From v4.1.0 of the documentation (2015-06-29), the HMAC SHA-1 key is
deprecated. Indeed, it should be considered as a legacy option to
guarantee compatibility with older skins.
The HMAC SHA-256 key should be used instead, and moreover, it seems that
any new skin created does not support SHA-1 at all, even if the option
is available.
The merchant signature code is inspired from the documentation example.
Source: https://docs.adyen.com/developers/hpp-manualFixes#9786
opw-676081
Nicolas Martinelli
7d25dd5618