Можно ли вычислить алгоритм SHA-1 в потоке? С низким объемом памяти?

Да, это вполне возможно. SHA-1 - это блочный алгоритм, поэтому он работает с одним блоком за раз. Точный размер блока зависит от размера создаваемого вами хэша, но он всегда довольно мал (например, 20-50 байт или около того). Это, конечно, предполагает, что вы хотите включить SHA-1 256, 384 и / или 512 (также известные как SHA-256, SHA-384 и SHA-512). Если вы включаете только исходную 160-битную версию, размер блока всегда составляет 20 байт (160 бит).

Изменить: упс - перечитывая спецификацию, размеры блока составляют 512 бит для SHA-1, SHA-224, SHA-256 и 1024 бит для SHA-384 и SHA-512. Спасибо, Чарльз!

Edit2: Я почти забыл о той части, где он ищет исходный код, а не просто совет. Вот код. Сначала заголовок:

// Sha.h:
#ifndef SHA_1_H_INCLUDED
#define SHA_1_H_INCLUDED

// This is a relatively straightforward implementation of SHA-1. It makes no particular
// attempt at optimization, instead aiming toward easy verification against the standard.
// To that end, many of the variable names are identical to those used in FIPS 180-2 and
// FIPS 180-3. 
//
// The code should be fairly portable, within a few limitations:
// 1. It requires that 'char' have 8 bits. In theory this is avoidable, but I don't think
// it's worth the bother.
// 2. It only deals with inputs in (8-bit) bytes. In theory, SHA-1 can deal with a number of 
// bits that's not a multiple of 8, but I've never needed it. Since the padding always results
// in a byte-sized stream, the only parts that would need changing would be reading and padding
// the input. The main hashing portion would be unaffected.
//
// Compiles cleanly with:
//    MS VC++ 9.0SP1 (x86 or x64): -W4 -Za
//    gc++ 3.4: -ansi -pedantic -Wall
//    comeau 4.3.3: --vc71 
// Appears to work corectly in all cases. 
// You can't use maximum warnings with Comeau though -- this code itself doesn't give problems
// (that I know of) but Microsoft's headers give it *major* heartburn.
// 
//
// Written by Jerry Coffin, February 2008
// 
// You can use this software any way you want to, with following limitations
// (shamelessly stolen from the Boost software license):
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
// SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
// FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
// 
// If you put this to real use, I'd be happy to hear about it. If you find a bug, 
// I'd be interested in hearing about that too. There's even a pretty good chance 
// that I'll try to fix it, though I certainly can't guarantee that.
// 
#include <algorithm>
#include <vector>
#include <string>
#include <assert.h>
#include <iostream>
#include <sstream>
#include <iomanip>

#if defined(_MSC_VER) && _MSC_VER < 1600
typedef unsigned int uint32_t;
typedef unsigned __int64 uint64_t;
#else
#include <stdint.h>
#endif

namespace crypto { 
namespace {
    struct ternary_operator { 
        virtual uint32_t operator()(uint32_t x, uint32_t y, uint32_t z) = 0;
    };
}

class sha1 { 
    static const size_t hash_size = 5;
    static const size_t min_pad = 64;
    static const size_t block_bits = 512;
    static const size_t block_bytes = block_bits / 8;
    static const size_t block_words = block_bytes / 4;

    std::vector<uint32_t> K;
    std::vector<uint32_t> H;
    std::vector<uint32_t> W;
    std::vector<ternary_operator *> fs;
    uint32_t a, b, c, d, e, T;
    static const size_t block_size = 16;
    static const size_t bytes_per_word = 4;
    size_t total_size;

    // hash a 512-bit block of input.
    //
    void hash_block(std::vector<uint32_t> const &block);

    // Pad the input to a multiple of 512 bits, and add the length
    // in binary to the end.
    static std::string pad(std::string const &input, size_t size);

    // Turn 64 bytes into a block of 16 uint32_t's.
    std::vector<uint32_t> make_block(std::string const &in);

public:

    // Construct a SHA-1 object. More expensive that typical 
    // ctor, but not expected to be copied a lot or anything
    // like that, so it should be fairly harmless.
    sha1();

    // The two ways to provide input for hashing: as a stream or a string.
    // Either way, you get the result as a vector<uint32_t>. It's a fairly
    // small vector, so even if your compiler doesn't do return-value 
    // optimization, the time taken for copying it isn't like to be 
    // significant.
    //
    std::vector<uint32_t> operator()(std::istream &in);
    std::vector<uint32_t> operator()(std::string const &input);

    friend std::ostream &operator<<(std::ostream &os, sha1 const &s);
};
}

#endif

И реализация:

// Sha1.cpp:
#include "sha.h"
// Please see comments in sha.h for licensing information, etc.
//

// Many people don't like the names I usually use for namespaces, so I've kept this one
// short and simple.
//
namespace crypto {
namespace {  
uint32_t ROTL(uint32_t const &value, unsigned bits) { 
    uint32_t mask = (1 << bits) - 1;
    return value << bits | (value >> (32-bits))&mask;
}

struct f1 : ternary_operator {
    uint32_t operator()(uint32_t x, uint32_t y, uint32_t z) { 
        return (x & y) ^ (~x&z);
    }
};

struct f2 : ternary_operator {
    uint32_t operator()(uint32_t x, uint32_t y, uint32_t z) {
        return x ^ y ^ z;
    }
};

struct f3 : ternary_operator {
    uint32_t operator()(uint32_t x, uint32_t y, uint32_t z) {
        return (x&y) ^ (x&z) ^ (y&z);
    }
};

uint32_t word(int a, int b, int c, int d) {
    a &= 0xff;
    b &= 0xff;
    c &= 0xff;
    d &= 0xff;
    int val =  a << 24 | b << 16 | c << 8 | d;
    return val;
}
}

// hash a 512-bit block of input.
//
void sha1::hash_block(std::vector<uint32_t> const &block) {
    assert(block.size() == block_words);

    int t;
    std::copy(block.begin(), block.end(), W.begin());
    for (t=16; t<80; t++) {
        W[t] = ROTL(W[t-3] ^ W[t-8] ^ W[t-14] ^ W[t-16], 1);
    }

    a = H[0]; b = H[1]; c = H[2]; d = H[3]; e = H[4];

    for (t=0; t<80; t++) {
        T = ROTL(a, 5) + (*fs[t])(b, c, d) + e + K[t] + W[t];
        e = d;
        d = c;
        c = ROTL(b, 30);
        b = a;
        a = T;
    }
    H[0] += a; H[1] += b; H[2] += c; H[3] += d; H[4] += e;
}

// Pad the input to a multiple of 512 bits, and put the length
// in binary at the end. 
std::string sha1::pad(std::string const &input, size_t size) {
    size_t length = size * 8 + 1;
    size_t remainder = length % block_bits;
    size_t pad_len = block_bits-remainder;

    if (pad_len < min_pad)
        pad_len += block_bits;
    ++pad_len;

    pad_len &= ~7;
    std::string padding(pad_len/8, '\0');

    for (size_t i=0; i<sizeof(padding.size()); i++)
        padding[padding.size()-i-1] = (length-1) >> (i*8) & 0xff;
    padding[0] |= (unsigned char)0x80;

    std::string ret(input+padding);
    return ret;
}

// Turn 64 bytes into a block of 16 uint32_t's.
std::vector<uint32_t> sha1::make_block(std::string const &in) { 
    assert(in.size() >= block_bytes);

    std::vector<uint32_t> ret(block_words);

    for (size_t i=0; i<block_words; i++) {
        size_t s = i*4;
        ret[i] = word(in[s], in[s+1], in[s+2], in[s+3]);
    }
    return ret;
}


// Construct a SHA-1 object. More expensive that typical 
// ctor, but not expected to be copied a lot or anything
// like that, so it should be fairly harmless.
sha1::sha1() : K(80), H(5), W(80), fs(80), total_size(0) {
    static const uint32_t H0[] = { 
        0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476, 0xc3d2e1f0
    };
    static const uint32_t Ks[] = {
        0x5a827999, 0x6ed9eba1, 0x8f1bbcdc, 0xca62c1d6
    };

    std::copy(H0, H0+hash_size, H.begin());

    std::fill_n(K.begin()+00, 20, Ks[0]);
    std::fill_n(K.begin()+20, 20, Ks[1]);
    std::fill_n(K.begin()+40, 20, Ks[2]);
    std::fill_n(K.begin()+60, 20, Ks[3]);

    static f1 sf1;
    static f2 sf2;
    static f3 sf3;

    std::fill_n(fs.begin()+00, 20, &sf1);
    std::fill_n(fs.begin()+20, 20, &sf2);
    std::fill_n(fs.begin()+40, 20, &sf3);
    std::fill_n(fs.begin()+60, 20, &sf2);
}

// The two ways to provide input for hashing: as a stream or a string.
// Either way, you get the result as a vector<uint32_t>. It's a fairly
// small vector, so even if your compiler doesn't do return-value 
// optimization, the time taken for copying it isn't like to be 
// significant.
// 
std::vector<uint32_t> sha1::operator()(std::string const &input) { 
    std::string temp(pad(input, total_size + input.size()));
    std::vector<uint32_t> block(block_size);

    size_t num = temp.size()/block_bytes;

    for (unsigned block_num=0; block_num<num; block_num++) {
        size_t s;
        for (size_t i=0; i<block_size; i++) {
            s = block_num*block_bytes+i*4;
            block[i] = word(temp[s], temp[s+1], temp[s+2], temp[s+3]);
        }
        hash_block(block);  
    }
    return H;
}

std::vector<uint32_t> sha1::operator()(std::istream &in) { 
    char raw_block[65];

    while (in.read(raw_block, block_bytes)) {
        total_size += block_bytes;
        std::string b(raw_block, in.gcount());
        hash_block(make_block(b));
    }
    std::string x(raw_block, in.gcount());
    return operator()(x);
}

std::ostream &operator<<(std::ostream &os, sha1 const &s) { 
    // Display a SHA-1 result in hex.
    for (size_t i=0; i<(s.H).size(); i++)
        os << std::fixed << std::setprecision(8) << std::hex << std::setfill('0') << (s.H)[i] << " ";
    return os << std::dec << std::setfill(' ') << "\n";
}

}

#ifdef TEST
#include <iostream>
#include <iomanip>
#include <string>
#include <sstream>

// A minimal test harness to check that it's working correctly. Strictly black-box
// testing, with no attempt at things like coverage analysis. Nonetheless, I believe
// it should cover most of the code -- the core hashing code all gets used for every
// possible value. The padding code should be tested fairly thoroughly as well -- the
// first test is a fairly simple case, and the second the more complex one (where the 
// padding requires adding another block).
class tester {
    bool verify(uint32_t *test_val, std::vector<uint32_t> const &hash, std::ostream &os) {
        // Verify that a result matches a test value and report result.
        for (size_t i=0; i<hash.size(); i++)
            if (hash[i] != test_val[i]) {
                os << "Mismatch. Expected: " << test_val[i] << ", but found: " << hash[i] << "\n";
                return false;
            }
            os << "Message digest Verified.\n\n";
            return true;
    }

public:

    bool operator()(uint32_t *test_val, std::string const &input) {
        std::cout << "Testing hashing from string:\n\"" << input << "\"\n";
        crypto::sha1 hasher1;
        std::vector<uint32_t> hash = hasher1(input);
        std::cout << "Message digest is:\n\t" << hasher1;
        bool verified = verify(test_val, hash, std::cerr);

        crypto::sha1 hasher2;
        std::cout << "Testing hashing from Stream:\n";
        std::istringstream buf(input);
        hash = hasher2(buf);
        std::cout << "Message digest is:\n\t" << hasher2;

        return verified & verify(test_val, hash, std::cerr);
    }
};

int main() {
    // These test values and results come directly from the FIPS pub.
    //
    char const *input1 = "abc";
    char const *input2 = "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq";
    uint32_t result1[] = {0xA9993E36, 0x4706816A, 0xBA3E2571, 0x7850C26C, 0x9CD0D89D};
    uint32_t result2[] = {0x84983E44, 0x1C3BD26E, 0xBAAE4AA1, 0xF95129E5, 0xE54670F1 };
    bool correct = tester()(result1, input1);
    correct &= tester()(result2, input2);
    if (correct)
        std::cerr << "All Tests passed!\n";
    else
        std::cerr << "Test Failed!\n";
}
#elif defined(MAIN) 

#include <sstream>
#include <fstream>
#include <iostream>

int main(int argc, char **argv) { 
    if (argc < 2) {
        std::cerr << "Usage: sha1 [filename]\n";
        return EXIT_FAILURE;
    }
    for (int i=1; i<argc; i++) {
        crypto::sha1 hash;
        std::ifstream in(argv[i], std::ios_base::binary);
        if (in.good()) {
            hash(in);
            std::cout << "SHA-1(" << argv[i] << ") = " << hash << "\n";
        }
    }
    return 0;
}
#endif