Questions about particular use cases (integrity verification w/o private key, add E flag to primary key, import secp256k1 key)

[Robert J. Hansen]

> I understand that the first one is ECDSA and the second is ECDH, but
> can't I use the same secp256k1 key (if I import it) but in
> different two representations (ECDSA representation for Sign and
> Certify and ECDH for Encrypt)?

Please re-read my message:

>> secp256k1 is a certain field of numbers in which elliptical curve 
>> operations may be defined.  It is not an algorithm.  You do not 
>> have a secp256k1 key.  You have an ECDSA key which operates in the 
>> curve defined by secp256k1.

What you want to do is like saying, "RSA and DSA each use prime numbers,
so can't I use the same prime numbers for each?"  And the answer is no,
not really, because RSA and DSA are different algorithms that work in
different ways.  Even if you were to use the same prime numbers for
both, your RSA keypair would be distinctly different from your DSA
keypair.  They are not interchangeable.

Please stop talking about "secp256k1 keys".  You do not have secp256k1
keys.  You have ExDSA or ECDH keys which are not interchangeable with
each other.