13 votes
Accepted

Would an encryption-only block cipher be useful at all?

Would it make any sense to design an irreversible block cipher where the encryption side wouldn't be reversible (and thus you can't implement decryption), Block cipher is a synonym for Pseudo Random ...
kelalaka's user avatar
  • 48.5k
7 votes

Is it possible to decrypt the 2nd byte of AES-256-CFB-8 ciphertext without decrypting the 1st byte?

Yes, you can do this. If you want to know how to do it (as opposed to just blindly copying a code snipped written by someone else and hoping it'll work), you'll need to understand a little bit about ...
Ilmari Karonen's user avatar
6 votes
Accepted

Why are ECB CBC and CFB parallelizable?

By parallelizable, what we mean is that the encryption/decryption process can be broken down into multiple tasks that can be run at the same time. For block cipher modes, this means that if we have ...
poncho's user avatar
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6 votes
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What is the advantage of CFB over OFB?

The only reason I can imagine why one would prefer CFB over OFB or CTR is error propagation. Another reason may be technical issues with the relieability of the transmission line. This comes from the ...
SEJPM's user avatar
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5 votes
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Is a chosen ciphertext attack on PGP still possible?

RFC 4880, OpenPGP (superseded RFC 2440 which was up to date in 2002) contains a chapter on security considerations, which also discusses the decryption oracle attack Jallad et al described: In late ...
Jens Erat's user avatar
  • 1,333
5 votes
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Is it possible to decrypt the 2nd byte of AES-256-CFB-8 ciphertext without decrypting the 1st byte?

Yes, CFB just takes the previous block as the IV for the next block making it a stream cipher. If you modify your iv_dec equal the result of the previous block, however long that is, it should work. ...
dave.zap's user avatar
  • 493
5 votes

OFB mode and CFB mode error propagation in 64bits

First, approximately nothing in modern cryptography cares about the notion of error propagation in block cipher modes of operation—it is an archaic relic of the dark ages of crypto engineering that ...
Squeamish Ossifrage's user avatar
4 votes
Accepted

Encrypting text using Caeser cipher in CFB mode

Caesar cipher You are confusing Caesar cipher with block ciphers. In Caesar cipher the ciphertext is calculated as $$c_i = p_i + 3 \bmod 26$$ There is no security in Caesar Cipher. A ciphertext ...
kelalaka's user avatar
  • 48.5k
4 votes

Self-synchronizing cipher recovery from dropped bits

First, when the textbook talks about 'dropped bits', they are not referring to bits that are received incorrectly; instead they are talking about transmission errors where certain bits are deleted. ...
poncho's user avatar
  • 147k
4 votes

Self-synchronizing cipher recovery from dropped bits

"Recovery", in this case, does not mean that the full plaintext can be recovered even if part of the ciphertext is missing.* Instead, the "eventual recovery" property of self-synchronizing stream ...
Ilmari Karonen's user avatar
4 votes

How do I check if the output of my CFB TEA encryption algorithm is correct?

No, there are of course other methods of verifying your implementation. Note that there are a few separate steps that can be distinguished: the TEA block cipher; the CFB mode (using the right number ...
Maarten Bodewes's user avatar
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4 votes
Accepted

does CFB support random read access?

Yes, with some more ciphertext. The CFB mode encryption equations; \begin{align} I_0 &= \text{IV}\\ I_i &= \big((I_{i-1} \ll s) + C_i\big) \bmod 2^b,\\ C_i &= \operatorname{MSB}_s\big(E_K(...
kelalaka's user avatar
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3 votes
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What is the difference between cfb and cfb1 and cfb8 ? ( like openssl's aria-128-cfb & aria-128-cfb1 & aria-128-cfb8 )

What is the difference between cfb and cfb1 and cfb8? Well, first of all, we need examine what CFB is. Here is what the encryption process is: Take the previous $n$ bits of ciphertext (where $n$ is ...
poncho's user avatar
  • 147k
3 votes

For CFB, if the IV is corrupted, can the system re-synchronise?

Depends on the type of corruption (mangled vs. lost data) and what the segment size is. If you use a segment size equal to the block size, which is most efficient from a computational point of view, ...
otus's user avatar
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3 votes

Number of cycles in cipher feedback mode (CFB)

In CFB-mode you effectively have a data-dependent stream cipher and you'll need $\lceil\frac{l}n\rceil$ block cipher calls or "cycles" as you call them to process the full message, where $l$ is the ...
SEJPM's user avatar
  • 46k
3 votes

does CFB support random read access?

simplified CFB supports random read access. but what about the real CFB where we have a shift register? Sure it does; actually, it's not that difficult. In CFB, the "plaintext" submitted ...
poncho's user avatar
  • 147k
3 votes

Why does CBC-MAC(M) = CFB-MAC(M)?

For a block cipher $E_k()$, consider the following process applied to a message $M$ of length $\ell$ blocks $M_0,\ldots,M_{\ell-1}$ $$X_0=0$$ $$Y_i=X_i\oplus M_i$$ $$X_{i+1}=E_k(Y_i).$$ In CBC ...
Daniel S's user avatar
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2 votes
Accepted

CFB-Mode biterror

SEJPM seems to guess right. If you'd loose a bit or byte then the ciphertext after that won't be aligned correctly and you would not be able to decrypt to the original plaintext. You could of course ...
Maarten Bodewes's user avatar
  • 92.6k
2 votes

Is it possible to decrypt the 2nd byte of AES-256-CFB-8 ciphertext without decrypting the 1st byte?

CFB works as a stream cipher, where the random stream is generated encrypting the previous block of ciphertext. Then, the plaintext is just xored with this random generated stream. To decrypt a ...
ddddavidee's user avatar
  • 3,324
2 votes
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Number of cycles in cipher feedback mode (CFB)

You seem to be reading the diagram wrong. There are no "cycles" in CFB mode, and each block of plaintext/ciphertext is encrypted only once. Admittedly, it's not entirely your fault: the diagram ...
Ilmari Karonen's user avatar
2 votes

requirements for CFB encryption mode

For CFB mode the input does not have to be a multiple of the block size; i.e. no padding or ciphertext stealing is required as with the ECB or CBC mode of operation. Maybe that has been badly ...
Maarten Bodewes's user avatar
  • 92.6k
2 votes

requirements for CFB encryption mode

CFB mode has no such length restrictions; it can be used to encrypt an arbitrary length plaintext. Perhaps you misunderstood what was said.
poncho's user avatar
  • 147k
2 votes

Cipher Feedback Mode - sizes of the blocks and shift registers

I dont understand which special case the first picture describes. Is really the entire ciphertext (after the XOR with the plaintext) is taken as the argument in for the next block or just s bits? It ...
poncho's user avatar
  • 147k
2 votes
Accepted

3DES CFB vs CFB8 modes

The main difference between CFB-1(CFB) and CFB-8 is the size of shifting. To understand this - $C_i = top_x(res_{i-1})\oplus P_i$ $res_0 = Enc_k(IV)$ $res_{i} = Enc_k(shift_x(res_{i-1}\oplus P_{i-1}))...
Mayank's user avatar
  • 447
2 votes

What is the advantage of CFB over OFB?

Advantages of CFB over OFB: Ability to parallel decryption, and hence it has better performance. Ability to random read access during decryption. There is no need to calculate all previous (expensive)...
backdround's user avatar
2 votes
Accepted

Struggling to understand CFB mode (with DES)

and so $E_k((IV << 8) | C_1 | C_2')$ gives an incorrect encryption That's your issue; CFB isn't designed to correct for errors in the plaintext - after all, if the ciphertext is not modified, ...
poncho's user avatar
  • 147k
1 vote

How many bytes of the last keystream block are used to XOR the final block of partial plaintext in CFB, OFB & CTR modes?

Suppose a block is $b$ bits long, say $b = 128$, and the message is $\ell$ bits long, say $\ell = 187$. Then the final plaintext block is $n = \ell - \lfloor\ell/b\rfloor \cdot b$ bits long. So use $...
Squeamish Ossifrage's user avatar
1 vote
Accepted

64 bit clock cipher with CFB mode. One-byte defected from ciphertext. What is the number of bits defected from plaintext

The decryption of CFB mode as follows; $$ P_i = E_k(C_{i-1}) \oplus C_i$$ $$C_0= \operatorname{IV}$$ Bit flipping attack in CFB If there is a Bit Flipping attack in the ciphertext then we have two ...
kelalaka's user avatar
  • 48.5k
1 vote

How do I check if the output of my CFB TEA encryption algorithm is correct?

I would recommend modularizing the implementation into these components: A generic interface abstracting the concept of a block cipher. A generic CFB mode implementation, that instead of hardcoding ...
Luis Casillas's user avatar
1 vote

Show number of affected blocks using CFB DES decryption of altered ciphertext

Block size = 1 byte for CFB thus 9 bytes are affected. Why this is true has been answered before. Even for CFB8 it takes 8 shifts to shift the wrong ciphertext value out of the buffer. To proof this ...
Maarten Bodewes's user avatar
  • 92.6k

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