Revisiting the RC4 / RC4_128 Cipher

Microsoft

Mar 23, 2019

First published on MSDN on Feb 09, 2011

The implementation of RC4/RC4_128 in SQL Server does not salt the key and this severely weakens the security of data that is encrypted using the RC4/RC4_128 algorithm.

In cryptography, an initialization vector (IV) is a fixed size input to a cryptographic algorithm that is typically required to be random or pseudorandom. Salting of cipher keys makes sure that the encryption algorithm always uses a randomized (IV) value. This leads to the following properties of the cipher-text data -

Encrypting the same piece of data two times by using the same key will produce two different cipher-text values. For example, a table might have a column value appearing multiple times. When encrypted, a user cannot recognize the presence of similar plain-text values by just comparing the cipher-text values.

This mechanism adds additional protection against cryptanalysis of the cipher-text data.

Figure: Encryption without salted keys

Figure: Encryption with salted keys

Since SQL Server does not salt RC4 or RC4_128 keys, similar data that is encrypted by using the same RC4/RC4_128 key repeatedly will result in the same cipher-text output.

Let us understand the implications of using the RC4 or RC4_128 cipher with the help of an example:

--Step (1) : Create a database testDB

CREATE DATABASE testDB;

USE testDB

--Step (2) : Create an RC4 symmetric key object protected by a password in testDB

CREATE SYMMETRIC KEY sym_key_RC4

WITH ALGORITHM = RC4

ENCRYPTION BY PASSWORD = 'SomeStr0ngPassword';

--Step (3) : Open the RC4 key to use for encryption

OPEN SYMMETRIC KEY sym_key_RC4 DECRYPTION BY PASSWORD = 'SomeStr0ngPassword';

--Step (4) : Experiment using the RC4 cipher to encrypt data

SELECT encryptbykey(key_guid('sym_key_RC4'), 'abc');

-- Output : 0x0053ED707ACDC54F83C4B273B29D819B01000000EADEA0D236B4D17BF321EB

SELECT encryptbykey(key_guid('sym_key_RC4'), 'abc');

-- Output : 0x0053ED707ACDC54F83C4B273B29D819B01000000EADEA0D236B4D17BF321EB

--Step (5) : Close the RC4 symmetric key

CLOSE SYMMETRIC KEY myRC4;

Notice that when data ‘abc’ is encrypted by using the RC4 symmetric key ‘sym_key_RC4’, the output representing the cipher-text is identical both times. It might appear intuitive to solve this problem by programmatically adding different salt values such as ‘0123456789ABCDEF’ and ‘FED6753925243232’ through the application code as shown below –

SELECT encryptbykey(key_guid('sym_key_RC4'), '0123456789ABCDEFabcdefg' );

--Output :

0x00E53ACDE34CAE4BA2140D6A246F6CBC 01000000 A6FC3B9FB44D4CC1 A8575A5FD06AFFA42FFCBD2DCF68F3F0 89FD6BC5947987

SELECT encryptbykey(key_guid('sym_key_RC4'), 'FED6753925243232abcdefg' );

--Output :

0x00E53ACDE34CAE4BA2140D6A246F6CBC 01000000 A6FC3B9FB44D4CC1 DE232C5AD36AFAAA25F0CE5BBF1E8584 89FD6BC5947987

However, because RC4/RC4_128 is a stream cipher, the additional salt does not help hide patterns across multiple usages of the key.

To detect the use of RC4/RC4_128 symmetric key objects, users can use the Microsoft Best Practices Analyzer tool for SQL Server 2008 R2 [2], Policy Based Management [3] or directly query the sys.symmetric_keys catalog view using the query below –

SELECT * FROM sys.symmetric_keys

WHERE algorithm_desc = 'RC4'

OR algorithm_desc = 'RC4_128';

To mitigate this problem, developers are advised to use stronger cipher algorithms such as the AES family of algorithms for protecting sensitive data as shown in the example below –

--Step (1) : Create an AES_256 symmetric key object protected by a password

CREATE SYMMETRIC KEY sym_key_aes256

WITH ALGORITHM = AES_256

ENCRYPTION BY PASSWORD = 'SomeStr0ngPassword';

--Step (2) : Open the AES-256 key to use for encryption

OPEN SYMMETRIC KEY sym_key_aes256 DECRYPTION BY PASSWORD = 'SomeStr0ngPassword';

--Step (3) : Experiment using the AES-256 cipher to encrypt data

SELECT encryptbykey(key_guid('sym_key_aes256'), 'abc');

-- Output call 1: 0x0067F1EFBC6DE347AB1C383CD1E1CBA801000000D95D9B9257F15A5B3F32EC8E2B11FB66B5EF589B240E31F72FA832BFF67BAE7A

SELECT encryptbykey(key_guid('sym_key_aes256'), 'abc');

-- Output call 2: 0x0067F1EFBC6DE347AB1C383CD1E1CBA80100000065A91373165552336A88CA70B6E6FFC61E84152D93BFCD834DD6F965DF22B475

--Step (4): Close the AES-256 symmetric key