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Also known as: CTR, Counter Mode
A block cipher mode that encrypts successive counter values to produce a keystream, enabling parallelizable, random-access encryption and decryption.
DESCRIPTION
Counter (CTR) mode forms keystream by encrypting a nonce-plus-counter input for each block and XORing it with plaintext. It supports parallel encryption/decryption and random access to any block, making it highly performant. Security critically depends on never reusing the same nonce and counter sequence with the same key. CTR provides confidentiality only and must be combined with a MAC (e.g., HMAC) or used in an AEAD mode like GCM for integrity. It is widely used in protocols and storage encryption.
ALTERNATIVE DEFINITIONS
NIST SP 800-38A
The Counter (CTR) mode is a confidentiality mode that generates the next keystream block by encrypting successive values of a counter and XORing with the plaintext.
Stresses counter input generation and XOR with plaintext.
IETF RFC 3686
AES-CTR is a confidentiality-only mode that combines a nonce with a counter to form input blocks to the AES algorithm; the output blocks are XORed with the plaintext.
Highlights nonce+counter construction and confidentiality-only property.
RELATED TERMS
AES-GCM Nonce Initialization Vector Rijndael Stream Cipher MAC Authenticated Encryption Random Access Parallel Processing TLS
COMPETING TERMS
Cipher Block Chaining Output Feedback
KNOWLEDGE AREAS
Data Security Software Engineering
TARGET AUDIENCE
Security Architect Cybersecurity Engineer Software Developer DevOps Engineer Cloud Architect
EXTERNAL SOURCES
- Wikipedia - Block cipher mode of operation#Counter (CTR)
- IETF - Using AES-CTR in IPsec ESP
- NIST - CTR mode specification
- MDN Web Docs - Web Crypto API context for AES-CTR
TOP QUESTIONS
Why is Counter (CTR) mode popular in modern systems?
CTR supports parallelism and random access, offering high throughput and low latency. It avoids padding and fits streaming and storage use cases well.
Category: Relevancy
What is the biggest risk when using Counter (CTR) mode?
Reusing the same nonce and counter sequence with a key repeats keystream, enabling plaintext recovery. Always ensure unique nonce and counters per key.
Category: Risks
Does Counter (CTR) mode provide integrity?
No. CTR offers confidentiality only. Pair it with a MAC or choose an AEAD mode like GCM to prevent tampering.
Category: Understanding
How is the input constructed in Counter (CTR) mode?
Each block input combines a nonce (or IV) with a counter value, then encrypts it to produce keystream.
Category: Understanding
What is a common authenticated alternative to Counter (CTR) mode?
Galois/Counter Mode (GCM) adds integrity via a polynomial MAC, offering combined authenticated encryption.
Category: Alternatives
Can Counter (CTR) mode be used for random-access decryption?
Yes. Any block can be decrypted independently by computing its corresponding counter input and keystream block.
Category: Example
How should nonces be generated for Counter (CTR) mode?
Nonces must be unique per key. Use counters, randomized nonces with collision avoidance, or system-level uniqueness guarantees.
Category: BestPractices
Is Counter (CTR) mode resistant to bit-flipping attacks?
Not by itself. Attackers can flip bits predictably; add a MAC or use AEAD to detect tampering.
Category: Risks
Does Counter (CTR) mode require padding?
No. It works as a stream using XOR with keystream, supporting any message length.
Category: Understanding