with standard padding options, but I can not solve it yet. When I encrypt with rsautl -encrypt -in msg.file -inkey tvfp.pem -out out.bin and decrypt with openssl rsautl -decrypt -in out.bin -inkey tvfp.pem it works properly. However, when I add -raw option for padding while decrypting, it prints out similar result to python decrypter. — mustafa.yavuz
– mustafa.yavuz, Commented Mar 14, 2014 at 9:29

mustafa.yavuz · Accepted Answer · 2014-03-14 09:50:24Z

3

The problem was about the padding. Python's rsa module decrypts result with PKCS1 padding and does not removes padding. With the function below which I have taken from here problem was solved:

def pkcs1_unpad(text):
if len(text) > 0 and text[0] == '\x02':
    # Find end of padding marked by nul
    pos = text.find('\x00')
    if pos > 0:
        return text[pos+1:]
return None

answered Mar 14, 2014 at 9:50

mustafa.yavuz

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Rounak Mehta · Accepted Answer · 2019-01-28 11:23:08Z

Is it possible to create a same pair of RSA key in Python and C . please find the code below and let me know if any modification needed to get it worked.

Code in python

   key = RSA.generate(2048)
   file_out_pub = open("pubkey.der", "wb")
   file_out_pub.write(key.publickey().exportKey())
   file_out_pub.close()
   file_out_pub = open("pubkey.der", "`enter code here`r")
   public_key = RSA.importKey(file_out_pub.read())
   cipher = PKCS1_OAEP.new(public_key)
   password = pw
   ciphertext = cipher.encrypt(password)

Code in C

 int clen = 0, num, ret;
 clen = strnlen_s(req->pw,2048);
 unsigned char ptext[2048];
 RSA *rsa = RSA_new();
 BIGNUM *e = BN_new();
 ret = RSA_generate_key_ex(rsa, 2048, e, NULL );
 num = RSA_private_decrypt(clen, req->pw , ptext, rsa, RSA_PKCS1_OAEP_PADDING);
 // Start authentication process
 strncpy(req->pw,ptext,MAX_PASSWORD_STR);

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