Microsoft Excel file formats have evolved significantly over the last two decades, moving from basic binary structures to sophisticated, compressed XML standards. 📊

The biggest shift occurred with Excel 2007, which introduced the "x" formats (like .xlsx), marking a transition from weak legacy security to modern, military-grade cryptography.

This guide breaks down every major extension, explaining which ones keep your data safe and which ones leave it vulnerable.

The Modern Era: Office Open XML (2007 - Microsoft 365)

Starting with Excel 2007, Microsoft introduced the Office Open XML format. These files are essentially zipped archives containing XML code, making them smaller, safer, and more robust against corruption.

Common Modern Extensions

Extension Format Name Macros Primary Use Case Encryption
.xlsx Excel Workbook ❌ No Standard default format for all modern Excel users. Best for sharing and compatibility. 🔒 AES-256
.xlsm Macro-Enabled Workbook ✅ Yes Same as .xlsx but allows VBA (Visual Basic for Applications) scripts to run. 🔒 AES-256
.xlsb Binary Workbook ✅ Yes Optimized for performance. Stores data in binary rather than XML. 🔒 AES-256
.xltx Excel Template ❌ No A blueprint for creating new .xlsx workbooks without overwriting the original. 🔒 AES-256
.xltm Macro-Enabled Template ✅ Yes A template that includes VBA macros. 🔒 AES-256
🔐 Security Guarantee: All modern formats support strong AES-256 encryption. When you set an "Open Password," the entire file structure is encrypted, making it completely unreadable without the correct key.

The Legacy Era: Binary Formats (97 - 2003)

Before 2007, Excel used proprietary binary formats (BIFF - Binary Interchange File Format). While modern Excel still reads them, these formats should be avoided for sensitive data due to fundamentally weak security architecture.

Extension Format Name Macros Security Rating Encryption
.xls Excel 97-2003 Workbook ✅ Yes ⚠️ High Risk 🔓 Weak RC4 (40/128-bit)
.xlt Legacy Template ✅ Yes ⚠️ High Risk 🔓 Weak RC4
.xla Legacy Add-in ✅ Yes ❌ Critical Risk 🔓 No file-level encryption
⚠️ Why Legacy Formats Fail: RC4 encryption was broken decades ago and can be cracked in minutes with modern hardware. Avoid .xls entirely for any confidential data.

OpenDocument & Plain-Text Formats (Non-Microsoft)

Excel can open formats like .ods, .csv, and .txt, but security expectations here are often misunderstood.

.ods (OpenDocument Spreadsheet)

  • Native to LibreOffice and OpenOffice
  • Supports encryption inside those applications
  • Excel ignores ODS encryption entirely
  • When opened in Excel, the file behaves as unencrypted data

.csv, .txt, .xml

  • Stored as plain text
  • No encryption, no password support
  • Suitable only for data exchange, never for protection

Modern Excel Encryption Standards (2007-Microsoft 365)

From Excel 2007 onward, all Office Open XML formats (.xlsx, .xlsm, .xlsb) use standardized, strong encryption when an "Open Password" is set.

Encryption Evolution by Version

Version Format Era Default Algorithm Key Strength Status
Excel 2007-2010 Early OOXML AES-128 (capable of AES-256) 128-256 bit Secure but older
Excel 2013-2019 Mature OOXML AES-256 (Standard/Agile modes) 256 bit ✅ Recommended
Microsoft 365 Current cloud + desktop AES-256 (Agile encryption) 256 bit ✅ Current standard
Key Insight: With a strong password (12+ characters, mixed case/numbers/symbols), Open password protection on modern formats is resistant to offline brute-force attacks. Because AES-256 encryption combined with key-derivation functions (PBKDF2, with salt and 100,000+ iterations) makes password guessing computationally expensive.

Complete File Format Reference Table

Comprehensive reference showing all major Excel extensions from 1997 to present:

Extension Era Format Type Macros Encryption Security Verdict
Modern & Secure Excel Formats (2007+)
.xlsx Excel 2007+ ZIP + XML (OOXML) No 🔒 AES (128/256-bit) ✅ Recommended
.xlsm Excel 2007+ ZIP + XML + VBA Yes 🔒 AES (128/256-bit) ✅ Recommended
.xlsb Excel 2007+ Binary blocks Yes 🔒 AES-256 ✅ Recommended
.xltx Excel 2007+ OOXML template No 🔒 AES (128/256-bit) ✅ Recommended
.xltm Excel 2007+ OOXML + VBA template Yes 🔒 AES (128/256-bit) ✅ Recommended
Add-ins & Special-Purpose Formats
.xlam Excel 2007+ OOXML add-in Yes ❌ Not designed for encryption ⚠️ Use caution
Legacy & Insecure Formats (Pre-2007)
.xls Excel 97-2003 Binary (BIFF8) Yes 🔓 RC4 (40/128-bit, broken) ❌ Avoid
.xlt Excel 97-2003 Binary (BIFF8) Yes 🔓 RC4 (weak) ❌ Avoid
.xla Excel 97-2003 Binary (BIFF8) Yes ❌ None ❌ Avoid
.xlw Excel 4.0 Legacy workspace No ❌ None ❌ Obsolete
Plain Text & Non-Excel Formats (No Security)
.csv All Plain text N/A ❌ None ❌ Not encrypted
.xml All Text XML N/A ❌ None ❌ Not encrypted
.txt All Plain text N/A ❌ None ❌ Not encrypted
Cross-Suite / Compatibility Risk
.ods LibreOffice 2010+ OpenDocument N/A ⚠️ Excel ignores ODS encryption ⚠️ Risky

Password Protection Features: Encryption vs. "Locking"

Excel provides multiple password dialogs, but they protect different things. Only one is true cryptographic security.

Feature What It Does Cryptographic? Actual Protection Use Case
Open Password Encrypts entire file with AES ✅ Yes 🔒 Strong - blocks file access completely Protect data confidentiality
Modify Password Requires password to save changes ❌ Policy-based ⚠️ Weak - can be removed by tools Prevent accidental edits
Sheet Protection Locks cells, ranges, formatting ❌ UI logic ⚠️ Weak - easily bypassed Control template editing
Workbook Structure Locks sheet add/move/hide actions ❌ UI logic ⚠️ Weak - easily bypassed Preserve workbook layout
Critical Point: Only Open Password is true security. The other three are convenience features and can usually be bypassed by specialized tools with zero decryption effort - they are just UI flags, not cryptographic controls.

Encryption in Practice: How It Works

Legacy Excel (.xls/.xlt) - Weak Architecture

  1. Password input → User sets an Open password in the Security dialog
  2. Weak key derivation → Excel derives a key from the password using MD5/SHA-1 (small key sizes: 40-128 bits)
  3. Partial encryption → Only specific BIFF8 workbook streams are RC4/XOR encrypted; metadata and structure often remain readable
  4. Storage → The encrypted stream is stored in the Compound Binary File (CBF); other structural data stays in plaintext
  5. Verification → On file open, a weak checksum (not strong cryptographic verification) determines if the password is correct

Result: 🔓 Cracking extracts the RC4 verifier and key-derivation data from the file; modern hardware cracks 40-128-bit RC4 in minutes to hours.

Modern Excel (.xlsx/.xlsm/.xlsb) - Strong Architecture

  1. Password input → User specifies an Open password in the Save/Protect dialog
  2. Strong key derivation → Excel runs PBKDF2 (Password-Based Key Derivation Function 2) with:
    • Salt: Random data to prevent rainbow tables
    • Iterations: 100,000+ iterations to slow brute-force attempts
    • Result: A robust 256-bit AES key
  3. Full encryption → The entire OOXML package (or binary workbook) is encrypted with AES-256 in Standard or Agile mode; nothing is readable without decryption
  4. Storage → The fully encrypted package is written to disk; the encryption metadata (salt, iteration count, algorithm) is stored in the package headers
  5. Verification → On file open, MSOffice Excel re-derives the key and attempts decryption; if the password is wrong, nothing meaningful decrypts - complete authentication failure

Result: 🔒 Cracking requires extracting the encryption info records (PBKDF2 salt, iteration count, AES IV, encrypted key) and performing offline dictionary/brute-force attacks against the key derivation, which is computationally expensive. With a strong password (12+ characters), this becomes infeasible without specialized hardware.

When to Use Which Format

✅ Use Modern Formats with Open Password (.xlsx/.xlsm/.xlsb)

  • Need real cryptographic encryption for confidential data
  • Want resistance to offline attacks with strong passwords
  • Sharing files in enterprise environments

⚡ Use .xlsb Specifically

  • Large, performance-critical workbooks that stay on desktop Excel
  • Data models with 100,000+ rows or complex calculations
  • Faster file I/O and reduced disk footprint

⚠️ Only Use Legacy .xls/.xlt for Compatibility

  • Old legacy systems that cannot read modern formats
  • Never for sensitive data - security is fundamentally broken
  • Plan migration to modern formats

❌ Never Treat These as Secure

  • .xla / .xlw (no file-level encryption)
  • .csv / .txt / .xml (plaintext only)
  • .ods (Excel ignores built-in ODS encryption)

For professionals and businesses handling confidential information, migrating to modern Excel formats with strong Open Passwords is no longer optional - it is a fundamental security requirement. Legacy formats expose you to unnecessary risks that modern encryption completely eliminates. 🛡️

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