1) Is it correct to say that the cryptographic base used by ASICs is the digital signature?
Nope. A digital signature generally means that a signature is generated that can be authenticated to a known key. ASICs do not do anything like that; they perform SHA256 hashes twice on the block header and they result in a hash.
2) Does this mean that encryption is asymmetric?
No. The hashing methods in SHA is
symmetric.
Sorry, I was wrong about this piece of info. SHA256 is a hashing algorithm and it is neither symmetrical nor asymmetrical.
3) Is the underlying algorithm SHA256 or ECDSA? I also read double SHA256... I think I've gone a little confused
Thanks in advance!
C.
SHA256D. Its used to hash block header and the hash is considered in the validity of a block (ie. Determine whether it meets the target).
ECDSA is used in the transaction signatures itself. Both is used in the protocol but the mining doesn't consist of ECDSA.