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Don't encrypt empty files in new repositories

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git has several problems with using smudge/clean filters
on empty files (see issue #53).  The easiest fix is to
just not encrypt empty files. Since it was already obvious
from the encrypted file length that a file was empty, skipping
empty files does not decrease security.

Since skipping empty files is a breaking change to the
git-crypt file format, we only do this on new repositories.
Specifically, we add a new critical header field to the key
file called skip_empty which is set in new keys.  We
skip empty files if and only if this field is present.

Closes: #53
Closes: #162
This commit is contained in:
Andrew Ayer
2020-07-29 08:57:22 -04:00
parent 7c129cdd38
commit 8ba75c4719
3 changed files with 26 additions and 1 deletions
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11
commands.cpp
View File

@@ -770,6 +770,10 @@ int clean (int argc, const char** argv)
return 1;
}
if (file_size == 0 && key_file.get_skip_empty()) {
return 0;
}
// We use an HMAC of the file as the encryption nonce (IV) for CTR mode.
// By using a hash of the file we ensure that the encryption is
// deterministic so git doesn't think the file has changed when it really
@@ -887,6 +891,11 @@ int smudge (int argc, const char** argv)
// Read the header to get the nonce and make sure it's actually encrypted
unsigned char header[10 + Aes_ctr_decryptor::NONCE_LEN];
std::cin.read(reinterpret_cast<char*>(header), sizeof(header));
if (std::cin.gcount() == 0 && key_file.get_skip_empty()) {
return 0;
}
if (std::cin.gcount() != sizeof(header) || std::memcmp(header, "\0GITCRYPT\0", 10) != 0) {
// File not encrypted - just copy it out to stdout
std::clog << "git-crypt: Warning: file not encrypted" << std::endl;
@@ -991,6 +1000,7 @@ int init (int argc, const char** argv)
std::clog << "Generating key..." << std::endl;
Key_file key_file;
key_file.set_key_name(key_name);
key_file.set_skip_empty(true);
key_file.generate();
mkdir_parent(internal_key_path);
@@ -1425,6 +1435,7 @@ int keygen (int argc, const char** argv)
std::clog << "Generating key..." << std::endl;
Key_file key_file;
key_file.set_skip_empty(true);
key_file.generate();
if (std::strcmp(key_file_name, "-") == 0) {

9
key.cpp
View File

@@ -232,6 +232,11 @@ void Key_file::load_header (std::istream& in)
key_name.clear();
throw Malformed();
}
} else if (field_id == HEADER_FIELD_SKIP_EMPTY) {
if (field_len != 0) {
throw Malformed();
}
skip_empty = true;
} else if (field_id & 1) { // unknown critical field
throw Incompatible();
} else {
@@ -256,6 +261,10 @@ void Key_file::store (std::ostream& out) const
write_be32(out, key_name.size());
out.write(key_name.data(), key_name.size());
}
if (skip_empty) {
write_be32(out, HEADER_FIELD_SKIP_EMPTY);
write_be32(out, 0);
}
write_be32(out, HEADER_FIELD_END);
for (Map::const_iterator it(entries.begin()); it != entries.end(); ++it) {
it->second.store(out);

7
key.hpp
View File

@@ -83,18 +83,23 @@ public:
void set_key_name (const char* k) { key_name = k ? k : ""; }
const char* get_key_name () const { return key_name.empty() ? 0 : key_name.c_str(); }
void set_skip_empty (bool v) { skip_empty = v; }
bool get_skip_empty () const { return skip_empty; }
private:
typedef std::map<uint32_t, Entry, std::greater<uint32_t> > Map;
enum { FORMAT_VERSION = 2 };
Map entries;
std::string key_name;
bool skip_empty = false;
void load_header (std::istream&);
enum {
HEADER_FIELD_END = 0,
HEADER_FIELD_KEY_NAME = 1
HEADER_FIELD_KEY_NAME = 1,
HEADER_FIELD_SKIP_EMPTY = 3 // If this field is present, empty files are left unencrypted (see issue #53)
};
enum {
KEY_FIELD_END = 0,