Security level
In cryptography, security level is a measure of the strength that a cryptographic primitive — such as a cipher or hash function — achieves. Security level is usually expressed as a number of "bits of security" (also security strength),^{[1]} where n-bit security means that the attacker would have to perform 2^{n} operations to break it,^{[2]} but other methods have been proposed that more closely model the costs for an attacker.^{[3]} This allows for convenient comparison between algorithms and is useful when combining multiple primitives in a hybrid cryptosystem, so there is no clear weakest link. For example, AES-128 (key size 128 bits) is designed to offer a 128-bit security level, which is considered roughly equivalent to a RSA using 3072-bit key.
In this context, security claim or target security level is the security level that a primitive was initially designed to achieve, although "security level" is also sometimes used in those contexts. When attacks are found that have lower cost than the security claim, the primitive is considered broken.^{[4]}^{[5]}
In symmetric cryptography
Symmetric algorithms usually have a strictly defined security claim. For symmetric ciphers, it is typically equal to the key size of the cipher — equivalent to the complexity of a brute-force attack.^{[5]}^{[6]} Cryptographic hash functions with output size of n bits usually have a collision resistance security level n/2 and a preimage resistance level n. This is because the general birthday attack can always find collisions in 2^{n/2} steps.^{[7]} For example, SHA-256 offers 128-bit collision resistance and 256-bit preimage resistance.
However, there are some exceptions to this. The Phelix and Helix are 256-bit ciphers offering a 128-bit security level.^{[5]}^{[8]} The SHAKE variants of SHA-3 are also different: for a 256-bit output size, SHAKE-128 provides 128-bit security level for both collision and preimage resistance.^{[9]}
In asymmetric cryptography
The design of most asymmetric algorithms (i.e. public-key cryptography) relies on neat mathematical problems that are efficient to compute in one direction, but inefficient to reverse by the attacker. However, attacks against current public-key systems are always faster than brute-force search of the key space. Their security level isn't set at design time, but represents a computational hardness assumption, which is adjusted to match the best currently known attack.^{[6]}
Various recommendations have been published that estimate the security level of asymmetric algorithms, which differ slightly due to different methodologies. For the RSA cryptosystem at 128-bit security level, NIST and ENISA recommend using 3072-bit keys^{[10]}^{[11]} and IETF 3253 bits.^{[12]}^{[13]} Elliptic curve cryptography requires shorter keys, so the recommendations are 256-383 (NIST), 256 (ENISA) and 242 bits (IETF).
Typical levels
The following table are examples of typical security levels for types of algorithms as found in s5.6.1.1 of the US NIST SP-800-57 Recommendation for Key Management.^{[14]}
Security Bits | Symmetric Key | Finite Field/Discrete Logarithm (i.e. DH) | Integer Factorization (i.e. RSA) | Elliptic Curve (i.e. x25519) |
---|---|---|---|---|
80 | 2TDEA^{†} | L=1024, N=160 | k=1024 | f=160-223 |
112 | 3TDEA^{†} | L=2048, N=224 | k=2048 | f=224-255 |
128 | AES | L=3072, N=256 | k=3072 | f=256-383 |
192 | AES | L=7680, N=384 | k=7680 | f=384-511 |
256 | AES | L=15360, N=511 | k=15360 | f=512+ |
^{†} DES was deprecated in 2003
Meaning of "broken"
A cryptographic primitive is considered broken when an attack is found to have less than its advertised level of security. However, not all such attacks are practical: most currently demonstrated attacks take fewer than 2^{40} operations, which translates to a few hours on an average PC. The costliest demonstrated attack on hash functions is the 2^{61.2} attack on SHA-1, which took 2 months on 900 GTX 970 GPUs, and cost US$75,000 (although the researchers estimate only $11,000 was needed to find a collision).^{[15]}
Aumasson draws the line between practical and impractical attacks at 2^{80} operations. He proposes a new terminology:^{[16]}
- A broken primitive has an attack taking ≤ 2^{80} operations. An attack can be plausibly carried out.
- A wounded primitive has an attack taking between 2^{80} and around 2^{100} operations. An attack is not possible right now, but future improvements are likely to make it possible.
- An attacked primitive has an attack that is cheaper than the security claim, but much costlier than 2^{100}. Such an attack is too far from being practical.
- Finally, an analyzed primitive is one with no attacks cheaper than its security claim.
References
- ↑ NIST Special Publication 800-57 Part 1, Revision 5. Recommendation for Key Management: Part 1 – General, p. 17.
- ↑ Lenstra, Arjen K.. "Key Lengths: Contribution to The Handbook of Information Security". https://infoscience.epfl.ch/record/164539/files/NPDF-32.pdf.
- ↑ Bernstein, Daniel J.; Lange, Tanja (4 June 2012). "Non-uniform cracks in the concrete: the power of free precomputation" (in en). Advances in Cryptology - ASIACRYPT 2013. Lecture Notes in Computer Science. pp. 321–340. doi:10.1007/978-3-642-42045-0_17. ISBN 9783642420443. https://cr.yp.to/nonuniform/nonuniform-20130914.pdf.
- ↑ Aumasson, Jean-Philippe (2011). "Cryptanalysis vs. Reality". Black Hat Abu Dhabi. https://media.blackhat.com/bh-ad-11/Aumasson/bh-ad-11-Aumasson-CryptanalysisVSReality_WP.pdf.
- ↑ ^{5.0} ^{5.1} ^{5.2} Bernstein, Daniel J. (25 April 2005). "Understanding brute force". ECRYPT STVL Workshop on Symmetric Key Encryption. https://cr.yp.to/snuffle/bruteforce-20050425.pdf.
- ↑ ^{6.0} ^{6.1} Lenstra, Arjen K. (9 December 2001). "Unbelievable Security: Matching AES Security Using Public Key Systems" (in en). Advances in Cryptology — ASIACRYPT 2001. Lecture Notes in Computer Science. 2248. Springer, Berlin, Heidelberg. pp. 67–86. doi:10.1007/3-540-45682-1_5. ISBN 978-3540456827. https://www.iacr.org/archive/asiacrypt2001/22480067.pdf.
- ↑ Alfred J. Menezes, Paul C. van Oorschot, Scott A. Vanstone. "Chapter 9 - Hash Functions and Data Integrity". Handbook of Applied Cryptography. pp. 336. http://cacr.uwaterloo.ca/hac/about/chap9.pdf.
- ↑ Ferguson, Niels; Whiting, Doug; Schneier, Bruce; Kelsey, John; Lucks, Stefan; Kohno, Tadayoshi (24 February 2003). "Helix: Fast Encryption and Authentication in a Single Cryptographic Primitive" (in en). Fast Software Encryption. Lecture Notes in Computer Science. 2887. Springer, Berlin, Heidelberg. pp. 330–346. doi:10.1007/978-3-540-39887-5_24. ISBN 978-3-540-20449-7. https://www.schneier.com/academic/paperfiles/paper-phelix.pdf.
- ↑ Dworkin, Morris J. (August 2015). SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions. p. 23. doi:10.6028/nist.fips.202. http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf.
- ↑ Barker, Elaine (January 2016). Recommendation for Key Management, Part 1: General. NIST. pp. 53. doi:10.6028/nist.sp.800-57pt1r4. http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57pt1r4.pdf.
- ↑ Algorithms, key size and parameters report – 2014. ENISA. Publications Office. 2013. p. 37. doi:10.2824/36822. ISBN 9789292041021. https://www.enisa.europa.eu/activities/identity-and-trust/library/deliverables/algorithms-key-size-and-parameters-report-2014/at_download/fullReport.
- ↑ Hilarie, Orman; Paul, Hoffman (April 2004). Determining Strengths For Public Keys Used For Exchanging Symmetric Keys. RFC 3766 (IETF). doi:10.17487/RFC3766. https://tools.ietf.org/html/rfc3766.
- ↑ Giry, Damien. "Keylength - Compare all Methods". https://www.keylength.com/en/compare/.
- ↑ Barker, Elaine (May 2020). Recommendation for Key Management, Part 1: General. NIST. pp. 158. doi:10.6028/nist.sp.800-57pt1r5. https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57pt1r5.pdf.
- ↑ Gaëtan Leurent; Thomas Peyrin (2020-01-08). SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and Application to the PGP Web of Trust. https://eprint.iacr.org/2020/014.pdf.
- ↑ Aumasson, Jean-Philippe (2020). "Too Much Crypto". Real World Crypto Symposium. https://eprint.iacr.org/2019/1492.pdf.
Further reading
- Aumasson, Jean-Philippe (2020). "Too Much Crypto". Real World Crypto Symposium. https://eprint.iacr.org/2019/1492.pdf.
See also
- Computational hardness assumption
- 40-bit encryption
- Cipher security summary
- Hash function security summary
Original source: https://en.wikipedia.org/wiki/Security level.
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