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I noticed this by stress testing my tls server implementation. From time to time curl (and other tools: ab, vegeta) will report invalid signature. I trace the problem to the way how std lib is encoding raw signature into der format. Using raw signature I got in some cases different encoding using std and openssl. Std is not producing minimal der when signature `r` or `s` integers has leading zero(es).
Here is an example to illustrate difference. Notice leading 00 in `s`
integer which is removed in openssl encoding but not in std encoding.
```Zig
const std = @import("std");
test "ecdsa signature to der" {
// raw signature r and s bytes
const raw = hexToBytes(
\\ 49 63 0c 94 95 2e ff 4b 02 bf 35 c4 97 9e a7 24
\\ 20 dc 94 de aa 1b 17 ff e1 49 25 3e 34 ef e8 d0
\\ c4 43 aa 7b a9 f3 9c b9 f8 72 7d d7 0c 9a 13 1e
\\
\\ 00 56 85 43 d3 d4 05 62 a1 1d d8 a1 45 44 b5 dd
\\ 62 9f d1 e0 ab f1 cd 4a 85 d0 1f 5d 11 d9 f8 89
\\ 89 d4 59 0c b0 6e ea 3c 19 6a f7 0b 1a 4a ce f1
);
// encoded by openssl
const expected = hexToBytes(
\\ 30 63 02 30
\\ 49 63 0c 94 95 2e ff 4b 02 bf 35 c4 97 9e a7 24
\\ 20 dc 94 de aa 1b 17 ff e1 49 25 3e 34 ef e8 d0
\\ c4 43 aa 7b a9 f3 9c b9 f8 72 7d d7 0c 9a 13 1e
\\
\\ 02 2f
\\ 56 85 43 d3 d4 05 62 a1 1d d8 a1 45 44 b5 dd
\\ 62 9f d1 e0 ab f1 cd 4a 85 d0 1f 5d 11 d9 f8 89
\\ 89 d4 59 0c b0 6e ea 3c 19 6a f7 0b 1a 4a ce f1
);
// encoded by std
const actual = hexToBytes(
\\ 30 64 02 30
\\ 49 63 0c 94 95 2e ff 4b 02 bf 35 c4 97 9e a7 24
\\ 20 dc 94 de aa 1b 17 ff e1 49 25 3e 34 ef e8 d0
\\ c4 43 aa 7b a9 f3 9c b9 f8 72 7d d7 0c 9a 13 1e
\\
\\ 02 30
\\ 00 56 85 43 d3 d4 05 62 a1 1d d8 a1 45 44 b5 dd
\\ 62 9f d1 e0 ab f1 cd 4a 85 d0 1f 5d 11 d9 f8 89
\\ 89 d4 59 0c b0 6e ea 3c 19 6a f7 0b 1a 4a ce f1
);
_ = actual;
const Ecdsa = std.crypto.sign.ecdsa.EcdsaP384Sha384;
const sig = Ecdsa.Signature.fromBytes(raw);
var buf: [Ecdsa.Signature.der_encoded_length_max]u8 = undefined;
const encoded = sig.toDer(&buf);
try std.testing.expectEqualSlices(u8, &expected, encoded);
}
pub fn hexToBytes(comptime hex: []const u8) [removeNonHex(hex).len / 2]u8 {
@setEvalBranchQuota(1000 * 100);
const hex2 = comptime removeNonHex(hex);
comptime var res: [hex2.len / 2]u8 = undefined;
_ = comptime std.fmt.hexToBytes(&res, hex2) catch unreachable;
return res;
}
fn removeNonHex(comptime hex: []const u8) []const u8 {
@setEvalBranchQuota(1000 * 100);
var res: [hex.len]u8 = undefined;
var i: usize = 0;
for (hex) |c| {
if (std.ascii.isHex(c)) {
res[i] = c;
i += 1;
}
}
return res[0..i];
}
```
Trimming leading zeroes from signature integers fixes encoding.