OFB
Output Feedback mode converts a block cipher into a stream cipher by encrypting the previous output block (or IV) to generate a keystream. Unlike CFB, errors do not propagate since the feedback uses the cipher output, not the ciphertext. 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 |
IBM/NIST |
| Year |
1981 |
| Origin |
🇺🇸 United States |
| Source |
algorithms/modes/ofb.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 |
| IV Reuse |
Reusing IV with same key creates identical keystream, enabling two-time pad attacks. Always use unique IVs. |
— |
| Short Cycle Risk |
In theory, output register could enter short cycle, but extremely unlikely with good block ciphers and proper IV selection. |
— |
| No Authentication |
OFB provides no authentication - use AEAD modes for applications requiring integrity protection. |
— |
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 OFB test vector
| Field |
Value |
cipher |
AES |
key |
2b7e151628aed2a6abf7158809cf4f3c |
iv |
000102030405060708090a0b0c0d0e0f |
input |
6bc1bee22e409f96e93d7e117393172a |
expected |
3b3fd92eb72dad20333449f8e83cfb4a |
Vector 2 — NIST SP 800-38A OFB multi-block
| Field |
Value |
cipher |
AES |
key |
2b7e151628aed2a6abf7158809cf4f3c |
iv |
000102030405060708090a0b0c0d0e0f |
input |
6bc1bee22e409f96e93d7e117393172a ae2d8a571e03ac9c9eb76fac45af8e51 30c81c46a35ce411e5fbc1191a0a52ef f69f2445df4f9b17ad2b417be66c3710 |
expected |
3b3fd92eb72dad20333449f8e83cfb4a 7789508d16918f03f53c52dac54ed825 9740051e9c5fecf64344f7a82260edcc 304c6528f659c77866a510d9c1d6ae5e |
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