# Modes of operation OAEP RSA

**URL:** <https://www.ruby-forum.com/t/modes-of-operation-oaep-rsa/74143>\
**Category:** Ruby\
**Created:** [October 31, 2006, 1:51am UTC](https://www.ruby-forum.com/t/modes-of-operation-oaep-rsa/74143 "2006-10-31T01:51:24Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![floye](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/43a26b/32.png) [@floye](https://www.ruby-forum.com/u/floye)\
**Post date:** [October 31, 2006, 1:51am UTC](https://www.ruby-forum.com/t/modes-of-operation-oaep-rsa/74143/1 "2006-10-31T01:51:24Z")

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Hi everyone,

I would really appreciate if someone could help me out with few  
issues/questions 🙂  
I’m writing a thesis and I can not find anything specific related to  
OEAP (modes of operation)

1. Can OAEP be used with symmetrical algorithms and i.e. CBC with RSA.  
If not why?

2. What would be a speed (throughput) of OAEP algorithm itself (without  
RSA)  
Maybe someone has done some tests.

3. How are OAEP and hush function work together.

Thank you in advance whoever would like to help 😉

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<div class="post-metadata">

**Author:** ![floye](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/43a26b/32.png) [@floye](https://www.ruby-forum.com/u/floye)\
**Post date:** [November 7, 2006, 10:48am UTC](https://www.ruby-forum.com/t/modes-of-operation-oaep-rsa/74143/2 "2006-11-07T10:48:55Z")

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Jerzy Urbanik wrote:

> Hi everyone,
> 
> I would really appreciate if someone could help me out with few  
> issues/questions 🙂  
> I’m writing a thesis and I can not find anything specific related to  
> OEAP (modes of operation)

Not really the right forum. sci.crypt is probably better. You will also  
want to know about the Handbook of Applied Cryptography, which is  
online at uwaterloo.ca.

> 1. Can OAEP be used with symmetrical algorithms and i.e. CBC with RSA.  
> If not why?

It can, but it’d be insanely slow. RSA is very very slow compared to  
AES and other block ciphers. That’s why we use block ciphers for bulk  
encryption.

> 1. What would be a speed (throughput) of OAEP algorithm itself (without  
> RSA)  
> Maybe someone has done some tests.

It’s very quick - it’s a 2 stage Feistel network. The use of OAEP is to  
prevent RSA from having a mathematical relationship with the output -  
it breaks up that relationship.

One of the problems with RSA by itself is that is must encrypt in  
blocks the same size as n (= p \* q), and the value encrypted must be  
smaller than n. Also, the output has a purely mathematical relationship  
with the input, n and e. If the attacker can choose plaintexts, theren  
there are certain attacks that become possible. OAEP mitigates these  
attacks by mixing up the input space for RSA, making it non-linear, and  
the matematical attacks harder, basiclly by bashing bits about first.  
Importantly, this is reversible at the other end. OAEP also provides  
evidence of someone having tampered with the ciphertext - if the  
ciphertext was tampered with, then the leading bits in OAEP will not be  
the correct pattern. This can be detected early in the OAEP decryption  
phase, but it is worth noting that this is not done in practice,  
because the early return can give the attacker timing information about  
where the ciphertext was rejected.

> 1. How are OAEP and hush function work together.

OAEP not not a hash function - it’s reversible. There’s no point in  
combining the two of them, either, as they have a similar job, except  
for the reversability of the OAEP.