How to sign a file via Crypto++ and RSA - c++

Question

I'm trying to sign a file with my private key using the following function:
void Signer::SignFile(const std::string& file)
{
RSASS<PSSR, Whirlpool>::Signer signer(rsaPrivate);
// Encrypt and write
FileSource ss1(file.c_str(), true,
new SignerFilter(rng, signer,
new FileSink(file.c_str(), true)
, true));
}
As an outcome my target file gets overwritten with the signature. Why does that happen? Obviously I'd like to append it to the file, that's why I also added an extra "true" parameter for the "putMessage".

Answer 1

FileSource ss1(file.c_str(), true,
new SignerFilter(rng, signer,
new FileSink(file.c_str(), true), true));
I [personally] have never seen someone use the same file as a source and sink. Usually the file data and signature are separate like with a detached signature.
It looks like one file/two streams is implementation defined: C++ read and write to same file using different streams. I guess you should expect seemingly random results on different platforms.
As an outcome my target file gets overwritten with the signature. Why does that happen?
The FileSink opens with std::ios_base::tunc. Also see FileSink on the Crypto++ wiki.
You should probably do something like this. It saves the signature to an intermediate ByteQueue, and then writes the queue to the file once the file is closed.
#include "cryptlib.h"
#include "filters.h"
#include "osrng.h"
#include "files.h"
#include "pssr.h"
#include "rsa.h"
#include "whrlpool.h"
#include <iostream>
#include <fstream>
#include <string>
int main(int argc, char* argv[])
{
using namespace CryptoPP;
AutoSeededRandomPool prng;
std::string fname("test.bin");
///// Create test message /////
{
std::string msg("Yoda said, Do or do not. There is no try.");
std::ofstream out(fname.c_str());
out.write(msg.data(), msg.size());
}
///// Generate a key /////
RSASS<PSSR, Whirlpool>::Signer signer;
signer.AccessKey().GenerateRandomWithKeySize(prng, 2048);
///// Sign the message /////
ByteQueue queue;
{
FileSource source(fname.c_str(), true,
new SignerFilter(prng, signer,
new Redirector(queue)));
}
///// Append the signature /////
{
std::ofstream out(fname.c_str(), std::ios_base::out | std::ios_base::binary | std::ios_base::app);
queue.TransferTo(FileSink(out).Ref());
}
///// Create a verifier /////
RSASS<PSSR, Whirlpool>::Verifier verifier(signer);
///// Verify the message /////
{
FileSource source(fname.c_str(), true,
new SignatureVerificationFilter(verifier, NULLPTR,
SignatureVerificationFilter::THROW_EXCEPTION));
}
std::cout << "Verified signature on message" << std::endl;
return 0;
}
It does not crash and it appends the message as expected:
$ ./test.exe
Verified signature on message
$ hexdump -C test.bin
00000000 59 6f 64 61 20 73 61 69 64 2c 20 44 6f 20 6f 72 |Yoda said, Do or|
00000010 20 64 6f 20 6e 6f 74 2e 20 54 68 65 72 65 20 69 | do not. There i|
00000020 73 20 6e 6f 20 74 72 79 2e 19 f2 1c 8f f9 cb 2f |s no try......./|
00000030 f2 38 9f a8 3b a9 0b 8b 62 25 56 a8 ea 81 7e 60 |.8..;...b%V...~`|
00000040 22 55 38 ce 79 7f 32 95 a5 1a 75 c1 80 ad b2 c2 |"U8.y.2...u.....|
00000050 6f ce a5 f7 bd 4b d3 3f e4 b3 69 00 21 60 d7 09 |o....K.?..i.!`..|
00000060 a8 71 9b 5f 41 d6 66 b1 80 f1 de 00 26 19 34 01 |.q._A.f.....&.4.|
00000070 b3 65 1b 78 e2 32 71 be bc 07 25 78 36 6b 56 4e |.e.x.2q...%x6kVN|
00000080 26 4e 12 9e a8 bb 72 86 ee 0d 70 b2 f1 bd a3 2c |&N....r...p....,|
00000090 14 fd 12 61 35 98 4a 80 9f ee 3c 31 d3 70 26 0f |...a5.J...<1.p&.|
000000a0 73 a0 5d 36 ef 96 56 65 f8 ac 3a fb 44 c3 04 76 |s.]6..Ve..:.D..v|
000000b0 e5 2f ae 92 84 be 40 34 f6 4b b8 84 aa bd 67 74 |./....#4.K....gt|
000000c0 05 43 91 d2 e6 b1 50 dd 6d 64 47 cc 3e 3c 3a 9d |.C....P.mdG.><:.|
000000d0 67 ff 4f 38 c1 a5 a6 d5 92 45 bc 2d ff 96 30 3a |g.O8.....E.-..0:|
000000e0 1d 3a 42 4f 8c 13 2d 4c 3f e9 ad 08 a6 b3 5e fa |.:BO..-L?.....^.|
000000f0 46 08 24 17 43 ce ed ec f7 1a 38 62 e7 bf 42 93 |F.$.C.....8b..B.|
00000100 84 44 b6 05 22 9e e3 bd 80 a6 08 b0 34 d0 a4 89 |.D..".......4...|
00000110 78 48 20 7f 7b 33 1c 51 9d 48 b5 b7 f7 de 2f dd |xH .{3.Q.H..../.|
00000120 d7 74 7b af 04 cd 92 fc 1c |.t{......|
I was not able to get this to work. I'm pretty sure it is a dead end.
std::fstream inout(fname.c_str(), std::ios_base::in | std::ios_base::out | std::ios_base::binary | std::ios_base::app);
FileSource fs(inout, true,
new SignerFilter(prng, signer,
new FileSink(inout), true));
Obviously I'd like to append it to the file, that's why ...
Also, because you are using PSSR, you don't need the original message. PSSR is "probabilistic signature scheme with recovery". The message is include in the signature with a mask function.
You would need the original message with a SSA scheme. SSA is "signature scheme with appendix". In a SSA scheme you need to provide both the original message and the signature.
(From the comments) Here is an example that uses a Signature Scheme with Appendix (SSA). It also uses std::iftream and std::ofstream directly so FileSource and FileSink work as you expect. std::ofstream includes std::ios_base::app so the signature is appended.
#include "cryptlib.h"
#include "filters.h"
#include "osrng.h"
#include "files.h"
#include "oaep.h"
#include "rsa.h"
#include "sha.h"
#include <iostream>
#include <fstream>
#include <string>
int main(int argc, char* argv[])
{
using namespace CryptoPP;
AutoSeededRandomPool prng;
std::string fname("test.bin");
///// Create test message /////
{
std::string msg("Yoda said, Do or do not. There is no try.");
std::ofstream out(fname.c_str());
out.write(msg.data(), msg.size());
}
///// Generate a key /////
RSASS<PKCS1v15, SHA256>::Signer signer;
signer.AccessKey().GenerateRandomWithKeySize(prng, 2048);
{
///// Create fstreams for input and output /////
std::ifstream fin(fname.c_str(), std::ios_base::in | std::ios_base::binary);
std::ofstream fout(fname.c_str(), std::ios_base::out | std::ios_base::binary | std::ios_base::app);
///// Sign the message /////
FileSource source(fin, true,
new SignerFilter(prng, signer,
new FileSink(fout)));
}
///// Create a verifier /////
RSASS<PKCS1v15, SHA256>::Verifier verifier(signer);
///// Verify the message /////
{
FileSource source(fname.c_str(), true,
new SignatureVerificationFilter(verifier, NULLPTR,
SignatureVerificationFilter::THROW_EXCEPTION));
}
std::cout << "Verified signature on message" << std::endl;
return 0;
}
And then:
$ ./test.exe
Verified signature on message
$ hexdump -C test.bin
00000000 59 6f 64 61 20 73 61 69 64 2c 20 44 6f 20 6f 72 |Yoda said, Do or|
00000010 20 64 6f 20 6e 6f 74 2e 20 54 68 65 72 65 20 69 | do not. There i|
00000020 73 20 6e 6f 20 74 72 79 2e c7 b3 6f 84 1d fd bf |s no try...o....|
00000030 c7 c8 38 7c 89 b1 f3 42 ee 5e f8 10 de a8 01 7f |..8|...B.^......|
00000040 7f a5 24 3d 27 7e 55 16 bc 80 8b 21 21 75 3d ed |..$='~U....!!u=.|
00000050 41 05 84 b1 3d bf d3 ae 3a 2f a8 81 7a e7 e4 ae |A...=...:/..z...|
00000060 50 d7 9b 25 04 17 a6 a3 1d 12 e2 8e cd 7a 02 42 |P..%.........z.B|
00000070 91 c0 d7 fc 43 09 94 a2 66 d9 67 95 55 5e dc 8c |....C...f.g.U^..|
00000080 eb bc 20 af e8 5c d4 63 05 d4 2c 48 57 6d f1 fe |.. ..\.c..,HWm..|
00000090 26 16 80 c3 41 11 58 8e 8d b0 cb 48 95 b9 ed 94 |&...A.X....H....|
000000a0 84 cc 86 0f a4 7e a3 6a ff 0d 0d 24 17 82 13 94 |.....~.j...$....|
000000b0 54 cb 8a ca 04 1e 65 18 c3 ab a2 3f 4d 44 de 42 |T.....e....?MD.B|
000000c0 32 07 29 e4 95 83 cc ff 39 85 08 bf d5 61 46 db |2.).....9....aF.|
000000d0 e0 96 d6 69 25 b9 ce 1e 3e bc 63 81 e5 16 bd 12 |...i%...>.c.....|
000000e0 a0 78 02 19 60 96 80 36 7d a5 79 be 0f 45 54 f4 |.x..`..6}.y..ET.|
000000f0 92 af f0 d8 74 65 7d 45 98 c7 bb 7f 6e 9b e3 cd |....te}E....n...|
00000100 c0 60 91 0f 78 aa 7c 77 a7 f5 4e 7d 6e ed e1 4c |.`..x.|w..N}n..L|
00000110 8e 5e 96 ac cd 30 16 e0 2d be 9e 2d 68 d4 25 46 |.^...0..-..-h.%F|
00000120 86 77 87 be 68 ef 06 26 55 |.w..h..&U|