# Is triple des similiar to RSA in that they message size is limited to the key size?

Is Triple DES similiar to RSA in that the message size you can encrypt is limited (unlike AES)?

Yes you can break the message size into parts and apply it, but I'm not interested in doing that so I want to know if Triple DES has this same "limitation" like RSA?

I know AES is better suited for my situation where the message size can be large, but I'm not the only one making the decision on the algorithm.

• TDES is similar to AES-256 in that it enciphers blocks of limited size (64-bit for TDES, 128-bit for AES-256), smaller than the key (like 168 or 192 bits depending on convention for TDES, 256 bits for AES-256). The size of an RSA-encrypted piece of data is also limited to less than the key size (slightly for naive RSA, more so for secure RSA) but comparison with block ciphers is mostly meaningless. For all three, large amounts of data can be safely encrypted, with appropriate operating modes (or hybrid encryption for RSA). – fgrieu Jun 5 '13 at 21:40

Triple DES and AES are the very same type of scheme: - symmetric block cypher, which also implies fixed size; block sizes are 64 bit for DES, 128 for AES.

RSA is asymmetric, and the key length determines the size of plain- and ciphertext. Therefore, with a fixed key length you also have an (almost; length may vary by 1 dependant on choice of primes) fixed size.

Your main error is, that you attribute the mode of operation to AES, which is false. In general, mode of operation describe how to use a block cipher of fixed length to encrypt large amounts of data or a stream of data (e.g. secure channels, etc.). Although they are defined on symmetric ciphers, you could also use them for asymmetric encryption(RSA, etc.), but it is not practical to do (see hybrid encryption)

Edit: Thanks @ poncho for pointing out, that AES has a fixed block size of 128 bit (and keys with 128, 192 or 256 bit), and the original Rijndael cipher supported all three sizes for keys and blocks.

• Using standard blockcipher modes of encryption with RSA isn't just impractical, it's totally broken for some modes. For other modes the non integral block-size of RSA prevetns use entirely. – CodesInChaos Jun 6 '13 at 14:49
• Correction: the block size for AES is fixed at 128 bits. Now, Rijndael (the AES submission that became AES) does allow other block sizes; AES is that subset of Rijndael that is limited to the NIST's original conditions (128 bit blocks, 128, 192 or 256 bit keys) – poncho Jun 6 '13 at 15:42
• poncho, you are right, of course. The block size is fixed and the key size supports the other formats. However, AES also has a limited size (without MoO), in contrast to the assumption in the question. – tylo Jun 10 '13 at 16:20

Is triple des similiar to rsa in that the message size you can encrypt is limited (unlike AES)?

If you mean they both are block ciphers, then yes.

• I don't see how this addresses the question, in fact I find it somewhat misleading. AES is a block cipher too and you can encrypt more than one block of data with 3DES. – Thomas Jun 6 '13 at 0:54
• Which applies to both AES and 3DES, so you're missing the "unlike AES" part. – Thomas Jun 7 '13 at 1:13
• Triple DES and RSA are not both block ciphers. The question was not about AES. – tylo Jun 10 '13 at 16:27
• No, RSA is NOT a block cipher (unless you're using a nonstandard definition of the term "block cipher") – poncho Jun 18 '13 at 14:39
• @SmitJohnth: if we look at, say, cacr.uwaterloo.ca/hac/about/chap7.pdf we see the assumption that we use the same key to encrypt and decrypt; that is, it is a symmetric cipher. RSA is, in normal usage, uses different keys (which is essentially the only reason we put up with the costs of doing RSA). I suppose you could propose an RSA variant for which keys could both encrypt and decrypt; I would claim that such a variant is no longer RSA. – poncho Jun 21 '13 at 1:33