What is difference between RSA and AES?

Is AES a symmetric key cipher?

I was looking at a presentation on crypto where the presenter talked about a symmetric key cipher called AES.

It was mentioned that AES is a symmetric key cipher. I was not able to find any good link for it on Wikipedia.

Can someone explain me in simple terms how AES works? AES is a symmetric block cipher. The block size is 128 bits.

The first step is a key expansion phase where you use a Key Schedule to generate a key. There are 16 rounds, each one encrypts 4 blocks of data. Each round is represented by a state which is a linear combination of previous rounds' states.

The rounds are defined by a function called the key schedule. For AES-128, the key schedule is a table of 16 4x4 matrices.

Each matrix is a different round of the encryption process. The table includes pre-computed values for the keys from 0 to 15 (inclusive). The key from 0 is used for the first round, the key from 1 is used for the second round, and so on. If the key was 5, the third round would be applied to the third matrix in the table. After that, the process is repeated.

The last step is an optional padding phase. There are two types of padding, byte-wise and word-wise.

Byte-wise padding means that the ciphertext starts with the block size in bits. It then repeats bytes in the same block until the total number of bits in the ciphertext is divisible by eight. So a 128 bit block cipher with word-wise padding will produce a ciphertext of 160 bits.

Word-wise padding means that the ciphertext starts with the word size in bits. It then repeats words in the same block until the total number of bits in the ciphertext is divisible by eight. So a 128 bit block cipher with word-wise padding will produce a ciphertext of 192 bits.

Yes. In a block cipher, the whole encryption process is based on a block of bytes or words. Depending on the implementation, the size of the block may vary.

Is DES and AES symmetric or asymmetric?

How does AES, used as an encryption algorithm, differ from DES, which is a symmetric encryption algorithm?

DES is a symmetric algorithm (ie, all key information is stored together with the plain text), and the process of encryption using a DES block cipher will be a function of both the key and the plaintext to be encrypted. In other words, the output of the encryption process will depend on the value of the plaintext (and, again, the particular key used).

AES, on the other hand, is an asymmetric algorithm (ie, the keys used for encryption and decryption differ). Since there are two keys involved, you need to use them both at the same time to encrypt/decrypt your plaintext (note that you need both the encryption and the decryption key to be correct in order to properly decrypt your ciphertext, since it's not obvious which one of the two will decrypt it).

There's a little confusion because of the different uses of the term "symmetric algorithm" or "asymmetric algorithm". There's asymmetric encryption, in which the encryption key and the decryption key are different; and symmetric encryption, in which both keys are the same (as in DES). However, symmetric encryption can involve a scheme in which the key and plaintext are both encrypted with a single secret key and then a subsequent decryption process (eg, in something like DES or RC4), or in which there's no key (eg, using a stream cipher like AES). So, for example, an application called Symmetric, which makes use of this confusion, can describe such encryption schemes asymmetric and symmetric algorithms.

Symmetric encryption means a shared secret is used to create the ciphers to be applied to the plain text to get the encrypted plain text. Symmetric encryption is a way to say that the secret can be reused for multiple messages.

Asymmetric encryption means different keys are used for the ciphers to be applied on the plain text to get the encrypted plain text. Asymmetric encryption is a way to say that there are unique keys that should not be reused across different messages.

What is "symmetric", and "asymmetric"?