Added whitepaper reference

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cc: @billmath

Author: MarileeTurscak-MSFT

articles/active-directory/hybrid/how-to-connect-password-hash-synchronization.md

@@ -51,7 +51,7 @@ The following section describes, in-depth, how password hash synchronization wor
 3. After the password hash synchronization agent has the encrypted envelope, it uses [MD5CryptoServiceProvider](/dotnet/api/system.security.cryptography.md5cryptoserviceprovider) and the salt to generate a key to decrypt the received data back to its original MD4 format. The password hash synchronization agent never has access to the clear text password. The password hash synchronization agent’s use of MD5 is strictly for replication protocol compatibility with the DC, and it is only used on-premises between the DC and the password hash synchronization agent.
 4. The password hash synchronization agent expands the 16-byte binary password hash to 64 bytes by first converting the hash to a 32-byte hexadecimal string, then converting this string back into binary with UTF-16 encoding.
 5. The password hash synchronization agent adds a per user salt, consisting of a 10-byte length salt, to the 64-byte binary to further protect the original hash.
-6. The password hash synchronization agent then combines the MD4 hash plus the per user salt, and inputs it into the [PBKDF2](https://www.ietf.org/rfc/rfc2898.txt) function. 1000 iterations of the [HMAC-SHA256](/dotnet/api/system.security.cryptography.hmacsha256) keyed hashing algorithm are used. 
+6. The password hash synchronization agent then combines the MD4 hash plus the per user salt, and inputs it into the [PBKDF2](https://www.ietf.org/rfc/rfc2898.txt) function. 1000 iterations of the [HMAC-SHA256](/dotnet/api/system.security.cryptography.hmacsha256) keyed hashing algorithm are used. For additional details, refer to the [Azure AD Whitepaper](https://aka.ms/aaddatawhitepaper).
 7. The password hash synchronization agent takes the resulting 32-byte hash, concatenates both the per user salt and the number of SHA256 iterations to it (for use by Azure AD), then transmits the string from Azure AD Connect to Azure AD over TLS.</br> 
 8. When a user attempts to sign in to Azure AD and enters their password, the password is run through the same MD4+salt+PBKDF2+HMAC-SHA256 process. If the resulting hash matches the hash stored in Azure AD, the user has entered the correct password and is authenticated.