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cencryptionopenssldesevp-cipher

Problem by Encryption & Decryption of text with EVP_des_ofb(), openSSL , C

I need to encrypt and decrypt txt-file with DES-ofb (libcrypto) using OpenSSL library, the key and Init Vector is given in one bin.file(key+iv). But after the decryption via EVP_DecryptUpdate(), decrypted text and plain text are not similar at all.

Decryption Difficulties

So I read plain.txt 8 bytes and a 'keyandIV.bin' files. Than I took first 8 bytes from keyandIVbuffer as a KEY for DES and the rest as IV. So I have 8 bytes key and 8 bytes IV, added '\0' at the end of both (Do I need '\0' here ? Key length must be 64 or 56 bits?).

This is my code for ercryption with DES ofb:

printf("ENCRYPTION:\n");
int howmany = 0, final1;
const EVP_CIPHER *CIPHER_TYPE = EVP_des_ofb();
EVP_CIPHER_CTX *ctx_encrypt = EVP_CIPHER_CTX_new();
EVP_CIPHER_CTX_init(ctx_encrypt);

EVP_EncryptInit(ctx_encrypt, CIPHER_TYPE, keybuf1, ivbuf1);
if(!EVP_EncryptUpdate(ctx_encrypt, ciphertextbuf1, &howmany, plaintextbuf1, plainlength1))return -1;
if(!EVP_EncryptUpdate(ctx_encrypt, ciphertextbuf1, &howmany, plaintextbuf1, plainlength1)) return -1;
EVP_EncryptFinal_ex(ctx_encrypt, ciphertextbuf1 + howmany , &final1);
EVP_CIPHER_CTX_cleanup(ctx_encrypt);

Than I took the encrypted buffer und decrypt it so:

printf("DECRYPTION:\n");
int final2;
EVP_CIPHER_CTX *ctx_decrypt = EVP_CIPHER_CTX_new();
EVP_CIPHER_CTX_init(ctx_decrypt);
EVP_DecryptInit(ctx_decrypt, CIPHER_TYPE, keybuf1, ivbuf1);
if(!EVP_DecryptUpdate(ctx_decrypt, decryptedtext, &howmany, ciphertextbuf1, strlen(ciphertextbuf1))) return -1;
if(!EVP_EncryptFinal_ex(ctx_decrypt, decryptedtext + howmany, &final2)) return -1;
EVP_CIPHER_CTX_cleanup(ctx_decrypt);

I definetly have understanding problem with DES. Maybe I did something wrong by creatimg key and IV from one file.I have seen plenty of examples but I still don't understand what I did wrong in my program.

Solution

  • The decryption sequence is EVP_DecryptInit_ex(), EVP_DecryptUpdate() and EVP_DecryptFinal_ex(). This follows EVP_EncryptInit_ex(), EVP_EncryptUpdate() and EVP_EncryptFinal_ex(). In your code you are calling EVP_EncryptFinal_ex() to decrypt, so obviously that's not going to work. Also, if something went wrong during an operation an error code should have been printed out to stderr.