	Definitions needed to implement a specific crypto library

This document offers some hints about implementing a new crypto library
interface.

A crypto library interface consists of at least a header file, defining
entities referenced from the libssh2 core modules.
Real code implementation (if needed), is left at the implementor's choice.

This document lists the entities that must/may be defined in the header file.

Procedures listed as "void" may indeed have a result type: the void indication
indicates the libssh2 core modules never use the function result.


0) Build system.

Add a new crypto backend to the autotools build system (./configure) as such:

* Add one new line to configure.ac:

m4_set_add([crypto_backends], [newname])

This automatically creates a new --with-crypto=newname option which users can
specify when invoking configure at compile-time to select the new backend.

* Add a new m4_case stanza to acinclude.m4 within LIBSSH2_CRYPTO_CHECK,
  with checks for library availability. A successful check should set
  library linking variables. The LIBSSH2_LIB_HAVE_LINKFLAGS macro creates
  such a variable automatically if the checked library can be found.

* Add a Makefile.newname.inc in the top-level directory which sets
  CRYPTO_CSOURCES and CRYPTO_HHEADERS to the new backend source files,
  and CRYPTO_LTLIBS to the libtool linking parameters for the library, set
  e.g. by a LIBSSH2_LIB_HAVE_LINKFLAGS call in LIBSSH2_CRYPTO_CHECK.

* Add a new block to src/Makefile.am:
  if NEWNAME
  include ../Makefile.newname.inc
  endif


1) Crypto library initialization/termination.

void libssh2_crypto_init(void);
Initializes the crypto library. May be an empty macro if not needed.

void libssh2_crypto_exit(void);
Terminates the crypto library use. May be an empty macro if not needed.


2) HMAC

libssh2_hmac_ctx
Type of an HMAC computation context. Generally a struct.
Used for all hash algorithms.

void libssh2_hmac_ctx_init(libssh2_hmac_ctx ctx);
Initializes the HMAC computation context ctx.
Called before setting-up the hash algorithm.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_hmac_update(libssh2_hmac_ctx ctx,
	                 const unsigned char *data,
	                 int datalen);
Continue computation of an HMAC on datalen bytes at data using context ctx.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_hmac_final(libssh2_hmac_ctx ctx,
	                unsigned char output[]);
Get the computed HMAC from context ctx into the output buffer. The
minimum data buffer size depends on the HMAC hash algorithm.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_hmac_cleanup(libssh2_hmac_ctx *ctx);
Releases the HMAC computation context at ctx.


3) Hash algorithms.

3.1) SHA-1
Must always be implemented.

SHA_DIGEST_LENGTH
#define to 20, the SHA-1 digest length.

libssh2_sha1_ctx
Type of an SHA-1 computation context. Generally a struct.

int libssh2_sha1_init(libssh2_sha1_ctx *x);
Initializes the SHA-1 computation context at x.
Returns 1 for success and 0 for failure

void libssh2_sha1_update(libssh2_sha1_ctx ctx,
	                 const unsigned char *data,
	                 size_t len);
Continue computation of SHA-1 on len bytes at data using context ctx.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_sha1_final(libssh2_sha1_ctx ctx,
                        unsigned char output[SHA_DIGEST_LEN]);
Get the computed SHA-1 signature from context ctx and store it into the
output buffer.
Release the context.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_hmac_sha1_init(libssh2_hmac_ctx *ctx,
	                    const void *key,
	                    int keylen);
Setup the HMAC computation context ctx for an HMAC-SHA-1 computation using the
keylen-byte key. Is invoked just after libssh2_hmac_ctx_init().

3.2) SHA-256
Must always be implemented.

SHA256_DIGEST_LENGTH
#define to 32, the SHA-256 digest length.

libssh2_sha256_ctx
Type of an SHA-256 computation context. Generally a struct.

int libssh2_sha256_init(libssh2_sha256_ctx *x);
Initializes the SHA-256 computation context at x.
Returns 1 for success and 0 for failure

void libssh2_sha256_update(libssh2_sha256_ctx ctx,
	                   const unsigned char *data,
	                   size_t len);
Continue computation of SHA-256 on len bytes at data using context ctx.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_sha256_final(libssh2_sha256_ctx ctx,
	                  unsigned char output[SHA256_DIGEST_LENGTH]);
Gets the computed SHA-256 signature from context ctx into the output buffer.
Release the context.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

int libssh2_sha256(const unsigned char *message,
                   unsigned long len,
                   unsigned char output[SHA256_DIGEST_LENGTH]);
Computes the SHA-256 signature over the given message of length len and
store the result into the output buffer.
Return 1 if error, else 0.
Note: Seems unused in current code, but defined in each crypto library backend.

LIBSSH2_HMAC_SHA256
#define as 1 if the crypto library supports HMAC-SHA-256, else 0.
If defined as 0, the rest of this section can be omitted.

void libssh2_hmac_sha256_init(libssh2_hmac_ctx *ctx,
	                      const void *key,
	                      int keylen);
Setup the HMAC computation context ctx for an HMAC-256 computation using the
keylen-byte key. Is invoked just after libssh2_hmac_ctx_init().

3.3) SHA-512
LIBSSH2_HMAC_SHA512
#define as 1 if the crypto library supports HMAC-SHA-512, else 0.
If defined as 0, the rest of this section can be omitted.

SHA512_DIGEST_LENGTH
#define to 64, the SHA-512 digest length.

void libssh2_hmac_sha512_init(libssh2_hmac_ctx *ctx,
	                      const void *key,
	                      int keylen);
Setup the HMAC computation context ctx for an HMAC-512 computation using the
keylen-byte key. Is invoked just after libssh2_hmac_ctx_init().

3.4) MD5
LIBSSH2_MD5
#define to 1 if the crypto library supports MD5, else 0.
If defined as 0, the rest of this section can be omitted.

MD5_DIGEST_LENGTH
#define to 16, the MD5 digest length.

libssh2_md5_ctx
Type of an MD5 computation context. Generally a struct.

int libssh2_md5_init(libssh2_md5_ctx *x);
Initializes the MD5 computation context at x.
Returns 1 for success and 0 for failure

void libssh2_md5_update(libssh2_md5_ctx ctx,
	                const unsigned char *data,
	                size_t len);
Continues computation of MD5 on len bytes at data using context ctx.
Returns 1 for success and 0 for failure.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_md5_final(libssh2_md5_ctx ctx,
	               unsigned char output[MD5_DIGEST_LENGTH]);
Gets the computed MD5 signature from context ctx into the output buffer.
Release the context.
Note: if the ctx parameter is modified by the underlying code,
this procedure must be implemented as a macro to map ctx --> &ctx.

void libssh2_hmac_md5_init(libssh2_hmac_ctx *ctx,
	                   const void *key,
	                   int keylen);
Setup the HMAC computation context ctx for an HMAC-MD5 computation using the
keylen-byte key. Is invoked just after libssh2_hmac_ctx_init().

3.5) RIPEMD-160
LIBSSH2_HMAC_RIPEMD
#define as 1 if the crypto library supports HMAC-RIPEMD-160, else 0.
If defined as 0, the rest of this section can be omitted.

void libssh2_hmac_ripemd160_init(libssh2_hmac_ctx *ctx,
	                         const void *key,
	                         int keylen);
Setup the HMAC computation context ctx for an HMAC-RIPEMD-160 computation using
the keylen-byte key. Is invoked just after libssh2_hmac_ctx_init().
Returns 1 for success and 0 for failure.


4) Bidirectional key ciphers.

_libssh2_cipher_ctx
Type of a cipher computation context.

_libssh2_cipher_type(name);
Macro defining name as storage identifying a cipher algorithm for
the crypto library interface. No trailing semicolon.

int _libssh2_cipher_init(_libssh2_cipher_ctx *h,
                         _libssh2_cipher_type(algo),
                         unsigned char *iv,
                         unsigned char *secret,
                         int encrypt);
Creates a cipher context for the given algorithm with the initialization vector
iv and the secret key secret. Prepare for encryption or decryption depending on
encrypt.
Return 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_cipher_crypt(_libssh2_cipher_ctx *ctx,
                          _libssh2_cipher_type(algo),
                          int encrypt,
                          unsigned char *block,
                          size_t blocksize);
Encrypt or decrypt in-place data at (block, blocksize) using the given
context and/or algorithm.
Return 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

void _libssh2_cipher_dtor(_libssh2_cipher_ctx *ctx);
Release cipher context at ctx.

4.1) AES
4.1.1) AES in CBC block mode.
LIBSSH2_AES
#define as 1 if the crypto library supports AES in CBC mode, else 0.
If defined as 0, the rest of this section can be omitted.

_libssh2_cipher_aes128
AES-128-CBC algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

_libssh2_cipher_aes192
AES-192-CBC algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

_libssh2_cipher_aes256
AES-256-CBC algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

4.1.2) AES in CTR block mode.
LIBSSH2_AES_CTR
#define as 1 if the crypto library supports AES in CTR mode, else 0.
If defined as 0, the rest of this section can be omitted.

void _libssh2_init_aes_ctr(void);
Initialize static AES CTR ciphers.
This procedure is already prototyped in crypto.h.

_libssh2_cipher_aes128ctr
AES-128-CTR algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

_libssh2_cipher_aes192ctr
AES-192-CTR algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

_libssh2_cipher_aes256ctr
AES-256-CTR algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

4.2) Blowfish in CBC block mode.
LIBSSH2_BLOWFISH
#define as 1 if the crypto library supports blowfish in CBC mode, else 0.
If defined as 0, the rest of this section can be omitted.

_libssh2_cipher_blowfish
Blowfish-CBC algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

4.3) RC4.
LIBSSH2_RC4
#define as 1 if the crypto library supports RC4 (arcfour), else 0.
If defined as 0, the rest of this section can be omitted.

_libssh2_cipher_arcfour
RC4 algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

4.4) CAST5 in CBC block mode.
LIBSSH2_CAST
#define 1 if the crypto library supports cast, else 0.
If defined as 0, the rest of this section can be omitted.

_libssh2_cipher_cast5
CAST5-CBC algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().

4.5) Tripple DES in CBC block mode.
LIBSSH2_3DES
#define as 1 if the crypto library supports TripleDES in CBC mode, else 0.
If defined as 0, the rest of this section can be omitted.

_libssh2_cipher_3des
TripleDES-CBC algorithm identifier initializer.
#define with constant value of type _libssh2_cipher_type().


5) Diffie-Hellman support.
If the crypto-library supports opaque Diffie-Hellman computations, symbol
`libssh2_dh_key_pair' should be #defined as described below and the rest of
this section applies.
Else, the Diffie-Hellman context MUST be defined as `_libssh2_bn *' and
the computation is emulated via calls to _libssh2_bn_rand() and
_libssh2_bn_mod_exp(): all other symbols in this section are unused in this
case.

5.1) Diffie-Hellman context.
_libssh2_dh_ctx
Type of a Diffie-Hellman computation context.
Must always be defined.

5.2) Diffie-Hellman computation procedures.
void libssh2_dh_init(_libssh2_dh_ctx *dhctx);
Initializes the Diffie-Hellman context at `dhctx'. No effective context
creation needed here.

int libssh2_dh_key_pair(_libssh2_dh_ctx *dhctx, _libssh2_bn *public,
                        _libssh2_bn *g, _libssh2_bn *p, int group_order,
                        _libssh2_bn_ctx *bnctx);
Generates a Diffie-Hellman key pair using base `g', prime `p' and the given
`group_order'. Can use the given big number context `bnctx' if needed.
The private key is stored as opaque in the Diffie-Hellman context `*dhctx' and
the public key is returned in `public'.
0 is returned upon success, else -1.
If defined, this procedure MUST be implemented as a #define'd macro.

int libssh2_dh_secret(_libssh2_dh_ctx *dhctx, _libssh2_bn *secret,
                      _libssh2_bn *f, _libssh2_bn *p, _libssh2_bn_ctx * bnctx)
Computes the Diffie-Hellman secret from the previouly created context `*dhctx',
the public key `f' from the other party and the same prime `p' used at
context creation. The result is stored in `secret'.
0 is returned upon success, else -1.

void libssh2_dh_dtor(_libssh2_dh_ctx *dhctx)
Destroys Diffie-Hellman context at `dhctx' and resets its storage.


6) Big numbers.
Positive multi-byte integers support is sufficient.

6.1) Computation contexts.
This has a real meaning if the big numbers computations need some context
storage. If not, use a dummy type and functions (macros).

_libssh2_bn_ctx
Type of multiple precision computation context. May not be empty. if not used,
#define as char, for example.

_libssh2_bn_ctx _libssh2_bn_ctx_new(void);
Returns a new multiple precision computation context.

void _libssh2_bn_ctx_free(_libssh2_bn_ctx ctx);
Releases a multiple precision computation context.

6.2) Computation support.
_libssh2_bn
Type of multiple precision numbers (aka bignumbers or huge integers) for the
crypto library.

_libssh2_bn * _libssh2_bn_init(void);
Creates a multiple precision number (preset to zero).

_libssh2_bn * _libssh2_bn_init_from_bin(void);
Create a multiple precision number intended to be set by the
_libssh2_bn_from_bin() function (see below). Unlike _libssh2_bn_init(), this
code may be a dummy initializer if the _libssh2_bn_from_bin() actually
allocates the number. Returns a value of type _libssh2_bn *.

void _libssh2_bn_free(_libssh2_bn *bn);
Destroys the multiple precision number at bn.

unsigned long _libssh2_bn_bytes(_libssh2_bn *bn);
Get the number of bytes needed to store the bits of the multiple precision
number at bn.

unsigned long _libssh2_bn_bits(_libssh2_bn *bn);
Returns the number of bits of multiple precision number at bn.

int _libssh2_bn_set_word(_libssh2_bn *bn, unsigned long val);
Sets the value of bn to val.
Returns 1 on success, 0 otherwise.

_libssh2_bn * _libssh2_bn_from_bin(_libssh2_bn *bn, int len,
	                           const unsigned char *val);
Converts the positive integer in big-endian form of length len at val
into a _libssh2_bn and place it in bn. If bn is NULL, a new _libssh2_bn is
created.
Returns a pointer to target _libssh2_bn or NULL if error.

int _libssh2_bn_to_bin(_libssh2_bn *bn, unsigned char *val);
Converts the absolute value of bn into big-endian form and store it at
val. val must point to _libssh2_bn_bytes(bn) bytes of memory.
Returns the length of the big-endian number.

void _libssh2_bn_rand(_libssh2_bn *bn, int bits, int top, int bottom);
Generates a cryptographically strong pseudo-random number of bits in
length and stores it in bn. If top is -1, the most significant bit of the
random number can be zero. If top is 0, it is set to 1, and if top is 1, the
two most significant bits of the number will be set to 1, so that the product
of two such random numbers will always have 2*bits length. If bottom is true,
the number will be odd.
This procedure is only needed if no specific Diffie-Hellman support is provided.

void _libssh2_bn_mod_exp(_libssh2_bn *r, _libssh2_bn *a,
	                 _libssh2_bn *p, _libssh2_bn *m,
	                 _libssh2_bn_ctx *ctx);
Computes a to the p-th power modulo m and stores the result into r (r=a^p % m).
May use the given context.
This procedure is only needed if no specific Diffie-Hellman support is provided.


7) Private key algorithms.
Format of an RSA public key:
a) "ssh-rsa".
b) RSA exponent, MSB first, with high order bit = 0.
c) RSA modulus, MSB first, with high order bit = 0.
Each item is preceded by its 32-bit byte length, MSB first.

Format of a DSA public key:
a) "ssh-dss".
b) p, MSB first, with high order bit = 0.
c) q, MSB first, with high order bit = 0.
d) g, MSB first, with high order bit = 0.
e) pub_key, MSB first, with high order bit = 0.
Each item is preceded by its 32-bit byte length, MSB first.

int _libssh2_pub_priv_keyfile(LIBSSH2_SESSION *session,
                              unsigned char **method,
                              size_t *method_len,
                              unsigned char **pubkeydata,
                              size_t *pubkeydata_len,
                              const char *privatekey,
                              const char *passphrase);
Reads a private key from file privatekey and extract the public key -->
(pubkeydata, pubkeydata_len). Store the associated method (ssh-rsa or ssh-dss)
into (method, method_len).
Both buffers have to be allocated using LIBSSH2_ALLOC().
Returns 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_pub_priv_keyfilememory(LIBSSH2_SESSION *session,
                                    unsigned char **method,
                                    size_t *method_len,
                                    unsigned char **pubkeydata,
                                    size_t *pubkeydata_len,
                                    const char *privatekeydata,
                                    size_t privatekeydata_len,
                                    const char *passphrase);
Gets a private key from bytes at (privatekeydata, privatekeydata_len) and
extract the public key --> (pubkeydata, pubkeydata_len). Store the associated
method (ssh-rsa or ssh-dss) into (method, method_len).
Both buffers have to be allocated using LIBSSH2_ALLOC().
Returns 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

7.1) RSA
LIBSSH2_RSA
#define as 1 if the crypto library supports RSA, else 0.
If defined as 0, the rest of this section can be omitted.

libssh2_rsa_ctx
Type of an RSA computation context. Generally a struct.

int _libssh2_rsa_new(libssh2_rsa_ctx **rsa,
                     const unsigned char *edata,
                     unsigned long elen,
                     const unsigned char *ndata,
                     unsigned long nlen,
                     const unsigned char *ddata,
                     unsigned long dlen,
                     const unsigned char *pdata,
                     unsigned long plen,
                     const unsigned char *qdata,
                     unsigned long qlen,
                     const unsigned char *e1data,
                     unsigned long e1len,
                     const unsigned char *e2data,
                     unsigned long e2len,
                     const unsigned char *coeffdata, unsigned long coefflen);
Creates a new context for RSA computations from key source values:
	pdata, plen	Prime number p. Only used if private key known (ddata).
	qdata, qlen	Prime number q. Only used if private key known (ddata).
	ndata, nlen	Modulus n.
	edata, elen	Exponent e.
	ddata, dlen	e^-1 % phi(n) = private key. May be NULL if unknown.
	e1data, e1len	dp = d % (p-1). Only used if private key known (dtata).
	e2data, e2len	dq = d % (q-1). Only used if private key known (dtata).
	coeffdata, coefflen	q^-1 % p. Only used if private key known.
Returns 0 if OK.
This procedure is already prototyped in crypto.h.
Note: the current generic code only calls this function with e and n (public
key parameters): unless used internally by the backend, it is not needed to
support the private key and the other parameters here.

int _libssh2_rsa_new_private(libssh2_rsa_ctx **rsa,
                             LIBSSH2_SESSION *session,
                             const char *filename,
                             unsigned const char *passphrase);
Reads an RSA private key from file filename into a new RSA context.
Must call _libssh2_init_if_needed().
Return 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_rsa_new_private_frommemory(libssh2_rsa_ctx **rsa,
                                        LIBSSH2_SESSION *session,
                                        const char *data,
					size_t data_len,
                                        unsigned const char *passphrase);
Gets an RSA private key from data into a new RSA context.
Must call _libssh2_init_if_needed().
Return 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_rsa_sha1_verify(libssh2_rsa_ctx *rsa,
                             const unsigned char *sig,
                             unsigned long sig_len,
                             const unsigned char *m, unsigned long m_len);
Verify (sig, siglen) signature of (m, m_len) using an SHA-1 hash and the
RSA context.
Return 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_rsa_sha1_signv(LIBSSH2_SESSION *session,
	                    unsigned char **sig, size_t *siglen,
	                    int count, const struct iovec vector[],
	                    libssh2_rsa_ctx *ctx);
RSA signs the SHA-1 hash computed over the count data chunks in vector.
Signature is stored at (sig, siglen).
Signature buffer must be allocated from the given session.
Returns 0 if OK, else -1.
Note: this procedure is optional: if provided, it MUST be defined as a macro.

int _libssh2_rsa_sha1_sign(LIBSSH2_SESSION *session,
                           libssh2_rsa_ctx *rsactx,
                           const unsigned char *hash,
                           size_t hash_len,
                           unsigned char **signature,
                           size_t *signature_len);
RSA signs the (hash, hashlen) SHA-1 hash bytes and stores the allocated
signature at (signature, signature_len).
Signature buffer must be allocated from the given session.
Returns 0 if OK, else -1.
This procedure is already prototyped in crypto.h.
Note: this procedure is not used if macro _libssh2_rsa_sha1_signv() is defined.

void _libssh2_rsa_free(libssh2_rsa_ctx *rsactx);
Releases the RSA computation context at rsactx.


7.2) DSA
LIBSSH2_DSA
#define as 1 if the crypto library supports DSA, else 0.
If defined as 0, the rest of this section can be omitted.


libssh2_dsa_ctx
Type of a DSA computation context. Generally a struct.

int _libssh2_dsa_new(libssh2_dsa_ctx **dsa,
                     const unsigned char *pdata,
                     unsigned long plen,
                     const unsigned char *qdata,
                     unsigned long qlen,
                     const unsigned char *gdata,
                     unsigned long glen,
                     const unsigned char *ydata,
                     unsigned long ylen,
                     const unsigned char *x, unsigned long x_len);
Creates a new context for DSA computations from source key values:
	pdata, plen	Prime number p. Only used if private key known (ddata).
	qdata, qlen	Prime number q. Only used if private key known (ddata).
	gdata, glen	G number.
	ydata, ylen	Public key.
	xdata, xlen	Private key. Only taken if xlen non-zero.
Returns 0 if OK.
This procedure is already prototyped in crypto.h.

int _libssh2_dsa_new_private(libssh2_dsa_ctx **dsa,
                             LIBSSH2_SESSION *session,
                             const char *filename,
                             unsigned const char *passphrase);
Gets a DSA private key from file filename into a new DSA context.
Must call _libssh2_init_if_needed().
Return 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_dsa_new_private_frommemory(libssh2_dsa_ctx **dsa,
                                        LIBSSH2_SESSION *session,
                                        const char *data,
					size_t data_len,
                                        unsigned const char *passphrase);
Gets a DSA private key from the data_len-bytes data into a new DSA context.
Must call _libssh2_init_if_needed().
Returns 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_dsa_sha1_verify(libssh2_dsa_ctx *dsactx,
                             const unsigned char *sig,
                             const unsigned char *m, unsigned long m_len);
Verify (sig, siglen) signature of (m, m_len) using an SHA-1 hash and the
DSA context.
Returns 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

int _libssh2_dsa_sha1_sign(libssh2_dsa_ctx *dsactx,
                           const unsigned char *hash,
                           unsigned long hash_len, unsigned char *sig);
DSA signs the (hash, hash_len) data using SHA-1 and store the signature at sig.
Returns 0 if OK, else -1.
This procedure is already prototyped in crypto.h.

void _libssh2_dsa_free(libssh2_dsa_ctx *dsactx);
Releases the DSA computation context at dsactx.


8) Miscellaneous

void libssh2_prepare_iovec(struct iovec *vector, unsigned int len);
Prepare len consecutive iovec slots before using them.
In example, this is needed to preset unused structure slacks on platforms
requiring it.
If this is not needed, it should be defined as an empty macro.

void _libssh2_random(unsigned char *buf, int len);
Store len random bytes at buf.