Understanding Hashed Password Storage and SQL Server

As a security-conscious developer, you’re likely familiar with the importance of storing hashed passwords securely. In this article, we’ll delve into the intricacies of hashing passwords in SQL Server and explore why converting between string representations can be tricky.

Introduction to Password Hashing

Password hashing is a process that transforms a plaintext password into a fixed-length string of characters, known as a hash value. This hash value is then stored in a database or other secure storage location, making it impossible for an attacker to recover the original password even if they gain access to the stored data.

In SQL Server, you can use built-in functions like hashbytes to generate a SHA-2 hash of a given input string. The resulting hash value is typically represented as a binary string, which needs to be converted into a human-readable format for storage in a database column.

The Problem with Storing Hashed Passwords

When you store hashed passwords in a SQL Server table, the resulting hash values may not match the expected format or representation. In the provided Stack Overflow question, the original code uses varchar(32) to store the hashed password, which leads to distorted output and issues when matching the stored hash with newly generated hashes.

Understanding the Issues

To understand why this happens, let’s break down the key concepts:

Hash values: Hash values are fixed-length binary strings that represent a unique value or message. In SQL Server, you can use functions like hashbytes to generate SHA-2 hash values.
Character encoding: Different character encodings (e.g., UTF-8, ASCII) can affect how text is represented in a database column. When storing hashed passwords, it’s essential to choose an encoding that matches the one used in your application or system.
Binary vs. string representation: When generating hash values using hashbytes, you get a binary string as output. However, when storing this value in a SQL Server column, you need to convert it into a human-readable format, like a string.

Solving the Issue

To avoid distorted output and ensure that hashed passwords match your expected format, follow these steps:

1. Convert Binary Hash Value to Hexadecimal String

You can use the Convert function in SQL Server to convert binary hash values into hexadecimal strings. This process involves specifying a length parameter to control the number of characters returned.

DECLARE @Pass AS VARCHAR(32) = 'Abc123()';
DECLARE @Hash AS VARBINARY(32) = HASHBYTES('SHA2_256', @Pass);

SELECT CONVERT(VARCHAR(66), @Hash, 1);

In this code snippet:

We use HASHBYTES to generate a SHA-2 hash value for the input string @Pass.
The Convert function is used to convert the binary hash value into a hexadecimal string, with a length of 66 characters.
By using parameter 1 in the Convert function, we ensure that the resulting hexadecimal string starts without a leading 0x.

2. Convert Binary Hash Value to Decimal String (Optional)

If you prefer a decimal representation of your hash values, you can use the following code snippet:

DECLARE @Pass AS VARCHAR(32) = 'Abc123()';
DECLARE @Hash AS VARBINARY(32) = HASHBYTES('SHA2_256', @Pass);

SELECT CONVERT(DECISION(66), @Hash);

However, this method has some limitations and should be used with caution.

Best Practices for Storing Hashed Passwords

When storing hashed passwords in a SQL Server table:

Choose the correct data type: Use varbinary to store binary hash values, or varchar(max) to store hexadecimal strings.
Select an encoding that matches your application or system’s character set.
Always use secure hashing algorithms like SHA-2 for generating hash values.

By following these guidelines and using the techniques outlined in this article, you can ensure that hashed passwords are stored securely in SQL Server.

Last modified on 2023-12-30