Add API for getting all of a key's fingerprints.

ssh2_all_fingerprints() and friends will return a small 'char **'
array, containing all the fingerprints of a key that we know how to
generate, indexed by the FingerprintType enum. The result requires
complex freeing, so there's an ssh2_free_all_fingerprints as well.

For SSH-1 RSA keys, we refuse to generate any fingerprint except the
old SSH-1 MD5 version, because there's no other fingerprint type I
know of that anyone else uses. So I've got a function that returns the
same 'char **' for an SSH-1 key, but it only fills in the MD5 slot,
and leaves the rest NULL.

As a result, I also need a dynamic function that takes a fingerprint
list and returns the id of the most preferred fingerprint type in it
_that actually exists_.

NFC: this API is introduced, but not yet used.

ssh.h

@@ -567,6 +567,7 @@ mp_int *rsa_ssh1_decrypt(mp_int *input, RSAKey *key);
 bool rsa_ssh1_decrypt_pkcs1(mp_int *input, RSAKey *key, strbuf *outbuf);
 char *rsastr_fmt(RSAKey *key);
 char *rsa_ssh1_fingerprint(RSAKey *key);
+char **rsa_ssh1_fake_all_fingerprints(RSAKey *key);
 bool rsa_verify(RSAKey *key);
 void rsa_ssh1_public_blob(BinarySink *bs, RSAKey *key, RsaSsh1Order order);
 int rsa_ssh1_public_blob_len(ptrlen data);
@@ -1337,6 +1338,10 @@ typedef enum {
 #define SSH_FPTYPE_DEFAULT SSH_FPTYPE_MD5
 #define SSH_N_FPTYPES (SSH_FPTYPE_SHA256 + 1)
 
+FingerprintType ssh2_pick_fingerprint(char **fingerprints,
+                                      FingerprintType preferred_type);
+FingerprintType ssh2_pick_default_fingerprint(char **fingerprints);
+
 char *ssh1_pubkey_str(RSAKey *ssh1key);
 void ssh1_write_pubkey(FILE *fp, RSAKey *ssh1key);
 char *ssh2_pubkey_openssh_str(ssh2_userkey *key);
@@ -1345,6 +1350,9 @@ void ssh2_write_pubkey(FILE *fp, const char *comment,
                        int keytype);
 char *ssh2_fingerprint_blob(ptrlen, FingerprintType);
 char *ssh2_fingerprint(ssh_key *key, FingerprintType);
+char **ssh2_all_fingerprints_for_blob(ptrlen);
+char **ssh2_all_fingerprints(ssh_key *key);
+void ssh2_free_all_fingerprints(char **);
 int key_type(const Filename *filename);
 int key_type_s(BinarySource *src);
 const char *key_type_to_str(int type);

sshpubk.c

@@ -1794,6 +1794,14 @@ char *ssh2_fingerprint_blob(ptrlen blob, FingerprintType fptype)
     return strbuf_to_str(sb);
 }
 
+char **ssh2_all_fingerprints_for_blob(ptrlen blob)
+{
+    char **fps = snewn(SSH_N_FPTYPES, char *);
+    for (unsigned i = 0; i < SSH_N_FPTYPES; i++)
+        fps[i] = ssh2_fingerprint_blob(blob, i);
+    return fps;
+}
+
 char *ssh2_fingerprint(ssh_key *data, FingerprintType fptype)
 {
     strbuf *blob = strbuf_new();
@@ -1803,6 +1811,22 @@ char *ssh2_fingerprint(ssh_key *data, FingerprintType fptype)
     return ret;
 }
 
+char **ssh2_all_fingerprints(ssh_key *data)
+{
+    strbuf *blob = strbuf_new();
+    ssh_key_public_blob(data, BinarySink_UPCAST(blob));
+    char **ret = ssh2_all_fingerprints_for_blob(ptrlen_from_strbuf(blob));
+    strbuf_free(blob);
+    return ret;
+}
+
+void ssh2_free_all_fingerprints(char **fps)
+{
+    for (unsigned i = 0; i < SSH_N_FPTYPES; i++)
+        sfree(fps[i]);
+    sfree(fps);
+}
+
 /* ----------------------------------------------------------------------
  * Determine the type of a private key file.
  */

sshrsa.c

@@ -319,6 +319,19 @@ char *rsa_ssh1_fingerprint(RSAKey *key)
     return strbuf_to_str(out);
 }
 
+/*
+ * Wrap the output of rsa_ssh1_fingerprint up into the same kind of
+ * structure that comes from ssh2_all_fingerprints.
+ */
+char **rsa_ssh1_fake_all_fingerprints(RSAKey *key)
+{
+    char **ret = snewn(SSH_N_FPTYPES, char *);
+    for (unsigned i = 0; i < SSH_N_FPTYPES; i++)
+        ret[i] = NULL;
+    ret[SSH_FPTYPE_MD5] = rsa_ssh1_fingerprint(key);
+    return ret;
+}
+
 /*
  * Verify that the public data in an RSA key matches the private
  * data. We also check the private data itself: we ensure that p >

utils.c

@@ -17,6 +17,7 @@
 
 #include "defs.h"
 #include "misc.h"
+#include "ssh.h"
 
 /*
  * Parse a string block size specification. This is approximately a
@@ -1099,3 +1100,23 @@ void memxor(uint8_t *out, const uint8_t *in1, const uint8_t *in2, size_t size)
         }
     }
 }
+
+FingerprintType ssh2_pick_fingerprint(
+    char **fingerprints, FingerprintType preferred_type)
+{
+    /*
+     * Keys are either SSH-2, in which case we have all fingerprint
+     * types, or SSH-1, in which case we have only MD5. So we return
+     * the default type if we can, or MD5 if that's all we have; no
+     * need for a fully general preference-list system.
+     */
+    FingerprintType fptype = fingerprints[preferred_type] ?
+        preferred_type : SSH_FPTYPE_MD5;
+    assert(fingerprints[fptype]);
+    return fptype;
+}
+
+FingerprintType ssh2_pick_default_fingerprint(char **fingerprints)
+{
+    return ssh2_pick_fingerprint(fingerprints, SSH_FPTYPE_DEFAULT);
+}