PGP5 and v4 GPG keys

[Hasnain Mujtaba]

Hello,

I read in an old GPG archives message (dated 2000) that PGP5.x cannot
encrypt a message using a GPG v4 public key. RFC2440 says something
about PGP5 not being able to properly encrypt to RSA v4 keys. Are these
two issues related?

Can anyone shed light on this problem. Why is PGP 5 picky about RSA v4
keys? 

Thanks
Hasnain. 
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