Cryptography Tools
What It Does
Encrypts and decrypts data using AES-CTR (Counter) mode — a stream cipher mode that requires no padding.
How to Use It
- Select “Encrypt” or “Decrypt” mode.
- Enter the plaintext or ciphertext.
- Enter or generate an encryption key (128, 192, or 256 bits).
- Provide or auto-generate a counter/nonce.
- Click “Encrypt” or “Decrypt”.
Options Explained
| Option | Description |
|---|---|
| Key size | 128, 192, or 256 bits — 256-bit is recommended |
| Key encoding | Hex, Base64, or UTF-8 |
| Counter / Nonce | 16-byte counter value — must never be reused with the same key |
| Output encoding | Hex or Base64 for the ciphertext |
⚠️ CRITICAL: Never reuse the same counter with the same key!
About AES-CTR Mode
AES-CTR (Counter Mode) turns the AES block cipher into a stream cipher by encrypting successive counter values and XOR-ing the resulting keystream with the plaintext. Each block uses a unique counter — typically a nonce concatenated with an incrementing integer — so no two blocks ever encrypt the same input, even within the same message.
Because each keystream block depends only on the key and the counter value, CTR mode is fully parallelizable: all blocks can be encrypted or decrypted simultaneously, making it extremely fast on multi-core processors and hardware accelerators. It also supports random-access decryption — any block can be decrypted independently without processing the preceding blocks.
CTR mode eliminates the need for padding since the keystream is truncated to match the plaintext length. However, like CBC, CTR provides only confidentiality — not integrity or authentication. Counter reuse with the same key is catastrophic, completely compromising the plaintext. For authenticated encryption, AES-GCM (which builds on CTR internally) is preferred.
Common Use Cases
- High-throughput disk and file encryption requiring parallel processing
- Streaming encryption of network traffic and real-time media
- Hardware-accelerated encryption using AES-NI instructions
- Random-access decryption of large encrypted archives or databases
- Building AEAD constructions (AES-GCM uses CTR internally)
- Generating deterministic pseudorandom streams from a key and nonce
What Is AES-CTR?
AES-CTR (Counter mode) turns the AES block cipher into a stream cipher by encrypting a sequence of incrementing counter values and XORing the resulting keystream with the plaintext. Because each block is encrypted independently with a unique counter, CTR mode supports parallel encryption and random-access decryption — you can decrypt any block without processing the blocks before it. The nonce (or counter initialization value) must be unique for every encryption under the same key; reusing a nonce completely compromises confidentiality. Like CBC, CTR provides encryption only — not authentication. AES-GCM builds on CTR internally and adds authentication.
Frequently Asked Questions
What happens if I reuse a nonce with the same key?
The same keystream is produced, allowing an attacker to XOR two ciphertexts together and recover the XOR of the two plaintexts. This is a catastrophic failure — always use a unique nonce for each encryption.
Can I decrypt a single block in the middle of a file?
Yes. CTR mode supports random-access decryption because each block depends only on the key and its counter value, not on previous blocks. This makes it ideal for encrypted databases and disk volumes.
How does AES-CTR compare to AES-GCM?
AES-GCM uses CTR mode internally for encryption and adds a Galois-field MAC for authentication. If you need authenticated encryption (recommended for most applications), use AES-GCM. Use bare CTR only when you handle authentication separately.
Is my data sent to a server?
No. All encryption and decryption runs locally in your browser. No data or keys leave your device.
All encryption and decryption is performed client-side in your browser. No data or keys are ever sent to an external server.