path: root/crypt_common.c

/* SPDX-License-Identifier: GPL-2.0 */

/** \file crypt_common.c Crypto functions independent of openssl/libgcrypt. */

#include "para.h"
#include "error.h"
#include "string.h"
#include "crypt.h"
#include "crypt_backend.h"
#include "portable_io.h"
#include "fd.h"

static const unsigned char base64_tab[256] = {
	255, 255, 255, 255, 255, 255, 255, 255, /* 00-07 */
	255, 255, 255, 255, 255, 255, 255, 255, /* 08-0f */
	255, 255, 255, 255, 255, 255, 255, 255, /* 10-17 */
	255, 255, 255, 255, 255, 255, 255, 255, /* 18-1f */
	255, 255, 255, 255, 255, 255, 255, 255, /* 20-2f */
	255, 255, 255,  62, 255, 255, 255,  63, /* 28-2f */
	52 ,  53,  54,  55,  56,  57,  58,  59, /* 30-37 */
	60 ,  61, 255, 255, 255, 255, 255, 255, /* 38-3f */
	255,   0,   1,   2,   3,   4,   5,   6, /* 40-47 */
	7  ,   8,   9,  10,  11,  12,  13,  14, /* 48-4f */
	15 ,  16,  17,  18,  19,  20,  21,  22, /* 50-57 */
	23 ,  24,  25, 255, 255, 255, 255, 255, /* 58-5f */
	255,  26,  27,  28,  29,  30,  31,  32, /* 60-6f */
	33 ,  34,  35,  36,  37,  38,  39,  40, /* 68-6f */
	41 ,  42,  43,  44,  45,  46,  47,  48, /* 70-77 */
	49 ,  50,  51, 255, 255, 255, 255, 255, /* 78-7f */
	255, 255, 255, 255, 255, 255, 255, 255, /* 80-87 */
	255, 255, 255, 255, 255, 255, 255, 255, /* 88-8f */
	255, 255, 255, 255, 255, 255, 255, 255, /* 90-97 */
	255, 255, 255, 255, 255, 255, 255, 255, /* 98-9f */
	255, 255, 255, 255, 255, 255, 255, 255, /* a0-a7 */
	255, 255, 255, 255, 255, 255, 255, 255, /* a8-af */
	255, 255, 255, 255, 255, 255, 255, 255, /* b0-b7 */
	255, 255, 255, 255, 255, 255, 255, 255, /* b8-bf */
	255, 255, 255, 255, 255, 255, 255, 255, /* c0-c7 */
	255, 255, 255, 255, 255, 255, 255, 255, /* c8-cf */
	255, 255, 255, 255, 255, 255, 255, 255, /* d0-d7 */
	255, 255, 255, 255, 255, 255, 255, 255, /* d8-df */
	255, 255, 255, 255, 255, 255, 255, 255, /* e0-e7 */
	255, 255, 255, 255, 255, 255, 255, 255, /* e8-ef */
	255, 255, 255, 255, 255, 255, 255, 255, /* f0-f7 */
	255, 255, 255, 255, 255, 255, 255, 255, /* f8-ff */
};

/** Maximal possible size of the decoded data. */
#define BASE64_MAX_DECODED_SIZE(_encoded_size) ((_encoded_size) / 4 * 3)

/*
 * The padding character which is appended to base64 encoded data.
 *
 * base64 encoded data is always an ascii string whose length is a multiple of
 * four. If the number of characters needed to encode the data is not a
 * multiple of four, the encoded string is padded with this character. The
 * number of pad characters is either 0, 1 or 2, but never 3, and this number
 * is a function of the number of bytes of the unencrypted data modulo 3.
 */
#define PAD64 '='

/*
 * This function is derived from openssh-5.2p1, Copyright (c) 1996 by Internet
 * Software Consortium. Portions Copyright (c) 1995 by International Business
 * Machines, Inc.
 *
 * The function skips all whitespace anywhere and converts four characters
 * at a time. It is OK to pass a NULL pointer as decoded_size. The result
 * is terminated with a NUL byte.
 */
static int base64_decode(char const *src, size_t encoded_size, char **result,
		size_t *decoded_size)
{
	size_t i, j, state; /* source/target indices */
	const char *end = src + encoded_size, *p;
	unsigned char *target, uch;

	target = alloc(BASE64_MAX_DECODED_SIZE(encoded_size) + 1);

	for (
		i = 0, j = 0, state = 0;
		i < encoded_size && (uch = src[i]) != '\0';
		i++
	) {
		if (para_isspace(uch)) /* Skip whitespace anywhere. */
			continue;
		if (uch == PAD64)
			break;
		if (base64_tab[uch] == 255) /* A non-base64 character. */
			goto fail;
		uch = base64_tab[uch];
		switch (state) {
		case 0:
			target[j] = uch << 2;
			break;
		case 1:
			target[j] |= uch >> 4;
			j++;
			target[j] = (uch & 0x0f) << 4;
			break;
		case 2:
			target[j] |= uch >> 2;
			j++;
			target[j] = (uch & 0x03) << 6;
			break;
		case 3:
			target[j] |= uch;
			j++;
			break;
		}
		state = (state + 1) % 4;
	}
	p = (i < encoded_size)? src + i : NULL;
	/*
	 * We are done decoding Base-64 chars.  Let's see if we ended
	 * on a byte boundary, and/or with erroneous trailing characters.
	 */
	if (p && *p == PAD64) { /* We got a pad char. Skip it, get next. */
		p++;
		switch (state) {
		case 0: /* Invalid = in first position */
		case 1: /* Invalid = in second position */
			goto fail;

		case 2: /* Valid, means one byte of info */
			/* Skip any number of spaces. */
			for (; p < end && *p != '\0'; p++)
				if (!para_isspace(*p))
					break;
			/* Make sure there is another trailing = sign. */
			if (*p != PAD64)
				goto fail;
			/* Fall through to "single trailing =" case. */
			p++;

		case 3: /* Valid, means two bytes of info */
			/*
			 * We know this char is an =.  Is there anything but
			 * whitespace after it?
			 */
			for (; p < end && *p != '\0'; p++)
				if (!para_isspace(*p))
					goto fail;
			/*
			 * Now make sure for cases 2 and 3 that the "extra"
			 * bits that slopped past the last full byte were
			 * zeros.  If we don't check them, they become a
			 * subliminal channel.
			 */
			if (target[j] != 0)
				goto fail;
		}
	} else {
		/*
		 * We ended by seeing the end of the string.  Make sure we
		 * have no partial bytes lying around.
		 */
		if (state != 0)
			goto fail;
	}
	/* success */
	target[j] = '\0'; /* just to be sure */
	if (decoded_size)
		*decoded_size = j;
	*result = (char *)target;
	return 1;
fail:
	free(target);
	return -E_BASE64;
}

/* Decode a buffer using the uuencode Base64 algorithm. */
static int uudecode(char const *src, size_t encoded_size, char **result,
		size_t *decoded_size)
{
	const char *end = src + encoded_size, *p;

	/* skip whitespace and data */
	for (p = src; p < end && (*p == ' ' || *p == '\t'); p++)
		;
	for (; p < end && *p != '\0' && *p != ' ' && *p != '\t'; p++)
		;
	/* and remove trailing whitespace because base64_decode needs this */
	return base64_decode(src, p - src, result, decoded_size);
}

/** If the key begins with this text, we treat it as an ssh key. */
#define KEY_TYPE_TXT "ssh-rsa"

/*
 * Check if the given buffer starts with an ssh rsa key signature.
 *
 * Returns number of header bytes to be skipped on success, zero if no ssh rsa
 * signature was found.
 */
static size_t is_ssh_rsa_key(char *data, size_t size)
{
	char *cp;

	if (size < strlen(KEY_TYPE_TXT) + 2)
		return 0;
	cp = memchr(data, ' ', size);
	if (cp == NULL)
		return 0;
	if (strncmp(KEY_TYPE_TXT, data, strlen(KEY_TYPE_TXT)))
		return 0;
	cp++;
	if (cp >= data + size)
		return 0;
	if (*cp == '\0')
		return 0;
	return cp - data;
}

/*
 * Perform some sanity checks on the decoded ssh key.
 *
 * This function returns the size of the header. Usually, the header is 11
 * bytes long: four bytes for the length field, and the string "ssh-rsa".
 */
static int check_ssh_key_header(const unsigned char *blob, int blen)
{
	const unsigned char *p = blob, *end = blob + blen;
	uint32_t rlen;

	if (p + 4 > end)
		return -E_SSH_KEY_HEADER;
	rlen = read_u32_be(p);
	p += 4;
	if (p + rlen < p)
		return -E_SSH_KEY_HEADER;
	if (p + rlen > end)
		return -E_SSH_KEY_HEADER;
	if (rlen < strlen(KEY_TYPE_TXT))
		return -E_SSH_KEY_HEADER;
	PARA_DEBUG_LOG("type: %s, rlen: %u\n", p, rlen);
	if (strncmp((char *)p, KEY_TYPE_TXT, strlen(KEY_TYPE_TXT)))
		return -E_SSH_KEY_HEADER;
	return 4 + rlen;
}

/**
 * Perform sanity checks and base64-decode an ssh-rsa key.
 *
 * \param filename The public key file (usually id_rsa.pub).
 * \param blob Pointer to base64-decoded blob is returned here.
 * \param decoded_size The size of the decoded blob.
 *
 * The memory pointed at by the returned blob pointer has to be freed by the
 * caller.
 *
 * \return On success, the offset in bytes of the start of the key values
 * (modulus, exponent..). This is the number of bytes to skip from the blob
 * until the start of the first encoded number. On failure, a negative error
 * code is returned.
 */
int decode_public_key(const char *filename, unsigned char **blob,
		size_t *decoded_size)
{
	int ret, ret2;
	void *map;
	size_t map_size;

	ret = mmap_full_file(filename, O_RDONLY, &map, &map_size, NULL);
	if (ret < 0)
		return ret;
	ret = is_ssh_rsa_key(map, map_size);
	if (ret == 0) {
		ret = -E_SSH_PARSE;
		goto unmap;
	}
	ret = uudecode(map + ret, map_size - ret, (char **)blob, decoded_size);
	if (ret < 0)
		goto unmap;
	ret = check_ssh_key_header(*blob, *decoded_size);
unmap:
	ret2 = para_munmap(map, map_size);
	if (ret >= 0 && ret2 < 0)
		ret = ret2;
	return ret;
}

/**
 * Check existence and permissions of a private key file.
 *
 * \param file The path of the key file.
 *
 * This checks whether the file exists and its permissions are restrictive
 * enough. It is considered an error if we own the file and it is readable for
 * others.
 *
 * \return Standard.
 */
int check_private_key_file(const char *file)
{
	struct stat st;

	if (stat(file, &st) != 0)
		return -ERRNO_TO_PARA_ERROR(errno);
	if ((st.st_uid == getuid()) && (st.st_mode & 077) != 0)
		return -E_KEY_PERM;
	return 1;
}

void hash_to_asc(const unsigned char *hash, char *asc)
{
	int i;
	const char hexchar[] = "0123456789abcdef";

	for (i = 0; i < HASH_SIZE; i++) {
		asc[2 * i] = hexchar[hash[i] >> 4];
		asc[2 * i + 1] = hexchar[hash[i] & 0xf];
	}
	asc[2 * HASH_SIZE] = '\0';
}

int hash_compare(const unsigned char *h1, const unsigned char *h2)
{
	int i;

	for (i = 0; i < HASH_SIZE; i++) {
		if (h1[i] < h2[i])
			return -1;
		if (h1[i] > h2[i])
			return 1;
	}
	return 0;
}

/**
 * Check header of an openssh private key and compute bignum offset.
 *
 * \param data The base64-decoded key.
 * \param len The size of the decoded key.
 *
 * Several assumptions are made about the key. Most notably, we only support
 * single unencrypted keys without comments.
 *
 * \return The offset at which the first bignum of the private key (the public
 * exponent n) starts. Negative error code on failure.
 */
int find_openssh_bignum_offset(const unsigned char *data, int len)
{
	/*
	 * Unencrypted keys without comments always start with the below byte
	 * sequence. See PROTOCOL.key of the openssh package.
	 */
	static const unsigned char valid_openssh_header[] = {
		/* string "openssh-key-v1" */
		0x6f, 0x70, 0x65, 0x6e, 0x73, 0x73, 0x68, 0x2d, 0x6b, 0x65,
			0x79, 0x2d, 0x76, 0x31,
		/* length of the cipher name */
		0x00, 0x00, 0x00, 0x00, 0x04,
		/* cipher name: "none" */
		0x6e, 0x6f, 0x6e, 0x65,
		/* length of the kdfname (only used for encrypted keys) */
		0x00, 0x00, 0x00, 0x04,
		/* kdfname: "none" */
		0x6e, 0x6f, 0x6e, 0x65,
		/* length of kdfoptions */
		0x00, 0x00, 0x00, 0x00,
		/* number of keys */
		0x00, 0x00, 0x00, 0x01,
	};
	uint32_t val;
	const unsigned char *p, *end = data + len;

	if (len <= sizeof(valid_openssh_header) + 4)
		return -E_OPENSSH_PARSE;
	if (memcmp(data, valid_openssh_header, sizeof(valid_openssh_header)))
		return -E_OPENSSH_PARSE;
	p = data + sizeof(valid_openssh_header);
	/* length of public key */
	val = read_u32_be(p);
	if (val > end - p - 4)
		return -E_OPENSSH_PARSE;
	p += val + 4;
	/* length of private key */
	val = read_u32_be(p);
	if (val > end - p - 4)
		return -E_OPENSSH_PARSE;
	p += 4;
	/* two equal random integers ("checkint") */
	if (p + 8 > end)
		return -E_OPENSSH_PARSE;
	if (read_u32_be(p) != read_u32_be(p + 4))
		return -E_OPENSSH_PARSE;
	p += 8;
	/* length of name of key type "ssh-rsa" */
	if (p + 11 > end)
		return -E_OPENSSH_PARSE;
	if (read_u32_be(p) != 7)
		return -E_OPENSSH_PARSE;
	if (memcmp(p + 4, "ssh-rsa", 7))
		return -E_OPENSSH_PARSE;
	p += 11;
	return p - data;
}

/** Private OPENSSH keys (RFC4716) start with this header. */
#define PRIVATE_OPENSSH_KEY_HEADER "-----BEGIN OPENSSH PRIVATE KEY-----"
/** Private OPENSSH keys (RFC4716) end with this footer. */
#define PRIVATE_OPENSSH_KEY_FOOTER "-----END OPENSSH PRIVATE KEY-----"

/**
 * Decode an openssh-v1 (aka RFC4716) private key.
 *
 * \param key_file The private key file (usually id_rsa).
 * \param result Pointer to base64-decoded blob is returned here.
 * \param blob_size The size of the decoded blob.
 *
 * This only checks header and footer and base64-decodes the part in between.
 * No attempt to read the decoded part is made.
 *
 * \return Standard.
 */
int decode_private_key(const char *key_file, unsigned char **result,
		size_t *blob_size)
{
	int ret, ret2, i, j;
	void *map;
	size_t map_size, key_size;
	unsigned char *blob = NULL;
	char *begin, *footer, *key;

	ret = mmap_full_file(key_file, O_RDONLY, &map, &map_size, NULL);
	if (ret < 0)
		goto out;
	ret = -E_KEY_MARKER;
	if (strncmp(map, PRIVATE_OPENSSH_KEY_HEADER,
			strlen(PRIVATE_OPENSSH_KEY_HEADER)) == 0) {
		begin = map + strlen(PRIVATE_OPENSSH_KEY_HEADER);
		footer = strstr(map, PRIVATE_OPENSSH_KEY_FOOTER);
		PARA_INFO_LOG("detected openssh key %s\n", key_file);
	} else
		goto unmap;
	if (!footer)
		goto unmap;
	/* skip whitespace at the beginning */
	for (; begin < footer; begin++) {
		if (para_isspace(*begin))
			continue;
		break;
	}
	ret = -E_KEY_MARKER;
	if (begin >= footer)
		goto unmap;

	key_size = footer - begin;
	key = alloc(key_size + 1);
	for (i = 0, j = 0; begin + i < footer; i++) {
		if (para_isspace(begin[i]))
			continue;
		key[j++] = begin[i];
	}
	key[j] = '\0';
	ret = base64_decode(key, j, (char **)&blob, blob_size);
	free(key);
	if (ret < 0)
		goto unmap;
	ret = 1;
unmap:
	ret2 = para_munmap(map, map_size);
	if (ret >= 0 && ret2 < 0)
		ret = ret2;
	if (ret < 0) {
		free(blob);
		blob = NULL;
	}
out:
	*result = blob;
	return ret;
}