/* Loading of PEM encoded files with optional encryption
 * Copyright (C) 2001-2004 Andreas Steffen, Zuercher Hochschule Winterthur
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the
 * Free Software Foundation; either version 2 of the License, or (at your
 * option) any later version.  See <http://www.fsf.org/copyleft/gpl.txt>.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * for more details.
 */

/* decrypt a PEM encoded data block using DES-EDE3-CBC
 * see RFC 1423 PEM: Algorithms, Modes and Identifiers
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stddef.h>
#include <sys/types.h>

#include <freeswan.h>

#include <library.h>
#include <crypto/hashers/hasher.h>

#define HEADER_DES_LOCL_H   /* stupid trick to force prototype decl in <des.h> */
#include <libdes/des.h>

#include "constants.h"
#include "defs.h"
#include "log.h"
#include "whack.h"
#include "pem.h"

/**
 * Check the presence of a pattern in a character string
 */
static bool present(const char* pattern, chunk_t* ch)
{
	u_int pattern_len = strlen(pattern);

	if (ch->len >= pattern_len && strneq(ch->ptr, pattern, pattern_len))
	{
		ch->ptr += pattern_len;
		ch->len -= pattern_len;
		return TRUE;
	}
	return FALSE;
}

/**
 * Compare string with chunk
 */
static bool match(const char *pattern, const chunk_t *ch)
{
	return ch->len == strlen(pattern) && strneq(pattern, ch->ptr, ch->len);
}

/**
 * Find a boundary of the form -----tag name-----
 */
static bool find_boundary(const char* tag, chunk_t *line)
{
	chunk_t name = chunk_empty;

	if (!present("-----", line))
	{
		return FALSE;
	}
	if (!present(tag, line))
	{
		return FALSE;
	}
	if (*line->ptr != ' ')
	{
		return FALSE;
	}
	line->ptr++;  line->len--;

	/* extract name */
	name.ptr = line->ptr;
	while (line->len > 0)
	{
		if (present("-----", line))
		{
			DBG(DBG_PARSING,
				DBG_log("  -----%s %.*s-----",
						tag, (int)name.len, name.ptr);
			)
			return TRUE;
		}
		line->ptr++;  line->len--;  name.len++;
	}
	return FALSE;
}

/**
 * Eat whitespace
 */
static void eat_whitespace(chunk_t *src)
{
	while (src->len > 0 && (*src->ptr == ' ' || *src->ptr == '\t'))
	{
		src->ptr++;  src->len--;
	}
}

/**
 * Extracts a token ending with a given termination symbol
 */
static bool extract_token(chunk_t *token, char termination, chunk_t *src)
{
	u_char *eot = memchr(src->ptr, termination, src->len);

	/* initialize empty token */
	*token = chunk_empty;

	if (eot == NULL) /* termination symbol not found */
	{
		return FALSE;
	}

	/* extract token */
	token->ptr = src->ptr;
	token->len = (u_int)(eot - src->ptr);

	/* advance src pointer after termination symbol */
	src->ptr = eot + 1;
	src->len -= (token->len + 1);

   return TRUE;
}

/**
 * Extracts a name: value pair from the PEM header
 */
static bool extract_parameter(chunk_t *name, chunk_t *value, chunk_t *line)
{
	DBG(DBG_PARSING,
		DBG_log("  %.*s", (int)line->len, line->ptr);
	)

	/* extract name */
	if (!extract_token(name,':', line))
	{
		return FALSE;
	}
	eat_whitespace(line);

	/* extract value */
	*value = *line;
	return TRUE;
}

/**
 * Fetches a new line terminated by \n or \r\n
 */
static bool fetchline(chunk_t *src, chunk_t *line)
{
	if (src->len == 0) /* end of src reached */
	{
		return FALSE;
	}
	if (extract_token(line, '\n', src))
	{
		if (line->len > 0 && *(line->ptr + line->len -1) == '\r')
		{
			line->len--;  /* remove optional \r */
		}	
	}
	else /*last line ends without newline */
	{
		*line = *src;
		src->ptr += src->len;
		src->len = 0;
	}
	return TRUE;
}

/**
 * Decrypts a DES-EDE-CBC encrypted data block
 */
static bool pem_decrypt_3des(chunk_t *blob, chunk_t *iv, char *passphrase)
{
	u_char des_iv[DES_CBC_BLOCK_SIZE];
	u_char key[24];
	des_cblock *deskey = (des_cblock *)key;
	des_key_schedule ks[3];
	u_char padding, *last_padding_pos, *first_padding_pos;
	hasher_t *hasher;
	chunk_t digest;
	chunk_t passphrase_chunk = { passphrase, strlen(passphrase) };

	/* Convert passphrase to 3des key */
	hasher = lib->crypto->create_hasher(lib->crypto, HASH_MD5);
	if (hasher == NULL)
	{
		plog("  passphrase could not be hashed, no MD5 hasher available");		
		return FALSE;
	}	
 	hasher->allocate_hash(hasher, passphrase_chunk, NULL);
	hasher->allocate_hash(hasher, *iv, &digest);

	memcpy(key, digest.ptr, digest.len);

	hasher->get_hash(hasher, digest, NULL);
 	hasher->get_hash(hasher, passphrase_chunk, NULL);
	hasher->get_hash(hasher, *iv, digest.ptr);
	hasher->destroy(hasher);

	memcpy(key + digest.len, digest.ptr, 24 - digest.len);
	free(digest.ptr);

	(void) des_set_key(&deskey[0], ks[0]);
	(void) des_set_key(&deskey[1], ks[1]);
	(void) des_set_key(&deskey[2], ks[2]);

	/* decrypt data block */
	memcpy(des_iv, iv->ptr, DES_CBC_BLOCK_SIZE);
	des_ede3_cbc_encrypt((des_cblock *)blob->ptr, (des_cblock *)blob->ptr,
		blob->len, ks[0], ks[1], ks[2], (des_cblock *)des_iv, FALSE);

	/* determine amount of padding */
	last_padding_pos = blob->ptr + blob->len - 1;
	padding = *last_padding_pos;
	first_padding_pos = (padding > blob->len)?
						 blob->ptr : last_padding_pos - padding;

	/* check the padding pattern */
	while (--last_padding_pos > first_padding_pos)
	{
		if (*last_padding_pos != padding)
		{
			return FALSE;
		}
	}

	/* remove padding */
	blob->len -= padding;
	return TRUE;
}

/**
 * Optionally prompts for a passphrase before decryption
 * currently we support DES-EDE3-CBC, only
 */
static err_t pem_decrypt(chunk_t *blob, chunk_t *iv, prompt_pass_t *pass,
						 const char* label)
{
	DBG(DBG_CRYPT,
		DBG_log("  decrypting file using 'DES-EDE3-CBC'");
	)
	if (iv->len != DES_CBC_BLOCK_SIZE)
	{
		return "size of DES-EDE3-CBC IV is not 8 bytes";
	}
	if (pass == NULL)
	{
		return "no passphrase available";
	}

	/* do we prompt for the passphrase? */
	if (pass->prompt && pass->fd != NULL_FD)
	{
		int i;
		chunk_t blob_copy;
		err_t ugh = "invalid passphrase, too many trials";

		whack_log(RC_ENTERSECRET, "need passphrase for '%s'", label);

		for (i = 0; i < MAX_PROMPT_PASS_TRIALS; i++)
		{
			int n;

			if (i > 0)
			{
				whack_log(RC_ENTERSECRET, "invalid passphrase, please try again");
			}
			n = read(pass->fd, pass->secret, PROMPT_PASS_LEN);

			if (n == -1)
			{
				err_t ugh = "read(whackfd) failed";

				whack_log(RC_LOG_SERIOUS,ugh);
				return ugh;
			}

			pass->secret[n-1] = '\0';
			
			if (strlen(pass->secret) == 0)
			{
				err_t ugh = "no passphrase entered, aborted";

				whack_log(RC_LOG_SERIOUS, ugh);
				return ugh;
			}

			blob_copy = chunk_clone(*blob);

			if (pem_decrypt_3des(blob, iv, pass->secret))
			{
				whack_log(RC_SUCCESS, "valid passphrase");
				free(blob_copy.ptr);
				return NULL;
			}
			
			/* blob is useless after wrong decryption, restore the original */
			free(blob->ptr);
			*blob = blob_copy;
		}
		whack_log(RC_LOG_SERIOUS, ugh);
		return ugh;
	}
	else
	{
		if (pem_decrypt_3des(blob, iv, pass->secret))
		{
			return NULL;
		}
		else
		{
			return "invalid passphrase";
		}
	}
}

/*  Converts a PEM encoded file into its binary form
 *
 *  RFC 1421 Privacy Enhancement for Electronic Mail, February 1993
 *  RFC 934 Message Encapsulation, January 1985
 */
err_t pemtobin(chunk_t *blob, prompt_pass_t *pass, const char* label, bool *pgp)
{
	typedef enum {
		PEM_PRE    = 0,
		PEM_MSG    = 1,
		PEM_HEADER = 2,
		PEM_BODY   = 3,
		PEM_POST   = 4,
		PEM_ABORT  = 5
	} state_t;

	bool encrypted = FALSE;

	state_t state  = PEM_PRE;

	chunk_t src    = *blob;
	chunk_t dst    = *blob;
	chunk_t line   = chunk_empty;
	chunk_t iv     = chunk_empty;

	u_char iv_buf[MAX_DIGEST_LEN];

	/* zero size of converted blob */
	dst.len = 0;

	/* zero size of IV */
	iv.ptr = iv_buf;
	iv.len = 0;

	while (fetchline(&src, &line))
	{
		if (state == PEM_PRE)
		{
			if (find_boundary("BEGIN", &line))
			{
				*pgp = FALSE;
				state = PEM_MSG;
			}
			continue;
		}
		else
		{
			if (find_boundary("END", &line))
			{
				state = PEM_POST;
				break;
			}
			if (state == PEM_MSG)
			{
				state = (memchr(line.ptr, ':', line.len) == NULL)?
							PEM_BODY : PEM_HEADER;
			}
			if (state == PEM_HEADER)
			{
				chunk_t name  = chunk_empty;
				chunk_t value = chunk_empty;

				/* an empty line separates HEADER and BODY */
				if (line.len == 0)
				{
					state = PEM_BODY;
					continue;
				}

				/* we are looking for a name: value pair */
				if (!extract_parameter(&name, &value, &line))
				{
					continue;
				}
				if (match("Proc-Type", &name) && *value.ptr == '4')
				{
					encrypted = TRUE;
				}
				else if (match("DEK-Info", &name))
				{
					const char *ugh = NULL;
					size_t len = 0;
					chunk_t dek;

					if (!extract_token(&dek, ',', &value))
					{
						dek = value;
					}

					/* we support DES-EDE3-CBC encrypted files, only */
					if (!match("DES-EDE3-CBC", &dek))
					{
						return "we support DES-EDE3-CBC encrypted files, only";
					}
					eat_whitespace(&value);
					ugh = ttodata(value.ptr, value.len, 16,
								  iv.ptr, MAX_DIGEST_LEN, &len);
					if (ugh)
					{
						return "error in IV";
					}
					iv.len = len;
				}
			}
			else /* state is PEM_BODY */
			{
				const char *ugh = NULL;
				size_t len = 0;
				chunk_t data;

				/* remove any trailing whitespace */
				if (!extract_token(&data ,' ', &line))
				{
					data = line;
				}

				/* check for PGP armor checksum */
				if (*data.ptr == '=')
				{
					*pgp = TRUE;
					data.ptr++;
					data.len--;
					DBG(DBG_PARSING,
						DBG_log("  Armor checksum: %.*s", (int)data.len, data.ptr);
					)
					continue;
				}

				ugh = ttodata(data.ptr, data.len, 64,
							  dst.ptr, blob->len - dst.len, &len);
				if (ugh)
				{
					DBG(DBG_PARSING,
						DBG_log("  %s", ugh);
					)
					state = PEM_ABORT;
					break;
				}
				else
				{
					dst.ptr += len;
					dst.len += len;
				}
			}
		}
	}
	/* set length to size of binary blob */
	blob->len = dst.len;

	if (state != PEM_POST)
	{
		return "file coded in unknown format, discarded";
	}
	if (encrypted)
	{
		return pem_decrypt(blob, &iv, pass, label);
	}
	else
	{	
		return NULL;
	}
}