PGP: is there such a thing as a "signature only key?"

[Dave Bird]

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In article <55ED5FD3B4D2D41193E60002A5090BCD01B3D878@clerkenwell.pres.co
>, Owen Blacker <owen.blacker@wheel.co.uk> writes
>Charles Lindsey, quoting Dave Bird:
>> 
>> >  It has been claimed to me there is such a thing as a "signature
>> >  only key", and that the PGP program will (automatically) refuse
>> >  to encrypt to it.
>> 
>> The feature was present even in PGP2.6.x. I.e., there is a bit in the key
>> somewhere that tells whether it is signature only, and I presume PGP will
>> refuse to encrypt to it in that case (I never actually tried, and for
>> sure a competent hacker could soon bypass it). But I  don't think the
>> options available to the ordinary user gave the ability to set that bit. 
>
>It seems, fwiw, that what is happening is that the PGPsdk functionality
>used by Turnpike is wrongly allowing the user to encrypt to a revoked
>ElGamal encryption subkey.  I would assume (but don't know and haven't got
>round to finding out yet) that the code is probably checking to see if the
>whole-key is revoked, rather than just the subkey in question.

 My version of PGP tells me that the main forward (signing) key 
 still has an UNREVOKED backward (encrypt) sub-key in there.  
 Note that it ** not ** just ** the SDK used by Turnpike. My PGP,
 launched directly from the PGP icon, also does the same thing.  

 I don't understand why, if it was revoked,  PGP did not 
 actually remove and over-write the key numbers for the sub-key.
>
>This thread is continuing on the PGP users list
><http://cryptorights.org/pgp-users/> rather than here as it's off topic for
>this list.

 What's the email address to subscribe? I'm not on it yet. (Or I consent
 for anyone to subscribe me at this email address).  

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