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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
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@@ -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) {