Write out precomputed hash with ECDSA or DSA

Question

Write out precomputed hash with ECDSA or DSA

I am playing with Crypto ++ subscribers and using the following code straight from the wiki:

ECDSA<ECP, SHA256>::PrivateKey privateKey;
const Integer D(string("8964e19c5ae38669db3047f6b460863f5dc6c4510d3427e33545caf9527aafcf").c_str());
privateKey.Initialize(CryptoPP::ASN1::secp256r1(), D);
if (!privateKey.Validate(rng, 3)) {
    cerr << "ECDSA privateKey key validation failed after setting private parameter." << endl;
    return 1;
}

ECDSA<ECP,SHA256>::Signer signer(privateKey);
StringSource ss1(message, true,
    new SignerFilter(rng, signer,
        new HexEncoder(new StringSink(signature), false)
    ) // SignerFilter
); // StringSource
int slen = signature.length() / 2;
// since it IEEE P1363 format to display r and s:
cout << signature.substr(0, slen) << "\n"
     << signature.substr(slen, slen) << endl;

Now, I would like to know how I can override SHA256 there to directly specify the digest value that I want to pass to the signature algorithm.

I dug up the wiki and doxigenic documentation but with no success. At first I thought that maybe NullHash could help there, but it really is only a zero hash per source. I also had hope with PK_MessageAccumulator , but it doesn't work as I expected.

So, is there some sort of "identity" function inheriting from the HashTransformation class that I completely missed?
If not, how would you go about it by building something like this to indicate that the digest should be signed directly?

+3

c ++ digital-signature crypto ++

Lery Jul 19 17 at 9:38 am

source to share

1 answer

jww · Accepted Answer · 2017-07-19T15:54:43+0000

H(M)=M

can work. Would it be possible to submit such a custom HashTransformation

one ECDSA<ECP,H>::Signer

?

Yes. The program is shown below. It provides a class IdentityHash

that copies the input for output. A template parameter is required to specify the size of the hash.

But be careful. The message is formatted after it has been hashed. In fact, we have to_sign = MF(H(M))

.

$ cat test.cxx
#include "cryptlib.h"
#include "secblock.h"
#include "eccrypto.h"
#include "osrng.h"
#include "oids.h"
#include "hex.h"

#include <iostream>
#include <string>

using namespace CryptoPP;

template <unsigned int HASH_SIZE = 32>
class IdentityHash : public HashTransformation
{
public:
    CRYPTOPP_CONSTANT(DIGESTSIZE = HASH_SIZE)
    static const char * StaticAlgorithmName()
    {
        return "IdentityHash";
    }

    IdentityHash() : m_digest(HASH_SIZE), m_idx(0) {}

    virtual unsigned int DigestSize() const
    {
        return DIGESTSIZE;
    }

    virtual void Update(const byte *input, size_t length)
    {
        size_t s = STDMIN(STDMIN<size_t>(DIGESTSIZE, length),
                                         DIGESTSIZE - m_idx);    
        if (s)
            ::memcpy(&m_digest[m_idx], input, s);
        m_idx += s;
    }

    virtual void TruncatedFinal(byte *digest, size_t digestSize)
    {
        if (m_idx != DIGESTSIZE)
            throw Exception(Exception::OTHER_ERROR, "Input size must be " + IntToString(DIGESTSIZE));

        ThrowIfInvalidTruncatedSize(digestSize);

        if (digest)
            ::memcpy(digest, m_digest, digestSize);

        m_idx = 0;
    }

private:
    SecByteBlock m_digest;
    size_t m_idx;
};

int main(int argc, char* argv[])
{
    AutoSeededRandomPool prng;

    ECDSA<ECP, IdentityHash<32> >::PrivateKey privateKey;
    privateKey.Initialize(prng, ASN1::secp256r1());

    std::string message;
    message.resize(IdentityHash<32>::DIGESTSIZE);
    ::memset(&message[0], 0xAA, message.size());

    ECDSA<ECP, IdentityHash<32> >::Signer signer(privateKey);
    std::string signature;

    StringSource ss(message, true,
                        new SignerFilter(prng, signer,
                            new HexEncoder(new StringSink(signature))
                        ) // SignerFilter
                    ); // StringSource

    std::cout << "Signature: " << signature << std::endl;

    return 0;
}

I know it compiles and produces output. I don't know if this is the correct way out:

skylake:cryptopp$ g++ test.cxx ./libcryptopp.a -o test.exe
skylake:cryptopp$ ./test.exe
Signature: cafb8fca487c7d5023fbc76ccf96f107f72a07fecca77254e8845a2c8f2ed0ee8b50b
8ee0702beb7572eaa30c8d250a7b082c79f2f02e58ccfb97d7091755e91

You can test IdentityHash

as follows. The class has IdentityHash

not changed from the previous example. Function completed main

.

$ cat test.cxx
#include "cryptlib.h"
#include "secblock.h"

#include <iostream>
#include <string>

using namespace CryptoPP;

template <unsigned int HASH_SIZE = 32>
class IdentityHash : public HashTransformation
{
public:
    CRYPTOPP_CONSTANT(DIGESTSIZE = HASH_SIZE)
    static const char * StaticAlgorithmName()
    {
        return "IdentityHash";
    }

    IdentityHash() : m_digest(HASH_SIZE), m_idx(0) {}

    virtual unsigned int DigestSize() const
    {
        return DIGESTSIZE;
    }

    virtual void Update(const byte *input, size_t length)
    {
        size_t s = STDMIN(STDMIN<size_t>(DIGESTSIZE, length),
                                         DIGESTSIZE - m_idx);    
        if (s)
            ::memcpy(&m_digest[m_idx], input, s);
        m_idx += s;
    }

    virtual void TruncatedFinal(byte *digest, size_t digestSize)
    {
        if (m_idx != DIGESTSIZE)
            throw Exception(Exception::OTHER_ERROR, "Input size must be " + IntToString(DIGESTSIZE));

        ThrowIfInvalidTruncatedSize(digestSize);

        if (digest)
            ::memcpy(digest, m_digest, digestSize);

        m_idx = 0;
    }

private:
    SecByteBlock m_digest;
    size_t m_idx;
};

int main(int argc, char* argv[])
{
    std::string message;
    message.resize(IdentityHash<32>::DIGESTSIZE);
    ::memset(&message[0], 'A', message.size());

    IdentityHash<32> hash;
    hash.Update((const byte*)message.data(), message.size());

    std::string digest(32, 0);
    hash.TruncatedFinal((byte*)digest.data(), digest.size());

    std::cout << "Message: " << message << std::endl;
    std::cout << " Digest: " << digest << std::endl;

    return 0;
}

It produces:

skylake:cryptopp$ g++ test.cxx ./libcryptopp.a -o test.exe
skylake:cryptopp$ ./test.exe
Message: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 Digest: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Write out precomputed hash with ECDSA or DSA

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