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author | 2015-11-23 17:48:48 +0100 | |
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committer | 2015-11-23 17:48:48 +0100 | |
commit | fca74d4bc3569506a6659880a89aa009dc11f552 (patch) | |
tree | 4cefd06af989608ea8ebd3bc6306889e2a1ad175 /moon-abe/pbc-0.5.14/doc/sigex.txt | |
parent | 840ac3ebca7af381132bf7e93c1e4c0430d6b16a (diff) |
moon-abe cleanup
Change-Id: Ie1259856db03f0b9e80de3e967ec6bd1f03191b3
Diffstat (limited to 'moon-abe/pbc-0.5.14/doc/sigex.txt')
-rw-r--r-- | moon-abe/pbc-0.5.14/doc/sigex.txt | 155 |
1 files changed, 0 insertions, 155 deletions
diff --git a/moon-abe/pbc-0.5.14/doc/sigex.txt b/moon-abe/pbc-0.5.14/doc/sigex.txt deleted file mode 100644 index dcfc8d5e..00000000 --- a/moon-abe/pbc-0.5.14/doc/sigex.txt +++ /dev/null @@ -1,155 +0,0 @@ -== Tutorial == - -This chapter walks through how one might implement the -Boneh-Lynn-Shacham (BLS) signature scheme using the PBC library. -It is based on the file `example/bls.c`. - -We have three groups 'G1', 'G2', 'GT' of prime order 'r', and a bilinear map -'e' that takes an element from 'G1' and an element from 'G2', and outputs an -element of 'GT'. We publish these along with the system parameter 'g', which is -a randomly chosen element of 'G2'. - -Alice wishes to sign a message. She generates her public and private keys as -follows. Her private key is a random element 'x' of 'Zr', and her corresponding -public key is 'g'^'x'^. - -To sign a message, Alice hashes the message to some element -'h' of 'G1', and then outputs the signature 'h'^'x'^. - -To verify a signature sigma, Bob checks that -'e'('h','g'^'x'^) = 'e'(sigma, 'g'). - -We now translate the above to C code using the PBC library. - -=== BLS signatures === - -First we include `pbc/pbc.h`: - - #include <pbc.h> - -Next we initialize a pairing: - - pairing_t pairing; - char param[1024]; - size_t count = fread(param, 1, 1024, stdin); - if (!count) pbc_die("input error"); - pairing_init_set_buf(pairing, param, count); - -Later we give pairing parameters to our program on standard input. Any file in -the `param` subdirectory will suffice, for example: - - $ bls < param/a.param - -We shall need several +element_t+ variables to hold the system parameters, keys -and other quantities. We declare them and initialize them, -.... -element_t g, h; -element_t public_key, secret_key; -element_t sig; -element_t temp1, temp2; - -element_init_G2(g, pairing); -element_init_G2(public_key, pairing); -element_init_G1(h, pairing); -element_init_G1(sig, pairing); -element_init_GT(temp1, pairing); -element_init_GT(temp2, pairing); -element_init_Zr(secret_key, pairing); -.... -generate system parameters, - - element_random(g); - -generate a private key, - - element_random(secret_key); - -and the corresponding public key. - - element_pow_zn(public_key, g, secret_key); - -When given a message to sign, we first compute its hash, using some standard -hash algorithm. Many libraries can do this, and this operation does not -involve pairings, so PBC does not provide functions for this step. For this -example, and our message has already been hashed, possibly using another -library. - -Say the message hash is "ABCDEF" (a 48-bit hash). We map these bytes to an -element h of G1, - - element_from_hash(h, "ABCDEF", 6); - -then sign it: - - element_pow_zn(sig, h, secret_key); - -To verify this signature, we compare the -outputs of the pairing applied to the signature and system parameter, -and the pairing applied to the message hash and public key. -If the pairing outputs match then the signature is valid. - -.... -pairing_apply(temp1, sig, g, pairing); -pairing_apply(temp2, h, public_key, pairing); -if (!element_cmp(temp1, temp2)) { - printf("signature verifies\n"); -} else { - printf("signature does not verify\n"); -} -.... - -=== Import/export === - -To be useful, at some stage the signature must be converted -to bytes for storage or transmission: - - int n = pairing_length_in_bytes_compressed_G1(pairing); - // Alternatively: - // int n = element_length_in_bytes_compressed(sig); - unsigned char *data = malloc(n); - element_to_bytes_compressed(data, sig); - -On the other end, the signature must be decompressed: - - element_from_bytes_compressed(sig, data); - -Eliding +_compressed+ in the above code -will also work but the buffer 'data' will be roughly twice as large. - -We can save more space by using the 'x'-coordinate of the signature only - - int n = pairing_length_in_bytes_x_only_G1(pairing); - // Alternative: - // int n = element_length_in_bytes_x_only(sig); - unsigned char *data = malloc(n); - element_to_bytes_compressed(data, sig); - -but then there is a complication during verification since two different -points have the same 'x'-coordinate. One way to solve this problem is to -guess one point and try to verify. If that fails, we try the other. -It can be shown that the pairing outputs of the two points are inverses -of each other, avoiding the need to compute a pairing the second time. -(In fact, there are even better ways to handle this.) -.... -int n = pairing_length_in_bytes_x_only_G1(pairing); -//int n = element_length_in_bytes_x_only(sig); -unsigned char *data = malloc(n); - -element_to_bytes_x_only(data, sig); - -element_from_bytes_x_only(sig, data) - -pairing_apply(temp1, sig, g, pairing); -pairing_apply(temp2, h, public_key, pairing); - -if (!element_cmp(temp1, temp2)) { - printf("signature verifies on first guess\n"); -} else { - element_invert(temp1, temp1); - if (!element_cmp(temp1, temp2)) { - printf("signature verifies on second guess\n"); - } else { - printf("signature does not verify\n"); - } -} -.... |