Compare the complete construction
| Mode | IV / nonce here | Padding here | Authentication |
|---|---|---|---|
| GCM | 12-byte nonce | None | 128-bit tag |
| CBC | 16-byte IV | PKCS7 | None |
| CTR | 16-byte counter; 64-bit counter field | None | None |
GCM is the password profile's fixed default. CBC and CTR are exposed for raw compatibility, with a warning. This is not a recommendation to replace an established protocol with a bare cipher mode.
Why GCM can refuse to decrypt
GCM verifies a tag over the ciphertext and any AAD. Changing the key, nonce, ciphertext, tag or AAD can prevent authentication. The tool returns no partial plaintext when this happens. Its output appends a 16-byte tag; if a system stores the tag separately, split or combine the bytes correctly rather than duplicating it.
The same key must not reuse a nonce. The tool generates fresh nonces by default. A manual nonce option exists only for deliberate interoperability and test vectors; a random-looking nonce copied from a previous operation must not become a permanent default.
CBC and CTR failure is different
CBC needs an unpredictable IV and a padding convention. A wrong key can cause a padding error, but occasional valid padding is not authentication. CTR has no padding and can return altered bytes without an error. Neither mode proves integrity or a correct key simply because processing completed.
CTR compatibility also depends on how counter blocks advance. This tool fixes the low-order counter field to 64 bits and rejects wrap for the input length. A peer using a different counter layout may not interoperate even with the same displayed IV.
An interoperability checklist
Before moving ciphertext between systems, record mode, key bytes, IV/nonce, encoding, tag placement, AAD and padding or counter length. Test with public data first. The site supports GCM, CBC and CTR only; ECB, CFB and OFB are not hidden options. For a new authenticated workflow, prefer the password GCM profile or a reviewed application protocol rather than inventing a CBC-plus-checksum scheme.
Related tools and reading
- Encrypt text
- Decrypt text
- Encrypt a file
- Generate a key
- What Is AES Encryption? Keys, Blocks and Recovery
- AES-GCM vs CBC vs CTR: Authentication and Parameters
- AES-128 vs AES-192 vs AES-256: Bytes and Encodings
Technical references
Updated October 16, 2026