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Diffstat (limited to 'g10/seskey.c')
| -rw-r--r-- | g10/seskey.c | 101 |
1 files changed, 101 insertions, 0 deletions
diff --git a/g10/seskey.c b/g10/seskey.c new file mode 100644 index 000000000..d81697296 --- /dev/null +++ b/g10/seskey.c @@ -0,0 +1,101 @@ +/* seskey.c - make sesssion keys + * Copyright (c) 1997 by Werner Koch (dd9jn) + * + * This file is part of G10. + * + * G10 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. + * + * G10 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. + * + * You should have received a copy of the GNU General Public License + * along with this program; if not, write to the Free Software + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA + */ + +#include <config.h> +#include <stdio.h> +#include <stdlib.h> +#include <string.h> +#include <assert.h> +#include "util.h" +#include "cipher.h" +#include "mpi.h" + + + +/**************** + * Make a session key and put it into DEK + */ +void +make_session_key( DEK *dek ) +{ + switch( dek->algo ) { + case CIPHER_ALGO_BLOWFISH: + dek->keylen = 20; + randomize_buffer( dek->key, dek->keylen, 1 ); + break; + + default: log_bug("invalid algo %d in make_session_key()\n"); + } +} + + +/**************** + * Encode the session key. NBITS is the number of bits which should be used + * for packing teh session key. + * returns: A mpi with the session key (caller must free) + */ +MPI +encode_session_key( DEK *dek, unsigned nbits ) +{ + int nframe = (nbits+7) / 8; + byte *p; + MPI frame; + int i,n,c; + u16 csum; + + /* the current limitation is, that we can only use a session key + * which length is a multiple of BITS_PER_MPI_LIMB + * I think we can live with that. + */ + if( dek->keylen + 7 > nframe || (nbits % BITS_PER_MPI_LIMB) || !nframe ) + log_bug("can't encode a %d bit key in a %d bits frame\n", + dek->keylen*8, nbits ); + + /* We encode the session key in this way: + * + * 0 2 RND(n bytes) 0 A DEK(k bytes) CSUM(2 bytes) + * + * RND are non-zero random bytes. + * A is the cipher algorithm ( 42 for Blowfish ) + * DEK is the encryption key (session key) length k depends on the + * cipher algorithm (20 is used with blowfish). + * CSUM is the 16 bit checksum over the DEK + */ + frame = mpi_alloc_secure( nframe / BYTES_PER_MPI_LIMB ); + csum = 0; + for( p = dek->key, i=0; i < dek->keylen; i++ ) + csum += *p++; + mpi_putbyte(frame, 0, csum ); + mpi_putbyte(frame, 1, csum >> 8 ); + for(n=2,i=dek->keylen-1, p = dek->key; i >= 0; i--, n++ ) + mpi_putbyte(frame, n, p[i] ); + mpi_putbyte(frame, n++, dek->algo ); + mpi_putbyte(frame, n++, 0 ); + while( n < nframe-2 ) { + while( !(c = get_random_byte(1)) ) + ; + mpi_putbyte(frame, n++, c ); + } + mpi_putbyte(frame, n++, 2 ); + mpi_putbyte(frame, n++, 0 ); + assert( n == nframe ); + return frame; +} + |