# Questions tagged [idea]

The International Data Encryption Algorithm (IDEA) is a block cipher developed in 1991 by James Massey and Xuejia Lai.

12 questions
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### IDEA (International Data Encryption Algorithm)

I don't know how to perform Multiplication Modulo and Addition Modulo in IDEA (International Data Encryption Algorithm).
42 views

### Why can't we use the IDEA's decryption rule for decrypting a block on CTR mode?

Suppose that a block is ciphered with IDEA using CTR mode operation. To get the original message, why can't we use the cyphered rule?
48 views

### Algebraic Attack on Mix Mode Modular Arithmetic

I have a toy cipher that comprises of Mix Mode Modular Arithmetic (bit similar to IDEA including, Addition Modulo $2^{n}$ , Multiplication Modulo $2^{n}$ , Subtraction Modulo $2^{n}$ and circular ...
42 views

### Modifying Mix Mode Modular Arithmetic in IDEA Cipher

IDEA uses Mix Mode Modular Arithmetic that includes Addition Modulo $2^{16}$ and Multiplication Modulo $2^{16}+1$. If Multiplication Modulo with $2^{16}$ is used instead of $2^{16}+1$ (where $2^{16}$...
724 views

### openssl_get_cipher_methods: which one to choose?

When calling openssl_get_cipher_methods we get several cipher methods. I noticed that some of these method have same prefix like AES-256-(CBC|CFB|CFB1) or IDEA-(...
161 views

### How to get the value of the subkeys in IDEA decryption?

So I saw this example of IDEA .. If the key is : 5a14 fb3e 021c 79e0 6081 46a0 117b ff03 The encryption subkeys are: http://imgur.com/a/bZGMF The decryption subkeys are: http://imgur.com/a/FnF2c ...
405 views

### Weak key schedule IDEA [closed]

Why was such a weak key schedule chosen for IDEA? The key schedule of IDEA works like this: Divide the key (128 bit) into 8 round keys, each 16 bit long. This are the first 8 "round" keys (6 keys per ...
754 views

### How is multiplication inverted in IDEA's decryption round?

As we can see in the picture we have a multiplication in this algorithm, we know that two 16 bit inputs should have a 32 bit output, but here we just use 16 bits of the 32 bit output. For decryption ...
1k views

### How do I generate decryption keys for the IDEA algorithm from the 128 bit key?

I am trying to implement IDEA algorithm in C#, just to learn how it works. I have taken a 128 bit binary key and generated the 52 encryption keys using the following code: ...
2k views

### How to perform Multiplicative Inverse Modulo in IDEA

How to perform the Multiplicative Inverse Modulo in International Data Encryption Algorithm? I don't understand on how to perform it… For example, let's say I have a value of ...