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Encryption can keep data secret without detecting changes to it. To know whether encrypted data was altered, use authenticated encryption—or a separate, properly keyed authentication mechanism—and verify it before trusting the data. A successful check provides strong assurance, not an absolute guarantee.
Does encryption detect tampering?
Not necessarily. Encryption and integrity are different protections. Encryption aims to keep content confidential; integrity checks help detect whether data has changed. Some encryption modes provide confidentiality without authenticating the data. NIST, for example, describes XTS-AES as a mode designed for storage-device confidentiality without authentication. An encrypted file extension, a lock icon, or successful decryption alone therefore does not prove that the data is intact. NIST’s overview of block-cipher modes distinguishes confidentiality modes from authenticated modes.
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Think of encryption as putting a message inside an opaque envelope. Authentication adds a tamper-evident seal that the recipient can check. The cryptographic tag is generated using secret key material and checked against the protected data and any associated inputs. The analogy has limits: a valid tag establishes authenticity relative to the relevant key and inputs, not the identity of a particular person or the truth of the message.
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What should I use to check data integrity?
For data that must remain confidential and be checked for changes, use an authenticated-encryption construction through a well-maintained cryptographic library’s documented API. NIST defines Galois/Counter Mode (GCM) as authenticated encryption with associated data. Its authentication tag lets the recipient check the ciphertext and associated data during authenticated decryption. NIST Special Publication 800-38D specifies GCM and GMAC.
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Associated data is information that needs authentication but does not need to be encrypted. For example, an application may need to bind visible metadata to a protected message so that changes to either are detected. The metadata remains visible; authentication checks whether it matches what was protected.
If the data does not need encryption but does need a cryptographic integrity check, GMAC is an authentication-only option specified by NIST. The right mechanism depends on the protocol and application; GCM is an example, not a universal setting for every system.
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How do the main approaches differ?
| Approach | Confidentiality | Cryptographically detects unauthorized changes? | What to know |
|---|---|---|---|
| Confidentiality-only encryption mode | Yes, when correctly used | Not necessarily | Encryption alone does not establish integrity. NIST identifies XTS-AES as a storage example that does not provide authentication. |
| Authenticated encryption, such as GCM | Yes | Yes, through authentication verification | Verify the data before trusting the decrypted result, and use the mode correctly. |
| Authentication-only mode, such as GMAC | No | Yes, subject to the key and construction assumptions | Can suit data that is public or protected separately but still needs an integrity check. |
What happens if an authentication check fails?
Treat the data as untrusted and do not use any plaintext produced by a failed or skipped check. In its GCM guidance, NIST says a FAIL result means at least one supplied input—the ciphertext, associated data, IV, or tag—is not authentic. A check that the application never performs cannot protect data that has already been accepted or acted upon. The authenticated-decryption discussion in SP 800-38D describes this failure behavior.
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Why does a successful check still depend on implementation?
A valid tag is strong evidence that the authenticated inputs have not been altered, but it is not an absolute guarantee. NIST discusses how assurance is affected by factors including tag length, the volume of messages, and repeated unsuccessful verification attempts. Its GCM guidance also stresses the importance of IV uniqueness. These details are reasons to use a documented library interface and follow authoritative guidance rather than invent a construction or casually combine encryption with a hash.
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NIST SP 800-38D was published on November 28, 2007. NIST has decided to revise it; its revision page records a second pre-draft call for comments published June 1, 2026, with a July 31, 2026 deadline, and says no draft document was available at that stage. The final publication remains the source for finalized GCM and GMAC guidance cited here; consult NIST’s revision status page for the development status.
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