Add android and ios information

Author: haldarmahesh

README.md

@@ -4,6 +4,23 @@ This plugin helps you by generating the assymetric RSA key pair.
 
 The keys are generated and persisted in android/ios keystore.
 
+## Assymetric Key details 
+The details of keys generated and store in android/iOS keystore is as follows:
+* type -> RSA
+* signature algorithm -> SHA256withRSA
+* key size -> 2048 bit
+
+### Android
+* The RSA key pair is stored android [Keystore](https://developer.android.com/training/articles/keystore.html)
+* AES encryption is used.
+* The AES secret is encrypted with RSA and stored in shared preference.
+* For Android 9 (API leverl 28) [StrongBox](https://developer.android.com/training/articles/keystore.html#HardwareSecurityModule) is enabled by default.
+* As keystore is available in Android 4.3 (API level 18) and hgher, hence this is minimum required Android.
+
+### iOS
+* The RSA key pair is stored in iOS [Keychain](https://developer.apple.com/documentation/security/keychain_services)
+* This plugin will run on iOS 10.0 and higher
+
 ## Using plugin
 This plugin exposes the following methods:
 * `getPublicKey`: returns the RSA public key
@@ -18,7 +35,6 @@ This key pair is persisted in key store, i.e it is generated once, and always re
 
 ```dart
 static Future<String> getPublicKey()
-
 ```
 #### usage: 
 
@@ -50,7 +66,7 @@ This function verifies the signature against the plain data, it returns `true` i
 static Future<bool> verify(String plainText, String signature)
 ```
 
-usage:
+#### usage:
 ```dart
 final bool result = await Cipher.verify('somePassword', 'SIGNATURE-XX');
 ```