AES encryption of 16 bytes without padding

问题:

This should be a simple question, but I can't find any examples or figure out the answer from the openssl docs.

I want to encrypt exactly 128 bits, which should fit in one encryption block.

So I call EVP_EncyptInit_ex, and then what?

Do I call EVP_EncryptUpdate_ex (to encrypt the 128 bit block) and EVP_EncryptFinal_ex (even though there is nothing more left to encrypt)?

Or only EVP_EncryptUpdate_ex? Or only EVP_EncryptFinal_ex?

回答1:

Here, you have already figured out the steps. So, it will be

1. EVP_encryptInit_ex
2. EVP_EncryptUpdate_ex
3. EVP_EncryptFinal_ex

EVP_EncryptFinal_ex also take care of the fact that data is not in multiple of block lengths.

In my opinion, if you have only to use AES with no padding (EVP_ interfaces takes care of padding), then go for AES_encrypt.

They are fairly easy to use.

//Step 1: Set encrypt key.
AES_KEY aeskey;
AES_set_encrypt_key(key, bits, &aeskey);
//Step2: Encrypt exactly 128 bits.
AES_encrypt(data, dataout, &aeskey);

回答2:

AES encryption of 16 bytes without padding

Use the EVP_* interfaces and disable padding on the block.

Use the EVP_* interface because it supports engines and hardware acceleration, like AES-NI. The AES_encrypt functions are software based and do not support alternate implementations. Also, its not readily apparent, but AES_encrypt is not portable - some platforms suffer endianess issues.

You need to call EVP_CIPHER_CTX_set_padding to ensure no padding is added. From the EVP_CIPHER_CTX_set_padding(3) man page:

EVP_CIPHER_CTX_set_padding() enables or disables padding. By default encryption operations are padded using standard block padding and the padding is checked and removed when decrypting. If the pad parameter is zero then no padding is performed, the total amount of data encrypted or decrypted must then be a multiple of the block size or an error will occur. This function should be called after the context is set up for encryption or decryption with EVP_EncryptInit_ex().

So your steps are:

1. Call EVP_CIPHER_CTX_new to create a context
2. Call EVP_EncryptInit_ex with the context
3. Call EVP_CIPHER_CTX_set_padding on the context
4. Call EVP_EncryptUpdate_ex to encrypt the data
5. Call EVP_EncryptFinal_ex to retrieve the cipher text

Also see EVP Symmetric Encryption and Decryption on the OpenSSL wiki.