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Cipher Block Size Calculator

Work out block count, padded length and the birthday-bound data limit per key for AES, 3DES and stream ciphers.

About Cipher Block Size Calculator

A block cipher can only process whole blocks, so CBC and ECB pad up to the next boundary — always adding at least one byte. The birthday bound sets a separate limit: after about 2^(n/2) blocks a repeated ciphertext block reveals the XOR of two plaintexts, which is 32 GiB for a 64-bit block and 256 EiB for AES. Sweet32 turned 3DES and Blowfish's 64-bit block into a practical attack on long-lived connections; knowing the block width tells you both your storage inflation and when you must rekey.

Formula

Cipher Block Size

blocks = ⌈data ÷ block size⌉; PKCS#7 padding = block size − (data mod block size); birthday-safe blocks per key ≈ 2^(block bits ÷ 2 − 1).

block size
Cipher block width in bytes (8 for 3DES, 16 for AES)
birthday bound
Where CBC ciphertext blocks start repeating and leaking XORs

Worked example

With the default values below, Cipher Block Size Calculator returns Block Size of 16. Change any input above and every figure here is recalculated in your browser.

Inputs used

Cipherone of AES (128-bit block), 3DES (64-bit block), Blowfish (64-bit block), ChaCha20 (512-bit stream block)
AES (128-bit block)
Data to encryptminimum 0 bytes
1,048,576 bytes

Results

Block Size
16 bytes
Blocks Required
65,536
Length After PKCS#7 Padding
1,048,592 bytes
Padding Bytes
16
Birthday-safe Data per Key
128 EiB per key
Assessment
128-bit block — comfortable margin for any realistic volume

Step by step

  1. 1Values used
  2. 2Cipher Block Size
  3. 3Block Size= 16 bytes
  4. 4Blocks Required= 65,536
  5. 5Length After PKCS#7 Padding= 1,048,592 bytes
  6. 6Padding Bytes= 16
  7. 7Birthday-safe Data per Key= 128 EiB per key
  8. 8Assessment= 128-bit block — comfortable margin for any realistic volume

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Cipher Block Size calculated?

blocks = ⌈data ÷ block size⌉; PKCS#7 padding = block size − (data mod block size); birthday-safe blocks per key ≈ 2^(block bits ÷ 2 − 1). A block cipher can only process whole blocks, so CBC and ECB pad up to the next boundary — always adding at least one byte. The birthday bound sets a separate limit: after about 2^(n/2) blocks a repeated ciphertext block reveals the XOR of two plaintexts, which is 32 GiB for a 64-bit block and 256 EiB for AES.

Why does Cipher Block Size matter?

Sweet32 turned 3DES and Blowfish's 64-bit block into a practical attack on long-lived connections; knowing the block width tells you both your storage inflation and when you must rekey.

What values do I need to enter?

This calculator takes 2 inputs: Cipher, Data to encrypt. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Why did TLS drop 3DES?

Its 64-bit block hits the birthday bound after roughly 32 GiB on one key, and Sweet32 showed an attacker who can force a long-lived connection to send that much can recover a repeated secret such as a session cookie.

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