On 19-02-2020 12:21, Arne Schwabe wrote:
> With modern Clients and server initialising the crypto cipher later
> and not when reading in the config, most users never the warning when
> having selected BF-CBC in the configuration.
>
> This patch adds the logic to print out warning to init_key_type.
>
> Main reason for this patch is a personal experience with someone who was
> strictly against putting 'cipher' into a config file because he did not
> like hardcoding a cipher and "OpenVPN will do AES-GCM anyway" and thinks
> that it is better to not have it in configuration even after told by me
> that 15 year defaults might not be good anymore.
>
> Patch V2: rebase on master, fix minor style issues
>
> Signed-off-by: Arne Schwabe <a...@rfc2549.org>
> ---
> src/openvpn/crypto.c | 26 ++++++++++++++++++--------
> 1 file changed, 18 insertions(+), 8 deletions(-)
>
> diff --git a/src/openvpn/crypto.c b/src/openvpn/crypto.c
> index 65e789ed..453cb20a 100644
> --- a/src/openvpn/crypto.c
> +++ b/src/openvpn/crypto.c
> @@ -736,6 +736,17 @@ crypto_max_overhead(void)
> +max_int(OPENVPN_MAX_HMAC_SIZE, OPENVPN_AEAD_TAG_LENGTH);
> }
>
> +static void warn_insecure_key_type(const char* ciphername, const cipher_kt_t
> *cipher)
> +{
> + if (cipher_kt_insecure(cipher))
> + {
> + msg(M_WARN, "WARNING: INSECURE cipher (%s) with block size less than
> 128"
> + " bit (%d bit). This allows attacks like SWEET32.
> Mitigate by "
> + "using a --cipher with a larger block size (e.g.
> AES-256-CBC).",
> + ciphername, cipher_kt_block_size(cipher)*8);
> + }
> +}
> +
> /*
> * Build a struct key_type.
> */
> @@ -779,6 +790,10 @@ init_key_type(struct key_type *kt, const char
> *ciphername,
> {
> msg(M_FATAL, "Cipher '%s' not allowed: block size too big.",
> ciphername);
> }
> + if (warn)
> + {
> + warn_insecure_key_type(ciphername, kt->cipher);
> + }
> }
> else
> {
> @@ -831,9 +846,10 @@ init_key_ctx(struct key_ctx *ctx, const struct key *key,
> cipher_ctx_init(ctx->cipher, key->cipher, kt->cipher_length,
> kt->cipher, enc);
>
> + const char* ciphername =
> translate_cipher_name_to_openvpn(cipher_kt_name(kt->cipher));
> msg(D_HANDSHAKE, "%s: Cipher '%s' initialized with %d bit key",
> prefix,
> - translate_cipher_name_to_openvpn(cipher_kt_name(kt->cipher)),
> + ciphername,
> kt->cipher_length *8);
>
> dmsg(D_SHOW_KEYS, "%s: CIPHER KEY: %s", prefix,
> @@ -841,13 +857,7 @@ init_key_ctx(struct key_ctx *ctx, const struct key *key,
> dmsg(D_CRYPTO_DEBUG, "%s: CIPHER block_size=%d iv_size=%d",
> prefix, cipher_kt_block_size(kt->cipher),
> cipher_kt_iv_size(kt->cipher));
> - if (cipher_kt_insecure(kt->cipher))
> - {
> - msg(M_WARN, "WARNING: INSECURE cipher with block size less than
> 128"
> - " bit (%d bit). This allows attacks like SWEET32. Mitigate
> by "
> - "using a --cipher with a larger block size (e.g.
> AES-256-CBC).",
> - cipher_kt_block_size(kt->cipher)*8);
> - }
> + warn_insecure_key_type(ciphername, kt->cipher);
> }
> if (kt->digest && kt->hmac_length > 0)
> {
> Tested, works as advertised. Acked-by: Steffan Karger <steffan.kar...@foxcrypto.com> -Steffan
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