git/hash.h

#ifndef HASH_H
#define HASH_H

#if defined(SHA1_PPC)
#include "ppc/sha1.h"
#elif defined(SHA1_APPLE)
#include <CommonCrypto/CommonDigest.h>
#elif defined(SHA1_OPENSSL)
#include <openssl/sha.h>
#elif defined(SHA1_DC)
#include "sha1dc/sha1.h"
#else /* SHA1_BLK */
#include "block-sha1/sha1.h"
#endif

#endif
hash.h: move SHA-1 implementation selection into a header file Many developers use functionality in their editors that allows for quick syntax checks, including warning about questionable constructs. This functionality allows rapid development with fewer errors. However, such functionality generally does not allow the specification of project-specific defines or command-line options. Since the SHA1_HEADER include is not defined in such a case, developers see spurious errors when using these tools. Furthermore, there are known implementations of "cc" whose '#include' is unhappy with this construct. Instead of using SHA1_HEADER, create a hash.h header and use #if and #elif to select the desired header. Have the Makefile pass an appropriate option to help the header select the right implementation to use. [jc: make BLK_SHA1 the fallback default as discussed on list, e.g. <20170314201424.vccij5z2ortq4a4o@sigill.intra.peff.net>; also remove SHA1_HEADER and SHA1_HEADER_SQ that are no longer used]. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com> 2017-03-12 01:28:18 +03:00			`#ifndef HASH_H`
			`#define HASH_H`

			`#if defined(SHA1_PPC)`
			`#include "ppc/sha1.h"`
			`#elif defined(SHA1_APPLE)`
			`#include <CommonCrypto/CommonDigest.h>`
			`#elif defined(SHA1_OPENSSL)`
			`#include <openssl/sha.h>`
Makefile: add DC_SHA1 knob This knob lets you use the sha1dc implementation from: https://github.com/cr-marcstevens/sha1collisiondetection which can detect certain types of collision attacks (even when we only see half of the colliding pair). So it mitigates any attack which consists of getting the "good" half of a collision into a trusted repository, and then later replacing it with the "bad" half. The "good" half is rejected by the victim's version of Git (and even if they run an old version of Git, any sha1dc-enabled git will complain loudly if it ever has to interact with the object). The big downside is that it's slower than either the openssl or block-sha1 implementations. Here are some timings based off of linux.git: - compute sha1 over whole packfile sha1dc: 3.580s blk-sha1: 2.046s (-43%) openssl: 1.335s (-62%) - rev-list --all --objects sha1dc: 33.512s blk-sha1: 33.514s (+0.0%) openssl: 33.650s (+0.4%) - git log --no-merges -10000 -p sha1dc: 8.124s blk-sha1: 7.986s (-1.6%) openssl: 8.203s (+0.9%) - index-pack --verify sha1dc: 4m19s blk-sha1: 2m57s (-32%) openssl: 2m19s (-42%) So overall the sha1 computation with collision detection is about 1.75x slower than block-sha1, and 2.7x slower than sha1. But of course most operations do more than just sha1. Normal object access isn't really slowed at all (both the +/- changes there are well within the run-to-run noise); any changes are drowned out by the other work Git is doing. The most-affected operation is `index-pack --verify`, which is essentially just computing the sha1 on every object. This is similar to the `index-pack` invocation that the receiver of a push or fetch would perform. So clearly there's some extra CPU load here. There will also be some latency for the user, though keep in mind that such an operation will generally be network bound (this is about a 1.2GB packfile). Some of that extra CPU is "free" in the sense that we use it while the pack is streaming in anyway. But most of it comes during the delta-resolution phase, after the whole pack has been received. So we can imagine that for this (quite large) push, the user might have to wait an extra 100 seconds over openssl (which is what we use now). If we assume they can push to us at 20Mbit/s, that's 480s for a 1.2GB pack, which is only 20% slower. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com> 2017-03-17 01:09:12 +03:00			`#elif defined(SHA1_DC)`
			`#include "sha1dc/sha1.h"`
hash.h: move SHA-1 implementation selection into a header file Many developers use functionality in their editors that allows for quick syntax checks, including warning about questionable constructs. This functionality allows rapid development with fewer errors. However, such functionality generally does not allow the specification of project-specific defines or command-line options. Since the SHA1_HEADER include is not defined in such a case, developers see spurious errors when using these tools. Furthermore, there are known implementations of "cc" whose '#include' is unhappy with this construct. Instead of using SHA1_HEADER, create a hash.h header and use #if and #elif to select the desired header. Have the Makefile pass an appropriate option to help the header select the right implementation to use. [jc: make BLK_SHA1 the fallback default as discussed on list, e.g. <20170314201424.vccij5z2ortq4a4o@sigill.intra.peff.net>; also remove SHA1_HEADER and SHA1_HEADER_SQ that are no longer used]. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com> 2017-03-12 01:28:18 +03:00			`#else /* SHA1_BLK */`
			`#include "block-sha1/sha1.h"`
			`#endif`

			`#endif`