From e705502ad014949c8eaebee7b4b5d56c05607f11 Mon Sep 17 00:00:00 2001
From: Peter Eckersley <pde@eff.org>
Date: Thu, 19 Nov 2015 13:30:16 -0800
Subject: [PATCH] This might be useful.

---
 tools/half-sign.c | 117 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 117 insertions(+)
 create mode 100644 tools/half-sign.c

diff --git a/tools/half-sign.c b/tools/half-sign.c
new file mode 100644
index 000000000..561fa22be
--- /dev/null
+++ b/tools/half-sign.c
@@ -0,0 +1,117 @@
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <openssl/evp.h>
+#include <openssl/rsa.h>
+#include <openssl/pem.h>
+
+// Sign with SHA1
+#define HASH_SIZE 20
+
+void usage() {
+	printf("half-sign <private key file> [binary hash file]\n");
+	printf("\n");
+	printf("    Computes and prints a binary RSA signature over data given the SHA1 hash of\n");
+	printf("    the data as input.\n");
+	printf("\n");
+	printf("    <private key file> should be PEM encoded.\n");
+	printf("\n");
+	printf("    The input SHA1 hash should be %d bytes in length. If no binary hash file is\n", HASH_SIZE);
+	printf("    specified, it will be read from stdin.\n");
+	exit(1);
+}
+
+void sign_hashed_data(EVP_PKEY *signing_key, unsigned char *md, size_t mdlen) {
+	// cribbed from the openssl EVP_PKEY_sign man page
+	EVP_PKEY_CTX *ctx;
+	unsigned char *sig;
+	size_t siglen;
+	
+	/* NB: assumes signing_key, md and mdlen are already set up
+	 * and that signing_key is an RSA private key
+	 */
+	ctx = EVP_PKEY_CTX_new(signing_key, NULL);
+	if ((!ctx) 
+	|| (EVP_PKEY_sign_init(ctx) <= 0) 
+	|| (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
+	|| (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha1()) <= 0)) {
+		fprintf(stderr, "Failure establishing ctx for signature\n");
+		exit(1);
+	}
+
+	/* Determine buffer length */
+	if (EVP_PKEY_sign(ctx, NULL, &siglen, md, mdlen) <= 0) {
+		fprintf(stderr, "Unable to determine buffer length for signature\n");
+		exit(1);
+	}
+
+	sig = OPENSSL_malloc(siglen);
+
+	if (!sig) {
+		fprintf(stderr, "Malloc failed\n");
+		exit(1);
+	}
+
+	if (EVP_PKEY_sign(ctx, sig, &siglen, md, mdlen) <= 0) {
+		fprintf(stderr, "Signature error\n");
+		exit(1);
+	}
+
+	/* Signature is siglen bytes written to buffer sig */
+	fwrite(sig, siglen, 1, stdout);
+}
+
+EVP_PKEY *read_private_key(char *filename) {
+	FILE *keyfile;
+	EVP_PKEY *privkey;
+	keyfile = fopen(filename, "r");
+	if (!keyfile) {
+		fprintf(stderr, "Failed to open private key.pem file %s\n", filename);
+		exit(1);
+	}
+	privkey = PEM_read_PrivateKey(keyfile, NULL, NULL, NULL);
+	if (!privkey) {
+		fprintf(stderr, "Failed to read PEM private key from %s\n", filename);
+		exit(1);
+	}
+	if (EVP_PKEY_type(privkey->type) != EVP_PKEY_RSA) {
+		fprintf(stderr, "%s was a non-RSA key\n", filename);
+		exit(1);
+	}
+	return privkey;
+}
+
+int main(int argc, char *argv[]) {
+	FILE *input;
+	unsigned char *buffer;
+	int test;
+	EVP_PKEY *privkey;
+	if (argc > 3 || argc < 2)
+		usage();
+	if (argc < 3 || strcmp(argv[2],"-") == 0) 
+		input = stdin;
+	else {
+		input = fopen(argv[2], "r");
+		if (!input) usage();
+	}
+	privkey = read_private_key(argv[1]);
+	buffer = malloc(HASH_SIZE); 
+	if (!buffer) {
+		fprintf(stderr, "Argh, malloc failed\n");
+		exit(1);
+	}
+	if (fread(buffer, HASH_SIZE, 1, input) != 1) {
+		perror("half-sign: Failed to read SHA1 from input\n");
+		exit(1);
+	}
+
+	test = fgetc(input);
+	if (test != EOF && test != '\n') {
+		fprintf(stderr,"Error, more than %d bytes fed to half-sign\n", HASH_SIZE);
+		fprintf(stderr,"Last byte was :%d\n" , (int) test);
+		exit(1);
+	}
+	sign_hashed_data(privkey, buffer, HASH_SIZE);
+	return 0;
+}