OpenSSL: can input and output buffers be the same for encryption / decryption routines?

Question

For example, in:

int EVP_EncryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl, unsigned char *in, int inl); 

... maybe out == in ?

Answer 1

I just stumbled upon this question because I was curious. Since no one answered, I tried this and it really works (at least with AES CTR 128 decryption), so I risk assuming it works for other types as well. Here is my sample code in case you are interested.

 /* Test Vector from http://www.inconteam.com/software-development/41-encryption/55-aes-test-vectors */ const unsigned char key[16] = { 0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6, 0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c }; const unsigned char IV[16] = { 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff }; unsigned char test[16] = { 0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a }; EVP_CIPHER_CTX mCtx; EVP_DecryptInit(&mCtx, EVP_aes_128_ctr(), key, IV); int out_size; EVP_DecryptUpdate(&mCtx, test, &out_size, test, 16);