Concept description updated #1

Author: nemothenoone

docs/concepts.md

@@ -1,3 +1,42 @@
 # Key Derivation Functions Concepts # {#kdf_concepts}
 
 ## Key Derivation Function Concepts ## {#kdf_concept}
+
+A ```KeyDerivationFunction``` is an object intended to compute non-isomorphic permutations from non-fixed size
+ elements integral field to fixed-size elements integral field,
+
+### Requirements ### {#block_ciphers_concepts_requirements}
+
+The type ```X``` satisfies ```KeyDerivationFunction``` if
+
+Given
+* ```MacType```, the type named by ```X::mac_type```
+* ```KeyType```, the type named by ```X::key_type```
+* ```SaltType```, the type named by ```X::salt_type```
+* ```LabelType```, the type named by ```X::label_type```
+* ```SecretType```, the type template named by ```X::secret_type```
+
+The following type members must be valid and have their specified effects
+
+|Expression                   |Type              |Requirements and Notes       |
+|-----------------------------|------------------|-----------------------------|
+|```X::mac_type```            |```MacType```     |```MacType``` type satisfies ```MessageAuthenticationCode``` concept|
+|```X::key_type```            |```KeyType```     |```KeyType``` type is a ```SequenceContainer``` of type ```T``` which satisfies ```Integral``` concept|
+|```X::salt_type```           |```SaltType```    |```SaltType``` type is a ```SequenceContainer``` of type ```T``` which satisfies ```Integral``` concept|
+|```X::label_type```          |```LabelType```   |```LabelType``` type is a ```SequenceContainer``` of type ```T``` which satisfies ```Integral``` concept|
+|```X::secret_type```         |```SecretType```  |```SecretType``` type is a ```SequenceContainer``` of type ```T``` which satisfies ```Integral``` concept|
+
+The following static data member definitions must be valid and have their specified effects
+
+|Expression          |Type             |Requirements and Notes                 |
+|--------------------|-----------------|---------------------------------------|
+|```X::key_bits```   |```std::size_t```|```Integral``` bits amount in ```KeyType```|
+|```X::salt_bits```  |```std::size_t```|```Integral``` bits amount in ```SaltType```|
+|```X::label_bits``` |```std::size_t```|```Integral``` bits amount in ```LabelType```|
+|```X::secret_bits```|```std::size_t```|```Integral``` bits amount in ```SecretType```|
+
+The following expressions must be valid and have their specified effects
+
+|Expression                 |Requirements      |Return Type                    |
+|---------------------------|------------------|-------------------------------|
+|```X(key_type)```|Constructs stateful ```BlockCipher``` object with input key of ```key_type```|```KeyDerivationFunction```|