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Re: [kitten] DES enctypes are not PKCS#7 padded (Re: Input on Paul's concerns about aes-cbc)

Tom Yu <tlyu@MIT.EDU> Wed, 09 October 2013 13:46 UTC

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To: Luke Howard <lukeh@padl.com>
References: <20131009022305.BD1FB1A9E6@ld9781.wdf.sap.corp> <B1237751-AD3C-4A09-93DF-8CE56B746499@padl.com> <ldvzjqik2qb.fsf_-_@cathode-dark-space.mit.edu> <A86BAEC9-A792-43BF-ACFE-93A4206EC01A@padl.com>
From: Tom Yu <tlyu@MIT.EDU>
Date: Wed, 09 Oct 2013 09:46:00 -0400
In-Reply-To: <A86BAEC9-A792-43BF-ACFE-93A4206EC01A@padl.com> (Luke Howard's message of "Thu, 10 Oct 2013 00:21:05 +1100")
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Cc: "kitten@ietf.org" <kitten@ietf.org>, Sam Hartman <hartmans-ietf@mit.edu>
Subject: Re: [kitten] DES enctypes are not PKCS#7 padded (Re: Input on Paul's concerns about aes-cbc)
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Luke Howard <lukeh@padl.com> writes:

>>> Because this sample was geared towards GSS-API interoperability (which was DES only at the time, hence required PKCS#7 padding), it's no surprise that it correctly uses SECBUFFER_PADDING.
>> 
>> The DES enctypes used in Kerberos are not PKCS#7 padding.  When the
>> plaintext is an exact multiple of the block size, there is no padding
>> for DES.  PKCS#7 type padding requires a full block of padding in that
>> case, because it is self-describing.
>
> Ah, my (long-term) bad.
>
> What about RFC 4757?
>
> Greg: in the MIT implementation, make_seal_token_v1_iov()/kg_fixup_padding_iov() may be  wrong then for DES, I believe I assumed PKCS#7 when implementing it.

Greg has reminded me that RFC 1964 uses PKCS#7 padding for single-DES;
I was mistakenly recalling that the DES CBC padding used in the RFC
1510 protocol (which is not PKCS#7) was the same used with DES for
GSS-Wrap.  Sorry for any confusion.

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