pushtls, tlsClient, tlsServer, initThumbprints, freeThumbprints, okThumbprint, readcert, readcertchain – attach TLS1 or SSL3 encryption to a communication channel

#include <u.h>
#include <libc.h>

int    pushtls(int fd, char *hashalg, char *encalg,
int isclient, char *secret, char *dir)

#include <mp.h>
#include <libsec.h>

int    tlsClient(int fd, TLSconn *conn)

int    tlsServer(int fd, TLSconn *conn)

uchar *readcert(char *filename, int *pcertlen)

PEMchain *readcertchain(char *filename)

Thumbprint *initThumbprints(char *ok, char *crl)

void freeThumbprints(Thumbprint *table)

int    okThumbprint(uchar *hash, Thumbprint *table)

Transport Layer Security (TLS) comprises a record layer protocol, doing message digesting and encrypting in the kernel, and a handshake protocol, doing initial authentication and secret creation at user level and then starting a data channel in the record protocol. TLS is nearly the same as SSL 3.0, and the software should interoperate with implementations of either standard.

To use just the record layer, as described in tls(3), call pushtls to open the record layer device, connect to the communications channel fd, and start up encryption and message authentication as specified in hashalg, encalg, and secret. These parameters must have been arranged at the two ends of the conversation by other means. For example, hashalg could be sha1, encalg could be rc4_128, and secret could be the base–64 encoding of two (client–to–server and server–to–client) 20–byte digest keys and two corresponding 16–byte encryption keys. Pushtls returns a file descriptor for the TLS data channel. Anything written to this descriptor will get encrypted and authenticated and then written to the file descriptor, fd. If dir is non–zero, the path name of the connection directory is copied into dir. This path name is guaranteed to be less than 40 bytes long.

Alternatively, call tlsClient to speak the full handshake protocol, negotiate the algorithms and secrets, and return a new data file descriptor for the data channel. Conn points to a (caller–allocated) struct:
typedef struct TLSconn {
char dir[40];         /* OUT      connection directory */
uchar *cert;          /* IN/OUT certificate */
uchar *sessionID;     /* IN/OUT session ID */
int    certlen, sessionIDlen;
void (*trace)(char*fmt, ...);
PEMChain *chain;
char *sessionType;    /* opt IN    session type */
uchar *sessionKey;    /* opt IN/OUT session key */
int    sessionKeylen; /* opt IN    session key length */
char *sessionConst; /* opt IN    session constant */
} TLSconn;

defined in tls.h. On input, the caller can provide options such as cert, the local certificate, and sessionID, used by a client to resume a previously negotiated security association. On output, the connection directory is set, as with listen (see dial(2)). The input cert is freed and a freshly allocated copy of the remote's certificate is returned in conn, to be checked by the caller according to its needs. One way to check the remote certificate is to use initThumbprints and freeThumbprints which allocate and free, respectively, a table of hashes from files of known trusted and revoked certificates. okThumbprint confirms that a particular hash is in the table.

TlsClient will optionally compute a session key for use by higher–level protocols. To compute a session key, the caller must set sessionType to a known session type; sessionKeylen to the desired key length; sessionKey to a buffer of length sessionKeylen; and sessionConst to the desired salting constant. The only supported session type is ttls, as used by 802.1x.

TlsServer executes the server side of the handshake. The caller must initialize conn–>cert, usually by calling readcert to read and decode the PEM–encoded certificate from filename, return a pointer to malloced storage containing the certificate, and store its length through pcertlen. The private key corresponding to cert.pem should have been previously loaded into factotum. (See rsa(8) for more about key generation.)

Readcertchain will read a PEM–encoded chain of certificates from filename and return a pointer to a linked list of malloced PEMChain structures, defined in tls.h:
typedef struct PEMChain PEMChain;
struct PEMChain {
uchar *pem;
int    pemlen;

By setting
conn–>chain = readcertchain("intermediate–certs.pem");

the server can present extra certificate evidence to establish the chain of trust to a root authority known to the client.

Conn is not required for the ongoing conversation and may be freed by the application whenever convenient.

Start the client half of TLS and check the remote certificate:
uchar hash[SHA1dlen];
conn = (TLSconn*)mallocz(sizeof *conn, 1);
fd = tlsClient(fd, conn);
sha1(conn–>cert, conn–>certlen, hash, nil);
exits("suspect server");

Run the server side:
fd = accept(lcfd, ldir);
conn = (TLSconn*)mallocz(sizeof *conn, 1);
conn–>cert = readcert("cert.pem", &conn–>certlen);
fd = tlsServer(fd, conn);

/sys/lib/tls   thumbprints of trusted services
/sys/lib/ssl   PEM certificate files


dial(2), tls(3), factotum(4), thumbprint(6)

Return –1 on failure.

Client certificates and client sessionIDs are not yet implemented.

Note that in the TLS protocol sessionID itself is public; it is used as a pointer to secrets stored in factotum.

Copyright © 2024 Plan 9 Foundation. All rights reserved.