[Cryptography] EDSA signing: Can you have an old (v2) signing of v2 hash and v3 signing of hash v3 (that is prefix+SHA256(v2 hash) cause some attacks, like lenghth extension and so on?

[Ondrej Mikle]

We have currenctly v2 signature scheme:

Take `hash_v2 = SHA256(binary_blob)` (blob has maximum size, fixed blob)

If we change the signing algorithm with the same keys to:

`hash_v3' = SHA256("Some_plaintext_prefix" + SHA256(hash_v2))

Which gives 2 signatures,

* ECDSA_v2 = secp256k1_sign(hash_v2)
* ECDSA_v3 = secp256k1_sign(hash_v3)

Both blob and prefix are plaintext.

Is it possible by some length extension or other attacks to malevolently 
manipulate the singatures?

E.g. If we check that either ECDSA_v2 or ECDSA_v3 are valid over the same 
`binary_blob`, would it allow any attacks?

Regards,
  O.M.