Zig Requires Chilkat v11.0.0+

Duplicate openssl rsautl -verify -inkey pubKey.pem -pubin -in rsautl.sig -out originalFile.dat

See more OpenSSL Examples

Demonstrates how to duplicate this OpenSSL command:

openssl rsautl -verify -inkey pubKey.pem -pubin -in rsautl.sig -out originalFile.dat

The signature file contains the contents of the original data, and the original data is output to "originalFile.dat".

The openssl rsautl command signs the data directly, and therefore the maximum amount of data that can signed is very small. (To sign any amount of data, use openssl dgst -sign.) The maximum amount of data that can be signed (with rsautl) is the key size minus the overhead for the padding. The overhead size depends on the padding. With OAEP padding, the overhead is 42 bytes. With the default PKCSv1.5 padding, the overhead is 11 bytes.

Given a 2048-bit RSA key (256 bytes) and using PKCSv1.5 padding, the max data size that can be signed is 256 - 11 = 245 bytes.

Zig

const std = @import("std");
const chilkat = @import("chilkat");

pub fn main(init: std.process.Init) !void {
    const alloc = init.arena.allocator();

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    const pub_key = try chilkat.PublicKey.init();
    defer pub_key.deinit();

    // Load the public key from an PEM file:
    pub_key.loadFromFile("pubKey.pem") catch {
        std.debug.print("{s}\n", .{try pub_key.getLastErrorText(alloc)});
        return;
    };

    // Load the signature.
    const bd = try chilkat.BinData.init();
    defer bd.deinit();
    try bd.loadFile("rsautl.sig");

    const rsa = try chilkat.Rsa.init();
    defer rsa.deinit();

    // Import the public key into the RSA component:
    rsa.usePublicKey(pub_key) catch {
        std.debug.print("{s}\n", .{try rsa.getLastErrorText(alloc)});
        return;
    };

    // OpenSSL uses big-endian.
    rsa.setLittleEndian(false);

    // If the signature is verified, then the signature contents of bd
    // are replaced with the extracted original data.
    rsa.verifyRawBd(bd) catch {
        std.debug.print("{s}\n", .{try rsa.getLastErrorText(alloc)});
        std.debug.print("The signature was invalid.\n", .{});
        return;
    };

    std.debug.print("The signature was verified.\n", .{});

    // Save the original data that was extracted from the signature:
    bd.writeFile("originalFile.dat") catch {
        std.debug.print("Failed to save extracted data.\n", .{});
        return;
    };
}