Excel Password Hashing and Security Algorithms Explained

Microsoft Excel is the standard for managing data, but have you ever wondered what actually happens when you click "Protect Workbook"? It is not just a simple lock. Under the hood, Excel uses a complex system of "hashing" and encryption to keep your data safe.

Understanding how these password hashes work is incredibly important for IT professionals, security auditors, and anyone trying to recover a lost password. The security level depends entirely on which version of Excel you are using and which type of protection you have applied.

The Three Levels of Excel Security

It is important to realize that not all password features in Excel are created equal. In fact, they function in very different ways and explained below.

Based on the detailed guide to Excel security we discussed above, here is a simple, human-readable explanation of what Hashing actually is.

What is Hashing? (The Digital Fingerprint)

In the world of Microsoft Excel security, Hashing is the mathematical magic trick that allows Excel to verify your password without ever actually saving your password.

Think of a hash like a digital fingerprint. Every person has a unique fingerprint. If the police find a fingerprint at a crime scene, they can match it to a person, but they cannot use that fingerprint to clone the person.

Hashing works the same way for your data. It is a "one-way" process.

How Hashing Works in Simple Terms

1. The Input

   You type your password (e.g., "MySecretPass").
2. The Process

   Excel runs this text through a hashing algorithm (like SHA-512 in modern Excel). This algorithm chops up the data, mixes it around, and performs complex math on it.
3. The Output

   The result is a long, random-looking string of characters called a "Hash."

Why Does Excel Use Hashing?

If Excel just saved your password in a text file inside the document, any hacker could open the file code and read it. Instead, Excel stores the Hash (the fingerprint).

1. When you set a password, Excel calculates the hash and saves only the hash.
2. When you try to open the file later, you type your password.
3. Excel instantly calculates the hash of what you just typed.
4. It compares your new hash with the saved hash.
5. If they match perfectly, Excel knows you typed the right password and opens the file.

Hashing in the XML Code

We can actually see this happening in the code blocks we looked at earlier. Excel doesn't hide the fact that it is hashing your data.

In the Worksheet Protection XML, you can see the result of the hashing process clearly labeled:

<sheetProtection 
    algorithmName="SHA-512" 
    hashValue="[Base64-encoded hash]" 
    saltValue="[Base64-encoded salt]" 
    spinCount="100000" ... />

• algorithmName="SHA-512": This tells the computer which "blender" to use.
• hashValue="...": This is the "smoothie." It is the scrambled result of your password.
• spinCount="100000": This is how many times Excel re-blended the mix to make it harder for hackers to reverse-engineer.

Key Takeaways

• It is unique: Even a tiny change to your password (like adding one space) will create a completely different hash.
• It is one-way: Hackers cannot simply "decrypt" a hash to get your password. They have to guess the password, hash their guess, and see if it matches.
• It is secure: Modern Excel uses SHA-512, which creates a massive, complex hash that is incredibly difficult to fake.

The Evolution of Excel Hashing

The way Excel handles passwords has changed a lot over the years. As computers became faster, Microsoft had to make their hashing algorithms much stronger to stop hackers from breaking in.

How the Hash is structured (The Technical Part)

Modern Excel files (.xlsx) are actually just ZIP folders filled with XML code. When you password protect a file, Excel creates a specific set of instructions inside that ZIP file.

The Encryption Info

For modern Excel files, the security "recipe" is stored in a stream of data called EncryptionInfo. This tells the computer exactly how to handle the password. It looks like this:

<encryption>
  <keyData 
    saltSize="16" 
    blockSize="16" 
    keyBits="256" 
    hashSize="64" 
    cipherAlgorithm="AES" 
    cipherChaining="ChainingModeCBC" 
    hashAlgorithm="SHA512" 
    saltValue="[Base64-encoded salt]"/>
  
  <dataIntegrity 
    encryptedHmacKey="[Base64-encoded]" 
    encryptedHmacValue="[Base64-encoded]"/>
  
  <keyEncryptors>
    <keyEncryptor uri="http://schemas.microsoft.com/office/2006/keyEncryptor/password">
      <encryptedKey 
        spinCount="100000" 
        saltSize="16" 
        blockSize="16" 
        keyBits="256" 
        hashSize="64" 
        cipherAlgorithm="AES" 
        cipherChaining="ChainingModeCBC" 
        hashAlgorithm="SHA512" 
        saltValue="[Base64-encoded salt]" 
        encryptedVerifierHashInput="[Base64-encoded]" 
        encryptedVerifierHashValue="[Base64-encoded]" 
        encryptedKeyValue="[Base64-encoded]"/>
    </keyEncryptor>
  </keyEncryptors>
</encryption>

The Worksheet Hash

When you lock a specific sheet, the hash is stored right there in the workbook XML. It looks a bit simpler:

How Excel Verifies Your Password (Without Storing It)

You might be wondering: if Excel doesn't store my password, how does it know I typed the right one? The answer lies in a clever process involving Salting, Spinning, and Verifiers.

Summary and Security Tips

The shift from the weak, 16-bit hashes of the 1990s to the massive 512-bit hashes of today has changed the game for document security.

By understanding how these hashes work, you can make better decisions about how you protect your personal and business data.