Let's assume this scenario:
- I sign a message using my subkey
- I send the signed message to someone who has my master public key
Is recipient be able to verify my message? Or do I have to provide my public-subkey?
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You have to provide the public subkey to the recipient in order for them to validate anything signed with the corresponding private subkey.
The Master key serves a purpose somewhat analogous to a Certificate Authority. It represents your authority to bless all your personal subkeys as valid keys that were issued by you. But just as having a root certificate doesn't mean you can use TLS to talk to any random server that was issued a public key - you still need that server's public key (which is delivered on the certificate.)
Digital signature is equivalent to handwritten signature where the authenticity of the person is assured (digital signature is the virtual equivalent of a written signature). It works as explained below:
You mat refer these sources: https://searchsecurity.techtarget.com/answer/Which-private-keys-and-public-keys-can-create-a-digital-signature