diff --git a/ssh.h b/ssh.h
index db694c58..139ea33f 100644
--- a/ssh.h
+++ b/ssh.h
@@ -353,8 +353,10 @@ struct ssh_mac {
 
 struct ssh_hash {
     void *(*init)(void); /* also allocates context */
+    void *(*copy)(const void *);
     void (*bytes)(void *, const void *, int);
     void (*final)(void *, unsigned char *); /* also frees context */
+    void (*free)(void *);
     int hlen; /* output length in bytes */
     const char *text_name;
 };   
diff --git a/sshsh256.c b/sshsh256.c
index 8e3d4fc1..b2bd862e 100644
--- a/sshsh256.c
+++ b/sshsh256.c
@@ -200,6 +200,24 @@ static void *sha256_init(void)
     return s;
 }
 
+static void *sha256_copy(const void *vold)
+{
+    const SHA256_State *old = (const SHA256_State *)vold;
+    SHA256_State *s;
+
+    s = snew(SHA256_State);
+    *s = *old;
+    return s;
+}
+
+static void sha256_free(void *handle)
+{
+    SHA256_State *s = handle;
+
+    smemclr(s, sizeof(*s));
+    sfree(s);
+}
+
 static void sha256_bytes(void *handle, const void *p, int len)
 {
     SHA256_State *s = handle;
@@ -212,12 +230,12 @@ static void sha256_final(void *handle, unsigned char *output)
     SHA256_State *s = handle;
 
     SHA256_Final(s, output);
-    smemclr(s, sizeof(*s));
-    sfree(s);
+    sha256_free(s);
 }
 
 const struct ssh_hash ssh_sha256 = {
-    sha256_init, sha256_bytes, sha256_final, 32, "SHA-256"
+    sha256_init, sha256_copy, sha256_bytes, sha256_final, sha256_free,
+    32, "SHA-256"
 };
 
 /* ----------------------------------------------------------------------
diff --git a/sshsh512.c b/sshsh512.c
index f0f13510..bdfc1e9d 100644
--- a/sshsh512.c
+++ b/sshsh512.c
@@ -331,6 +331,24 @@ static void *sha512_init(void)
     return s;
 }
 
+static void *sha512_copy(const void *vold)
+{
+    const SHA512_State *old = (const SHA512_State *)vold;
+    SHA512_State *s;
+
+    s = snew(SHA512_State);
+    *s = *old;
+    return s;
+}
+
+static void sha512_free(void *handle)
+{
+    SHA512_State *s = handle;
+
+    smemclr(s, sizeof(*s));
+    sfree(s);
+}
+
 static void sha512_bytes(void *handle, const void *p, int len)
 {
     SHA512_State *s = handle;
@@ -343,12 +361,12 @@ static void sha512_final(void *handle, unsigned char *output)
     SHA512_State *s = handle;
 
     SHA512_Final(s, output);
-    smemclr(s, sizeof(*s));
-    sfree(s);
+    sha512_free(s);
 }
 
 const struct ssh_hash ssh_sha512 = {
-    sha512_init, sha512_bytes, sha512_final, 64, "SHA-512"
+    sha512_init, sha512_copy, sha512_bytes, sha512_final, sha512_free,
+    64, "SHA-512"
 };
 
 static void *sha384_init(void)
@@ -370,7 +388,8 @@ static void sha384_final(void *handle, unsigned char *output)
 }
 
 const struct ssh_hash ssh_sha384 = {
-    sha384_init, sha512_bytes, sha384_final, 48, "SHA-384"
+    sha384_init, sha512_copy, sha512_bytes, sha384_final, sha512_free,
+    48, "SHA-384"
 };
 
 #ifdef TEST
diff --git a/sshsha.c b/sshsha.c
index 2b2eead1..07a71ca2 100644
--- a/sshsha.c
+++ b/sshsha.c
@@ -230,6 +230,24 @@ static void *sha1_init(void)
     return s;
 }
 
+static void *sha1_copy(const void *vold)
+{
+    const SHA_State *old = (const SHA_State *)vold;
+    SHA_State *s;
+
+    s = snew(SHA_State);
+    *s = *old;
+    return s;
+}
+
+static void sha1_free(void *handle)
+{
+    SHA_State *s = handle;
+
+    smemclr(s, sizeof(*s));
+    sfree(s);
+}
+
 static void sha1_bytes(void *handle, const void *p, int len)
 {
     SHA_State *s = handle;
@@ -242,12 +260,11 @@ static void sha1_final(void *handle, unsigned char *output)
     SHA_State *s = handle;
 
     SHA_Final(s, output);
-    smemclr(s, sizeof(*s));
-    sfree(s);
+    sha1_free(s);
 }
 
 const struct ssh_hash ssh_sha1 = {
-    sha1_init, sha1_bytes, sha1_final, 20, "SHA-1"
+    sha1_init, sha1_copy, sha1_bytes, sha1_final, sha1_free, 20, "SHA-1"
 };
 
 /* ----------------------------------------------------------------------