Thks.
But, I also meet a problem when decrypt data(the encrypted data is a 16 bytes
long ). The code is below:
int ret;
EVP_CIPHER_CTX ctx;
EVP_CIPHER_CTX_init(&ctx);
ret = EVP_DecryptInit_ex(&ctx, EVP_aes_128_cbc(), 0, key, iv); // ret=1
cout<<"EVP_CIPHER_CTX_block_size: "<<EVP_CIPHER_CTX_block_size(&ctx)<<endl;
// 16
cout<<"EVP_CIPHER_CTX_key_length: "<<EVP_CIPHER_CTX_key_length(&ctx)<<endl;
// 16
cout<<"EVP_CIPHER_CTX_iv_length: "<<EVP_CIPHER_CTX_iv_length(&ctx)<<endl;
//16
int outl;
int len=0;
ret = EVP_DecryptUpdate(&ctx, buffer, &outl, (unsigned char *)in, 16); //
ret = 1, outl=0 why?
len += outl;
ret = EVP_DecryptFinal_ex(&ctx, buffer+len, &outl); // ret = 0 why?
here len=0
len += outl;
ret = EVP_CIPHER_CTX_cleanup(&ctx);
But, when I call EVP_DecryptUpdate with param inl = 17
ret = EVP_DecryptUpdate(&ctx, buffer, &outl, (unsigned char *)in, 17); // ret
= 1, outl = 16
I get correct decrypted data. What's wrong with me?
Bian
> Hi
>
> This is my piece of program:
>
> EVP_CIPHER_CTX_init(&ctx);
> ret = EVP_CIPHER_CTX_set_padding(&ctx, 0); //0 for no
> padding, 1 for padding // ret ==1 here
> unsigned char *key = GetKeyPtr();
> ret = EVP_EncryptInit(&ctx, EVP_aes_128_cbc(), NULL, NULL);
> // ret ==1 here
> ret = EVP_CIPHER_CTX_set_key_length(&ctx, 16); // ret ==1 here
> ret = EVP_EncryptInit(&ctx, NULL, key, NULL); // ret ==1 here
> ///...
> unsigned char buffer[128];
> int outl;
> int len=0;
>
> ret = EVP_EncryptUpdate(&ctx, buffer, &outl, (unsigned char
> *)in, 16); // ret ==1 here
> len += outl; // == 16
Why is this line here? The output buffer doesn't contain the final output
yet, so there's no reason to add the length to the final length.
> ret = EVP_EncryptFinal(&ctx, buffer+len, &outl); // ret ==1 here
> len += outl; // == 32
> ///...
>
> Should len equals 16 at last ? But , why the result is 32? Can
> anyone tell me the reason?
You added the same data twice. If you want to find *new* data that has been
output, you have to look at the *change* in 'outl'. It is both an input and
an output parameter.
If you always want 'len' to contain the amount of ouput data, you need a
'len -= outl' before you call EVP_EncryptFinal. Whether it makes sense to do
that or just eliminate the 'len += outl' above depends on what you're doing
with the intermediate results. Since your example does nothing with the
intermediate results, it makes no sense to add thier size to 'len'.
DS
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