On a NIST-sponsored hash function mailing list, Jesse Walker (from Intel; also a member of the Skein team) did some back-of-the-envelope calculations to estimate how long it will be before we see a practical collision attack against SHA-1. I’m reprinting his analysis here, so it reaches a broader audience.

According to E-BASH, the cost of one block of a SHA-1 operation on already deployed commodity microprocessors is about 214 cycles. If Stevens’ attack of 260 SHA-1 operations serves as the baseline, then finding a collision costs about 214 * 260 ~ 274 cycles.

A core today provides about 231 cycles/sec; the state of the art is 8 = 23 cores per processor for a total of 23 * 231 = 234 cycles/sec. A server typically has 4 processors, increasing the total to 22 * 234 = 236 cycles/sec. Since there are about 225 sec/year, this means one server delivers about 225 * 236 = 261 cycles per year, which we can call a “server year.”

There is ample evidence that Moore’s law will continue through the mid 2020s. Hence the number of doublings in processor power we can expect between now and 2021 is:

3/1.5 = 2 times by 2015 (3 = 2015 – 2012) 6/1.5 = 4 times by 2018 (6 = 2018 – 2012) 9/1.5 = 6 times by 2021 (9 = 2021 – 2012)

So a commodity server year should be about:

261 cycles/year in 2012 22 * 261 = 263 cycles/year by 2015 24 * 261 = 265 cycles/year by 2018 26 * 261 = 267 cycles/year by 2021

Therefore, on commodity hardware, Stevens’ attack should cost approximately:

274 / 261 = 213 server years in 2012 274 / 263 = 211 server years by 2015 274 / 265 = 29 server years by 2018 274 / 267 = 27 server years by 2021

Today Amazon rents compute time on commodity servers for about $0.04 / hour ~ $350 /year. Assume compute rental fees remain fixed while server capacity keeps pace with Moore’s law. Then, since log 2 (350) ~ 8.4 the cost of the attack will be approximately:

213 * 28.4 = 221.4 ~ $2.77M in 2012 211 * 28.4 = 219.4 ~ $700K by 2015 29 * 28.4 = 217.4 ~ $173K by 2018 27 * 28.4 = 215.4 ~ $43K by 2021

A collision attack is therefore well within the range of what an organized crime syndicate can practically budget by 2018, and a university research project by 2021.

Since this argument only takes into account commodity hardware and not instruction set improvements (e.g., ARM 8 specifies a SHA-1 instruction), other commodity computing devices with even greater processing power (e.g., GPUs), and custom hardware, the need to transition from SHA-1 for collision resistance functions is probably more urgent than this back-of-the-envelope analysis suggests.