RC4 Weak Key checks

Markus Sander msander at gmx.de
Mon Apr 18 11:17:53 EDT 2011

Am 24.03.2011 15:33, schrieb Jeffrey Altman:
> In k5_arcfour_init() src/lib/crypto/enc_provider/rc4.c MIT's
> implementation of RC4 has a check for two families of weak keys.
> Those beginning with either of the following three octet sequences:
>   arcfour_weakkey1[] = {0x00, 0x00, 0xfd};
>   arcfour_weakkey2[] = {0x03, 0xfd, 0xfc};
> If a key in either of these families is detected in k5_arcfour_init(),
> the error KRB5DES_WEAK_KEY is returned to the caller.
> In my reading of RFC 3961 and RFC 4757 I do not come across any
> indication that a weak key test should be applied when RC4 is used with
> Kerberos or GSS.  When used with GSS the weak key test is especially
> problematic.  For example, each call to gss_wrap() generates a new per
> message RC4 key as each message is treated by RFC 4757 as a new RC4
> keystream.  The RC4 key is generated using the sequence number as input
> using the following call sequence:
>   gss_wrap -> gss_seal -> k5glue_seal -> krb5_gss_seal -> kg_seal ->
>   make_seal_token_v1 -> kg_make_seq_num -> kg_arcfour_docrypt ->
>   k5_arcfour_docrypt -> k5_arcfour_init
> An application calling gss_wrap() experiences a random behavior.  When a
> KRB5DES_WEAK_KEY error is returned from k5_arcfour_init(), it is never
> handled and the gss_wrap() call fails.  This can happen relatively
> quickly or can require hundreds of thousands of messages.  Regardless,
> if the GSS context is used to send enough messages, the error will be
> thrown.
> In examining other implementations of RC4 I can find no weak key check.
>  I am questioning whether MIT's implementation should have one and if so
> whether it should be used in conjunction with GSS.
> If the weak key check is maintained, it implies that in any given GSS
> context certain sequence numbers cannot be used.  Since there is no
> standard for how these sequence numbers should be skipped, the inclusion
> of the weak key check is an interoperability problem with existing
> deployed GSS implementations.
> I would appreciate the feedback of the members of this list.

I was bitten by this. I detailed the problem as we/our customer
experienced it on the OpenLDAP mailing list. The post is available from

Removing the weak key check made the problem go away.

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