A block cipher is an encryption algorithm which encrypts fixed-size blocks of plaintext to same-sized blocks of ciphertext. For good ciphers every bit of the ciphertext block depends on every bit of the plaintext block and every bit of the key.
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pseudo random sequence and permutation functions [closed]
Iam doing my Master thesis in enhancing of an encryption algorithm by using of a pseudo random sequence generators to create a 16*16 shared table and apply a permutation function on it. one of the ...
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2answers
185 views
AES - plaintext is smaller 128 bit - how to expaned?
we are currently developing a little AES implementation in a crypto-course at university.
As far as I know, AES uses 128 bit blocklength, so all data that will be encrypted will get split up into ...
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115 views
AES AddRoundKey
Looking at the first step of AES encryption I see that we XOR the key with the plaintext block. Why is the actual key involved at all, why not just use the round keys derived from the key schedule?
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1answer
56 views
Why are the Davies-Meyer and Miyaguchi-Preneel constructions secure?
The Davies-Meyer compression function $h(H, m) = E_m(H) \oplus H$ is said to be secure. So too is the Miyaguchi-Preneel compression function $h(H, m) = E_m(H) \oplus m \oplus H$. Why are these ...
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2answers
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Why is $h(H, m) = E(m, H) \oplus m$ insecure?
I am taking a cryptography class on Coursera. I learned that the compression function $h(H, m) = E_m(H) \oplus m$ is insecure (even though other variants like Davies-Meyer or Miyaguchi-Preneel are ...
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1answer
104 views
Ciphertext-only attack on Simplified DES
Is it possible to deduce the plaintext block or the key, given only a Simplified-DES ciphertext block (e.g. c=01110110)? I'm reading Cryptography And Network Security, by William Stallings and I'm bit ...
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2answers
89 views
Do I need to know the “mode of operation” to decrypt a message encrypted with a block cipher?
If I have received an AES encrypted message, and if I do know the key with which it was encrypted, do I also need to know the mode of operation with which it was encrypted in order to decrypt it? My ...
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1answer
65 views
Is there a preferred way/standard on how to transmit an initialization vector for AES encryption
Is there a preferred way on how to transmit the initiliazation vector with witch AES encryption was made? I could imagine that, if the encrypted data is stored to a file, the IV is preppended at the ...
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1answer
212 views
Is AES-256 a post-quantum secure cipher or not?
We know Grover's algorithm speedup brute-force attacks two time faster in block ciphers (e.g brute-forcing 128 bit keys take $2^{64}$ operations not $2^{128}$).
That explains why we are using 256 bit ...
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1answer
96 views
Is there an efficient way to hide the encrypted plaintext length with a block cipher?
In block cipher modes of operation for encryption on input of a plaintext of $N$ blocks (We assume that the input size is always a multiple of the blockcipher mode: $N·16$ bytes) the size of the ...
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1answer
63 views
What is the usual block cipher mode that goes with CTS
I am just wondering what is the most common, or most used, block cipher mode used to goes with CTS?
ECB or CBC?
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1answer
67 views
How to secure a mental poker protocol?
I would like to implement a mental poker protocol in a secure fashion. How should I go about that without (preferably) infringing on the Mental Poker Framework patent?
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Are there any secure commutative ciphers?
This answer lists two commutative cipher algorithms - Pohlig-Hellman and SRA. However, they don't appear to be too secure.
My question is, here there any commutative ciphers out there that are secure ...
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1answer
94 views
Security of Pohlig-Hellman exponentation cipher?
I am looking into implementing Pohlig-Hellman exponentation cipher and I would like to know how secure that algorithm is? I am guessing it's security relates greatly to the prime number used in it. ...
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1answer
489 views
Why is CAMELLIA suddenly so widely used?
When nowadays I point my browser to https sites, the cipher that is on most occasions used is Camellia. My browsers (Chrome and Firefox) seem to prefer it, even when AES is available.
Is that not ...
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1answer
127 views
AES key/ciphertext space sizes
This is giving me a brain ache now... If I have AES-128, block is 128 bit, then every plaintext (128-bit) can be encrypted to some ciphertext that is also 128-bit. This is the block size. But: 128-bit ...
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1answer
94 views
Low complexity implementation of a small blocksize cipher (< 64 bit)
Searching for "small blocksize cipher" finds a number of discussions on the topic, mostly refering to FPE.
This one in particular suggests using AES as the round function of a Feistel network.
The ...
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1answer
171 views
Can we replace the XOR operation in DES with some other operation?
Can we replace the XOR operation in the DES algorithm with some other operation? If so, does it work for both encryption and decryption?
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1answer
61 views
What are rounds in the context of ciphers?
Another recent question on security margin included in its answer the discussion of how many rounds are broken so far. What is the definition of a round?
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3answers
146 views
What is a block cipher?
I have read about block ciphers, but I do not understand the topic completely.
Is a block cipher a way to send data or a way to encrypt data?
How is the data divided into multiply blocks before it ...
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1answer
377 views
Difference between Rijndael 128 / 256 blocksize implementations? (and impact of block size in general)
Can anyone shed some light onto the advantages/disadvantages of using Rijndael with 256-bit block size, as opposed to the 128-bit (AES) implementation? (please note: I'm not referring to key-size ...
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2answers
159 views
Is it possible to break a hash-based block cipher?
Let's define the following block cipher:
$C_n = M_n \oplus H(k + n)$ where $C_n$ is the nth block of ciphertext, $M_n$ is the nth block of plaintext, $H$ is a cryptographic hash function, and $k$ is ...
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171 views
Break double encryption
Let $E_k$ : {0,1}$^l$ be a block cipher encryption function with block-size $l$ and key-length $n$.
In class, we saw that a double encryption with two independent keys $E{}'_{k_1k_2}(x)$ = ...
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2answers
143 views
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1answer
129 views
Does Grover's algorithm effect block size or only key size?
We know that Grover's algorithm can speed up cracking symmetric keys. Basically the key space is halved. This means we have to use at least a 256 bit key (to get 128 bit security).
I heard somewhere ...
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1answer
153 views
Can a nested block cipher avoid the meet in the middle attack by using a secret initialization vector for the inner encryption?
It seems to be believed that encrypting twice with a block cipher using an independent key each time is not as secure as you might expect because of the "meet in the middle" attack.
This is an attack ...
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2answers
138 views
AES Key Length vs Block Length
This answer points out that certain key and block lengths were a requirement for the AES submissions:
The candidate algorithm shall be capable of supporting key-block combinations with sizes of ...
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1answer
102 views
What is an oracle adversary?
I'm trying to figure out block ciphers and this is the first time I've encountered the term "oracle adversary". It appears in the context of defining a PRP and a PRF. Can someone please explain what ...
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2answers
108 views
What cipher mode is suitable for independantly decryptable short messages?
I'm building a small VoIP client which transmits encrypted audio data via UDP, and some packet loss is expected, so each packet will need to be decrypted separately even if others in the stream don't ...
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2answers
126 views
Will varying plaintext compensate for a fixed initialisation vector?
This is a follow-up question to Relative merits of AES ECB and CBC modes for securing data at rest.
I need to store encrypted Personal Account Numbers (PANs) in a database. The only encryption option ...
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5answers
322 views
Using CBC with a fixed IV and a random first plaintext block
What if, instead of using CBC mode in the normal way with a random IV, I used this approach:
Use a fixed IV (like a block of 0's).
Before encrypting, generate a random block and prepend it to the ...
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1answer
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Difference between stream cipher and block cipher
A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time.
A block cipher ...
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297 views
How can I implement the “Multiplication Modulo” and “Addition Modulo” operations in IDEA?
I am currently working on IDEA (International Data Encryption Algorithm), and I don't know how to perform Multiplication Modulo and Addition Modulo.
This is how IDEA operates:
IDEA operates on ...
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136 views
How could block encryption in counter mode be secure from viewpoint of entropy?
A good block cipher e.g. AES running in counter mode is commonly considered to be secure. Assuming a perfectly random key, one could in this case from $128$ bits generate up to $2^{128}·128$ bits for ...
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Why is there a strong distinction between stream and block ciphers?
If I don't err, in the literature a stream cipher is one in which each plaintext bit is processed individually, commonly via xor-ing with one bit of a random or pseudo-random bit stream, while a block ...
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1answer
180 views
What are the safe ways to derive HMAC key using block cipher?
Suppose we have a state of block cipher initialized with some key unknown to us, that is, we have the state after running key schedule, but we have no access to actual key or subkeys, all we can do is ...
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1answer
125 views
How is block cipher padding verified for consistancy?
Under what circumstances is a decryption routine able to tell that the padding of a message is invalid? If a cipher text block where to be randomly modified, what is the probability that the padding ...
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3answers
324 views
Malleability attacks against encryption without authentication
Suppose there is a message that is encrypted with AES-128-CBC. The message is as follows, new lines are used to delimit the 16 byte boundary for each block:
...
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2answers
248 views
Difference in one time key and one time pad and many time key
These terms are confusing me. One time pad is when you use one key for one message. That is what One time key is.
Secondly, what is the connection of many time key, can i use one time pad many ...
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1answer
98 views
How do I calculate the maximum plain text length allowable for a certain cipher text length?
I am encrypting and storing sensitive values in the database. I have a set max length in the database, but in order to provide useful user feedback, I'd like to know the max input I should allow for a ...
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4answers
279 views
Which block cipher modes of operation allow a predictable IV?
Recently I found out that in the modes CBC and PCBC the IV may be passed in cleartext but never must be predictable. However for this part of my app I rather have the IV be predictable and unique ...
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117 views
Why does .NET's ECB mode implementation append a constant block to my ciphertext?
Consider the following code and output:
...
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1answer
120 views
Encrypt array of int for individual retrieval
I have limited exposure to cryptographic terminology, so please bear with me.
My end goal is to encrypt integer IDs, before transmitting them to a web client in a list of search results, in a way ...
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Pseudo preimage for a hash made from a cipher
Consider the Miyaguchi–Preneel construction:
$H_0 = E(0,m_0) \oplus m_0$ (0 here means a vector filled with zeros)
$H_1 = E(H_0,m_1) \oplus H_0 \oplus m_1$
where $E(K,M)$ is a block cipher (for ...
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246 views
Can I build a secure tweakable block cipher from a normal one by adding key and tweak?
Let (E,D) be a secure block cipher.
Consider the following tweakable block cipher:
...
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3answers
203 views
Why is a MAC needed?
I agree that for certain encryption systems or modes of operation, a MAC is indispensible.
The best example are probably stream ciphers (and therefore also block ciphers in OFB or CTR mode) that ...
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495 views
Padding methods for block ciphers - PKCS7 vs ANSI X.923
I was looking through block cipher padding methods, and found two good candidates:
ANSI X.923 - pad with zeros, then a final byte for the padding length, e.g. ...
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1answer
277 views
Hash function from narrower block cipher operated in CBC-encryption mode?
I am trying to build a public hash function (thus collision-resistant and preimage-resistant, and more generally behaving like a random oracle), with input a message $M$ of fixed size $|M|=m\cdot b$ ...
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318 views
Are Stream Ciphers Less Secure?
This is by no means a scientific observation, but it seems to me that stream ciphers receive a lot less attention than block ciphers. Is there any reason for this? (Is it because block ciphers are ...
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2answers
105 views
Byte location of plaintext vs ciphertext when using a block cipher or stream cipher
I know that when you use a stream cipher you are just XOR'ing the plain text with the PRNG stream, such that you know if a specific byte is in position 3 of the plain text it will still be in ...
