diff --git a/ssh.h b/ssh.h
index 02c4a1ae..6dbe31bf 100644
--- a/ssh.h
+++ b/ssh.h
@@ -535,11 +535,10 @@ struct ssh_rsa_kex_extra {
 struct RSAKey *ssh_rsakex_newkey(ptrlen data);
 void ssh_rsakex_freekey(struct RSAKey *key);
 int ssh_rsakex_klen(struct RSAKey *key);
-void ssh_rsakex_encrypt(const struct ssh_hashalg *h,
-                        unsigned char *in, int inlen,
-                        unsigned char *out, int outlen, struct RSAKey *key);
-mp_int *ssh_rsakex_decrypt(const struct ssh_hashalg *h, ptrlen ciphertext,
-                              struct RSAKey *rsa);
+strbuf *ssh_rsakex_encrypt(
+    struct RSAKey *key, const struct ssh_hashalg *h, ptrlen plaintext);
+mp_int *ssh_rsakex_decrypt(
+    struct RSAKey *key, const struct ssh_hashalg *h, ptrlen ciphertext);
 
 /*
  * SSH2 ECDH key exchange functions
diff --git a/ssh2kex-client.c b/ssh2kex-client.c
index f2e5dc13..15ab9eb7 100644
--- a/ssh2kex-client.c
+++ b/ssh2kex-client.c
@@ -557,9 +557,7 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
             int klen = ssh_rsakex_klen(s->rsa_kex_key);
             int nbits = klen - (2*s->kex_alg->hash->hlen*8 + 49);
             int i, byte = 0;
-            strbuf *buf;
-            unsigned char *outstr;
-            int outstrlen;
+            strbuf *buf, *outstr;
 
             s->K = mp_power_2(nbits - 1);
 
@@ -579,22 +577,19 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
             /*
              * Encrypt it with the given RSA key.
              */
-            outstrlen = (klen + 7) / 8;
-            outstr = snewn(outstrlen, unsigned char);
-            ssh_rsakex_encrypt(s->kex_alg->hash, buf->u, buf->len,
-                               outstr, outstrlen, s->rsa_kex_key);
+            outstr = ssh_rsakex_encrypt(s->rsa_kex_key, s->kex_alg->hash,
+                                        ptrlen_from_strbuf(buf));
 
             /*
              * And send it off in a return packet.
              */
             pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH2_MSG_KEXRSA_SECRET);
-            put_string(pktout, outstr, outstrlen);
+            put_stringpl(pktout, ptrlen_from_strbuf(outstr));
             pq_push(s->ppl.out_pq, pktout);
 
-            put_string(s->exhash, outstr, outstrlen);
+            put_stringsb(s->exhash, outstr); /* frees outstr */
 
             strbuf_free(buf);
-            sfree(outstr);
         }
 
         ssh_rsakex_freekey(s->rsa_kex_key);
diff --git a/ssh2kex-server.c b/ssh2kex-server.c
index a5c3fb5c..5d8cd410 100644
--- a/ssh2kex-server.c
+++ b/ssh2kex-server.c
@@ -279,7 +279,7 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
             ptrlen encrypted_secret = get_string(pktin);
             put_stringpl(s->exhash, encrypted_secret);
             s->K = ssh_rsakex_decrypt(
-                s->kex_alg->hash, encrypted_secret, s->rsa_kex_key);
+                s->rsa_kex_key, s->kex_alg->hash, encrypted_secret);
         }
 
         if (!s->K) {
diff --git a/sshrsa.c b/sshrsa.c
index 8c768646..a1abcfae 100644
--- a/sshrsa.c
+++ b/sshrsa.c
@@ -743,9 +743,8 @@ static void oaep_mask(const struct ssh_hashalg *h, void *seed, int seedlen,
     }
 }
 
-void ssh_rsakex_encrypt(const struct ssh_hashalg *h,
-                        unsigned char *in, int inlen,
-                        unsigned char *out, int outlen, struct RSAKey *rsa)
+strbuf *ssh_rsakex_encrypt(
+    struct RSAKey *rsa, const struct ssh_hashalg *h, ptrlen in)
 {
     mp_int *b1, *b2;
     int k, i;
@@ -783,10 +782,12 @@ void ssh_rsakex_encrypt(const struct ssh_hashalg *h,
     k = (7 + mp_get_nbits(rsa->modulus)) / 8;
 
     /* The length of the input data must be at most k - 2hLen - 2. */
-    assert(inlen > 0 && inlen <= k - 2*HLEN - 2);
+    assert(in.len > 0 && in.len <= k - 2*HLEN - 2);
 
     /* The length of the output data wants to be precisely k. */
-    assert(outlen == k);
+    strbuf *toret = strbuf_new();
+    int outlen = k;
+    unsigned char *out = strbuf_append(toret, outlen);
 
     /*
      * Now perform EME-OAEP encoding. First set up all the unmasked
@@ -806,8 +807,8 @@ void ssh_rsakex_encrypt(const struct ssh_hashalg *h,
     /* A bunch of zero octets */
     memset(out + 2*HLEN + 1, 0, outlen - (2*HLEN + 1));
     /* A single 1 octet, followed by the input message data. */
-    out[outlen - inlen - 1] = 1;
-    memcpy(out + outlen - inlen, in, inlen);
+    out[outlen - in.len - 1] = 1;
+    memcpy(out + outlen - in.len, in.ptr, in.len);
 
     /*
      * Now use the seed data to mask the block DB.
@@ -835,10 +836,11 @@ void ssh_rsakex_encrypt(const struct ssh_hashalg *h,
     /*
      * And we're done.
      */
+    return toret;
 }
 
-mp_int *ssh_rsakex_decrypt(const struct ssh_hashalg *h, ptrlen ciphertext,
-                              struct RSAKey *rsa)
+mp_int *ssh_rsakex_decrypt(
+    struct RSAKey *rsa, const struct ssh_hashalg *h, ptrlen ciphertext)
 {
     mp_int *b1, *b2;
     int outlen, i;