RSA encryption and decryption for data which is larger than RSA_SIZE

[Michael Schmid]

  Hi,

I'm trying to encrypt some data with rsa. The RSA_SIZE of my key is 128.
The data can be more than 128 bit. So my idea was to encrypt the data using
a
loop which encrypts 128 bytes at once.
I've read that there could be problems if the length is bigger than 
RSA_SIZE-1.
So I set blockLength to RSA_SIZE-11.
The length of data in this example is less than 256 so the encrypted data
has a
size of 256 (with padding).


This is the encryption-code:

  int blockLength = RSA_size(rsa)-11;  // (see below)
  int padding = RSA_PKCS1_PADDING;
  int clen=blockLength;

  // encrypt blockwise, size: RSA_SIZE-11 (( see documentation)
  for (unsigned int left=0; left< flen ; left+=blockLength)
  {
    if(left+clen>flen)
    {
      clen = flen - left;
    }
    l=RSA_private_encrypt(blockLength,                    /// <-- how big
should this be ??????????
                (unsigned char*) value.mid(left,clen).data(),
                to+left , rsa, padding);
    if(l<=0)
    {
      printf("RSA Encrpytion Error.\n");
    }
    else
    {
      *len+=l;
    }
  }


And this is the decryption code:

  int blockLength = RSA_size(rsa);  // -11
  int padding = RSA_PKCS1_PADDING;
  unsigned char to[KEYLEN+1]=""; // KEYLEN = 256
  int flen = KEYLEN;//RSA_size(rsa);

  for (unsigned int left=0; left< flen ; left+=blockLength)
  {
    l = RSA_public_decrypt(blockLength-11, binkey+left, to+left, rsa,
padding);
    if(l<=0)
    {
      printf("RSA Decryption Error.");
    }
    else
    {
      len+=l;
    }
    printf("Decrypted %d bytes",len);
  }
  ERR_print_errors_fp(stdout);



Encryption is working fine, but I get -1 in as result of RSA_public_decrypt.

This is the message from ERR_print_errors():
6364:error:0407006A:lib(4):func(112):reason(106):rsa_pk1.c:100:
6364:error:04067072:lib(4):func(103):reason(114):rsa_eay.c:430:
6364:error:0407006A:lib(4):func(112):reason(106):rsa_pk1.c:100:
6364:error:04067072:lib(4):func(103):reason(114):rsa_eay.c:430:

What am I doing wrong? Is there a problem with padding?

Thanks in advance
      M.Schmid

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