BMCF definition for PHC string format

[ademarre]

Apparently related to the Password Hashing Competition that selected Argon2 as the winner in 2015, PHC string format can be considered a subset or profile of MCF. It is more thoroughly specified than MCF. It has nice properties that enable the creation of a BMCF definition that could be reused for all PCH string format–compatible schemes:

• it defines a concrete format with four fields: $<id>[$<param>=<value>(,<param>=<value>)*][$<salt>[$<hash>]]
• it defines the legal characters for id, param, and value
• it specifies a max length for id and param
• it designates RFC 4648 Base64 (without = padding) as the encoding to use for the hash digest
• although it does not require the same encoding for the salt, it does define the legal characters for the salt