diff options
Diffstat (limited to 'g10/seskey.c')
| -rw-r--r-- | g10/seskey.c | 28 |
1 files changed, 17 insertions, 11 deletions
diff --git a/g10/seskey.c b/g10/seskey.c index e79faf8c9..507eea111 100644 --- a/g10/seskey.c +++ b/g10/seskey.c @@ -31,9 +31,11 @@ #include "i18n.h" -/**************** - * Make a session key and put it into DEK - */ +/* Generate a new session key in *DEK that is appropriate for the + algorithm DEK->ALGO (i.e., ensure that the key is not weak). + + This function overwrites DEK->KEYLEN, DEK->KEY. The rest of the + fields are left as is. */ void make_session_key( DEK *dek ) { @@ -67,11 +69,12 @@ make_session_key( DEK *dek ) } -/**************** - * Encode the session key. NBITS is the number of bits which should be used - * for packing the session key. - * returns: A mpi with the session key (caller must free) - */ +/* Encode the session key stored in DEK as an MPI in preparation to + encrypt it with the public key algorithm OPENPGP_PK_ALGO with a key + whose length (the size of the public key) is NBITS. + + On success, returns an MPI, which the caller must free using + gcry_mpi_release(). */ gcry_mpi_t encode_session_key (int openpgp_pk_algo, DEK *dek, unsigned int nbits) { @@ -136,14 +139,15 @@ encode_session_key (int openpgp_pk_algo, DEK *dek, unsigned int nbits) 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: + /* We encode the session key according to PKCS#1 v1.5 (see section + * 13.1.1 of RFC 4880): * - * 0 2 RND(n bytes) 0 A DEK(k bytes) CSUM(2 bytes) + * 0 2 RND(i bytes) 0 A DEK(k bytes) CSUM(2 bytes) * * (But how can we store the leading 0 - the external representaion * of MPIs doesn't allow leading zeroes =:-) * - * RND are non-zero random bytes. + * RND are (at least 1) non-zero random bytes. * A is the cipher algorithm * DEK is the encryption key (session key) length k depends on the * cipher algorithm (20 is used with blowfish160). @@ -154,6 +158,8 @@ encode_session_key (int openpgp_pk_algo, DEK *dek, unsigned int nbits) n = 0; frame[n++] = 0; frame[n++] = 2; + /* The number of random bytes are the number of otherwise unused + bytes. See diagram above. */ i = nframe - 6 - dek->keylen; assert( i > 0 ); p = gcry_random_bytes_secure (i, GCRY_STRONG_RANDOM); |