[openssl-users] help on des_cblock

[Scott Neugroschl]

I suspect the use of std::string and c_str().  Use a std::vector<char> instead.

From: openssl-users [mailto:openssl-users-bounces at openssl.org] On Behalf Of Jason Qian
Sent: Friday, March 18, 2016 1:19 PM
To: openssl-users at openssl.org
Subject: [openssl-users] help on des_cblock

I am new on openSSl and run  into a issue need some help.


In our application, the client and server perform a Diffie Hellman Key exchange and then encrypt the data  The client is written in C++(using openSSL), and server is in java.

 Most of time, it is running correctly, but occasionally the server(java) throw a  "Given final block not properly padded" exception.

I added more log on the both side. When the exception happen,  the keys are offset by one(for the working case, they are the same)


Server -- java  get from getEncoded()

DES Key  size (8)    (1,-83,-113,-74,-77,109,84,88)

Client -- openSSL  get from des_cblock struct

DES Key  size (8)   (-83,-113,-74,-77,109,84,88,8)

Thanks
Jason

Here is the C++ code

void DiffieHellmanCipher::init(const std::string &Y){
    if (Y.length() == 0) {
        return;
    }
    if (m_DH == NULL) {
        return;
    }

    // convert the Y to BIGNUM
    BIGNUM *bnY = NULL;
    // Memory for bnY is allocated in BN_dec2bn call.
    if (!BN_dec2bn(&bnY, Y.c_str())) {
        if (bnY)
            BN_free(bnY);
        printf("Could not convert Diffie-Hellman Y value to BIGNUM");
    }

    // compute the secret key
    int dhSize = DH_size(m_DH);
    unsigned char *secretKey = (unsigned char*) new char[dhSize + 1];
    int secretKeyLen = DH_compute_key(secretKey, bnY, m_DH);
    BN_free(bnY);

    if (secretKeyLen < 8) {
        delete [] secretKey;
        printf("Error computing secret key: key length is too short");
    }

    // convert from raw form to odd parity DES key
    des_cblock desKey;
    memcpy(desKey, secretKey, 8);
    delete [] secretKey;
    DES_set_odd_parity(&desKey);

      //just print out des_cblock
    secretKeyString="(";
    char ch[10]="\0";
    for(int i=0;i<8;i++){
        sprintf(ch,"%d",(char)desKey[i]);
      secretKeyString+=ch;
      if(i != 7){
        secretKeyString+=",";
      }
    }
    secretKeyString+=")";


    int skRet;
    if ((skRet = DES_set_key(&desKey, &m_DESKey)) != 0) {
        delete [] secretKey;
        printf("Error computing secret key: generated key is weak");
    }

    m_bInited = true;
}
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