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Questions tagged [padding]

Padding has two primary uses in cryptography, ensuring messages are the proper length necessary for certain ciphers (e.g., block ciphers) or to provide assurances not built into the core cipher (e.g., semantic security)

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2answers
7k views

Which attacks are possible against raw/textbook RSA?

The PKCS#1 standard defines multiple padding schemes for signature generation/verification (EMSA-PSS and EMSA-PKCS1-v1_5), and encryption/decryption (EME-OAEP and the less safe EME-PKCS1-v1_5). ...
37
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2answers
22k views

Understanding the length extension attack

I have been trying to understand exactly how a length extension attack works on SHA-1. I'll detail below what I've understood so far, so that I can convey my understanding of the same and hopefully ...
22
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3answers
5k views

Why does the padding in Merkle–Damgård hash functions like MD5 contain the message length?

I understand the need for padding in MD5. But why do we append the message length to the padding? I heard it strengthens the hash but how? Please provide an example if possible and how it applies to ...
67
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1answer
94k views

What is the difference between PKCS#5 padding and PKCS#7 padding

One runtime platform provides an API that supplies PKCS#5 padding for block cipher modes such as ECB and CBC. These modes have been defined for the triple DES, AES and Blowfish block ciphers. The ...
25
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5answers
42k views

Why is padding used for RSA encryption given that it is not a block cipher?

In AES we use some padded bytes at end of message to fit 128/256 byte blocks. But as RSA is not a block cipher why is padding used? Can the message size be any byte length (is the encrypting agent ...
6
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2answers
2k views

Why can the last block contain a full block of padding in CBC Encryption?

I'm trying to understand the SSL Poodle Attack and I'm wondering why the last block of a CBC Record can be full of padding? Wouldn't that mean that the useful data was already a multiple of the key ...
6
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2answers
1k views

How does compression before encryption leak info about the input?

Apparently current best practices recommend that you do not compress before you encrypt. For example in this blog entry (*): http://sockpuppet.org/blog/2013/07/22/applied-practical-cryptography/ It ...
6
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2answers
2k views

Is RSA of a random nonce with no padding safe?

Consider the following protocol: Bob has a private RSA key $B_{priv}$, and Alice knows the public key $B_{pub}$. Alice wants to send confidential messages to Bob (no integrity intended). To send a ...
8
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1answer
510 views

The 9 lives of Bleichenbacher's CAT, it puts another scratch again

Bleichenbacher demonstrated a padding oracle attack against RSA implementations that follow the PKCS #1 v1.5. Through the years, there are various mitigation techniques developed as OAEP and limiting ...
8
votes
1answer
506 views

Why does SHA-2 call for doing 10* padding in addition to appending the message length?

Appending the length of the message when padding of a SHA-2 message is sufficient to satisfy the Merkle-Damgård construction. However the padding in SHA-2 also pads with 10* between the end of the ...
8
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2answers
3k views

RSA blind signatures in practice

Hi I have a problem with moving my blind signature implementation from educational (textbook RSA) to more practical (padded RSA) side. David Chaums paper gives a following figure: $r$ - blinding ...
4
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1answer
10k views

How is input message for SHA-2 padded?

I read about how is an input message prepared to be hashed by MD4,MD5 or SHA-1: Step1 Append padding bits The input message is "padded" (extended) so that its length (in bits) equals to 448 ...
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1answer
13k views

Removing Padded Value in Decrypted Message

How to remove padded value in Decrypted Message? I am using AES Algorithm. Let's take this as the decrypted message: "abcdefghijklmn " There is a 2 space in ...
12
votes
1answer
3k views

Why was ISO10126 Padding Withdrawn?

Wikipedia mentions ISO10126 Padding has been withdrawn, but doesn't say why. Also there were no news reports about this, as far as I can see. Why was it withdrawn? Are there security flaws? Is there ...
11
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3answers
2k views

Is RSA padding needed for single recipient, one-time, unique random message?

I want a way to encrypt files using this process: https://crypto.stackexchange.com/a/15 . That is: generate a random password, use that to AES-encrypt a file, and use an RSA public key to encrypt the ...
3
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0answers
154 views

IND-CCA1 RSA padding?

I've found a way to complete a task which I'd solve with passwords or by sending keys over the wire (otherwise) by using RSA's homomorphic property. I'm restricted to RSA (any padding; for hardware ...
5
votes
1answer
100 views

Random Masking of Padded RSA Ciphertext through homomorphism

I had asked a question related to this before: Oblivious Decryption: Decrypting with a private key, without knowing the message @rikhavshah has an answer, which I would like to discuss the security ...
4
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2answers
365 views

Why verify padding at all?

Whenever a ciphertext is decrypted using a block-cypher, we need to remove the padding. There are different ways to add padding, but they usually set the last byte of the last block to the number of ...
4
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1answer
457 views

What prevents a padding standard to cause a data loss?

In here different byte padding standards are listed as ANSI X.923, ISO 101.26, PKCS7, ISO/IEC 7816-4 and zero padding. All of them seems to apply padding to the last byte blocks. Odds are rare but ...
3
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2answers
252 views

Usability of padding scheme in blinded RSA signature?

In Wikipedia's article on Blind signature, in the section Dangers of blind signing which describes RSA blinding attack one reads the following This attack works because in this blind signature ...
3
votes
1answer
344 views

How does padding oracle attack work with bytes larger than possible length?

I am trying to understand how the POODLE attack would actually be implemented. In the original security advisory from Möller, Duong and Kotowicz [POODLE 14] the following example is presented: ...
2
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1answer
4k views

Computing the padding of MD5

So I'm reading this question to learn more about length extension attack, and I want to make sure I understand the basics of padding. It says in the post that given the hash and the length of the ...
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2answers
844 views

Selection of cryptographic functions for OAEP

I can't understand how the "cryptographic functions" are to be selected RSA + OAEP which are used in OAEP. How to choose these "cryptographic functions"(G and H)?
11
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3answers
9k views

In the SHA hash algorithm, why is the message always padded?

In the SHA hash algorithm the message is always padded, even if initially the correct length without padding; the padding is of the form "1" followed by the necessary number of 0s. Why is it ...
6
votes
2answers
1k views

RSA Signature Weak Padding Attack

Assume that we have a message $m$ of size $n$, and it is padded with two 01 bytes in front. Then the signature $s$ is computed using a private key $ks$. Can we ...
3
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1answer
6k views

How much safer is RSA-OAEP compared to RSA with PKCS#1 v1.5 padding?

Is there a way to calculate how much safer RSA-OAEP is compared to RSA with PKCS#1 v1.5 compliant padding? Or is there a good rule of thumb?
1
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1answer
119 views

Help understanding how to decrypt cipher

I am trying to create a padding oracle attack to decrypt a cipher that is encrypted with cbc and PKCS#7 padding. My first step is just to try and decrypt the second block. I have read a lot about it,...
4
votes
1answer
1k views

RSA Encryption and Signature - Weak Padding

Assume that I have an plaintext $m$ and it is padded with $randompad||00||m$ and then it is encrypted with RSA and a public encryption key so we get the encrypted $Sm$. Then to assure its integrity ...
4
votes
3answers
2k views

Hash function in PBKDF2

From this excellent answer I learned (correct me if I am wrong) that when writing a block cipher with say key size 128 bit, one has to pad the password given (variable size) so that it becomes exactly ...
10
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0answers
586 views

Are there attacks against broken RSA signature pad checking with $e = 65537$?

Let's say that an RSA implementation of PKCS #1 signatures fails to validate that the 00 01 FF FF FF ... FF 00 portion of the decrypted signature is exactly as long ...
7
votes
2answers
286 views

Which attacks are possible against raw/textbook NTRU encryption?

In the same fashion that these questions about attacks to textbook RSA and ECC, I was wondering what are the immediate drawbacks of applying NTRU Encryption directly, without any padding scheme, such ...
6
votes
2answers
604 views

What is the key strength reduction encrypting only 160 bits of data using RSA1024 for signatures?

I am attempting to determine the strength of an incorrectly implemented 1024 bit RSA signature scheme. The weakness in the implementation is that the padding data lacks random numbers. As a result, ...
5
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2answers
1k views

Does RSA padding have to be unpredictable if the payload is?

I'm trying to understand the precise requirements on padding when using RSA for encryption. Suppose Alice uses RSA to encrypt a payload $M$ that cannot be guessed (say, a random nonce): Alice send $\...
3
votes
1answer
429 views

Bitwise method of generating r for RSA-KEM

It seems that RSA-KEM has a very troublesome method of generating the secret $r$. It seems that the random value needs to be in the range $0 \le r \le {n - 1}$. Now most cryptographic environments ...
2
votes
1answer
2k views

How does OAEP improve the security of RSA?

The heart of OAEP algorithm used for RSA encryption are the cryptographic hash functions $H$ and $G$. Does everybody (so also an adversary) know these functions? If YES: How does it help the ...
0
votes
1answer
888 views

What's the best way to pad a message?

Currently my padding is adding a string of random characters $ l(N)/3 - (l(M) + 1) $ Long and just adding it to the messages, is this sufficient? $l(x)$ = length of a variable
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1answer
1k views

How to perform the padding oracle attack on AES CBC?

Assume I am the attacker. I want to break the plaintext by performing the padding oracle attack on AES CBC mode. I know the following information: ...
9
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2answers
1k views

How does PKCS 1.5 solve the insecureness of Textbook RSA?

As we know, Textbook RSA is not enough safe to use because of the following issues; Guessable plaintext problem Small $e$ issue I want to know how PKCS 1.5 solved these problems.
7
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1answer
317 views

What encryption schemes to hide the plaintext length exist besides padding?

From this answers comments a statement came up about padding that I don't understand: "If we need to keep the length of the plaintext confidential to some degree, there are better methods." Are there ...
4
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1answer
199 views

Role of lHash field in RSAES-OAEP

The RSA encryption padding of RSAES-OAEP has a lHash field set to the hash of a label. That label is typically the empty string, or some public description of the intended use of the message. Does ...
3
votes
2answers
876 views

SHA-3 maximal number of zeros (Padding)

What is the maximum number of zeros, which are appended at the input- padding in messages? How large must the message be for this?
2
votes
1answer
312 views

poodle padding multiple blocks?

I'm quite new to the topic and read a bit about poodle and padding oracle. I quite understand the "regular" padding oracle attack, substituting the last byte of block(n-1) to determine the last byte ...
2
votes
1answer
2k views

Why does HMAC need a fixed length padding?

As everybody knows in order to calculate HMAC we have to concatenate padding to the message. I am just curious why the padding needs to be fixed-length. Why do we need the blocksize parameter here? ...
2
votes
1answer
129 views

OAEP Cryptanalysis

If $M_1$ and $M_2$ are $\operatorname{OAEP}$ padded, encrypted and sent and an adversary somehow manages to get around the encryption and retrieve $\operatorname{OAEP}(M_1) \oplus \operatorname{OAEP}(...
1
vote
1answer
1k views

RSA padding: what is it and how does that work?

If I am not mistaken, plaintext RSA is not secure. So, I have read that padding and hashing is needed to make the RSA algorithm secure. However, I am confused on what does padding or armoring mean in ...
0
votes
1answer
140 views

Weird size of data transmitted in SSLv3

Note - I know SSLv3 is dangerous but that is why I am using it to do demo of POODLE attack using server and client written in python. So the question is I am using "ECDHE-RSA-AES128-SHA" cipher for ...