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NIST SP 800-57 §5.6.1 p.62–64 specifies a correspondence between RSA modulus size $n$ and expected security strength $s$ in bits:

Strength  RSA modulus size
  80        1024
 112        2048
 128        3072
 192        7680
 256       15360

This works out to approximately $s \approx 4 n^{0.43}$ (but I have no idea whether my extrapolation means anything; strengths up to 112 are indexed on DES while strengths 128 and above are indexed on AES, i.e. the strength is the difficulty of brute-forcing the corresponding symmetric algorithm with the specified key size).

What are these numbers based on? Do they come from the expected complexity of the best known factorization methods? Or are they extrapolation from the amount of computation in specific factorization efforts such as for RSA-768 (which “required more than $10^{20}$ operations”)?

(I'm asking for more precise information than Difficulty of breaking RSA for a given key size. How big an RSA key is considered secure today? has a nice history of RSA factorizations, but doesn't answer my question — is that what the strength estimates are based on?)

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The numbers on the left are symmetric keys of corresponding strength. I.e. a symmetric key of 80 bits provides roughly as much security as a 1024-bit RSA key. For posterity, I'll mention that the table is on page 62 of the linked NIST document. –  pg1989 Jun 11 '13 at 17:26
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up vote 8 down vote accepted

Those appear to be based on the complexity of the General Number Field Sieve, one of the fastest (if not the fastest) classical factoring algorithms. I confirmed this in Mathematica.

Here is the complexity for the GNFS (pulled from the linked Wikipedia article):

$$\exp\left( \left(\sqrt[3]{\frac{64}{9}} + o(1)\right)(\ln n)^{\frac{1}{3}}(\ln \ln n)^{\frac{2}{3}}\right)$$

where $n$ is a number to factor. Evaluating the above expression at $2^b$ is a rough approximation of the the time needed to factor a $b$-bit integer. Here's a table showing the bit-length of the evaluation at $2^{1024},2^{2048},\dots$:

Strength  RSA modulus size   Complexity bit-length
  80        1024              86.76611925028119
 112        2048              116.8838132958159
 128        3072              138.7362808527251
 192        7680              203.01873594417665
 256       15360              269.38477262128384

I generated these numbers with the following Mathematica code:

({#, Log[2, g[#]]} &) /@ {1024, 2048, 3072, 7680, 15360}

where g is defined as:

g[b_] := N@Exp[CubeRoot[64/9]*CubeRoot[Log[2^b]]*(Log[Log[2^b]])^(2/3)]

which is not the prettiest thing I've ever written, but it seems accurate.

As for the reasoning behind the larger key sizes for RSA, the explanation's not too difficult. If you look at the document in the question, you will notice that the "bits of security" for block ciphers correlate almost perfectly with the size (in bits) of the keys for that block cipher (with rare exceptions). This is because our best attack on a secure block cipher essentially is a brute-force search for the key, which, on average, takes $2^{n-1}$ time where $n$ is the bit-length of the key.

The exception is 3DES, which in its most common form takes about $2^{112}$ time instead of $2^{168}$ thanks to a meet-in-the-middle attack. Regardless, secure block ciphers have a security that is approximately the key's bit-length; for example, AES-128 offers approximately 128 bits of security. (In fact, the real figure for a secure implementation is $2^{126.1}$, but the loss of two bits is inconsequential.)

However, for RSA, our best line of attack is not to execute a brute-force search for the key; instead, we "simply" factor the (public) modulus, so the security of the scheme revolves around the efficiency of factoring. The GNFS (linked above) has a sub-exponential, but still super-polynomial, time complexity, and one can use this time complexity to roughly approximate the security offered by the scheme. I believe that's what the NIST is doing here.

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Could you explain why the strength is 80 and not 86 for RSA-1024? –  user129789 Aug 30 '13 at 13:35
    
"The value of k is commonly considered to be the key size." See page 64 of NIST SP 800-57 –  user129789 Aug 30 '13 at 13:46
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@user129789: First, directly evaluating asymptotic expressions is quite imprecise. But more importantly, the NIST is an administrative organization, and their purpose in that table was to define a set of key sizes for various primitives that give a specific level of security. The intent is that you might decide 112 bits of security is enough for your application, then use the key sizes/algorithms suggested in the table. So they picked commonly-used key sizes that were slightly above the desired security level. The extra margin also gives you a bit of leeway in terms of Moore's law. –  Reid Aug 30 '13 at 14:01
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