Re: [pkix] Question about Curve P-192

Denis <denis.ietf@free.fr> Fri, 11 May 2018 10:09 UTC

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From: Denis <denis.ietf@free.fr>
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Date: Fri, 11 May 2018 12:09:13 +0200
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Subject: Re: [pkix] Question about Curve P-192
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Michael,

I don't see how RFC 5480 can be used to obtain an *identifier* for a 
cryptographic *suite* for both P-192 and
a SHA-256 hash function truncated to 192 bits.

In between, I got a response why the leftmost bits shall be used.

Close to the end of section 6.4NIST FIPS 186-4 
<http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf> states:

    When the length of the output of the hash function is greater than
    the bit length of /n/, then the *leftmost **/n/**bits* of the hash
    function
    output block *shall be used* in any calculation using the hash
    function output during the generation or verification of a digital
    signature.

    A hash function that provides a lower security strength than the
    security strength associated with the bit length of n ordinarily
    *should not*
    be used, since this would reduce the security strength of the
    digital signature process to a level no greater than that provided
    by the hash function.

Denis

> Actually see RFC5480.  It describes a set of suggested pairings of 
> signature strengths and hashes and includes recommendations for P-192.
>
> Re Russ’s comment, the ECDSAWithShaxxx identifiers can be used with 
> any curve, (but follow the 5480 and other similar document pairing 
> recommendations)   so it’s not exactly correct that there are no 
> algorithm identifiers.
>
> Lastly, AFAICT NIST didn’t originally define the P192 curve - it just 
> incorporated a previously defined curve in a set of acceptable 
> parameters when it was NISTifying EC cryptography.
>
> Mike
>
> On Thu, May 10, 2018 at 17:47 Denis <denis.ietf@free.fr 
> <mailto:denis.ietf@free.fr>> wrote:
>
>     Hi Ernst, Russ and Dan,
>
>     Thank your for your replies. This is what I feared : there is no
>     cryptographic suite defined for P-192.
>     Quite strange that NIST defined the algorithm and didn't defined a
>     hash function to go with it.
>
>     Key sizes need to be appreciated relative to the environment where
>     they are used.
>
>     P-192 would be used in a constrained environment where the size of
>     the digital signature matters (i.e. the smaller, the better).
>
>     The verification of the digital signature would be real time. The
>     private key should resist one year, because it would be changed
>     every year.
>
>     P-192 seems to be a good trade-off between the security level and
>     the size of the digital signature.
>
>     A SHA-192 function has been defined in a paper available at:
>     http://www.ijctee.org/files/VOLUME2ISSUE3/IJCTEE_0612_24.pdf.
>     The title of this paper is : Performance Analysis of SHA
>     Algorithms (SHA-1 and SHA-192): A Review
>     However, I don't believe that any crypto-library supports it.
>
>     So Ernst's method would certainly be one way to do it, but why not
>     take the 192 low bits ?
>
>     The last question would be for Ernst who wrote:
>
>         The corresponding signature suite can be defined with ISO
>         14888-3, which allows the specification of the algo
>
>         (e.g. EC-DSA, EC-KCDSA or whatsoever), the curve and the hash
>         function.
>
>     What would the OID or the URI for this suite, if we take the 192
>     left bits ? Same question if we take the 192 low bits ?
>
>
>     Denis
>
>>     Hi Denis,
>>
>>      Please, do not use P-192, unless there are some severe constraints.
>>
>>     Even if you credit EC with a very generous 16 extra bits in
>>     security (compared to hashes & ciphers), P-192 would only reach
>>     96+16=112-bit security, which does not meet the current best
>>     practice of 128 bit security.
>>
>>      History as I understand it: NIST P-192 was meant for the 80-bit
>>     level (though it looks like 96-bit). This low security level has
>>     been widely deprecated since 2010, at least informally - to what
>>     extent it is formally deprecated, I don’t recall off-hand. I
>>     recall added text to ANSI X9.62/63 deprecating this security level.
>>
>>     Anyway, originally, the idea was to use P-192 with SHA-1, P-224
>>     with SHA-224, etc.
>>
>>     I think that there were also OIDs for P-192, e.g. secp192k1, and
>>     OIDs for ECDSA-with-SHA1, which could be combined in some ways. 
>>     I do not recall how far these OIDs made into IETF, i.e. as
>>     algorithm identifiers.
>>
>>     Using 160-bit hash in ECDSA with P-192 renders the EU-CMA
>>     security to 80 bits, which is waste considering that P-192
>>     potentially provides 96-bit security.  As noted in the thread
>>     below, the standards have options to truncate a longer hash,
>>     which should correct this.
>>
>>      Arguably, the security of P-192 has fared far better than SHA-1
>>     in some ways, yet SHA-1 is probably much more widely used than
>>     P-192, though admittedly hashes are considered a general purpose
>>     tool.
>>
>>      Best regards,
>>
>>     Dan
>>
>>
>>     *From:* pkix [mailto:pkix-bounces@ietf.org] *On Behalf Of *Russ
>>     Housley
>>     *Sent:* Thursday, May 10, 2018 1:30 PM
>>     *To:* Ernst G Giessmann <giessman@informatik.hu-berlin.de>;
>>     <mailto:giessman@informatik.hu-berlin.de>
>>     *Cc:* IETF PKIX <pkix@ietf.org>; <mailto:pkix@ietf.org>
>>     *Subject:* Re: [pkix] Question about Curve P-192
>>
>>      Ernst:
>>
>>      Of course, this technique works.  That said, I am not aware of
>>     any algorithm identifiers that make use of the P-192 curve for
>>     digital signature or key agreement.
>>
>>      Russ
>>
>>       On May 10, 2018, at 1:24 PM, Ernst G Giessmann
>>     <giessman@informatik.hu-berlin.de
>>     <mailto:giessman@informatik.hu-berlin.de>> wrote:
>>
>>         Yes, there is a standardized way:
>>         Pick up a corresponding hash function, in case of P-192 it
>>         should be SHA-224 and take the 192 left most bits of the hash
>>         value as the input to the EC sign primitive.
>>         The correspondig signature suite can be defined with ISO
>>         14888-3, which allows the specification of the algo (e.g.
>>         EC-DSA, EC-KCDSA or whatsoever), the curve and the hash function.
>>         Kind regards,
>>         /Ernst.
>>
>>         Am 2018-05-10 um 19:07 schrieb Denis:
>>
>>             Hello everybody,
>>
>>             Curve P-192 is specified in FIPS PUB 186-4 (Digital
>>             Signature Standard (DSS)).
>>
>>             There is no "SHA-192" hash function defined in FIPS PUB
>>             180-4 (Secure Hash Standard (SHS)).
>>
>>             Is there any standardized way to use a hash function with
>>             Curve P-192 ?
>>
>>             Is there any RFC or any another document that specifies a
>>             cryptographic suite for Curve P-192 ?
>>
>>             Denis
>>
>>