CTR
Counter mode converts a block cipher into a stream cipher by encrypting successive counter values to generate a keystream. Allows parallel processing and random access. The counter typically combines a nonce (number used once) with an incrementing counter value. Both encryption and decryption use the block cipher in encryption mode only.
Properties
| Property |
Value |
| Category |
Cipher Modes |
| Sub-category |
Stream Cipher Mode |
| Security status |
🛡️ Secure |
| Complexity |
Intermediate |
| Inventor |
Whitfield Diffie, Martin Hellman |
| Year |
1979 |
| Origin |
🇺🇸 United States |
| Source |
algorithms/modes/ctr.js |
Parameters
| Parameter |
Supported values |
| IV sizes |
8 bytes (64 bits) to 32 bytes (256 bits) in steps of 8 bytes |
Capabilities
| Flag |
Value |
RequiresIV |
Yes |
Security
Status: 🛡️ Secure
Known vulnerabilities
| Issue |
Description |
Mitigation |
| Nonce Reuse |
Reusing nonce/counter combination reveals XOR of plaintexts. Ensure unique nonces and proper counter management. |
— |
| Counter Overflow |
If counter overflows and wraps around, keystream may repeat. Use sufficiently large counter space. |
— |
| No Authentication |
CTR provides no integrity protection. Use AEAD modes or combine with MAC for authentication. |
— |
Documentation
References
Test vectors
2 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.
Vector 1 — NIST SP 800-38A CTR test vector
| Field |
Value |
cipher |
AES |
key |
2b7e151628aed2a6abf7158809cf4f3c |
iv |
f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff |
input |
6bc1bee22e409f96e93d7e117393172a |
expected |
874d6191b620e3261bef6864990db6ce |
Vector 2 — NIST SP 800-38A CTR multi-block
| Field |
Value |
cipher |
AES |
key |
2b7e151628aed2a6abf7158809cf4f3c |
iv |
f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff |
input |
6bc1bee22e409f96e93d7e117393172a ae2d8a571e03ac9c9eb76fac45af8e51 30c81c46a35ce411e5fbc1191a0a52ef f69f2445df4f9b17ad2b417be66c3710 |
expected |
874d6191b620e3261bef6864990db6ce 9806f66b7970fdff8617187bb9fffdff 5ae4df3edbd5d35e5b4f09020db03eab 1e031dda2fbe03d1792170a0f3009cee |
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