path: root/sha1dc
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authorJeff King <>2017-03-16 22:09:12 (GMT)
committerJunio C Hamano <>2017-03-17 17:40:25 (GMT)
commit8325e43b82dd0bd00c37abed45861bb8c155b022 (patch)
treef5a4c16bfbf0862e7ab59ff04c92202957f4c15c /sha1dc
parentc0c20060af3dfc55258e0b9168e0d9c564b5bdfd (diff)
Makefile: add DC_SHA1 knob
This knob lets you use the sha1dc implementation from: 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 <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 'sha1dc')
2 files changed, 35 insertions, 0 deletions
diff --git a/sha1dc/sha1.c b/sha1dc/sha1.c
index 8ff2321..6dd0da3 100644
--- a/sha1dc/sha1.c
+++ b/sha1dc/sha1.c
@@ -1786,3 +1786,23 @@ int SHA1DCFinal(unsigned char output[20], SHA1_CTX *ctx)
output[19] = (unsigned char)(ctx->ihv[4]);
return ctx->found_collision;
+void git_SHA1DCFinal(unsigned char hash[20], SHA1_CTX *ctx)
+ if (!SHA1DCFinal(hash, ctx))
+ return;
+ die("SHA-1 appears to be part of a collision attack: %s",
+ sha1_to_hex(hash));
+void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *vdata, unsigned long len)
+ const char *data = vdata;
+ /* We expect an unsigned long, but sha1dc only takes an int */
+ while (len > INT_MAX) {
+ SHA1DCUpdate(ctx, data, INT_MAX);
+ data += INT_MAX;
+ len -= INT_MAX;
+ }
+ SHA1DCUpdate(ctx, data, len);
diff --git a/sha1dc/sha1.h b/sha1dc/sha1.h
index 7d4d423..bd8bd92 100644
--- a/sha1dc/sha1.h
+++ b/sha1dc/sha1.h
@@ -100,6 +100,21 @@ void SHA1DCUpdate(SHA1_CTX*, const char*, size_t);
/* returns: 0 = no collision detected, otherwise = collision found => warn user for active attack */
int SHA1DCFinal(unsigned char[20], SHA1_CTX*);
+ * Same as SHA1DCFinal, but convert collision attack case into a verbose die().
+ */
+void git_SHA1DCFinal(unsigned char [20], SHA1_CTX *);
+ * Same as SHA1DCUpdate, but adjust types to match git's usual interface.
+ */
+void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *data, unsigned long len);
+#define platform_SHA_CTX SHA1_CTX
+#define platform_SHA1_Init SHA1DCInit
+#define platform_SHA1_Update git_SHA1DCUpdate
+#define platform_SHA1_Final git_SHA1DCFinal
#if defined(__cplusplus)