generalise transaction header/footer validation

[iltoga]

Description

implment util function to generalise tx validation for header and footer (this last if tx is signed)

it should, at least contain:

• transaction type: must be
  • not nil
  • a valid tx type
• sender account type: must be
  • not nil
  • a valid account type
• sender account: must be a valid account
  • not nil
  • a valid account (validation depends on the account type):
    • type 0: b64decode and check that is 32 bytes long
• recipient account: if present (depends on tx type), must be a valid account
  • a valid account (validation depends on the account type):
    • type 0: b64decode and check that is 32 bytes long
• fee: must be
  • greater than 0 (beside if genesis block transaction?)
• transaction body length: must be
  • not nil (?)
  • greater than 0 (?)
• transaction body bytes: must be
  • len(TransactionBodyBytes) == TransactionBodyLength
• transaction signature: if present, must be
  • a valid signature (verify against account public key...)

Breakdown

• Implement new method in util package and add above tx validation rules
• call it at the beginning of every transaction Validate() implementation
• refactor VerifySignature method to use account public key instead of account ID (to be discussed)

[andy-shi88]

Looks good, and in my #74 PR I already update the signature verification to accept AccountType+AccountAddress instead, it'll check the type and do proper signature validation according to it type.