Add Get-StringHash

Author: iSazonov

1-Draft/RFC-Get-StringHash.md

@@ -0,0 +1,93 @@
+---
+RFC: 
+Author: Ilya Sazonov
+Status: Draft
+SupercededBy: n/a
+Version: 0.1
+Area: Cmdlet
+Comments Due:
+---
+
+# Get-StringHash cmdlet
+
+Add the new cmdlet to PowerShell to get cryptographically strong hashes for strings.
+
+Current .Net implementation of GetHashCode method calculates a hash that is not a global (across application domains) safe and is not a permanent value.
+According to the documentation, this method has many practical limitations.
+https://msdn.microsoft.com/en-us/library/system.object.gethashcode%28v=vs.110%29.aspx
+https://msdn.microsoft.com/en-us/library/system.string.gethashcode%28v=vs.110%29.aspx
+>As a result, hash codes should never be used outside of the application domain in which they were created, they should never be used as key fields in a collection, and they should never be persisted.
+
+Currently Powershell only has Get-FileHash cmdlet to get a file hashes.
+Users are forced to use a workaround to get a string hashes:
+```powershell
+Get-FileHash -InputStream ([System.IO.MemoryStream]::new([System.Text.Encoding]::UTF8.GetBytes("test string")))
+```
+
+## Motivation
+
+With the new cmdlet users can use native Powershell cmdlet syntax without a workaround.
+
+With the new cmdlet users can get hashes which:
+
+* is a cryptographically strong hashes
+* is across-platform
+* may be sended across application domains
+* may be saved in databases
+* may be used as key in collections
+* may be used to compare strings and texts
+
+## Specification
+
+### Parameters
+
+1. InputString
+
+    Type = array of strings
+
+    Position = 0
+
+    ##### Attributes
+
+    ValueFromPipeline = true
+
+    AllowNull()
+
+    AllowEmptyString()
+
+2. Algorithm
+
+    Type = string
+
+    Position = 1
+
+    ##### Attributes
+
+    ValidateSet = SHA1, SHA256, SHA384, SHA512, MD5
+
+3. Encoding
+
+    Type = [Microsoft.PowerShell.Commands.FileSystemCmdletProviderEncoding]
+
+    Position = 2
+
+### Encoding
+
+.Net hash algorithm methods work on byte buffer. So the cmdlet must convert a input string to byte array.
+This process is a encoding sensitive.
+To correctly process the user must specify the encoding of incoming strings.
+
+### InputString accept null and empty input strings
+
+.Net hash algorithm methods can calculate a hash for empty string without exception.
+
+.Net hash algorithm methods throw for null input buffer.
+This behavior is not convenient when processing an array of strings by using a pipeline:
+```powershell
+    "test1", $null, "test2" | Get-StringHash
+```
+Best behavior is to create null as result hash for null string and generate non-terminating error.
+
+## Alternate Proposals and Considerations
+
+None