Re: [FIPS compliance] ssl reneg when counter overflows(AES_GCM)

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On 04/11/2016 09:26, Marcus Meissner wrote:
On Fri, Nov 04, 2016 at 10:03:21AM +0530, Akshar Kanak wrote:
Dear team
     as per the documnet http://csrc.nist.gov/groups/
STM/cmvp/documents/fips140-2/FIPS1402IG.pdf
     page 150 , Its mentioned
     The implementation of the nonce_explicit management logic inside the
module shall ensure that
     when the nonce_explicit part of the IV exhausts the maximum number of
possible values for a given
     session key (e.g., a 64-bit counter starting from 0 and increasing,
when it reaches the maximum value
     of 2 64 -1),
*either party (the client or the server) that encounters this condition
triggers a handshake to    establish a new encryption key – see Sections
7.4.1.1 and 7.4.1.2 in RFC 5246*.

     is this being handled by openssl ? in the source code of openssl i am
not able find out the
     exact location where this renegotiation is initiated when the counter
over flows ?
(my understanding might be limited)


I think we currently do an error if the calling frontend does not initiate renegotiation.

Code is here:
crypto/modes/gcm128.c

CRYPTO_gcm128_encrypt_ctr32

These kind of checks avoid the 32bit counter overflow:

     if (mlen > ((U64(1) << 36) - 32) || (sizeof(len) == 8 && mlen < len))
             return -1;

The calling instance needs to re-iv with CRYPTO_gcm128_setiv.

For TLS, if I understand correctly, the stack does intiailize the GCM cipher
with a new IV on every TLS record and these cannot be that large.

Ciao, Marcus
As I understand the GCM mode, the limitation is that the same
IV must not be used twice, and only a limited number of all
the possible IVs may be used before changing the *key*.
Setting a "new" IV for each TLS record is always needed,
regardless if it comes from a simple record counter or is
transmitted for each record (the TLS RFCs presumable have a
fixed choice for this).

But to get a new *key* before too much data has been encrypted
with it (such limits exist for *all* algorithms and modes), I
believe that an actual TLS renegotiation is needed, involving
at least a partial handshake without closing the connection.

Enjoy

Jakob
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