Cryptography Stack Exchange
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Questions tagged [ctr]

Counter Mode (CTR) is an encryption mode, that builds a random-access stream-cipher from a block-cipher.

Counter Mode (CTR), also known as Segmented Integer Counter (SIC) or Integer Counter Mode (ICM), makes a block-cipher into a random-access stream cipher by generating a keystream using the block-cipher that is XORed with the plaintext to produce the ciphertext. CTR mode generates keystream blocks by encrypting successive values of a non-repeating counter with the block-cipher.

See the wikipedia page for more information.

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Why does CTR mode become insecure after $2^{0.5 \times n}$ blocks?

The question is exactly what it says on the tin. If $n$ is the block size in bits of a block cipher, then why does outputting $2^{0.5 \times n}$ block in counter mode become a problem when there is no repetition of the keystream until after $2^{n}$…
Melab
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What is the maximum number of messages that can be encrypted using the same key for CTR mode?

In the counter mode of encryption, the nonce cannot be used again unless a new block cipher key is chosen. What is the maximum number of messages that can be encrypted using the same key? The maximum length of messages that can be encrypted using…
geektobe
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What does counter mean in Counter (CTR) Mode? Is it the same as nonce?

As asked above, what does "counter" mean exactly? Is it the same as nonce? Also, the book Network Security Essentials (6ed.) from William Stallings states, "Typically the counter is initialized to some value and then incremented by 1 for each…
Baldovín Cadena Mejía
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CTR Mode and Chosen Plaintext Attacks

P.71 of Cryptography Engineeering states "Any weakness in CTR encryption mode immediately implies a chosen plaintext attack on the block cipher." It seems to me that "any weakness" is vague. I'm not quite getting what the author has in mind. Can…
Yizop
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How many different keystreams can I produce with AES in Counter Mode?

I'm using a 64-bit nonce (incremented between the streams) and a 64-bit counter (incremented within a stream) for AES in CTR mode. How many different key streams can I produce with this setup?
goldroger
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Size of the counter in CTR mode

If I understood it correctly, in CTR mode I encrypt the nonce or IV together with the counter as a block, which is then XORed with the plaintext. For the next block, I increment the counter. Is there a fixed size of the counter to be able to iterate…
Luqus
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