SHACAL

Key takeaways

SHACAL belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SHACAL to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SHACAL from memory before moving on to harder problems.

Reference excerpt

SHACAL-1 (originally simply SHACAL) is a 160-bit block cipher based on SHA-1, and supports keys from 128-bit to 512-bit. SHACAL-2 is a 256-bit block cipher based upon the larger hash function SHA-256. Both SHACAL-1 and SHACAL-2 were selected for the second phase of the NESSIE project. However, in 2003, SHACAL-1 was not recommended for the NESSIE portfolio because of concerns about its key schedule, while SHACAL-2 was finally selected as one of the 17 NESSIE finalists.

Design SHACAL-1 is based on the following observation of SHA-1: The hash function SHA-1 is designed around a compression function. This function takes as input a 160-bit state and a 512-bit data word and outputs a new 160-bit state after 80 rounds. The hash function works by repeatedly calling this compression function with successive 512-bit data blocks and each time updating the state accordingly. This compression function is easily invertible if the data block is known, i.e. given the data block on which it acted and the output of the compression function, one can compute that state that went in. SHACAL-1 turns the SHA-1 compression function into a block cipher by using the state input as the data block and using the data input as the key input. In other words, SHACAL-1 views the SHA-1 compression function as an 80-round, 160-bit block cipher with a 512-bit key. Keys shorter than 512 bits are supported by padding them with zeros. SHACAL-1 is not intended to be used with keys shorter than 128 bits.

Security of SHACAL-1 In the paper "Related-key rectangle attack on the full SHACAL-1", 2006, Orr Dunkelman, Nathan Keller and Jongsung Kim presented a related-key rectangle attack on the full 80 rounds of SHACAL-1. In the paper "Differential and Rectangle Attacks on Reduced-Round SHACAL-1", Jiqiang Lu, Jongsung Kim, Nathan Keller and Orr Dunkelman presented rectangle attacks on the first 51 rounds and a series of 52 inner rounds of SHACAL-1 and presented differential attacks on the first 49 rounds and a series of 55 inner rounds of SHACAL-1. These are the best currently known cryptanalytic results on SHACAL-1 in a single key attack scenario.

Security of SHACAL-2 In the paper "Related-Key Rectangle Attack on 42-Round SHACAL-2", Jiqiang Lu, Jongsung Kim, Nathan Keller, Orr Dunkelman presented a related-key rectangle attack on 42-round SHACAL-2. In 2008 Lu and Kim presented a related-key rectangle attack on 44-round SHACAL-2. This is the best currently known cryptanalytic result on SHACAL-2.

References Eli Biham, Orr Dunkelman, Nathan Keller (February 2003). Rectangle Attacks on 49-Round SHACAL-1 (PDF). 10th International Workshop on Fast Software Encryption (FSE '03). Lund: Springer-Verlag. pp. 22–35. Archived from the original (PDF) on 2007-09-26. Retrieved 2007-07-02.{{cite conference}}: CS1 maint: multiple names: authors list (link) Helena Handschuh, Lars R. Knudsen, Matthew J. B. Robshaw (April 2001). Analysis of SHA-1 in Encryption Mode (PDF/PostScript). CT-RSA 2001, The Cryptographer's Track at RSA Conference 2001. San Francisco, California: Springer-Verlag. pp. 70–83. Retrieved 2007-07-02.{{cite conference}}: CS1 maint: miscellaneous url (link) CS1 maint: multiple names: authors list (link) Seokhie Hong; Jongsung Kim; Guil Kim; Jaechul Sung; Changhoon Lee; Sangjin Lee (December 2003). Impossible Differential Attack on 30-Round SHACAL-2. 4th International Conference on Cryptology in India (INDOCRYPT 2003). New Delhi: Springer-Verlag. pp. 97–106. Jongsung Kim; Guil Kim; Sangjin Lee; Jongin Lim; Junghwan Song (December 2004). Related-Key Attacks on Reduced Rounds of SHACAL-2. INDOCRYPT 2004. Chennai: Springer-Verlag. pp. 175–190. Jongsung Kim; Guil Kim; Seokhie Hong; Sangjin Lee; Dowon Hong (July 2004). The Related-Key Rectangle Attack — Application to SHACAL-1. 9th Australasian Conference on Information Security and Privacy (ACISP 2004). Sydney: Springer-Verlag. pp. 123–136. Jongsung Kim; Dukjae Moon; Wonil Lee; Seokhie Hong; Sangjin Lee; Seokwon Jung (December 2002). Amplified Boomerang Attack against Reduced-Round SHACAL. ASIACRYPT 2002. Queenstown, New Zealand: Springer-Verlag. pp. 243–253. Markku-Juhani Olavi Saarinen (February 2003). Cryptanalysis of Block Ciphers Based on SHA-1 and MD5 (PDF). FSE '03. Lund: Springer-Verlag. pp. 36–44. Archived from the original (PDF) on 2006-12-24. Retrieved 2007-07-02. YongSup Shin; Jongsung Kim; Guil Kim; Seokhie Hong; Sangjin Lee (July 2004). Differential-Linear Type Attacks on Reduced Rounds of SHACAL-2. ACISP 2004. Sydney: Springer-Verlag. pp. 110–122. Jiqiang Lu; Jongsung Kim; Nathan Keller; Orr Dunkelman (2006). Related-Key Rectangle Attack on 42-Round SHACAL-2 (PDF). Information Security, 9th International Conference (ISC 2006). Samos: Springer-Verlag. pp. 85–100. Archived from the original (PDF) on 2006-09-25. Retrieved 2007-07-02. Jiqiang Lu; Jongsung Kim; Nathan Keller; Orr Dunkelman (December 2006). Differential and Rectangle Attacks on Reduced-Round SHACAL-1 (PDF). INDOCRYPT 2006. Kolkata: Springer-Verlag. pp. 17–31. Retrieved 2007-07-02. O. Dunkelman; N. Keller; J. Kim (August 2006). Related-key rectangle attack on the full SHACAL-1 (PostScript). Selected Areas in Cryptography (SAC 2006), to appear. Montreal: Springer-Verlag. pp. 16 pages. Retrieved 2007-07-02. Jiqiang Lu; Jongsung Kim (September 2008). "Attacking 44 Rounds of the SHACAL-2 Block Cipher Using Related-Key Rectangle Cryptanalysis" (PDF). IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences. IEICE: 2599–2596. Retrieved 2012-01-30.

External links Nathan Keller's homepage

Illustrations

Worked examples

Example 1 — a first encounter with SHACAL

Start with the simplest possible case. Write down what SHACAL claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SHACAL before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SHACAL ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SHACAL

In research: SHACAL appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SHACAL in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SHACAL is common in secondary-school and first-year university syllabi. It links to neighbouring topics Block ciphers, so understanding it makes those chapters shorter.
In everyday life: Look for SHACAL outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study SHACAL in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SHACAL means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SHACAL out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SHACAL in simple terms?

SHACAL-1 (originally simply SHACAL) is a 160-bit block cipher based on SHA-1, and supports keys from 128-bit to 512-bit. SHACAL-2 is a 256-bit block cipher based upon the larger hash function SHA-256.

Why does SHACAL matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SHACAL?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SHACAL.