Generate Random Passwords in Excel with Entropy

You can generate random strings in Excel using RANDARRAY for simple PINs or VBA macros for alphanumeric passwords, but these methods rely on pseudo-random number generators that are not cryptographically secure. For low-stakes internal codes, Excel is sufficient; for passwords protecting real accounts, you should generate them in a dedicated tool and use Excel only to store or organize them.

Why Excel Password Generators Are Risky

Excel’s native random functions, such as RAND() and RANDBETWEEN(), rely on the standard pseudo-random number generator (PRNG) built into the spreadsheet engine. These generators are designed for statistical simulations, not security. They have a relatively short period before the sequence repeats, and if an attacker knows the seed or a few previous outputs, they can often predict future values.

For a six-digit PIN used on a low-value internal form, this risk is negligible. For a 12-character password protecting a bank account or an admin dashboard, it is a liability. The core issue is that Excel does not use a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG). It uses logic similar to Math.random(), which is vulnerable to state recovery attacks.

When you generate passwords in bulk for credential rotation, you need assurance that no two passwords are correlated and that the sequence cannot be reverse-engineered. Excel cannot provide this guarantee natively. You can mitigate this by using VBA to access better random sources, but even then, it is safer to generate the passwords elsewhere and paste them into Excel for record-keeping.

The Math: Calculating Entropy in Excel

Entropy measures the unpredictability of a password in bits. The formula for a random password generated from a specific character set is:

$$ \text{Entropy} = \text{Length} \times \log_2(\text{Charset Size}) $$

You can calculate this directly in Excel to verify if your generated strings are strong enough. Let’s say you want to check a 12-character password using a standard alphanumeric set (A-Z, a-z, 0-9).

1. Determine Charset Size:
2. Uppercase (26) + Lowercase (26) + Digits (10) = 62 characters.
3. Calculate Log Base 2:
4. In Excel, use =LOG(62, 2) which returns approximately 5.954.
5. Multiply by Length:
6. =12 * LOG(62, 2) returns approximately 71.45 bits.

Here is a practical table for common configurations:

You can build a simple entropy calculator in Excel by placing the length in cell A1 and the charset size in cell B1, then entering =A1*LOG(B1,2) in C1. This helps you decide how long your generated strings need to be before you even start generating them.

Method 1: Using RANDARRAY for Simple PINs

If you only need numeric PINs or simple alphanumeric codes for non-critical uses, Excel’s dynamic array functions are the easiest route. This method uses RANDARRAY, available in Excel 365 and Excel 2021+.

To generate a 6-digit numeric PIN in cell A1:

=TEXTJOIN("", TRUE, RANDARRAY(6, 1, 0, 9, TRUE))

How it works:

• RANDARRAY(6, 1, 0, 9, TRUE) creates a 6x1 array of random integers between 0 and 9.
• TEXTJOIN("", TRUE, ...) concatenates these numbers into a single string without separators.

To generate a 10-character alphanumeric string, you need to map random numbers to characters. This is clunky in pure Excel formulas because you must manually define the character set.

=LET(
    chars, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789",
    len, 10,
    rnd, RANDARRAY(len, 1, 1, LEN(chars), TRUE),
    TEXTJOIN("", TRUE, INDEX(MID(chars, rnd, 1), SEQUENCE(len)))
)

Worked Example: If RANDARRAY produces indices 12, 45, 2, 60, 33, 19, 55, 8, 41, 27, the MID function extracts the corresponding characters from the chars string (where index 1 is 'A', index 2 is 'B', etc.). Index 12 maps to 'L', 45 to 's', 2 to 'B', 60 to '7', 33 to 'g', 19 to 'S', 55 to '2', 8 to 'H', 41 to 'o', and 27 to 'a'. The result is LsB7gS2Hoa.

Limitation: This relies on RANDARRAY, which uses the same weak PRNG as RAND(). It is fast and requires no coding, but it is not secure for high-value accounts.

Method 2: Generating Complex Strings with VBA

For better randomness without leaving Excel, use VBA. VBA can access the Windows CryptoAPI or use the Rnd function with a better seed, though true CSPRNG access is limited. A more robust approach in VBA is to use the System.Security.Cryptography library via COM, but that is complex. A practical middle ground is to use VBA’s Rnd function seeded by Timer and Randomize, which is better than Excel’s static seed but still not cryptographically secure.

Here is a VBA macro that generates a 12-character alphanumeric password using a custom random function.

1. Press Alt + F11 to open the VBA Editor.
2. Go to Insert > Module.
3. Paste the following code:

Function GeneratePassword(Length As Integer) As String
    Dim chars As String
    Dim i As Integer
    Dim result As String
    Dim rndNum As Integer
    
    ' Define character set: Uppercase, Lowercase, Digits
    chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
    
    Randomize ' Seeds the random number generator using Timer
    
    For i = 1 To Length
        ' Generate random number between 1 and length of chars
        rndNum = Int((Len(chars) * Rnd) + 1)
        result = result & Mid(chars, rndNum, 1)
    Next i
    
    GeneratePassword = result
End Function

1. Close the VBA Editor.
2. In any Excel cell, type =GeneratePassword(12).

Worked Example: If you enter =GeneratePassword(12) in cell A1, Excel might return kR7pQ2mX9vL4. The Randomize statement ensures the seed changes each time the function runs, unlike Excel’s built-in RAND() which can repeat sequences in certain contexts. However, Rnd in VBA is still a linear congruential generator, so it is not truly secure for high-stakes credentials.

Verifying Strength with PasswordForge

Since Excel’s generators are not cryptographically secure, the best practice is to generate passwords in a dedicated tool and use Excel only for storage or bulk organization. PasswordForge generates passwords using crypto.getRandomValues(), the same CSPRNG your browser uses for TLS keys. This ensures true unpredictability.

For bulk credential rotation, PasswordForge’s bulk mode allows you to generate 1–1000 passwords at once. You can export these as CSV, which you can then open directly in Excel. This workflow ensures the passwords are secure from the start, rather than trying to fix weak Excel-generated strings after the fact. The entropy display lets you confirm strength before you commit the passwords to your systems.

Best Practices for Bulk Credential Rotation

When managing many accounts, follow this workflow to balance Excel’s convenience with security requirements:

1. Generate Elsewhere: Use a CSPRNG-based tool to generate your passwords. Do not rely on Excel’s RAND() or VBA Rnd for high-value accounts.
2. Import for Organization: Paste the generated passwords into Excel to organize them by service, username, and creation date. Use Excel’s sorting and filtering to track which accounts have been rotated.
3. Validate Entropy: Before deploying passwords from Excel, paste a sample into a verification tool to confirm the entropy matches your expectations (e.g., 70+ bits for 12-character alphanumeric).
4. Secure Storage: If you keep passwords in Excel, protect the file with a strong password and encryption. Remember that Excel’s file encryption is separate from the password generation process.
5. Avoid Copy-Paste Errors: When moving bulk passwords from Excel to a password manager, use CSV export to avoid formatting issues. Ensure that leading zeros in PINs are preserved by formatting the cells as Text before pasting.

By treating Excel as a database rather than a generator, you avoid the pitfalls of weak random number generation while still leveraging its powerful data management features.