Re: Internal key management system

On Wed, 19 Feb 2020 at 09:29, Cary Huang <cary(dot)huang(at)highgo(dot)ca> wrote:
>
> Hi
>
> I have tried the attached kms_v3 patch and have some comments:
>
> 1) In the comments, I think you meant hmac + iv + encrypted data instead of iv + hmac + encrypted data?
>
> ---> in kmgr_wrap_key( ):
> + /*
> + * Assemble the wrapped key. The order of the wrapped key is iv, hmac and
> + * encrypted data.
> + */

Right, will fix.

>
>
> 2) I see that create_keywrap_ctx function in kmgr_utils.c and regular cipher context init will both call ossl_aes256_encrypt_init to initialise context for encryption and key wrapping. In ossl_aes256_encrypt_init, the cipher method always initialises to aes-256-cbc, which is ok for keywrap because under CBC block cipher mode, we only had to supply one unique IV as initial value. But for actual WAL and buffer encryption that will come in later, I think the discussion is to use CTR block cipher mode, which requires one unique IV for each block, and the sequence id from WAL and buffer can be used to derive unique IV for each block for better security? I think it would be better to allow caller to decide which EVP_CIPHER to initialize? EVP_aes_256_cbc(), EVP_aes_256_ctr() or others?
>
> +ossl_aes256_encrypt_init(pg_cipher_ctx *ctx, uint8 *key)
> +{
> + if (!EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, NULL, NULL))
> + return false;
> + if (!EVP_CIPHER_CTX_set_key_length(ctx, PG_AES256_KEY_LEN))
> + return false;
> + if (!EVP_EncryptInit_ex(ctx, NULL, NULL, key, NULL))
> + return false;
> +
> + /*
> + * Always enable padding. We don't need to check the return
> + * value as EVP_CIPHER_CTX_set_padding always returns 1.
> + */
> + EVP_CIPHER_CTX_set_padding(ctx, 1);
> +
> + return true;
> +}

It seems good. We can expand it to make caller decide the block cipher
mode of operation and key length. I removed such code from the
previous patch to make it simple since currently we support only
AES-256 CBC.

>
> 3) Following up point 2), I think we should enhance the enum to include not only the Encryption algorithm and key size, but also the block cipher mode as well because having all 3 pieces of information can describe exactly how KMS is performing the encryption and decryption. So when we call "ossl_aes256_encrypt_init", we can include the new enum as input parameter and it will initialise the EVP_CIPHER_CTX with either EVP_aes_256_cbc() or EVP_aes_256_ctr() for different purposes (key wrapping, or WAL encryption..etc).
>
> ---> kmgr.h
> +/* Value of key_management_cipher */
> +enum
> +{
> + KMGR_CIPHER_OFF = 0,
> + KMGR_CIPHER_AES256
> +};
> +
>
> so it would become
> +enum
> +{
> + KMGR_CIPHER_OFF = 0,
> + KMGR_CIPHER_AES256_CBC = 1,
> + KMGR_CIPHER_AES256_CTR = 2
> +};
>
> If you agree with this change, several other places will need to be changed as well, such as "kmgr_cipher_string", "kmgr_cipher_value" and the initdb code....

KMGR_CIPHER_XXX is relevant with cipher mode used by KMS and KMS would
still use AES256 CBC even if we had TDE which would use AES256 CTR.

After more thoughts, I think currently we can specify -e aes-256 to
initdb but actually this is not necessary. When
--cluster-passphrase-command specified, we enable the internal KMS and
always use AES256 CBC. Something like -e option will be needed after
supporting TDE with several cipher options. Thoughts?

>
> 4) the pg_wrap_key and pg_unwrap_key SQL functions defined in kmgr.c
> I tried these new SQL functions and found that the pg_unwrap_key will produce the original key with 4 bytes less. This is because the result length is not set long enough to accommodate the 4 byte VARHDRSZ header used by the multi-type variable.
>
> the len variable in SET_VARSIZE(res, len) should include also the variable header VARHDRSZ. Now it is 4 byte short so it will produce incomplete output.
>
> ---> pg_unwrap_key function in kmgr.c
> + if (!kmgr_unwrap_key(UnwrapCtx, (uint8 *) VARDATA_ANY(data), datalen,
> + (uint8 *) VARDATA(res), &len))
> + ereport(ERROR,
> + (errmsg("could not unwrap the given secret")));
> +
> + /*
> + * The size of unwrapped key can be smaller than the size estimated
> + * before unwrapping since the padding is removed during unwrapping.
> + */
> + SET_VARSIZE(res, len);
> + PG_RETURN_BYTEA_P(res);
>
> I am only testing their functionalities with random key as input data. It is currently not possible for a user to obtain the wrapped key from the server in order to use these wrap/unwrap functions. I personally don't think it is a good idea to expose these functions to user

Thank you for testing. I'm going to include regression tests and
documentation in the next version patch.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services