crypto: add/fix algo guards and extend NO options

Add new guard `LIBSSH2_RSA_SHA1`. Add missing guards for `LIBSSH2_RSA`,
`LIBSSH2_DSA`.

Fix warnings when all options are disabled.

This is still not complete and it's possible to break a build with
certain crypto backends (e.g. mbedTLS) and/or combination of options.
It's not guaranteed that all bits everywhere get disabled by these
settings. Consider this a "best effort".

Add these new options to disable certain crypto elements:
- `LIBSSH2_NO_3DES`
- `LIBSSH2_NO_AES_CTR`
- `LIBSSH2_NO_BLOWFISH`
- `LIBSSH2_NO_CAST`
- `LIBSSH2_NO_ECDSA`
- `LIBSSH2_NO_RC4`
- `LIBSSH2_NO_RSA_SHA1`
- `LIBSSH2_NO_RSA`

The goal is to offer a way to disable legacy/obsolete/insecure ones.

See also: 146a25a06d `LIBSSH2_NO_HMAC_RIPEMD`
See also: 38015f4e46 `LIBSSH2_NO_DSA`
See also: be31457f30 `LIBSSH2_NO_MD5`

Closes #986

src/crypto.h

@@ -67,6 +67,46 @@
 #define LIBSSH2_DSA 0
 #endif
 
+#ifdef LIBSSH2_NO_RSA
+#undef LIBSSH2_RSA
+#define LIBSSH2_RSA 0
+#endif
+
+#ifdef LIBSSH2_NO_RSA_SHA1
+#undef LIBSSH2_RSA_SHA1
+#define LIBSSH2_RSA_SHA1 0
+#endif
+
+#ifdef LIBSSH2_NO_ECDSA
+#undef LIBSSH2_ECDSA
+#define LIBSSH2_ECDSA 0
+#endif
+
+#ifdef LIBSSH2_NO_AES_CTR
+#undef LIBSSH2_AES_CTR
+#define LIBSSH2_AES_CTR 0
+#endif
+
+#ifdef LIBSSH2_NO_BLOWFISH
+#undef LIBSSH2_BLOWFISH
+#define LIBSSH2_BLOWFISH 0
+#endif
+
+#ifdef LIBSSH2_NO_RC4
+#undef LIBSSH2_RC4
+#define LIBSSH2_RC4 0
+#endif
+
+#ifdef LIBSSH2_NO_CAST
+#undef LIBSSH2_CAST
+#define LIBSSH2_CAST 0
+#endif
+
+#ifdef LIBSSH2_NO_3DES
+#undef LIBSSH2_3DES
+#define LIBSSH2_3DES 0
+#endif
+
 #define LIBSSH2_ED25519_KEY_LEN 32
 #define LIBSSH2_ED25519_PRIVATE_KEY_LEN 64
 #define LIBSSH2_ED25519_SIG_LEN 64
@@ -92,6 +132,7 @@ int _libssh2_rsa_new_private(libssh2_rsa_ctx ** rsa,
                              LIBSSH2_SESSION * session,
                              const char *filename,
                              unsigned const char *passphrase);
+#if LIBSSH2_RSA_SHA1
 int _libssh2_rsa_sha1_verify(libssh2_rsa_ctx * rsa,
                              const unsigned char *sig,
                              size_t sig_len,
@@ -102,6 +143,7 @@ int _libssh2_rsa_sha1_sign(LIBSSH2_SESSION * session,
                            size_t hash_len,
                            unsigned char **signature,
                            size_t *signature_len);
+#endif
 #if LIBSSH2_RSA_SHA2
 int _libssh2_rsa_sha2_sign(LIBSSH2_SESSION * session,
                            libssh2_rsa_ctx * rsactx,